\r\n\tTopics covered include but are not limited to: Hydrologic Cycle (Precipitation, Runoff, Infiltration and their Measurement, Land surface interaction); Hydrologic Analysis (Hydrograph, Wave routing, Hydrologic statistics, Frequency Analysis); Applied Hydrology (Applications in Engineering, Sciences and Agriculture, Design storms, Risk analysis, Case studies); Computational Hydrology (Numerical modeling, Hydrologic modeling and forecasting, Flow visualization, Model validation, Parameter estimation); Interdisciplinary Hydrology (Hydrometeorology, Impact of Climate Change, Precipitation data analysis, Mathematical concepts, Natural hazards); Radar Hydrology (Precipitation estimation techniques, Promise and Challenges in Radar technology, Uncertainty in radar precipitation estimates).
\r\n
\r\n\tThe contents covered in this book will serve as a valuable reference guide to students, researchers, government agencies and practicing engineers who work in hydrology and related areas. We hope that this book will open new directions in basic and applied research in hydrological science.
",isbn:"978-1-83962-330-1",printIsbn:"978-1-83962-329-5",pdfIsbn:"978-1-83962-331-8",doi:null,price:0,priceEur:0,priceUsd:0,slug:null,numberOfPages:0,isOpenForSubmission:!1,hash:"02925c63436d12e839008c793a253310",bookSignature:"Dr. Theodore Hromadka and Dr. Prasada Rao",publishedDate:null,coverURL:"https://cdn.intechopen.com/books/images_new/9864.jpg",keywords:"Runoff, Land Surface Interaction, Hydrograph, Wave Routing, Design Storms, Risk Analysis, Numerical Modeling, Hydrologic Modeling and Forecasting, Flow Visualization, Hydrometeorology, Precipitation Data Analysis, Radar Hydrology",numberOfDownloads:293,numberOfWosCitations:0,numberOfCrossrefCitations:0,numberOfDimensionsCitations:0,numberOfTotalCitations:0,isAvailableForWebshopOrdering:!0,dateEndFirstStepPublish:"June 8th 2020",dateEndSecondStepPublish:"September 11th 2020",dateEndThirdStepPublish:"November 10th 2020",dateEndFourthStepPublish:"January 29th 2021",dateEndFifthStepPublish:"March 30th 2021",remainingDaysToSecondStep:"4 months",secondStepPassed:!0,currentStepOfPublishingProcess:4,editedByType:null,kuFlag:!1,biosketch:"Principal and founder of Hromadka & Associates, professor United States Military Academy, Professor Emeritus at the California State University, and a Member of Board of Directors and an Adjunct Professor at Wessex Institute of Technology.",coeditorOneBiosketch:"Prasada Rao, Ph.D. is a professor in the Civil and Environmental Engineering Department at California State University, Fullerton. His current research areas relate to Climate Change, Surface and Subsurface flow modeling, and Computational Mathematics. He is also the Associate Director for International Institute for Computational Engineering Mathematics.\r\nCo1 - Biosketch",coeditorTwoBiosketch:null,coeditorThreeBiosketch:null,coeditorFourBiosketch:null,coeditorFiveBiosketch:null,editors:[{id:"181008",title:"Dr.",name:"Theodore",middleName:null,surname:"Hromadka",slug:"theodore-hromadka",fullName:"Theodore Hromadka",profilePictureURL:"https://mts.intechopen.com/storage/users/181008/images/system/181008.jpg",biography:"Hromadka & Associates’ Principal and Founder, Theodore Hromadka II, PhD, PhD, PhD, PH, PE, has extensive scientific, engineering, expert witness, and litigation support experience. 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He has worked extensively on developing innovative, hydraulic and hydrological modelling solutions to better predict surface flow phenomena along with its impact on groundwater levels. He has also worked on developing parallel hydraulic models for large scale applications. He has taught undergraduate and graduate level courses in hydraulics, hydrology, open channel flow, and hydraulic structures.",institutionString:"California State University, Fullerton",position:null,outsideEditionCount:0,totalCites:0,totalAuthoredChapters:"2",totalChapterViews:"0",totalEditedBooks:"0",institution:{name:"California State University, Fullerton",institutionURL:null,country:{name:"United States of America"}}},coeditorTwo:null,coeditorThree:null,coeditorFour:null,coeditorFive:null,topics:[{id:"10",title:"Earth and Planetary Sciences",slug:"earth-and-planetary-sciences"}],chapters:[{id:"74366",title:"Perspectives of Hydrologic Modeling in Agricultural Research",slug:"perspectives-of-hydrologic-modeling-in-agricultural-research",totalDownloads:7,totalCrossrefCites:0,authors:[null]},{id:"74795",title:"Ecohydrology: An Integrative Sustainability Science",slug:"ecohydrology-an-integrative-sustainability-science",totalDownloads:2,totalCrossrefCites:null,authors:[null]},{id:"74255",title:"Hydrometeorology: Review of Past, Present and Future Observation Methods",slug:"hydrometeorology-review-of-past-present-and-future-observation-methods",totalDownloads:63,totalCrossrefCites:0,authors:[null]},{id:"73259",title:"Statistical Analysis of the Precipitation Isotope Data with Reference to the Indian Subcontinent",slug:"statistical-analysis-of-the-precipitation-isotope-data-with-reference-to-the-indian-subcontinent",totalDownloads:92,totalCrossrefCites:0,authors:[null]},{id:"73550",title:"Interlinking of River: Issues and Challenges",slug:"interlinking-of-river-issues-and-challenges",totalDownloads:54,totalCrossrefCites:0,authors:[null]},{id:"73830",title:"Examination of Hydrologic Computer Programs DHM and EDHM",slug:"examination-of-hydrologic-computer-programs-dhm-and-edhm",totalDownloads:78,totalCrossrefCites:0,authors:[{id:"181008",title:"Dr.",name:"Theodore",surname:"Hromadka",slug:"theodore-hromadka",fullName:"Theodore Hromadka"},{id:"192274",title:"Dr.",name:"Prasada",surname:"Rao",slug:"prasada-rao",fullName:"Prasada Rao"}]}],productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"},personalPublishingAssistant:{id:"247865",firstName:"Jasna",lastName:"Bozic",middleName:null,title:"Ms.",imageUrl:"https://mts.intechopen.com/storage/users/247865/images/7225_n.jpg",email:"jasna.b@intechopen.com",biography:"As an Author Service Manager, my responsibilities include monitoring and facilitating all publishing activities for authors and editors. From chapter submission and review to approval and revision, copyediting and design, until final publication, I work closely with authors and editors to ensure a simple and easy publishing process. I maintain constant and effective communication with authors, editors and reviewers, which allows for a level of personal support that enables contributors to fully commit and concentrate on the chapters they are writing, editing, or reviewing. I assist authors in the preparation of their full chapter submissions and track important deadlines and ensure they are met. I help to coordinate internal processes such as linguistic review, and monitor the technical aspects of the process. As an ASM I am also involved in the acquisition of editors. 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\n
1. Volume Bragg gratings
\n
\n
1.1. Description and properties
\n
Volume grating as its name suggest is a grating that occupies the volume of a medium. Typically, for such gratings the term volume Bragg gratings (VBGs) is used in relation to Sir William Bragg who in 1915 used diffraction of light propagating through a crystal to determine the crystal’s lattice structure [1]. What he found was that at certain conditions the light is strongly diffracted by the crystal. Such condition is called “resonant condition” or also “Bragg condition.” Here is also the place to make the distinction between surface and volume Bragg gratings. If we start with a surface grating and start increasing its thickness at some point the different diffraction orders will reduce to a moment where there will be only one order. This defines the transition to a volume grating behavior [2].
\n
There are two basic types of VBGs which is shown in \nFigure 1\n. The first one is a transmission Bragg grating (TBG) for which if the incident light satisfies the Bragg condition it is not transmitted but also diffracted. The second type is a reflection Bragg grating (RBG) which behaves like a mirror for incoming light that matches the Bragg condition.
\n
Figure 1.
Beam geometries for transmission Bragg grating (a) and for reflective Bragg grating (b).
\n
For simplicity, in \nFigure 1\n, for both types of volume gratings, the angle of incidence is the same θ\n\ni\n and the tilt of the volume grating inside the medium is θ\ntilt. Resonant diffraction from each of these VBGs occurs upon satisfaction of their Bragg conditions which are defined by Eqs. (1) and (2).
Here, Λ is the period of the VBG and λ\nVBG is the wavelength of the incident light which for the particular θ\n\ni\n and θ\ntilt satisfies the Bragg condition. The gratings depicted in \nFigure 1\n are uniform VBGs that can be recorded in a photosensitive material by simple interference of two collimated laser beams. The recording wavelength, angle of interference, and the refractive index of the material determine the grating’s parameters. There are techniques capable of recording more complex volume gratings which have nonuniform period and for which the Bragg condition will be different depending on the space coordinates [2]. Regardless, if the variation of the period is negligible when compared to the probe beam size used for characterization of the VBG; Eqs. (1) and (2) can still provide the resonance wavelength.
\n
\n\nFigure 2\n exhibits one example of beam geometry for recording uniform transmitting and reflecting volume gratings. In this recording approach, two plane waves (purple beams) illuminate the sample from one side at a half-angle of interference φ. Depending on the direction from which the grating is used, it can either work as a TBG (green beams) or as an RBG (orange beams). Before recording, the grating parameters such as period and modulation need to be calculated so the recording is carried out accordingly.
\n
Figure 2.
Recording geometry used for recording reflective and transmitting Bragg gratings. The recording beams are shown in purple and the half-angle of interference φ determines the properties of the grating. The VBG is utilized as an RBG if probed as the orange beams or as TBG if probed as the green beams.
\n
The main model describing volume Bragg gratings was introduced by Kogelnik [3] in 1969. His model describes that diffraction from a VBG is based on coupled wave theory (CWT) and provides analytical solutions for RBGs and TBGs including tilted ones. Figures 3(a, b) and \n4(a, b) present examples of the wavelength and angular responses of an RBG and a TBG correspondingly, calculated using Kogelnik’s theoretical approach.
\n
Figure 3.
Wavelength (a) and angular (b) response for an RBG. The VBG is 5.5 mm thick, it is 20° tilted, and has a 240 ppm refractive index modulation.
\n
Figure 4.
Wavelength (a) and angular (b) response for a TBG. The VBG is 1.5 mm thick, it is 20° tilted, and has a 333 ppm refractive index modulation.
\n\n
\n\nFigures 3\n and \n4\n give good overview of the main properties of TBGs and RBGs. In particular, RBGs are much more suited for implementation as narrow wavelength filters. For example, the full-width half-maximum (FWHM) wavelength selectivity of the reflective grating simulated in \nFigure 3(a)\n is around 225 pm but it can reach down to 15–20 pm if designed accordingly. TBGs, in contrast, have much wider wavelength acceptance starting at a few hundred picometers and reaching several nanometers. \nFigure 4(a)\n shows the spectral response of 1.5 mm thick TBG with nodulation of 330 ppm. For these parameters, its FWHM of 2.3 nm is close to an order magnitude larger if compared to the RBG one.
\n
TBGs, alternatively, can be used as narrow angular filters with acceptance values as low as 0.1 mrad, whereas RBGs have typical angular acceptance of more than 10 mrad all the way to 100 mrad. These properties of the two types of VBGs define their use in different applications. For example, the narrow angular selectivity of the TBGs makes them a great angular filter that can be used to suppress higher order modes generation in laser cavities while keeping them very compact [4]. Alternatively, RBGs with their narrow wavelength selectivity can be used for narrow wavelength beam combining where the diffracted by and the transmitted through an RBG beams can be separated by only a few hundred picometers [5]. Regardless of the close wavelength separation, the RBG does not diffract the transmitted beam even though both beams have a common propagating direction.
\n
\n\nTable 1\n summarizes the TBG’s and RBG’s characteristics and their typical range. Until now, we have discussed wavelength and angular selectivity of VBGs but the third very important parameter is the VBG’s efficiency. There are two generally accepted ways to define a VBG’s efficiency and the more widely used on is the so-called “relative diffraction efficiency.” It is defined as normalization of the diffracted to the transmitted by a VBG power. Its advantage is that it removes any losses introduced by the medium. The second way is called “absolute diffraction efficiency,” where the diffracted power is normalized to the incident power.
\n
\n
\n
\n
\n\n
\n
\n
Transmitting VBG (TBG)
\n
Reflecting VBG (RBG)
\n
\n\n\n
\n
Angular selectivity
\n
0.1–10 mrad
\n
10–100 mrad
\n
\n
\n
Spectral selectivity
\n
0.3–20 nm
\n
0.02–2 nm
\n
\n
\n
Diffraction efficiency
\n
Up to 100%
\n
>99.9%
\n
\n\n
Table 1.
TBG and RBG characteristics.
\n
\n
\n
1.2. Recording materials
\n
Recoding of a volume grating requires the use of a material that is photosensitive. The modulation of the recording light intensity should create a corresponding refractive index change in the recording material which on a macroscopic level will be in fact the volume Bragg grating. There is a wide variety of photosensitive materials that can be used for recording VBGs [6, 7]. The main requirement that they need to fulfill is to have enough spatial resolution that will allow the recording of gratings with particular periods. The other two factors are the photosensitivity of the material and its dynamic range. The photosensitivity determines the exposure duration and given that VBGs are most commonly recorded by interference of light, it is of great benefit to keep the exposure time as short as possible. The material’s dynamic range provides the maximum refractive index change that can be achieved. This property affects the VBGs thickness and the maximum number of volume gratings that can be multiplexed together in the same volume. Other properties that depending on the particular application may be important are the optical damage threshold, the maximum physical dimensions of the material, its losses, its environmental sensitivity, and others.
\n
The most common recording materials are dichromated gelatins, photopolymers, photorefractive crystals, photosensitive fibers, and photothermo refractive glasses. We will not discuss in detail the properties of each of these materials because they have been well investigated in the literature [8–16]. The applications and experimental results shown further in the chapter are based on using photothermo-refractive glass (PTR) due to its capabilities of handling high-power laser radiation because of its low losses, its environmental robustness, and extremely high resolution [14–16].
\n
Photothermo-refractive glass is a relatively new photosensitive material well suited for phase hologram recording. It combines high sensitivity achieved by two-step hologram formation process and high-optical quality resulting from its technological development. The PTR glass is a Na2O-ZnO-Al2O3-SiO2 glass doped with silver (Ag), cerium (Ce), and fluorine (F). It is transparent from 350 to 2500 nm. The chain of processes, which produce refractive index variation, is as follows: the first step is the exposure of the glass to UV radiation, somewhere in the range from 280 to 350 nm. This exposure results in photoreduction of silver ions Ag+ to atomic state Ag0. This stage is similar to formation of a latent image in a conventional photo film because no significant changes in the optical properties of the glass occur. The final formation of the holographic recording is secured by subjecting the glass to thermal development. During this step, at elevated temperatures, a number of silver containing clusters are formed in the exposed regions of the glass due to the increased mobility of Ag0 atoms. These silver-containing clusters serve as nucleation centers for the growth of NaF crystals. Interaction of these nanocrystals with the surrounding glass matrix causes the decrease of refractive index. Refractive index change Δn of about 1.5 × 10−3 (1500 ppm) can be achieved and is enough to allow the recoding of high-efficiency hologram into glass wafers with thickness exceeding several hundred microns.
\n
The second consequence of the crystalline phase precipitation in PTR glass is related to its physical properties and is extremely valuable. The NaF crystalline particles in the glass matrix are almost impossible to destroy by any type of radiation which makes PTR holograms stable under exposure to IR, visible, UV, X-ray, and gamma-ray irradiation. For example, laser damage threshold for 8 ns laser pulses at 1064 nm is in the range of 40 J/cm2. Also, the nonlinear refractive index of PTR glass is the same as that for fused silica which allows the use of PTR diffractive elements in all types of pulsed lasers. Another PTR advantage is its very low losses—on the order of 10−5 cm−1. Testing of VBG recorded in the PTR glass performed under irradiation of 9 kW CW with a 6-mm-diameter spot showed heating that did not exceed 15 K [17]. Even though small heating effects lead to thermal variations of the refractive index of the glass (dn/dt = 5 × 10−8 K−1). In the case of Bragg grating written inside a PTR glass, this feature leads to thermal shift of the Bragg wavelength of around 10 pm/K. It is worth mentioning also that due to the melting temperature of the NaF crystals being almost 1000°C, PTR holograms are stable at elevated temperatures and could tolerate thermal cycling up to 400°C. This temperature is determined by the plasticity point of the glass matrix.
\n
Typically, Bragg gratings in the PTR glass are recorded by an exposure to interference pattern of radiation from a He-Cd laser operating at 325 nm. The spatial frequency of the gratings can vary from 50 up to about 10,000 mm−1, their thickness from 0.5 to 25 mm, and a diffraction efficiency of up to 99.9%.
\n
\n
\n
\n
2. Applications of volume Bragg gratings
\n
\n
2.1. Spectral and coherent laser beam combining by volume Bragg gratings
\n
Single laser sources are limited in terms of maximum power by thermal and nonlinear effects and can achieve no more than few kW. Laser systems that can generate from 10 to 100 kW CW power integrate from several to tens of laser sources. There are several approaches for integrating/combining laser beams but the most common ones are using either volume Bragg gratings, diffractive optical elements, or surface diffraction gratings [18]. This chapter discusses the use of VBGs for laser beam combining including spectral and coherent combining.
\n
Spectral beam combining (SBC) and coherent beam combining (CBC) are two complimentary methods leading toward multi-kilowatt diffraction limited laser sources. In SBC, multiple channels of different wavelengths are superimposed spatially to generate a single output beam. The main advantage of SBC if compared to CBC is the simplified optical setup due to the fact that there is no need to monitor and adjust the phase of the individual beams. The drawback of using SBC is the fact that the spectrum of the combined beam is much broader when compared to the individual input beams. Regardless of this, the final combined output could still have diffraction limited quality. To minimize the wider spectrum issue, it is necessary to use very narrow spectrally selective beam combining elements that will deliver an output beam with minimum spectral bandwidth.
\n
Reflective volume Bragg gratings are holograms that are not angularly dispersive and depending on their design they can be made to be very wavelength selective. Also, they can have diffraction efficiencies close to 100%, and if recorded in a suitable material that can have very low losses, which makes them suitable for high power laser applications. All these facts together make them a very good optical element for implementation in the SBC system. Figure 5 demonstrates the concept for using a VBG for spectral combining: diffraction efficiency is close to 100% when the Bragg condition is met and is close to 0% at wavelengths shifted away from the Bragg condition and corresponding to the grating’s minima in its characteristic curve.
\n
Figure 5.
Spectral dependence of the diffraction efficiency of an RBG.
\n
In the example shown in \nFigure 6\n, two beams with shifted wavelengths are brought to interact with a reflective VBG with characteristic efficiency versus wavelength curve shown in Figure 5. This VBG reflects wavelength λ1 when it satisfies the Bragg condition at a given angle but transmits wavelength λ2 with minimal losses if it matches with one the VBG’s minima (e.g., the forth one). In this way, the diffracted beam λ1 and the transmitted beam λ2 can emerge overlapped and collinear. When using reflective VBGs for spectral beam combining, it is imperative to ensure as high as possible diffraction efficiency for the diffracted beam and minimal diffraction efficiency for the transmitted beam.
\n
Figure 6.
Schematic description of two-beam spectral combining setup using RBG as a combining element.
\n
This approach for SBC can be extended where several VBGs are used to combine more than two laser beams. Such system was presented in [5] and demonstrated the combining of five lasers, each generating 150 W CW to give a total combined power of 750 W (\nFigure 7\n). Spectrally, the beams were 250 pm apart so the combined spectrum had width of 1 nm in total. For many applications, it is not only the total power that is of significance but also to final beam quality. In this particular example, the M\n2 of the combined beam was 1.6 and the beam combining efficiency was greater than 90%. The VBGs were recorded in the PTR glass which, as already mentioned, possesses very low intrinsic losses and therefore can handle high power fluxes with minimal light being absorbed and converted to heat. Regardless, the authors had to implement a thermal tuning and compensation scheme in order to control the resonant conditions of each grating and to manage thermal distortions. As shown in \nFigure 7\n, the five-beam combining system is quite complex and scaling it to a larger number of channels will scale the mechanical and also thermal management complexity. In addition, the footprint of the systems will also be quite substantial and will make it impractical for use out of laboratory environment.
\n
Figure 7.
Five -beam combining setup using RBGs as combining elements [32].
\n
An approach where a single diffracting optical element that is capable of diffracting several beams simultaneously and substitutes several single beam reflecting elements can reduce the complexity and the space that the beam combining system occupies. Such element can be a computer-generated DOE or one consisting of several mutually aligned VBGs that occupy the same volume. We will discuss in detail the latter where several reflective VBGs are multiplexed such that each laser channel is redirected into a single, common output.
\n
\n\nFigure 8\n shows the design of a spectral beam combining system capable of combining five laser beams by using a multiplexed VBG (MVBG) element. The MVBG contains four volume Bragg gratins that reflect four beams with different wavelengths correspondingly while transmitting the fifth beam which is out of resonance with any of the four multiplexed VBGs. This system is fully analogous to the one shown in \nFigure 7\n but with the benefits of being more compact and simpler to align. The feasibility of the approach was proven and demonstrated in [20] where a double-multiplexed VBG recorded in the PTR glass was used to combine three laser beams. As a first step, the authors combined the two reflected by the MVBG beams to realize a total power of 282 W with combining efficiency of 99%. The M\n2 of the combined beam was very close to the lasers’ original M\n2 of 1.05 and was measured to be 1.15 in the ‘X’ direction and 1.08 in the ‘Y’ direction. The MVBG was kept at constant temperature by a placing it in a copper housing in which thermo-electric elements attached to it. This approach allowed keeping the MVBG into resonance with the lasers in the case of heating due to absorption occurred. Heating leads to expansion of the glass and therefore to change of the gratings’ periods which, on its own, leads to the lasers falling out of resonance with their corresponding VBGs. At the power density of approximately 3 kW/cm2, the authors did not observe any heating problems and therefore no beam quality degradation. Next, the third beam, in this case the transmitted one, was added to the system to achieve a total combined power of 420 W. While the final three beam combining efficiency of 96.5 % was still very high, the total beam quality parameter got worse and reached 1.38 in the ‘X’ and 1.20 in the ‘Y’ directions. The worse M\n2 was due to heating of the glass introduced by the transmitted beam. Such thermal effect was not observed when combining only reflected beams because they penetrated the MVBG significantly less and therefore much less of their power was absorbed and dissipated as heat into the glass. Using better cooling techniques such as surface air-flow can eliminate the thermal effect and the resulting beam quality degradation observed [17]. In conclusion, the use of multiplexed volume Bragg gratings for spectral beam combining is excellent alternative and addition to the current state of the art combining techniques. The capability of reducing the number of combining elements in the system while being able to manage the thermal load is especially valuable especially when combining kilowatt level laser sources.
\n
Figure 8.
Five-beam combining setup using a single multiplexed VBG as combining element [19].
\n
Volume Bragg gratings can be used for coherent beam combining (CBC) as well [21]. In coherent combining, the lasers are phased to emit coherently at the same wavelength and in phase. Depending on how the phase-locking of the lasers is achieved, CBC can be either passive or active. In the active case, the phase of each of the lasers that are being combined is controlled with high precision using feedback loop. This drastically complicates the whole system from optical and electronics perspective. In the passive approach, the sources share a common resonator and due to this they emit coherently without the need for external phase control. Such system is very simple and compact. Volume Bragg gratings and especially multiplexed ones are the ideal option for use in passive CBC because a system implementing such MVBG will have a single coherently combined output beam. The MVBG is used as 1:N splitter and combiner and it is important that there is equal radiation exchange between each laser.
\n
Such approach was used by [22] where two lasers were coherently combined using a double MVBG. Two identical reflecting VBGs were symmetrically recorded in the PTR glass in such a way that they have a degenerate output. \nFigure 9\n presents the optical setup of the two-channel system. The output of each of the fiber lasers is reflected in the common/degenerate direction toward the output coupler (OC). The part of the combined output reflected by the OC is split by the MVBG for feedback to the two lasers. Small part of the emission that leaks through the MVBG is used to confirm the mutual coherence of the two lasers by interfering the two lasers. Using this scheme for CBC, the authors reported a combining efficiency of more than 90%, slope efficiency of almost 50%, and fringe contrast of 96%, which indicates a significant degree of coherence between the two channels. A similar scheme can be realized using multiplexed transmitting Bragg grating as well.
\n
Figure 9.
Coherent beam combining setup using double RBG as a combining element [22].
\n
The design of the multiplexed VBG is quite important in order for the systems as whole to operate with high combining and slope efficiencies. For example, losses due to lower than 99% diffraction efficiency of the gratings lead to losses in both the combining and the splitting processes in a given resonator round trip.
\n
In conclusion, an approach for passive coherent beam combining capable of delivering from kW to tens of kW narrow-linewidth laser power is highly desired and sought after. Passive CBC using multiplexed volume Bragg gratings is a very promising technique that can deliver such power levels with great efficiency and minimal system complexity.
\n
\n
\n
2.2. Holographic phase masks and their applications
\n
Phase masks have found numerous applications in areas such as beam shaping, laser mode conversion, encryption, and others. The two most common ways to make phase masks are either by using a contoured surface where the path length for different parts of the beam is different or by using a bulk material inside which localized refractive index changes are made. In both cases, the phase shift is done by changing the local optical propagation path and therefore the phase masks are limited to use at a specific wavelength. This limits substantially their potential for use in different applications. The solution will be to make an achromatic phase mask and some attempts have been performed in the past [23–25]. For example, multiplexing many computer-generated holograms is one approach where arbitrary wavefronts can be generated if the mask is illuminated with a suitable beam. For this approach to work, a separate hologram needs to be recorded for every desired wavelength which makes the method quite complicated. A simpler and more flexible approach for making achromatic phase masks will be described in this section of the chapter.
\n
The foundation of the approach is to encode phase mask profile into transmitting Bragg grating, which as whole works as a so-called “holographic phase mask” (HPM). This HPM can be implemented for a broad range of wavelengths and for each one of them it produces the desired diffracted phase profile, as long as the Bragg condition of the TBG is met for the given wavelength. As it will be shown, such HPM can be made fully achromatic by introducing two surface diffraction gratings with particular periods, before and after the HPM. In this way, no angular Bragg angle tuning is required. As expected, HPMs utilize the diffraction properties of regular TBGs and therefore they can diffract up to 100% of a beam into a single order. Another specific property of TBGs is that they have relatively narrow angular selectivity and that allows the multiplexing several HPMs into one piece of recording materials, while having little or no cross-talk between them.
\n
To help understand the way HPMs work, it is important to note that a volume Bragg grating is the simplest volume hologram that can diffract different wavelengths without distorting the initial beam profile (as long as they satisfy the Bragg condition) which sets it apart from more complex holograms, which are capable of changing the beam wavefront. Also, this leads to the fact that HPM can be tested with wavelength different or the same as the recording one.
\n
The encoding of a phase profile into a TBG can be carried out by using a holographic setup shown in \nFigure 10\n [26]. In the setup, a standard binary phase mask (see \nFigure 11\n) is placed into one of the arms (object beam) of a two-beam-recording system. It is important to emphasize that the phase mask must have the desired phase transitions for the hologram recording wavelength and not for the reconstructing wavelength. The beams interfere at an angle θ relative to the normal of the sample to create a fringe pattern inside the sample following the equation:
\n
Figure 10.
Recording setup used for encoding phase profile into a TBG [26].
\n
Figure 11.
Binary phase distribution for a four-sector phase mask.
where I is the intensity, \n\n\n\n\n\nk\n⇀\n\n\ni\n\n\n\n is the wavevector for each beam, and φ is the phase change introduced by the phase mask after the object beam has propagated to the recording material. As described in [26], the recorded hologram will have a refractive index profile described by:
where n\n0 is the background refractive index, n\n1 is the refractive index modulation, and \n\n\n\n\nK\n⇀\n\n\n=\n\n\n\nk\n⇀\n\n\n1\n\n−\n\n\n\nk\n→\n\n\n2\n\n\n\n is the grating vector.
\n
By applying coupled wave theory, one can determine the diffracted beam phase profile and diffraction efficiency of a beam satisfying the Bragg condition of an HPM [3]. The model was applied for the binary phase profile encoded in a TBG as the one shown in \nFigure 11\n.
\n
Applying binary π phase shift to a regular TBG along both the x- and the y-axis will determine if there are any orientation-dependent variations in the output beam’s phase profile or diffraction efficiency. The performance of the HPM was simulated for 632 and 975 nm. The parameters of the TBG were the same for both wavelengths just the incident angle was adjusted in order to match the corresponding Bragg condition.
\n
As shown in \nFigure 12(b)\n, the diffraction efficiency of an HPM can reach the one of a uniform TBG illuminated with a plane wave. The observed lower efficiency for smaller probe beam sizes is due to the fact that the parts of the beam that cross the phase discontinuity will not satisfy the Bragg condition and this affects smaller beams more when their area is comparable to the phase discontinuity total area. For larger beams, this effect is negligible and its influence on the diffraction efficiency is very little. \nFigure 12(c)\n shows that the π phase shift is present for all three wavelengths when it is along the x-axis. The slight offset of the phase shift from the origin is due to the propagation of the test beam through the HPM and the diffraction that occurs only in the x-direction. In the case when the phase jump is along the y-axis, as shown in \nFigure 12(d)\n, it is as well present for all wavelengths but the discontinuity is centered exactly at the origin because the test beam does not have a component propagating in the y-direction. In conclusion, HPMs have the wavelength and angular diffraction properties of regular TBGs, while capable of encoding a desired phase profile on to a test beam over its whole bandwidth as long as it satisfies the Bragg condition.
\n
Figure 12.
(a) Schematic representation of beam incident on HPM, (b) diffraction efficiency of an HPM at 1064 nm as a function of beam diameter when a binary phase dislocation is encoded along the x-axis, (c) the diffracted beam phase profile when a binary phase dislocation is encoded along the x-axis for beams of different wavelength, (d) the diffracted beam phase profile when a binary phase dislocation is encoded along the y-axis for beams of different wavelength.
\n
Experimentally, HPMs were fabricated and characterized using PTR glass and some of the results will be discussed here [27]. As a master phase mask was used a four-sector one, as shown in \nFigure 11\n. The mask was designed to give π phase shift for the hologram recording wavelength of 325 nm. For comparison purposes, a regular TBG was also recorded in the same piece of PTR glass by removing the phase mask and rotating the sample. Such multiplication guarantees that both diffractive elements share the same glass volume and therefore any localized glass inhomogeneity will influence both of them in the same way. \nFigure 13\n shows the measured diffraction efficiencies of the HPM and the standard TBG. The results match very well the theoretically predicted small difference in efficiency due to the phase shift areas and also show that HPMs behave as regular TBGs in terms of angular selectivity properties.
\n
Figure 13.
Diffraction efficiency angular spectrum of an HPM and homogenous grating.
\n
HPM, as the one just theoretically discussed, was experimentally realized and used to demonstrate its capabilities as an optical mode converter that can operate at different wavelengths. To show the unique properties of HPMs, the fabricated element was tested with three different wavelengths (632.8, 975, and 1064 nm). A standard binary four-sector phase mask can convert a Gaussian beam to a TEM11 mode if properly aligned to the center of the phase jump boundaries. \nFigure 14\n demonstrates the far-field intensity pattern for a simulated mode conversion through a binary phase mask (a) and the patterns experimentally observed for three different wavelengths after diffraction by the HPM (b–d). The diffracted beam profiles clearly exhibit the four-lobed pattern which confirms the notion that the phase profile imprinted in the HPM is present in the diffracted beam and this fact applies for very broad range of wavelengths. Unlike standard phase masks which can only operate for one predetermined wavelength for which the corresponding phase shift is as required. This example demonstrates the great potential that HPMs possess given that laser and fiber modes are present in almost any optical system and their conversion to other modes is of great interest.
\n
Figure 14.
(a) Simulated far-field profile of a beam after passing through an ideal four-sector binary mask and the diffracted beam from a four-sector HPM at (b) 632.8 nm, (c) 975 nm, and (d) 1064 nm. The sizes shown here are not to scale [27].
\n
We already described that multiplexing of volume Bragg gratings can be used for laser beam combining. The same approach can be applied to HPMs and as a result fabricate an element with unique functionality. The property of the HPMs to do mode conversion is not unique [28, 29], but if this is integrated with the capability of VBGs to do beam combining, an element will be created that can simultaneously convert multiple beams into different modes while combining them to a single beam. For example, fiber lasers that operate at higher order mode are of interest because they are considered to overcome the power limitations of fiber lasers operating at the fundamental mode. Therefore, beam combining several lasers operating at higher order modes into one high-power fundamental mode beam will be very beneficial and of interest as a power scaling approach.
\n
As an example, \nFigure 15\n shows how a double multiplexed HPM can convert individually two TEM11 modes (\nFigure\n\n\n15a\n and \n15b\n) to TEM00 modes (Figure\n\n15c\n and \n15d\n) while also spectrally beam combining the beams into one beam (\nFigure 15e\n) [27]. The lasers were operating at 1061 and 1064 nm and the HPM consisted two four-sector phase masks integrated in two TBGs that had a generate output. The authors attributed the difference between the far-field profiles of the two laser beams after their conversion to different collimations. In the final combined beam, there are wings present but these were credited to the generation of the initial TEM11 modes which was done by a set of HPMs. This brought some alignment challenges as shown in Figure 15(c) and. Nevertheless, it is evident that the integration of VBGs and phase plates could open new optical design spaces in areas such as high-power beam combining, mode multiplexing in communication systems, and others.
\n
Figure 15.
Demonstration of conversion from the TEM11 mode to TEM00 for 1061 and 1064 nm lasers separately (a, c and b, d); a multiplexed four-sector HPM spectrally combines the two beams and converts them to TEM00 (e).
\n
\n
\n
\n
3. Achromatization of HPM with surface diffraction gratings
\n
As discussed, HPMs can successfully imprint their phase pattern as long as the wavelength satisfies the Bragg condition but to achieve this, the HPM needs to be angle tuned which cannot be considered pure achromatization. Such achromatization of HPMs can be accomplished with the concept of pairing the Bragg grating with two surface gratings [30]. According to the grating dispersion equation (Eq. (5)), a surface grating with a given period (Λ\nSG) will diffract normally incident light at an angle (θ) as a function of its wavelength (λ):
Based on coupled wave theory [2], a VBG will diffract light if the Bragg condition (Eq. (6)) is met and can reach diffraction efficiencies as high as ≈100% [3]:
Since both of these diffraction angles are dependent on the corresponding grating periods, if the surface grating period is double the period of the volume Bragg grating (Eq. (7)), then any first-order diffraction by normally incident light will be at the corresponding Bragg condition of the volume Bragg grating and that will hold for any wavelength [30]:
Therefore, a surface grating with twice the period of a TBG can make different wavelengths get diffracted by the TBG at the same time as long as they have the same incident angle. In order to recollimate the diffracted beams, an identical surface grating needs to be added in a mirror orientation to the transmitting volume Bragg grating, as shown in \nFigure 16\n. This grating completely cancels out the dispersion of the first surface grating and recollimates the outgoing beam. Applying this concept to an HPM will eliminate the need for angle tuning in order to meet the Bragg condition for different wavelengths, making, therefore, the device a fully achromatic phase element.
\n
Figure 16.
Concept of using surface grating pairs to meet the Bragg condition for various wavelengths regardless of angle tuning [30].
\n
The experimental proof was carried out by using two surface gratings with a grove spacing of 150 lines/mm (a period of 6.66 µm) aligned to an HPM with a period of 3.4µm in setup shown in \nFigure 17\n [31]. The goal of the experiment was to achieve successful broadband mode conversion from a Gaussian to a TEM11 mode without the need to angularly tune the HPM. Three different TEM00 tunable diode laser sources were used in order to get a wavelength range of over 300 nm (765–1071 nm).
\n
Figure 17.
Experimental setup for observing Gaussian to TEM11 conversion of the HPM surface grating system with three different diode sources [31].
\n
\n\nFigure 18\n shows as an example, the far-field profiles of three different wavelengths ((a) 765, (b) 978, and (c) 1071 nm) that were converted to the TEM11 mode without any angular adjustment of the HPM. This successfully demonstrates that full achromatization of a holographic phase mask can be achieved with the combination of surface gratings and phase-encoded transmitting volume Bragg grating.
\n
Figure 18.
Far-field profile of the diffracted beam after propagating through a holographic four-sector mode converting mask aligned to two surface gratings at (a) 765 nm, (b) 978 nm, and (c) 1071 nm. The sizes shown are not to scale.
\n
In conclusion, this is a demonstration of a way to make phase masks fully achromatic—something not possible until recently. This is achieved by the combination of surface gratings and phase-encoded transmitting volume Bragg grating.
\n
\n
\n
4. Conclusion
\n
The chapter discussed the nature and properties of volume Bragg gratings and presented several of the broad number of applications where VBGs find use. The opportunities to multiplex VBGs and integrate them with specific phase profiles were shown to bring unique capabilities that are hard or impossible to achieve by other means. VBGs are and will continue to benefit the development and the abilities of many laser and optical systems.
\n
\n\n',keywords:"holography, volume Bragg gratings, beam combining, phase plates, photothermo refractive glass, multiplexed volume gratings",chapterPDFUrl:"https://cdn.intechopen.com/pdfs/53837.pdf",chapterXML:"https://mts.intechopen.com/source/xml/53837.xml",downloadPdfUrl:"/chapter/pdf-download/53837",previewPdfUrl:"/chapter/pdf-preview/53837",totalDownloads:2235,totalViews:1426,totalCrossrefCites:0,totalDimensionsCites:0,hasAltmetrics:1,dateSubmitted:"May 24th 2016",dateReviewed:"November 16th 2016",datePrePublished:null,datePublished:"March 22nd 2017",dateFinished:null,readingETA:"0",abstract:"Two major volume Bragg grating (VBG) applications will be presented and in particular laser beam combining and holographically encoded phase masks. Laser beam combining is an approach where multiple lasers are combined to produce more power. Spectral beam combining is a technique in which different wavelengths are superimposed spatially (combined) using a dispersive element such as a volume Bragg grating. To reduce the complexity of such combining system instead of multiple individual VBGs, it will be demonstrated that a single holographic element with multiple VBGs recorded inside could be used for the same purpose. Similar multiplex volume holographic elements could be used for coherent beam combining. In this case, the gratings operate at the same wavelength and have degenerate output. Such coherent combining using gratings written in photothermo-refractive (PTR) glass will be discussed. The chapter also demonstrates that binary phase profiles may be encoded into volume Bragg gratings, and that for any probe beam capable of satisfying the Bragg condition of the hologram, this phase profile will be present in the diffracted beam. A multiplexed set of these holographic phase masks (HPMs) can simultaneously combine beams while also performing mode conversion. An approach for making HPMs fully achromatic by combining them with a pair of surface gratings will be outlined.",reviewType:"peer-reviewed",bibtexUrl:"/chapter/bibtex/53837",risUrl:"/chapter/ris/53837",book:{slug:"holographic-materials-and-optical-systems"},signatures:"Ivan Divliansky",authors:[{id:"192442",title:"Dr.",name:"Ivan",middleName:null,surname:"Divliansky",fullName:"Ivan Divliansky",slug:"ivan-divliansky",email:"ibd1@creol.ucf.edu",position:null,institution:{name:"University of Central Florida",institutionURL:null,country:{name:"United States of America"}}}],sections:[{id:"sec_1",title:"1. Volume Bragg gratings",level:"1"},{id:"sec_1_2",title:"1.1. Description and properties",level:"2"},{id:"sec_2_2",title:"1.2. Recording materials",level:"2"},{id:"sec_4",title:"2. Applications of volume Bragg gratings",level:"1"},{id:"sec_4_2",title:"2.1. Spectral and coherent laser beam combining by volume Bragg gratings",level:"2"},{id:"sec_5_2",title:"2.2. Holographic phase masks and their applications",level:"2"},{id:"sec_7",title:"3. Achromatization of HPM with surface diffraction gratings",level:"1"},{id:"sec_8",title:"4. Conclusion",level:"1"}],chapterReferences:[{id:"B1",body:'\nW. H. Bragg and W. L. Bragg, X Rays and Crystal Structure. London: G Bell and Sons, Ltd., 1915.\n'},{id:"B2",body:'\nR. R. Syms, Practical Volume Holography. Oxford, UK: Clarendon Press, pp. 48, 64, 132–162, 1990.\n'},{id:"B3",body:'\nH. Kogelnik, “Coupled Wave Theory for Thick Hologram Grating,” Bell Syst. Tech. J., vol. 48, no. 9, pp. 2909–2945, 1969.\n'},{id:"B4",body:'\nB. M. Anderson, G. Venus, D. Ott, E. Hale, I. Divliansky, D. R. Drachenberg, J. Dawson, M. J Messerly, P. H. Pax, J. B. Tassano, and L. B. Glebov, “Higher order mode selection for power scaling in laser resonators using transmitting Bragg gratings”, Proc. SPIE 9466, Laser Technology for Defense and Security XI, 94660C, 2015.\n'},{id:"B5",body:'\nD. Drachenberg, I. Divliansky, G. Venus, V. Smirnov, and L. Glebov, “High-power spectral beam combining of fiber lasers with ultra high-spectral density by thermal tuning of volume Bragg gratings”, SPIE Photonics West 2011, Proc. SPIE 7914, 79141F, 2011.\n'},{id:"B6",body:'\nL. H. Lin, “Method of Characterizing Hologram-Recording Materials,” J. Opt. Soc. Am., vol. 61, no. 2, pp. 203–208, 1971.\n'},{id:"B7",body:'\nR. Collier, C. Burchardt, and L. Lin, Optical Holography. New York: Academic, 1971.\n'},{id:"B8",body:'\nB. J. Chang and C. D. Leonard, “Dichromated gelatin for the fabrication of holographic optical elements.,” Appl. Opt., vol. 18, no. 14, pp. 2407–17, 1979.\n'},{id:"B9",body:'\nW. S. Colburn and K. A. Haines, “Volume hologram formation in photopolymer materials.,” Appl. Opt., vol. 10, no. 7, pp. 1636–41, 1971.\n'},{id:"B10",body:'\nS.-D. Wu and E. N. 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Lett., vol. 25, no. 23, pp. 1693–5, 2000.\n'},{id:"B16",body:'\nO. M. Efimov, L. B. Glebov, and V. I. Smirnov, “Diffractive optical elements in photosensitive inorganic glasses,” Inorg. Opt. Mater., vol. 4452, pp. 39–47, 2001.\n'},{id:"B17",body:'\nB. Anderson, S. Kaim, G. Venus, J. Lumeau, V. Smirnov, B. Zeldovich, and L. Glebov, “Forced air cooling of volume Bragg gratings for spectral beam combination,” Proc. SPIE. 8601, Fiber Lasers X: Technology, Systems, and Applications, 86013D. (March 22, 2013) doi: 10.1117/12.2005951.\n'},{id:"B18",body:'\nT. Y. Fan , “Laser beam combining for high-power, high-radiance sources,” IEEE J. Sel. Top. Quantum Electron., vol. 11, no. 3, pp. 567–577, 2005.\n'},{id:"B19",body:'\nI. Divliansky, D. Ott, B. Anderson, D. Drachenberg, V. Rotar, G. Venus, and L. Glebov, “Multiplexed volume Bragg gratings for spectral beam combining of high power fiber lasers”, SPIE Photonics West 2012, Proc. SPIE 8237, 823705, 2012.\n'},{id:"B20",body:'\nD. Ott, I. Divliansky, B. Anderson, G. Venus, and L. Glebov, “Scaling the spectral beam combining channels in a multiplexed volume Bragg grating”, Opt. Express, vol. 21, no. 24, pp. 29620, 2013.\n'},{id:"B21",body:'\nA. Jain, D. Drachenberg, O. Andrusyak, G. Venus, V. Smirnov, and L. Glebov, “Coherent and spectral beam combining of fiber lasers using volume Bragg gratings,” Proc. SPIE, vol. 7686, p. 768615, 2010.\n'},{id:"B22",body:'\nA. Jain, O. Andrusyak, G. Venus, V. Smirnov, and L. Glebov, “Passive coherent locking of fiber lasers using volume Bragg gratings,” Proc. SPIE 7580, 2010, vol. 7580, p. 75801S.\n'},{id:"B23",body:'\nD. Mawet, C. Lenaerts, V. Moreau, Y. Renotte, D. Rouan and J. Surdej, “Achromatic four quadrant phase mask coronagraph using the dispersion of form birefringence,” in Astronomy with High Contrast Imaging, C. Aime and R. Soummer, ed., EAS Publications Series, vol. 8, pp. 117–128, 2003.\n'},{id:"B24",body:'\nJ. Rosen, M. Segev, and A. Yariv, “Wavelength-multiplexed computer-generated volume holography,” Opt. Lett. Vol. 18, no. 9, pp. 744–746, 1993.\n'},{id:"B25",body:'\nT. D. Gerke and R. Piestun, “Aperiodic volume optics,” Nat. Phot. vol. 4, pp. 188–193, 2010.\n'},{id:"B26",body:'\nM. SeGall, I. Divliansky, C. Jollivet, A. Schülzgen, and L.B. Glebov, “Holographically encoded volume phase masks”, Optical Engineering, vol. 54, no. 7, pp. 076104–076104, 2015.\n'},{id:"B27",body:'\nM. SeGall, I. Divliansky, C. Jollivet, A. Schulzgen, and L. Glebov, “Simultaneous laser beam combining and mode conversion using multiplexed volume phase elements,” SPIE LASE, Photonics West, 89601F-89601F-6, 2014.\n'},{id:"B28",body:'\nA. Shyouji, K. Kurihara, A. Otomo, and S. Saito, “Diffraction-grating-type phase converters for conversion of Hermite-Laguerre-Gaussian mode into Gaussian mode,” Appl. Opt. vol. 49, no. 9, pp. 1513–1517, 2010.\n'},{id:"B29",body:'\nM. Beresna, M. Gecevičius, P. G. Kazansky, and T. Gertus, “Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass,” Appl. Phys. Lett. vol. 98, pp. 201101, 2011.\n'},{id:"B30",body:'\nL. Glebov, V. Smirnov, N. Tabirian and B. Zeldovich, “Implementation of 3D angular selective achromatic diffraction optical grating device”, Frontiers in Optics 2003, Talk WW3.\n'},{id:"B31",body:'\nI. Divliansky, E. Hale, M. SeGall, D. Ott, B. Y. Zeldovich, B. E. A. Saleh, and L. B. Glebov, “Achromatic phase elements based on a combination of surface and volume diffractive gratings”, Proc. SPIE 9346, Photonics West: Components and Packaging for Laser Systems, 93460Q, 2015.\n'},{id:"B32",body:'\nI. Divliansky, A. Jain, D. Drachenberg, A. Podvyaznyy, V. Smirnov, G. Venus, L. Glebov, “Volume Bragg lasers,” Proc. SPIE 7751, XVIII International Symposium on Gas Flow, Chemical Lasers, and High-Power Lasers, 77510Z, 2010.\n'}],footnotes:[],contributors:[{corresp:"yes",contributorFullName:"Ivan Divliansky",address:"ibd1@creol.ucf.edu",affiliation:'
The College of Optics & Photonics (CREOL), University of Central Florida, Orlando, FL, USA
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1. Introduction
Multiple pregnancies are the result of one of the three possibilities: a fertilization of two or more oocytes from different spermatozoids, a single fertilization followed by a splitting of the zygote, or a combination of both [1]. These pregnancies have an increased risk of several complications for both mother and fetuses, such as diabetes mellitus, hypertensive disorders associated with pregnancy, preeclampsia, anemia, hyperemesis, hemorrhage, and cesarean delivery [2, 3, 4, 5] in the maternal side and higher risk of fetal anomalies, fetal demise, neonatal death [6], and preterm birth in the fetal side [7].
It is known that monochorionic (MC) pregnancies have higher rates of fetal morbidity and mortality when compared to dichorionic (DC) ones [1, 8, 9]. Besides that, the MC pregnancies have specific complications such as the twin to twin transfusion syndrome (TTTS), the selective fetal growth restriction (sFGR), the twin anemia polycythemia sequence (TAPS), and the twin reversed arterial perfusion sequence (TRAPS). Most of these complications can be managed and treated in order to decrease the fetal morbimortality.
2. Importance of multiple pregnancy
In the last years, the rate of multiple pregnancies has raised all over the globe. In the USA, it rose from 18.9in 1980 to 33.4 twins per 1000 births in 2016. The twin birth rates were higher in black women, followed by non-Hispanic white women. The triplet and high-order multiple birth rate has decreased about 48% in the last 8 years, from 193.5 in 1998 to 101.4 twins per 100.000 births in 2016 [7]. This decrease in high-order multiple pregnancies illustrates the reproductive medicine societies’ strategies for reducing the risk of high-order pregnancies, like single-embryo transfer and multifetal pregnancy reduction [10, 11, 12].
In England, there is also an increase in multiple births. From 1998 to 2016, the multiple maternity rate rose from 14.4 to 15.9 twins per 1000 births. Since 1993, women aged 45 and over have consistently recorded the highest multiple maternity rate. These changes in the multiple pregnancy rates are due to the increase in ART. It is estimated that in vitro fertilization (IVF) conceptions are 11 times more likely to result in a multiple birth than natural conceptions. In 2014, 16% of IVF pregnancies resulted in multiple birth, with nearly 19,000 IVF babies born in the UK in 2014 [13].
This trend was largely attributed to an elevated amount of dizygotic pregnancies, without significant variations in monozygotic births over the past few decades. The dizygotic twinning rate is affected by many factors such as race, previous multiple pregnancy, maternal age and parity, lifestyle, season, use of fertility drugs and treatments, genetics, and others [14, 15, 16].
The high number of multiple births impacts directly in rate of preterm birth and low birthweight. Data from 2016 show that among twin pregnancies, 59.9% are born before complete 37 weeks of gestation, while in singletons, only 8% are preterm births. In singleton births, 6.4% were born with weight less than 2500 g. This percentage is 55.4 in twins and more than 95% in triplets [7].
3. Complications
The MC pregnancies have several unique and serious complications that contribute to a perinatal mortality rate of 11% [17, 18]. The pathophysiology of most of these complications is related to the placental angio-architecture [19]. Placental anastomoses are described since the 1600s. The term “third circulation” that represents an “area of transfusion” and the potential harmful effect of vascular connections between the fetuses was first described by Schatz in 1896 [20]. In 1965, Naeye [21] identified the effect of chronic nutritional deprivation on the size of organs in one twin while appreciating that transfusion to the other increased the hemoglobin concentration and hematocrit, with subsequent cardiomyopathy and hypertension. Since then, several authors have proposed diagnosis criteria and different kinds of treatments of the MC pregnancy problems. In this session, the main complications of the MC gestations will be discussed.
3.1. Twin to twin transfusion syndrome
One of the first suggestions of this disease in history lies in a Dutch painting from 1617 named the Early-Deceased Children of Jacob de Graeff and Aeltge Boelens that illustrates two children. One of them is pale and the other plethoric (Figure 1). Twin to twin transfusion syndrome is one of the main complications that occurs in about 10–15% of the MC pregnancies with an overall incidence of 3 in 10,000 pregnancies [22, 23].
Figure 1.
The Dutch painting the Early-Deceased Children of Jacob de Graeff and Aeltge Boelens shows two male twins: one pale and the other plethoric.
If left untreated, TTTS mortality rates are about 70–100%. Perinatal mortality is the result of either miscarriage or very preterm delivery as a consequence of severe polyhydramnios and uterine distention or fetal demise due to severe cardiovascular disturbances [24, 25].
3.1.1. Pathophysiology
The pathophysiology underlies in the placental angio-architecture which is characterized by individual placental territory size, cord insertion location, and the quantity, size, and direction of intertwin anastomoses which are the most important factors in the pathogenesis because when unbalanced, they may cause hemodynamic changes that end in TTTS [26].
All MC placentas have intertwin anastomoses that are formed in the first trimester. They are important because they allow transfer of volume, red blood cells, vasoactive substances, and hormones. There are three type of intertwin anastomoses, and their flow may be unidirectional or bidirectional. Arteriovenous (AV) anastomoses are unidirectional but they exist in both directions (from donor to recipient or from recipient to donor). AV anastomoses end in a shared cotyledon where the arterial villous circulation of one twin links to the venous villous return of the other at the level of the intervillous space. Artery-to-artery (AA) and vein-to-vein (VV) are more superficial and bidirectional anastomoses (Figure 2). The flow direction depends on the types of connection, vessel calibers, and the pulse pressure. TTTS results from an unbalanced chronic perfusion from donor to recipient twin across placental anastomoses. This blood transfer is more likely in those placentas with more AV anastomoses and a lack of superficial balancing AA or VV anastomoses or when these bidirectional anastomoses are unusually small [26, 28].
Figure 2.
Monochorionic placenta of not complicated twin pregnancy. The blue, white, and yellow arrows represent AA, VV, and AV anastomoses, respectively. Adapted from twin research and human genetics, Zhao et al. [27].
3.1.2. Clinical manifestations of TTTS
The principal clinical feature in TTTS is hypervolemia in the recipient and hypovolemia in the donor twin that may progress to cardiovascular impairment, hydrops, and fetal death. In the first trimester, diagnosis is difficult, since the amniotic fluid is usually normal in both fetuses. Some sonographic markers such as discordance in nuchal translucency thickness (NT) and abnormalities in ductus venosus (DV) may be early signs of TTTS, but they have a low predictive value [29, 30, 31]. The sonographic manifestations usually may be noted as early as 16 weeks of gestation, but they can appear in the third trimester as well. TTTS manifestations are rare after 28 weeks of gestation.
In the second trimester, the oligohydramnios in the donor twin, as well as the polyhydramnios in the recipient twin, can easily be noted by ultrasound examination. The donor becomes hypovolemic; therefore, renal perfusion decreases. This hypoperfusion activates the renin-angiotensin system (RAS), producing vasoconstriction, oliguria, and oligohydramnios. As the disease progresses, the fetus becomes anuric and gets “stuck” against the uterine walls (Figure 3). The circulation becomes hyperdynamic with an increased vascular resistance in the fetus and in the placenta, leading to fetal growth restriction (FGR), cerebral redistribution, and abnormal arterial Doppler assessment. The recipient twin becomes hypervolemic and, by myocardial stretching, releases atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), which are also biomarkers associated with heart failure. Elevated levels of these biomarkers and troponin are found in the amniotic fluid of the recipient, suggesting the presence of myocardial damage [26, 32, 33]. Despite the hypervolemia, vascular resistance in the recipient twin is increased. This hypertension is attributed to vasoactive mediators such as endothelin and also a paradoxically high level of renin. The source of endothelin and renin is probably partly from the placenta and partly from the donor via the vascular communications [34, 35]. These changes in fetal hemodynamics may cause a progressive cardiomyopathy that increases the heart size, reduces the myocardial compliance, and causes atrioventricular valvar regurgitation and abnormal venous Doppler findings. Several studies show that in early Quintero stages or even before the diagnostic of TTTS, the cardiac function in the recipient twin may be impaired [36, 37, 38]. A recent study noted that in the recipient twin, left ventricular filling pressures are elevated and systolic function is decreased before abnormalities in the right heart become apparent. They also described an improvement after fetoscopic laser photocoagulation (FLPC) in these fetuses [38].
Figure 3.
Two fetal abdomens. The smaller one (short arrow) is stuck in the anterior uterine wall and has no amniotic fluid. The bigger fetus (long arrow) has polyhydramnios. Adapted from: https://radiologykey.com/complications-of-multiple-gestations/.
In the third trimester, fetal discordance in amniotic fluid and growth may occur, increasing uterine distension and causing shortened cervical length and preterm birth. Also, the mirror syndrome, a rare condition that presents itself as a sudden maternal edema, loss of renal and cardiac function, hypertension, and fetal hydrops, may appear in women with TTTS [26, 39, 40].
There is another rare form of TTTS described as “acute peripartum TTTS” which is defined as the intertwin hemoglobin difference at birth >8 g/dl. Since it is a rare condition (2.5% of all the MC pregnancies), there are a few studies and the pathogenesis remains unclear. Some studies says that in theory, acute fetal blood loss from the donor twin into the circulation of the recipient twin may occur as a result of variations in blood pressure due to uterine contractions or fetal positions [41].
3.1.3. Diagnostic criteria and staging
In the past, TTTS was diagnosed at the time of birth based on neonatal criteria that included a growth discordance of 15–20% associated with discordant cord or neonatal hemoglobin concentrations of ≥5 g/dl [42]. In 1992, another study showed that these criteria are present in other conditions such as uteroplacental insufficiency, infection, and malformations and therefore should not be used as diagnostic criteria for TTTS [43].
The screening for TTTS should begin with an early ultrasound in order to confirm the chorionicity. The first trimester scan should be performed to look for morphology abnormalities and discordance in the NT measurement, abnormalities in the DV, and even crown-rump length discordances [44]. Unlike dichorionic pregnancies, where ultrasound can be performed every 4 weeks until the end of the second trimester, monochorionic pregnancies should be examined by ultrasound every 2 weeks beginning in the 16th week. An analysis of fetal growth, amniotic fluid deepest vertical pocket (DVP), umbilical artery pulsatility index (UA-PI), medium cerebral artery pulsatility index (MCA-PI), and peak systolic velocity (MCA-PSV) should be obtained [45, 46]. Besides that, a fetal echocardiography should be performed, since cardiac abnormalities are the most common defect in MC pregnancies. The fetal growth and the MCA-PSV are important parameters in the differential diagnosis of sFGR and TAPS, respectively. The early diagnosis is extremely important, since it allows timely treatment with FLPC.
In 1999, Quintero et al. standardized the diagnostic criteria and classification system of TTTS (Table 1) [47]. The diagnosis is made when a discordance in the DVP of the twins is visualized. The DVP of the donor twin should be <2 cm; meanwhile the DVP of the recipient, before 20 weeks, should be >8 cm, and after 20 weeks, it should be >10 cm in the European criteria and > 8 cm in the US criteria. The fetal bladders should also be evaluated since there might be a discordance in the size of the fetal bladders (larger in the recipient and smaller in the donor). It is worth reminding that weight discordance is not a diagnostic criterion for TTTS, but it also can be noted in the ultrasound examination.
Stage
Sonographic findings
I
DVP > 8 cm in the recipient* and < 2 cm in the donor twin
II
Absent bladder filling in the donor
III
Critically abnormal Doppler studies of either fetus**
IV
Hydrops of either fetus
V
Intrauterine fetal demise of either fetus
Table 1.
TTTS staging system. Adapted from Journal of Perinatology, Quintero et al. [44].
Before 20 weeks the universal cutoff is 8 cm, and between 21 and 26 weeks, the cutoff is 8 cm in the USA and 10 cm in Europe.
Absent-reverse diastolic flow in the umbilical artery and/or absent/reverse flow in the ductus venosus or pulsatile flow in the umbilical vein.
There are some critics about it because this staging system is not progressive (e.g., stage I can go to stage IV without passing through stages II and III) [45], and it does not correlate well with survival chance in twins treated with FLPC [48]. Nevertheless, these criteria are the most used to classify TTTS.
3.1.4. Management of TTTS
The natural history of TTTS shows high rates of fetal morbidity and mortality. The perinatal death in some series of cases is about 70–100%, depending on the stage of disease [26]. In stage I, it is known that nearly 70% of the pregnancies remain stable or regress, but in 5% of cases of stages I or II, there is fetal death of one or both twins without warning. Besides that, only 30% of pregnancies managed expectantly have double survivors. In the other stages, mortality increases and treatment is necessary [49]. There are several ways to manage TTTS, which include FLPC, amnioreduction, selective reduction, and pregnancy termination.
The FLPC is the preferred option because its outcomes are better when compared to serial amnioreduction [50, 51]. For stage I, there is no consensus regarding the use of FLPC, so the cases should be individualized [52]. For stages II to IV, FLPC of placental anastomoses is the primary treatment between 16 and 26 weeks of gestation. In 2004, Senat et al. have shown that the mortality rate of fetuses treated with FLPC when compared with serial amnioreduction is significantly lower (RR 0.71; 95% CI 0.55; 0.92). This study also showed a decreased risk of intraventricular hemorrhage and neurological impairment in the laser group. Probably it is because there is a higher rate of prematurity in the amnioreduction group [51]. The procedure consists in inserting a fetoscope in the amniotic sac of the recipient, locating the donor twin and the intertwin membrane, coagulating (with Nd:YAG or diode laser) the intertwin anastomoses along the placental vascular equator, and, after that, removing amniotic fluid from the recipient sac [26, 51].
The quality of fetoscopy images in the early 1990s, when the first FLPC for TTTS was performed, was not good; therefore, the vascular anastomoses were not so easy to identify. The so-called nonselective technique for vessel coagulation was proposed [53]. This technique consisted in coagulating all of the vessels that crossed the intertwin membrane. It did not attempt to differentiate anastomotic from non-anastomotic vessels but rather to catch as many anastomoses as possible (Figure 4). With the development of new techniques and advance in fetoscopy technology, another approach was proposed: the selective fetoscopic laser photocoagulation (SFLP) [54, 55]. In this method, the vascular equator is visualized and only intertwin anastomoses are coagulated. This technique differs from the “nonselective” FLPC because the equator does not always coincide with the membrane; therefore, not all the vessels that cross the intertwin membrane should be coagulated; thus, theoretically, more placental tissue will be available for the donor twin after the procedure (Figure 4). In 2000, Quintero et al. compared the SFLP with the “nonselective” FLPC and found that the selective method yielded superior results, with survival of at least 1 infant in 83% of patients against 61% in the “nonselective” group [56]. The order of anastomoses coagulation was also studied. Some authors claim that the sequential method, which is a technique where the AV (donor to recipient) are coagulated before the VA, improves the survival rate of both fetuses [57, 58, 59] and the survival rate of at least on fetus [58, 59, 60]. A recent meta-analysis showed that there may be an improved double neonatal survival as well as a decreased donor and recipient fetal demise with the use of the sequential technique, although all the studies are small and underpowered to confirm the hypothesis [61]. Although the SFLP improved neonatal outcomes, there is about 18% of surgical failure, defined as postoperative symptomatic patent anastomoses (Figure 5) [62, 63, 64, 65], which could result in several complications such as recurrent TTTS (7–9%) [61, 65], TAPS (13–16%) [66, 67], and fetal death. This is a very delicate situation, because repeating the procedure is more difficult for several reasons such as the size of the uterus and the fetuses. Furthermore, it is associated with an overall perinatal survival rate of 50% [67].
Figure 4.
Types of fetoscopic laser techniques in the treatment of TTTS. The nonselective method coagulates all vessels crossing the intertwin membrane. SFLP occlude anastomoses where they occur, sparing placental tissue of the donor. The equatorial laser dichorionization or Solomon technique separates the fetal circulations by coagulating the vascular equator. Adapted from Am J Perinatol. Benoit et al. [26].
Figure 5.
Digitally modified image of placenta with recurrent TTTS with missed AV and VA anastomoses. Adapted from Am J Obstet Gynecol. Lewi et al. [62].
Recently, a new fetoscopic technique in which superficial coagulation of microvasculature on the chorionic plate between ablated anastomotic sites following SFLP was described [68] (Figure 4). Some authors compared the SFLP with this new technique in cohort studies and showed a trend toward the latter group [69, 70]. A subsequent randomized trial by Slaghekke et al. compared this new approach called the Solomon technique versus the SFLP and found no difference in the overall survival rates. However, a decrease in recurrent TTTS and TAPS after the procedure was observed in the Solomon group (4 vs. 21%) [66].
The main early complications of laser photocoagulation are unintentional septostomy in 8–12%, premature rupture of membranes (PROM) in about 1–9%, and amnion dehiscence (membrane separation) in 5–10% of cases.
The time of delivery in cases of laser photocoagulation varies between 31 and 34 weeks and most of them (60–80%) are not elective. The most common indication is the onset of labor followed by nonreassuring fetal testing and PROM. The mode of delivery is usually by cesarean section, in 57–70% of cases [26, 51, 60, 69, 70].
Unfortunately, in low-income countries, the laser therapy is not widely known and there are no teaching facilities. One Brazilian study showed the initial experience of a single center and found a single twin and both twins’ survival rate 1 month after birth is 87.5 and 45.8%, respectively. These reported data are in line with those obtained in major centers worldwide, considering the learning curves and infrastructures [71]. In order to extend the range of the laser therapy to all the MC pregnancies, more teaching centers should be opened, and telemedicine should be used to aid low-income places to achieve the excellence in fetoscopy techniques.
3.1.5. Perinatal outcomes after treatment
The perinatal outcomes after the use of SFLP or Solomon technique are very satisfactory. Baschat et al. [70] found that the double survival rates at 6 months of age were 68% in the Solomon group and 50% in the SFLP. Ruano et al. [69] showed an overall neonatal survival rate from 61.8% in the SFLP group to 86.5% when Solomon technique was used. This difference could be due to the increased experience with fetoscopic laser in general and not to the use of the Solomon technique. In the only randomized trial, the single twin and both twins’ survival rates after 1 month in the SFLP were 87% and 60%, while in the Solomon group these rate were 85% and 64%, respectively [66].
The neurologic outcomes in the neonatal period following laser procedures, such as intraventricular hemorrhage, periventricular leukomalacia, cerebral white matter cysts, ventricular dilatation, and cerebral atrophy, range from 8 to 18% [51, 72, 73]. The long-term neurodevelopmental outcomes vary between 3 and 12% for cerebral palsy and 4 and 18% for neurodevelopmental impairment [73]. In one study, the neurodevelopmental scores in preterm-born children treated with laser therapy for TTTS were similar in preterm-born DC children, suggesting that prematurity has the main role in the neurologic impairment in fetus treated with laser photocoagulation [74]. Other authors have suggested risk factors for poorer neurodevelopmental outcomes [75, 76]. Lopriore et al. analyzed 212 pregnancies treated with fetoscopic laser surgery and found that advanced gestational age at laser surgery, low gestational age at birth, low birthweight, and high Quintero stage are risk factors of poor neurological development at 2 years of age [76].
Several studies report a rapid cardiac function recovery in the recipient and in the donor twin [36, 38, 77, 78, 79, 80]. The coagulation of vascular anastomoses stops the volume exchange, as well as the vasoactive mediators, allowing cardiac output, cardiac size, valvular regurgitation, and ventricular inflow to normalize in the recipient twin in about half of the cases [38, 77]. The donor twin shows an increase in left ventricular filling pressure and cardiac output, which can temporarily cause a relative volume overload. It can worsen the cardiac function and cause ductus venosus alterations and even hydrops; however, these changes tend to disappear by 2 to 4 weeks after the laser procedure [79, 81, 82].
There are other types of treatment, such as septostomy. This procedure increases the risk of severe complications like cord entanglement and disruption of the membrane. This procedure has generally been abandoned [64, 83]. The selective reduction is another therapeutic option that tries to improve the outcome of the surviving twin whenever there is an imminent risk of spontaneous intrauterine death of one fetus. It can be performed either by ultrasound-guided vascular embolization or cord clamping through fetoscopy. A maximum of 50% survival is reached and most services have not supported this technique [68].
The fetoscopic laser coagulation is the gold standard treatment in stage II to stage IV TTTS affected pregnancies; the SFLP and Solomon technique are the best options for lowering the mortality and morbidity in theses fetuses. For Quintero stage I, there is not enough data that favors laser surgery, and more powered studies should be done comparing it to other kinds of treatment; therefore, the treatment for this stage has to be individualized.
3.2. Selective intrauterine growth restriction
Selective intrauterine growth restriction happens in 10–25% of MC gestations and it considerably increases perinatal morbidity and mortality [84, 85, 86]. The diagnostic criteria for sFGR differ among clinicians; therefore, it is hard to compare the findings of existing studies, to combine their results, or to establish robust evidence-based management. The pathophysiology in sFGR in MC and DC twins seems to be different. While DC sFGR have conventionally been managed as FGR in a singleton pregnancy, MC twin pregnancies sFGR is thought to result mainly from an unequal placental share. In most cases the origin is in the placental territory discrepancy (Figure 6). Vascular anastomoses between both fetuses intrinsically justify IUGR, and one twin receives better oxygenated blood [87].
Figure 6.
Macroscopic photograph demonstrates the measurement of the vascular anastomoses. There is a 2-mm arterioarterial anastomosis (dashed arrow) and 5 AV anastomoses (arrows). A macroscopic placental surface discordance is also visible (green dashed line: Vascular equator). Adapted from Am J Obstet Gynecol. Lewi et al. [86].
3.2.1. Diagnostic criteria and staging
Since many authors have proposed different diagnostic criteria, in 2017, the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) published a guideline for the sFGR diagnostic. It is defined as a condition in which one fetus has estimated fetal weight (EFW) < 10th centile and the intertwin EFW discordance is >25%. EFW discordance is calculated by the following formula: (weight of larger twin – weight of smaller twin) × 100)/weight of larger twin [45]. This weight discordance was proposed by an expert consensus, mainly based on data that show that an 18% EFW discordance reflects poorer outcomes both in DC and MC pregnancies [88]. Curiously, the charts used to monitor the fetal growth should be the same as those used in singleton pregnancies [45, 89], although specific multiple pregnancy charts are available [90]. However, there is a reduction in fetal growth in twin compared with singleton pregnancy, particularly in the third trimester. The key question for clinicians is whether this difference in growth represents adaptation or restriction [91]. Once the diagnosis is made, a detailed anomaly scan and screening for viral infections (cytomegalovirus, rubella, and toxoplasmosis) should be made. Amniocentesis may also be required to exclude chromosomal abnormalities as a cause of FGR [45, 92].
In order to follow up the sFGR pregnancies, as well as in singleton pregnancies, umbilical artery Doppler waveforms and UA-PI are accessed. In pregnancies complicated with sFGR, there are particularities in the umbilical artery Doppler probably because of the variability in the intertwin vascular anastomoses resistance [93, 94]. Three patterns are observed in the umbilical artery Doppler: positive end-diastolic flow, absent or reversed end-diastolic flow (AREDF), and intermittent absent or reversed end-diastolic flow (iAREDF) [95]. The latter pattern though is to result from the presence of transmitted waveforms from the larger into the smaller twin’s cord due to the existence of placental large AA anastomoses (Figure 6) [93, 94, 95]. Based on these three Doppler types, Gratacós et al. proposed a three-stage classification system of the sFGR fetuses. In stage I, the umbilical artery in the smaller twin has a positive end-diastolic flow; in stage II, there is an AREDF; and stage III is characterized by iAREDF (Figure 7) [95].
Figure 7.
Classification of selective fetal growth restriction in monochorionic twin pregnancy. In type I, the umbilical artery Doppler waveform has positive end-diastolic flow, while in type II there is absent or reversed end-diastolic flow (AREDF). In type III there is a cyclical/intermittent pattern of AREDF. Extracted from ISUOG. https://www.isuog.org/uploads/assets/uploaded/b4ce0129-a7e8-40a9-8543c4243fb7638f.pdf [45].
The stage I prognosis is better, with an overall intrauterine mortality rate of 3–4% and a 97% rate of intact survival-free from neurological complications according to two recent meta-analyses. The neonatal morbidity, defined as abnormal brain imaging, respiratory distress syndrome (RDS), admission to the neonatal intensive care unit (NICU), or retinopathy of prematurity (ROP), was reported in about 9% of newborns. The neurologic outcome in this stage seems to be better when compared to the others as well as the gestational age at delivery [84, 93, 94, 95, 97]. Stage II sFGR has a poorer prognostic. It is reported that these fetuses tend to have a high risk of hypoxic deterioration and consequently overall, single, and double intrauterine death rates of 16.6%, 8.2%, and 10.4% of cases managed expectantly [97] and a 21% perinatal mortality [84]. The double survival rate in this stage is about 25% [98]. The iAREDF pattern has an intrauterine mortality rate similar to stage II. The overall, single, and double intrauterine death occurred in 13.2%, 7.2%, and 5.5% of cases managed expectantly although this stage is more unpredictable than the others [86, 93, 95, 97, 98]. Some ultrasound markers can be used as adverse predictors such as ductus venosus Z score [98], velamentous cord insertion (Figure 8) [99, 100], and weight discordance. A recent meta-analysis found that, in MC twin pregnancies, excluding cases affected by twin to twin transfusion syndrome, twins with birthweight discordance ≥25% were at higher risk of intrauterine death (OR 3.2, 95%CI, 1.5–6.7) and neonatal death (OR 4.66, 95% CI, 1.8–12.4) compared with controls [101]. Gratacós et al. [93] found a 15% unexpected intrauterine death rate in the smaller twin on stage III sFGR compared with 2.6% and 0 in stages I and II, respectively. On the other hand, other authors show a better prognosis. Rustico et al. [102] showed a 0% rate in double fetal death at stage III as well as better rates in overall survival and lower neonatal death in the smaller twin (8% and 62%, respectively).
Figure 8.
Monochorionic placenta with velamentous cord insertion in the smaller twin side. AV and VA anastomoses are seen and on big AA anastomoses. Adapted from Am J Obstet Gynecol. Lewi et al. [96].
3.2.2. Management of sFGR
When sFGR presents with an umbilical artery positive end-diastolic flow, the prognosis is good, and therefore it is a consensus that the expectant management based on a weekly fetal growth and UA-PI evaluation should be done to look for progression to more severe stages which can occur in up to 25% of cases. For stages II and III, several studies compared SFLP with cord occlusion or expectant management, but there are not powered studies to support a gold standard treatment. In a retrospective study with 142 stage II sFGR fetuses treated with SFLP, there was survival rate of the smaller, larger, and both twins of 38.7, 67.6, and 34.5%, respectively. The survival rate of at least one twin was 71.8% [103]. When compared to expectant management, SFLP for stage III sFGR showed a higher overall intrauterine death (14.5 vs. 36%, respectively) as well as a higher death rate in the smaller twin which is 19% for the expectant group and 66% for the SFLP group [104]. Other prospective trial with ten pregnant women with sFGR stages II or III and oligohydramnios treated with SFLP showed that only three newborns of the restricted group survived and all of the newborns in the larger twin group were well and alive at 28 days of age [105].
Cord occlusion of the smaller twin is an option for early diagnosed sFGR, when the spontaneous death of the restricted fetus is most likely to happen, but it is the most difficult decision for the parents to make since they give up the life of one child to protect the other. Chalouhi et al. [106] found a 90% survival rate in the larger twin after cord occlusion and a 4.5% neurologic complication rate which is much lower than the 26% rate when a spontaneous intrauterine death occurs [107].
The sFGR treatment is not yet defined. Several factors should be evaluated together with parents such as weight discordance, time of diagnosis (early vs. late), hemodynamic state of the restricted fetus at the time of diagnosis, and the will to protect the larger twin since the adverse outcomes are very low after a cord occlusion [94]. If FLPC or expectant management is elected, parent counseling should be made regarding complications and outcomes to both fetus.
3.3. Twin anemia polycythemia sequence
The placental angio-architecture is responsible for most of the complications in MC pregnancies. The intertwin vascular anastomoses have a key role in the pathogenesis of TTTS and sFGR. In 2007, a new MC pregnancy complication was described by Lopriore et al. [108] that involves a discordance in postnatal hemoglobin and hematocrit levels, a difference in neonate reticulocyte levels, and small AV anastomoses in the placenta after colored dye injection (Figure 9). This condition was named twin anemia polycythemia sequence. TAPS happens when blood from one twin is slowly transfused to the other by small AV anastomoses at a 5–15 ml/ 24 h rate [108]. Unlike TTTS, there is a less acute and well-compensated intertwin transfusion process leading to a discordance in hemoglobin levels without hemodynamic or amniotic fluid alterations [110]. The reticulocyte levels are also increased in the donor newborn and decreased in the recipient, which differ from other acute diseases, such as acute peripartum TTTS [41]. Another characteristic of TAPS is that after colored dye injection in MC placentas after TAPS, AA anastomoses are observed in about 11% and all of them are small (<1 mm). In comparison, the incidence of AA anastomoses in uncomplicated MC pregnancies and TTTS pregnancies is 80 and 25%, respectively [111, 112], which suggests that AA anastomoses protect against TAPS and TTTS. The maternal side of the TAPS placenta also shows an important color difference. The donor side is more white than the recipient side that shows a plethoric aspect like the respective twin (Figure 10) [113].
Figure 9.
TAPS placenta after colored dye injection (blue or green for arteries and pink or yellow for veins). The white arrows indicate the small AV and VA anastomoses. Adapted from placenta. de Villiers et al. [109].
Figure 10.
Maternal side of the TAPS placenta showing the difference in color between the plethoric share of the recipient (left side of the placenta) and the anemic share of the donor (right side of the placenta). Adapted from twin research and human genetics. Tollenaar et al. [113].
TAPS may occur spontaneously or post-laser surgery. The prevalence of spontaneous TAPS is about 1.6–5% [18, 68, 114], while post-laser TAPS occurs in 3–16% [66], depending on the technique used. The possible pathophysiology for the latter is the inability to identify all AV anastomoses, therefore leaving some small AV anastomoses without coagulation. The Solomon trial showed a significant decrease in post-laser TAPS in the placental dichorionization group, supporting this hypothesis [66].
3.3.1. Diagnostic criteria and classification
TAPS can be diagnosed either antenatally or postnatally. Antenatal diagnosis (Table 2) is based in MCA-PSV measurement in both fetuses showing an increased velocity in the anemic and a decreased velocity in the polycythemic twin. The most used criteria of TAPS diagnosis are an MCA-PSV > 1.5 MoM for the donor twin and <1.0 MoM for the recipient twin [111, 115]. Slaghekke et al. analyzed 43 twin pregnancies complicated by TAPS and found that a MCA-PSV > 1.5 MoM correlated with anemia (hemoglobin levels >5 SD below the mean) with a 94% sensitivity, a 74% specificity, a 76% positive predictive value, and a 94% negative predictive value. In the same study, MCA-PSV ≤ 1.0 MoM correlated with polycythemia (hemoglobin levels >5 SD above the mean) with a 97% sensitivity, a 96% specificity, a 93% positive predictive value, and a 99% negative predictive value [115]. In some TAPS cases, other ultrasound findings have been reported. The first one is the difference in placental thickness, and echodensity on ultrasound examination was detected [110]. Another ultrasound finding described in TAPS is the so-called starry sky liver [116] which is characterized by clearly identified portal venules and diminished parenchymal echogenicity. More studies are needed to further investigate the validity and significance of these antenatal ultrasound findings for the diagnosis of TAPS.
Antenatal criteria
Postnatal criteria
Donor MCA-PSV ≥ 1.5 MoM
Intertwin hemoglobin difference > 8 g/dl
AND
AND 1 of the following
Recipient MCA-PSV ≤ 1.0 MoM
Reticulocyte count ratio > 1.7
Placenta with only small (diameter < 1 mm) vascular anastomoses
Table 2.
Antenatal and postnatal diagnostic criteria for TAPS. Adapted from ultrasound Obstet Gynecol. Slaghekke et al. [111].
The postnatal criteria (Table 2) can be used when TAPS is not diagnosed by MCA Doppler. It is based on the finding of discordant hemoglobin levels (Hb difference > 8.0 g/dl) associated with an increased intertwin reticulocyte count ratio > 1.7 that is pathognomonic for TAPS and placental evidence of only small vascular anastomoses [111, 117].
The classification for TAPS was proposed by Slaghekke et al. in 2010 [111] based on the difference in hemoglobin levels postnatally (Table 3).
Antenatal stage
Doppler ultrasound
Stage I
MCA-PSV donor >1.5 MoM and MCA-PSV recipient <1.0 MoM, without other signs of fetal compromise
Stage II
MCA-PSV donor >1.7 MoM and MCA-PSV recipient <0.8 MoM, without other signs of fetal compromise
Stage III
As stage I or II, with cardiac compromise of donor, defined as critically abnormal flow*
Stage IV
Hydrops of donor
Stage V
Intrauterine demise of one or both fetuses preceded by TAPS
Table 3.
Antenatal TAPS classification. Adapted from Ultrasound Obstet Gynecol. Slaghekke et al. [111].
Critically abnormal Doppler is defined as absent or reversed end-diastolic flow in umbilical artery, pulsatile flow in the umbilical vein, and increased pulsatility index or reversed flow in ductus venosus.
3.3.2. Management of TAPS
There is no optimal treatment for TAPS. Options include expectant management and early delivery; intrauterine transfusion (IUT) in the donor, with or without partial exchange transfusion (PET) in the recipient; selective feticide; and fetoscopic laser surgery.
Expectant management is made with closing ultrasound monitoring with serial MCA-PVS evaluation and an early delivery when necessary. It leads to a 75 to 83% survival rate [111, 118].
Another kind of treatment is IUT that can be performed intravascularly or intraperitoneal. It seems the latter may be superior to intravascular intrauterine transfusions because it is technically easier and can be performed as early as 15 weeks [119]. Although this method is commonly used, it is a palliative option, since it temporarily meliorates the donor anemia. Furthermore, the raise in blood viscosity in the recipient twin can lead to embolic complications [67]. These complications can be managed by partial exchange transfusion (PET) that decreases the viscosity of the blood of the polycythemic recipient. The perinatal survival rate in some studies is generally good, reaching 85–100% [111, 118].
The only causal treatment for both spontaneous and post-laser TAPS is laser surgery. It is technically more difficult because of the absence of polyhydramnios and a stuck twin, which makes the visualization of the vascular equator more challenging as well as the size of anastomoses, which is difficult to visualize during fetoscopy [111]. The results in small studies are satisfactory, with a survival rate of 94–100% [111, 118, 120, 121] and an apparent improvement in perinatal outcome by prolonging pregnancy and reducing respiratory distress syndrome [117].
The TAPS management should be made after evaluation of different factors, including TAPS stage, gestational age, and the clinician experience in the different types of treatments. In early stages, TAPS can be managed expectantly. If gestational age is below 26–28 weeks, laser treatment should be considered [113]. When laser treatment is not possible, IUT should be considered. When repeated IUT is expected or in case of severe polycythemia in the recipient, PET of the recipient can be done.
3.4. Twin reversed arterial perfusion sequence
Twin reversed arterial perfusion sequence resulting in an acardiac twin is a rare condition and occurs in 1:35,000 births or 1% of all monozygotic twins [122]. It consists in one health twin (the “pump” twin) and one acardiac mass which is perfused by the other fetus’ heart. This acardiac twin most often has an underdeveloped head and upper body and impressive edema also mostly of the upper body. In some cases, there might be fetal movements. In rare cases, a rudimentary pulsating cardiac structure may be seen. It is though that the VV and AA bidirectional anastomoses are responsible for the perfusion of the acardiac fetus. One study analyzed the TRAPS placenta and found big AA anastomoses as well as veins in direct continuity with each other. They also noted that umbilical cords were attached, with insertion adjacent to each other [123]. The blood from the pump twin flows through the umbilical artery to the umbilical artery of the acardiac twin and then it flows back to the recipient twin through the umbilical vein. The returning blood bypasses the placenta and returns to the pump twin via VV anastomoses, without passing through the placenta. This condition may cause a hyperdynamic circulation and progressive high output cardiac failure in the pump twin causing fetal death in about half of cases if not treated [122, 124, 125].
The diagnostic is made by turning on the color Doppler and showing the inverse direction of blood flow in the aorta of the acardiac twin [92] (Figure 11). TRAPS is usually diagnosed in the 11–13 weeks scan or even in the early endovaginal ultrasound [126, 127, 128]. Given the fact that 50% of pump twin dies if expectant management is made and that in 33% of the TRAPS pregnancies diagnosed at the first trimester the healthy twin dies before 18 weeks [123, 129], several intrauterine interventions have been tested in order to improve the perinatal outcomes. The overall survival of the treatment methods is similar among several studies and varies between 71 and 86% [130, 131, 132, 133, 134]. The methods used to manage TRAPS are cord ligation; monopolar, bipolar, or laser cord coagulation; and fetoscopic laser coagulation of placental anastomoses. However, intrafetal techniques such as intrafetal laser ablation and intrafetal radiofrequency ablation (RFA) are preferred because, when compared to cord occlusion techniques, they are associated with a lower technical failure rate (13 vs. 35%), lower rate of preterm birth or rupture of membranes before 32 weeks (23 vs. 58%), and higher rate of clinical success (77 vs. 50%) [135].
Figure 11.
Upper image: Acardiac twin with retrograde flow in the umbilical cord. Lower image: Normal recipient twin. Adapted from ultrasound Obstet Gynecol. Pagani et al. [124].
There are some doubts about the optimal time to do the treatment. Performing any procedure before the obliteration of the coelomic cavity increases the risk of talipes and miscarriage [136]; therefore, most of the authors perform the intervention between 13 and 16 weeks [124]. In one study in which the median gestational age at intervention (intrafetal laser ablation) was 13.2 weeks, there was a 41% mortality rate in the first 72 h after the procedure; therefore, surgery before 13 weeks of gestation should be avoided [136]. Some studies showed that expectant management could be offered in special cases. Jelin et al. [125] found a 100% survival when the acardiac twin had less than 50% of the pump twin’s weight. Other studies suggested that discordance between crown-rump length of the pump twin and upper pole-rump length of the TRAP twin could be potential predictors of pregnancy outcome [137].
The optimal approach should be an early diagnosis and a proper parental counseling and an intrafetal intervention, by laser or RFA in 13–16 weeks. The expectant management could be considered if the TRAP twin is smaller (about half the size) than the pump twin.
4. Conclusion
Monochorionic pregnancies are at a great risk of complications such as preterm birth, fetal and neonatal death, and neurological injury. The early sonographic screening is extremely important to diagnose some of the most important complications which can lead to death of one or both siblings. It should begin in the first trimester, where the confirmation of chorionicity should be done and the search for potential predictors of adverse outcomes such as NT discordance should be accessed. Some complications such as TRAPS can be diagnosed and managed in this period. Beginning in the 16th week, a biweekly detailed ultrasound examination is extremely important since it can detect early stages of TTTS, sFGR, and TAPS. Most of these complications can be treated in the mid-trimester improving the survival rate of one or both fetuses.
The fetoscopic approach is the main method to manage MC twin complications and should be available in specialized fetal medicine centers with trained staff to perform the laser surgery. Several laser techniques have been tested in the last years and the improvement in the outcomes is clear. Although the results are satisfactory, the complication rates, such as PROM and unintentional septostomy, are still relatively high as well as the both twins’ survival rate.
Future directions in the management of TTTS are likely to involve refinements in the prediction of the disease, clarification of the optimum frequency of surveillance, technique of laser therapy, prediction of adverse outcome after treatment, and development of other vascular ablative techniques.
Although the treatment efficacy is rapidly improving in big centers, in most parts of the world, there is a lack of specialized centers and trained personnel. In order to achieve an optimal management in MC pregnancy complications, it is important to improve the early screening and diagnosis and the referral system, mainly in low-income countries.
Conflict of interest
There are no conflicts of interest in this chapter.
\n',keywords:"monochorionic, twin to twin transfusion syndrome, TTTS, twin anemia polycythemia sequence, TAPS, selective fetal growth restriction, multiple pregnancy",chapterPDFUrl:"https://cdn.intechopen.com/pdfs/65160.pdf",chapterXML:"https://mts.intechopen.com/source/xml/65160.xml",downloadPdfUrl:"/chapter/pdf-download/65160",previewPdfUrl:"/chapter/pdf-preview/65160",totalDownloads:577,totalViews:172,totalCrossrefCites:0,dateSubmitted:"October 11th 2018",dateReviewed:"December 7th 2018",datePrePublished:"December 31st 2018",datePublished:"January 30th 2019",dateFinished:null,readingETA:"0",abstract:"Monochorionic (MC) pregnancies have higher rates of fetal morbidity and mortality when compared to dichorionic (DC) ones. Therefore, the early diagnostic of chorionicity is of great importance. Monochorionic pregnancies have specific complications such as twin to twin transfusion syndrome (TTTS), selective fetal growth restriction (sFGR), twin anemia polycythemia sequence (TAPS), and twin reversed arterial perfusion sequence (TRAPS). MC pregnancies have several unique and serious complications that contribute to a perinatal mortality rate of 11%. The pathophysiology of most of these complications is related to the placental angio-architecture, and it results from an unbalanced perfusion between the fetuses. The screening of TTTS starts in 16 weeks with a sonographic follow-up every 2 weeks. In the last decade, there was an improvement in the treatment of TTTS. With the advent of the fetoscopic laser photocoagulation (FLPC), there was a drastic increase in the survival rate of the fetuses with TTTS when compared with serial amnioreduction. Besides that, in TRAPS, fetoscopic procedures such as cord occlusion improve the outcome of the normal fetus. We will also discuss sFGR and its classification and management. The aim of this chapter is to review the most important complications in MC pregnancies.",reviewType:"peer-reviewed",bibtexUrl:"/chapter/bibtex/65160",risUrl:"/chapter/ris/65160",signatures:"Bruno Rodrigues Toneto",book:{id:"7173",title:"Multiple Pregnancy",subtitle:"New Challenges",fullTitle:"Multiple Pregnancy - New Challenges",slug:"multiple-pregnancy-new-challenges",publishedDate:"January 30th 2019",bookSignature:"Julio Elito Jr.",coverURL:"https://cdn.intechopen.com/books/images_new/7173.jpg",licenceType:"CC BY 3.0",editedByType:"Edited by",editors:[{id:"35132",title:"Prof.",name:"Julio",middleName:null,surname:"Elito Jr.",slug:"julio-elito-jr.",fullName:"Julio Elito Jr."}],productType:{id:"1",title:"Edited Volume",chapterContentType:"chapter",authoredCaption:"Edited by"}},authors:[{id:"280542",title:"Dr.",name:"Bruno",middleName:null,surname:"Toneto",fullName:"Bruno Toneto",slug:"bruno-toneto",email:"brunotoneto@yahoo.com.br",position:null,institution:null}],sections:[{id:"sec_1",title:"1. Introduction",level:"1"},{id:"sec_2",title:"2. Importance of multiple pregnancy",level:"1"},{id:"sec_3",title:"3. Complications",level:"1"},{id:"sec_3_2",title:"3.1. Twin to twin transfusion syndrome",level:"2"},{id:"sec_3_3",title:"3.1.1. Pathophysiology",level:"3"},{id:"sec_4_3",title:"3.1.2. Clinical manifestations of TTTS",level:"3"},{id:"sec_5_3",title:"Table 1.",level:"3"},{id:"sec_6_3",title:"3.1.4. Management of TTTS",level:"3"},{id:"sec_7_3",title:"3.1.5. Perinatal outcomes after treatment",level:"3"},{id:"sec_9_2",title:"3.2. Selective intrauterine growth restriction",level:"2"},{id:"sec_9_3",title:"3.2.1. Diagnostic criteria and staging",level:"3"},{id:"sec_10_3",title:"3.2.2. Management of sFGR",level:"3"},{id:"sec_12_2",title:"3.3. Twin anemia polycythemia sequence",level:"2"},{id:"sec_12_3",title:"Table 2.",level:"3"},{id:"sec_13_3",title:"3.3.2. Management of TAPS",level:"3"},{id:"sec_15_2",title:"3.4. Twin reversed arterial perfusion sequence",level:"2"},{id:"sec_17",title:"4. Conclusion",level:"1"},{id:"sec_21",title:"Conflict of interest",level:"1"}],chapterReferences:[{id:"B1",body:'Gary Cunningham F. Multifetal pregnancy. In: Williams Obstetrics. 24th ed. McGraw-Hill Education; 2014. pp. 891-924'},{id:"B2",body:'Schwartz DB, Daoud Y, Zazula P, Goyert G, Bronsteen R, Wright D. Gestational diabetes mellitus: Metabolic and blood glucose parameters in singleton versus twin pregnancies. American Journal of Obstetrics and Gynecology. 1999;181(4):912-914'},{id:"B3",body:'Bailit JL. Hyperemesis gravidarum: Epidemiologic findings from a large cohort. American Journal of Obstetrics and Gynecology. 2005;193(3 Pt 1):811-814. DOI: 10.1016/j.ajog.2005.02.132'},{id:"B4",body:'Conde-Agudelo A, Belizán JM, Lindmark G. Maternal morbidity and mortality associated with multiple gestations. 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Diagnosis of twin-to-twin transfusion syndrome, selective fetal growth restriction, twin anemia-polycythaemia sequence, and twin reversed arterial perfusion sequence. Best practice and research. Best Practice & Research. Clinical Obstetrics & Gynaecology. 2014;28:215-226'},{id:"B93",body:'Gratacós E, Lewi L, Muñoz B, Acosta-Rojas R, Hernandez-Andrade E, Martinez JM, et al. A classification system for selective intrauterine growth restriction in monochorionic pregnancies according to umbilical artery Doppler flow in the smaller twin. Ultrasound in Obstetrics & Gynecology. 2007;30(1):28-34'},{id:"B94",body:'Wee LY, Taylor MJ, Vanderheyden T, Talbert D, Fisk NM. Transmitted arterio-arterial anastomosis waveforms causing cyclically intermittent absent/reversed end-diastolic umbilical artery flow in monochorionic twins. Placenta. 2003;24(7):772-778'},{id:"B95",body:'Gratacos E, Lewi L, Carreras E, Becker J, Higueras T, Deprest J, et al. Incidence and characteristics of umbilical artery intermittent absent and/or reversed end-diastolic flow in complicated and uncomplicated monochorionic twin pregnancies. Ultrasound in Obstetrics & Gynecology. 2004;23:456-460. DOI: 10.1002/uog.1013'},{id:"B96",body:'Lewi L, Gucciardo L, Huber A, Jani J, Van Mieghem T, Doné E, et al. Clinical outcome and placental characteristics of monochorionic diamniotic twin pairs with early- and late-onset discordant growth. American Journal of Obstetrics and Gynecology. 2008;199(5):1-7. DOI: 10.1002/uog.18966'},{id:"B97",body:'Townsend R, D’Antonio F, Sileo FG, Kumbay H, Thilaganathan B, Khalil A. Perinatal outcome of monochorionic twin pregnancies complicated by selective fetal growth restriction according to management: A systematic review and meta-analysis. Ultrasound in Obstetrics & Gynecology. 2018. DOI: 10.1002/uog.20114'},{id:"B98",body:'Monaghan C, Kalafat E, Binder J, Thilaganathan B, Khalil A. Prediction of adverse pregnancy outcome in monochorionic- diamniotic twin pregnancies complicated by selective fetal growth restriction. Ultrasound in Obstetrics & Gynecology. 2018. DOI: 10.1002/uog.19078'},{id:"B99",body:'Couck I, Mourad Tawfic N, Deprest J, De Catte L, Devlieger R, Lewi L. Does site of cord insertion increase risk of adverse outcome, twin-to-twin transfusion syndrome and discordant growth in monochorionic twin pregnancy? Ultrasound in Obstetrics & Gynecology. 2018;52(3):385-389. DOI: 10.1002/uo.18926'},{id:"B100",body:'Kalafat E, Thilaganathan B, Papageorghiou A, Bhide A, Khalil A. Significance of placental cord insertion site in twin pregnancy. Ultrasound in Obstetrics & Gynecology. 2018;52(3):378-384. DOI: 10.1002/uog.18914'},{id:"B101",body:'D’Antonio F, Odibo AO, Prefumo F, Khalil A, Buca D, Flacco ME, et al. Weight discordance and perinatal mortality in twin pregnancy: Systematic review and meta-analysis. Ultrasound in Obstetrics & Gynecology. 2018;52(1):11-23. DOI: 10.1002/uog.18966'},{id:"B102",body:'Rustico MA, Consonni D, Lanna M, Faiola S, Schena V, Scelsa B, et al. Selective intrauterine growth restriction in monochorionic twins: Changing patterns in umbilical artery Doppler flow and outcomes. Ultrasound in Obstetrics & Gynecology. 2017;49(3):387-393. DOI: 10.1002/uog.15933'},{id:"B103",body:'Peeva G, Bower S, Orosz L, Chaveeva P, Akolekar R, Nicolaides KH. Endoscopic placental laser coagulation in monochorionic diamniotic twins with type II selective fetal growth restriction. Fetal Diagnosis and Therapy. 2015;38(2):86-93. DOI: 10.1002/uog.15933'},{id:"B104",body:'Gratacós E, Antolin E, Lewi L, Martínez JM, Hernandez-Andrade E, Acosta-Rojas R, et al. Monochorionic twins with selective intrauterine growth restriction and intermittent absent or reversed end-diastolic flow (type III): Feasibility and perinatal outcome of fetoscopic placental laser coagulation. Ultrasound in Obstetrics & Gynecology. 2008;31(6):669-675. DOI: 10.1002/uog.15933'},{id:"B105",body:'Ishii K, Nakata M, Wada S, Murakoshi T, Sago H. Feasibility and preliminary outcomes of fetoscopic laser photocoagulation for monochorionic twin gestation with selective intrauterine growth restriction accompanied by severe oligohydramnios. The Journal of Obstetrics and Gynaecology Research. 2015;41(11):1732-1737. DOI: 10.1002/uog.15933'},{id:"B106",body:'Chalouhi GE, Marangoni MA, Quibel T, Deloison B, Benzina N, Essaoui M, et al. Active management of selective intrauterine growth restriction with abnormal Doppler in monochorionic diamniotic twin pregnancies diagnosed in the second trimester of pregnancy. Prenatal Diagnosis. 2013;33(2):109-115. DOI: 10.1002/uog.15933'},{id:"B107",body:'Hillman SC, Morris RK, Kilby MD. Co-twin prognosis after single fetal death: A systematic review and meta-analysis. Obstetrics and Gynecology. 2011;118(4):928-940. DOI: 10.1002/uog.15933'},{id:"B108",body:'Lopriore E, van den Wijngaard JPHM, Middeldorp JM, Oepkes D, Walther FJ, van Gemert MJ, et al. Assessment of feto-fetal transfusion flow through placental arterio-venous anastomoses in a unique case of twin-to-twin transfusion syndrome. Placenta. 2007;28(2-3):209-211. DOI: 10.1016/j.placenta.2006.03.006'},{id:"B109",body:'De Villiers SF, Slaghekke F, Middeldorp JM, Walther FJ, Oepkes D, Lopriore E. Placental characteristics in monochorionic twins with spontaneous versus post-laser twin anemia-polycythemia sequence. Placenta. 2013;34(5):456-459. DOI: 10.1016/j.placenta.2013.02.005'},{id:"B110",body:'Lopriore E, Deprest J, Slaghekke F, Oepkes D, Middeldorp JM, Vandenbussche FPHA, et al. Placental characteristics in monochorionic twins with and without twin anemia–Polycythemia sequence. Obstetrics and Gynecology. 2008;112(4):753-758. DOI: 10.1097/AOG.0b013e318187e1ff'},{id:"B111",body:'Slaghekke F, Kist WJ, Oepkes D, Pasman SA, Middeldorp JM, Klumper FJ, et al. Twin anemia-polycythemia sequence: Diagnostic criteria, classification, perinatal management and outcome. Fetal Diagnosis and Therapy. 2010;27(4):181-190. DOI: 10.1159/000304512'},{id:"B112",body:'Zhao DP, de Villiers SF, Slaghekke F, Walther FJ, Middeldorp JM, Oepkes D, et al. Prevalence, size, number and localization of vascular anastomoses in monochorionic placentas. Placenta. 2013;34(7):589-593. DOI: 10.1159/000304512'},{id:"B113",body:'Tollenaar LSA, Slaghekke F, Middeldorp JM, Klumper FJ, Haak MC, Oepkes D, et al. Twin anemia polycythemia sequence: Current views on pathogenesis, diagnostic criteria, perinatal management, and outcome. Twin Research and Human Genetics. 2016;19(3):222-233. DOI: 10.1017/thg.2016.18'},{id:"B114",body:'Yokouchi T, Murakoshi T, Mishima T, Yano H, Ohashi M, Suzuki T, et al. Incidence of spontaneous twin anemia-polycythemia sequence in monochorionic-diamniotic twin pregnancies: Single-center prospective study. The Journal of Obstetrics and Gynaecology Research. 2015;41(6):857-860. DOI: 110.1111/jog.12641'},{id:"B115",body:'Slaghekke F, Pasman S, Veujoz M, Middeldorp JM, Lewi L, Devlieger R, et al. Middle cerebral artery peak systolic velocity to predict fetal hemoglobin levels in twin anemia-polycythemia sequence. Ultrasound in Obstetrics & Gynecology. 2015;46(4):432-436. DOI: 10.1002/uog.14925'},{id:"B116",body:'Soundararajan LP, Howe DT. Starry sky liver in twin anemia-polycythemia sequence. Ultrasound in Obstetrics & Gynecology. 2014;43(5):597-599. DOI: 10.1002/uog.13276'},{id:"B117",body:'Lopriore E, Slaghekke F, Oepkes D, Middeldorp JM, Vandenbussche FPHA, Walther FJ. Hematological characteristics in neonates with twin anemia-polycythemia sequence (TAPS). Prenatal Diagnosis. 2010;30(3):251-255. DOI: 10.1002/pd.2453'},{id:"B118",body:'Slaghekke F, Favre R, Peeters SHP, Middeldorp JM, Weingertner AS, van Zwet EW, et al. Laser surgery as a management option for twin anemia-polycythemia sequence. Ultrasound in Obstetrics & Gynecology. 2014;44(3):304-310. DOI: 10.1002/uog.6334'},{id:"B119",body:'Herway C, Johnson A, Moise K, Moise KJ. Fetal intraperitoneal transfusion for iatrogenic twin anemia-polycythemia sequence after laser therapy. Ultrasound in Obstetrics & Gynecology. 2009;33(5):592-594. DOI: 10.1002/uog.6334'},{id:"B120",body:'Groussolles M, Sartor A, Connan L, Vayssire C. Evolution of middle cerebral artery peak systolic velocity after a successful laser procedure for iatrogenic twin anemia-polycythemia sequence. Ultrasound in Obstetrics & Gynecology. 2012;39(3):354-356. DOI: 10.1002/uog.6334'},{id:"B121",body:'Ishii K, Hayashi S, Mabuchi A, Taguchi T, Yamamoto R, Murata M, et al. Therapy by laser equatorial placental dichorionization for early-onset spontaneous twin anemia-polycythemia sequence. Fetal Diagnosis and Therapy. 2013;35(1):65-68. DOI: 10.1002/uog.6334'},{id:"B122",body:'Moore TR, Gale S, Benirschke K. Perinatal outcome of forty-nine pregnancies complicated by acardiac twinning. American Journal of Obstetrics and Gynecology. 1990;163:907-912'},{id:"B123",body:'Steffensen TS, Gilbert-Barness E, Spellacy W, Quintero RA. Placental pathology in trap sequence: Clinical and pathogenetic implications. Fetal and Pediatric Pathology. 2008;27(1):13-29. DOI: 10.1002/uog.6334'},{id:"B124",body:'Pagani G, D’Antonio F, Khalil A, Papageorghiou A, Bhide A, Thilaganathan B. Intrafetal laser treatment for twin reversed arterial perfusion sequence: Cohort study and meta-analysis. Ultrasound in Obstetrics & Gynecology. 2013;42(1):6-14. DOI: 10.1002/uog.12495'},{id:"B125",body:'Jelin E, Hirose S, Rand L, Curran P, Feldstein V, Guevara-Gallardo S, et al. Perinatal outcome of conservative management versus fetal intervention for twin reversed arterial perfusion sequence with a small acardiac twin. Fetal Diagnosis and Therapy. 2010;27(3):138-141. DOI: 10.1159/000295176'},{id:"B126",body:'Zucchini S, Borghesani F, Soffriti G, Chirico C, Vultaggio E, Di Donato P. Transvaginal ultrasound diagnosis of twin reversed arterial perfusion syndrome at 9 weeks’ gestation. Ultrasound in Obstetrics & Gynecology. 1993;3(3):209-211. DOI: 10.1046/j.1469-0705.1993.03030209.x'},{id:"B127",body:'Schwärzler P, Ville Y, Moscosco G, Tennstedt C, Bollmann R, Chaoui R. Diagnosis of twin reversed arterial perfusion sequence in the first trimester by transvaginal color Doppler ultrasound. Ultrasound in Obstetrics & Gynecology. 1999;13(2):143-146. DOI: 10.1046/j.1469-0705.1999.13020143.x'},{id:"B128",body:'Coulam CB, Wright G. First trimester diagnosis of acardiac twins. Early Pregnancy. 2000;4(4):261-270'},{id:"B129",body:'Lewi L, Valencia C, Gonzalez E, Deprest J, Nicolaides KH. The outcome of twin reversed arterial perfusion sequence diagnosed in the first trimester. American Journal of Obstetrics and Gynecology. 2010;203(3):213.e1-213.e4. DOI: 10.1002/uog.12495'},{id:"B130",body:'Lee H, Wagner AJ, Sy E, Ball R, Feldstein VA, Goldstein RB, et al. Efficacy of radiofrequency ablation for twin-reversed arterial perfusion sequence. American Journal of Obstetrics and Gynecology. 2007;196(5):1-4. DOI: 10.1016/j.ajog.2006.11.039'},{id:"B131",body:'Hecher K, Lewi L, Gratacos E, Huber A, Ville Y, Deprest J. Twin reversed arterial perfusion: Fetoscopic laser coagulation of placental anastomoses or the umbilical cord. Ultrasound in Obstetrics & Gynecology. 2006;28(5):688-691. DOI: 10.1002/uog.3816'},{id:"B132",body:'Sugibayashi R, Ozawa K, Sumie M, Wada S, Ito Y, Sago H. Forty cases of twin reversed arterial perfusion sequence treated with radio frequency ablation using the multistep coagulation method: A single-center experience. Prenatal Diagnosis. 2016;36(5):437-443. DOI: 10.1002/pd.4800'},{id:"B133",body:'Cabassa P, Fichera A, Prefumo F, Taddei F, Gandolfi S, Maroldi R, et al. The use of radiofrequency in the treatment of twin reversed arterial perfusion sequence: A case series and review of the literature. European Journal of Obstetrics, Gynecology, and Reproductive Biology. 2013;166(2):127-132. DOI: 10.1016/j.ejogrb.2012.10.009'},{id:"B134",body:'Lee H, Bebbington M, Crombleholme TM. The north American fetal therapy network registry data on outcomes of radiofrequency ablation for twin-reversed arterial perfusion sequence. Fetal Diagnosis and Therapy. 2013;33(4):224-229. DOI: 10.1159/000343223'},{id:"B135",body:'Tan TYT, Sepulveda W. Acardiac twin: A systematic review of minimally invasive treatment modalities. Ultrasound in Obstetrics & Gynecology. 2003;22(4):409-419'},{id:"B136",body:'Nicolaides K, Brizot Mde L, Patel F, Snijders R. Comparison of chorionic villus sampling and amniocentesis for fetal karyotyping at 10-13 weeks’ gestation. Lancet. 1994;344(8920):435-439'},{id:"B137",body:'Roethlisberger M, Strizek B, Gottschalk I, Mallmann MR, Geipel A, Gembruch U, et al. First-trimester intervention in twin reversed arterial perfusion sequence: Does size matter? Ultrasound in Obstetrics & Gynecology. 2017;50(1):40-44'}],footnotes:[],contributors:[{corresp:"yes",contributorFullName:"Bruno Rodrigues Toneto",address:"brunotoneto@yahoo.com.br",affiliation:'
Federal University of São Paulo, São Paulo, Brazil
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The company was founded in Vienna in 2004 by Alex Lazinica and Vedran Kordic, two PhD students researching robotics. While completing our PhDs, we found it difficult to access the research we needed. So, we decided to create a new Open Access publisher. A better one, where researchers like us could find the information they needed easily. The result is IntechOpen, an Open Access publisher that puts the academic needs of the researchers before the business interests of publishers.
",metaTitle:"Our story",metaDescription:"The company was founded in Vienna in 2004 by Alex Lazinica and Vedran Kordic, two PhD students researching robotics. While completing our PhDs, we found it difficult to access the research we needed. So, we decided to create a new Open Access publisher. A better one, where researchers like us could find the information they needed easily. The result is IntechOpen, an Open Access publisher that puts the academic needs of the researchers before the business interests of publishers.",metaKeywords:null,canonicalURL:"/page/our-story",contentRaw:'[{"type":"htmlEditorComponent","content":"
We started by publishing journals and books from the fields of science we were most familiar with - AI, robotics, manufacturing and operations research. Through our growing network of institutions and authors, we soon expanded into related fields like environmental engineering, nanotechnology, computer science, renewable energy and electrical engineering, Today, we are the world’s largest Open Access publisher of scientific research, with over 4,200 books and 54,000 scientific works including peer-reviewed content from more than 116,000 scientists spanning 161 countries. Our authors range from globally-renowned Nobel Prize winners to up-and-coming researchers at the cutting edge of scientific discovery.
\\n\\n
In the same year that IntechOpen was founded, we launched what was at the time the first ever Open Access, peer-reviewed journal in its field: the International Journal of Advanced Robotic Systems (IJARS).
\\n\\n
The IntechOpen timeline
\\n\\n
2004
\\n\\n
\\n\\t
Intech Open is founded in Vienna, Austria, by Alex Lazinica and Vedran Kordic, two PhD students, and their first Open Access journals and books are published.
\\n\\t
Alex and Vedran launch the first Open Access, peer-reviewed robotics journal and IntechOpen’s flagship publication, the International Journal of Advanced Robotic Systems (IJARS).
\\n
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2005
\\n\\n
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IntechOpen publishes its first Open Access book: Cutting Edge Robotics.
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2006
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IntechOpen publishes a special issue of IJARS, featuring contributions from NASA scientists regarding the Mars Exploration Rover missions.
\\n
\\n\\n
2008
\\n\\n
\\n\\t
Downloads milestone: 200,000 downloads reached
\\n
\\n\\n
2009
\\n\\n
\\n\\t
Publishing milestone: the first 100 Open Access STM books are published
\\n
\\n\\n
2010
\\n\\n
\\n\\t
Downloads milestone: one million downloads reached
\\n\\t
IntechOpen expands its book publishing into a new field: medicine.
\\n
\\n\\n
2011
\\n\\n
\\n\\t
Publishing milestone: More than five million downloads reached
\\n\\t
IntechOpen publishes 1996 Nobel Prize in Chemistry winner Harold W. Kroto’s “Strategies to Successfully Cross-Link Carbon Nanotubes”. Find it here.
\\n\\t
IntechOpen and TBI collaborate on a project to explore the changing needs of researchers and the evolving ways that they discover, publish and exchange information. The result is the survey “Author Attitudes Towards Open Access Publishing: A Market Research Program”.
\\n\\t
IntechOpen hosts SHOW - Share Open Access Worldwide; a series of lectures, debates, round-tables and events to bring people together in discussion of open source principles, intellectual property, content licensing innovations, remixed and shared culture and free knowledge.
\\n
\\n\\n
2012
\\n\\n
\\n\\t
Publishing milestone: 10 million downloads reached
\\n\\t
IntechOpen holds Interact2012, a free series of workshops held by figureheads of the scientific community including Professor Hiroshi Ishiguro, director of the Intelligent Robotics Laboratory, who took the audience through some of the most impressive human-robot interactions observed in his lab.
\\n
\\n\\n
2013
\\n\\n
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IntechOpen joins the Committee on Publication Ethics (COPE) as part of a commitment to guaranteeing the highest standards of publishing.
\\n
\\n\\n
2014
\\n\\n
\\n\\t
IntechOpen turns 10, with more than 30 million downloads to date.
\\n\\t
IntechOpen appoints its first Regional Representatives - members of the team situated around the world dedicated to increasing the visibility of our authors’ published work within their local scientific communities.
\\n
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2015
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\\n\\t
Downloads milestone: More than 70 million downloads reached, more than doubling since the previous year.
\\n\\t
Publishing milestone: IntechOpen publishes its 2,500th book and 40,000th Open Access chapter, reaching 20,000 citations in Thomson Reuters ISI Web of Science.
\\n\\t
40 IntechOpen authors are included in the top one per cent of the world’s most-cited researchers.
\\n\\t
Thomson Reuters’ ISI Web of Science Book Citation Index begins indexing IntechOpen’s books in its database.
\\n
\\n\\n
2016
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\\n\\t
IntechOpen is identified as a world leader in Simba Information’s Open Access Book Publishing 2016-2020 report and forecast. IntechOpen came in as the world’s largest Open Access book publisher by title count.
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2017
\\n\\n
\\n\\t
Downloads milestone: IntechOpen reaches more than 100 million downloads
\\n\\t
Publishing milestone: IntechOpen publishes its 3,000th Open Access book, making it the largest Open Access book collection in the world
We started by publishing journals and books from the fields of science we were most familiar with - AI, robotics, manufacturing and operations research. Through our growing network of institutions and authors, we soon expanded into related fields like environmental engineering, nanotechnology, computer science, renewable energy and electrical engineering, Today, we are the world’s largest Open Access publisher of scientific research, with over 4,200 books and 54,000 scientific works including peer-reviewed content from more than 116,000 scientists spanning 161 countries. Our authors range from globally-renowned Nobel Prize winners to up-and-coming researchers at the cutting edge of scientific discovery.
\n\n
In the same year that IntechOpen was founded, we launched what was at the time the first ever Open Access, peer-reviewed journal in its field: the International Journal of Advanced Robotic Systems (IJARS).
\n\n
The IntechOpen timeline
\n\n
2004
\n\n
\n\t
Intech Open is founded in Vienna, Austria, by Alex Lazinica and Vedran Kordic, two PhD students, and their first Open Access journals and books are published.
\n\t
Alex and Vedran launch the first Open Access, peer-reviewed robotics journal and IntechOpen’s flagship publication, the International Journal of Advanced Robotic Systems (IJARS).
\n
\n\n
2005
\n\n
\n\t
IntechOpen publishes its first Open Access book: Cutting Edge Robotics.
\n
\n\n
2006
\n\n
\n\t
IntechOpen publishes a special issue of IJARS, featuring contributions from NASA scientists regarding the Mars Exploration Rover missions.
\n
\n\n
2008
\n\n
\n\t
Downloads milestone: 200,000 downloads reached
\n
\n\n
2009
\n\n
\n\t
Publishing milestone: the first 100 Open Access STM books are published
\n
\n\n
2010
\n\n
\n\t
Downloads milestone: one million downloads reached
\n\t
IntechOpen expands its book publishing into a new field: medicine.
\n
\n\n
2011
\n\n
\n\t
Publishing milestone: More than five million downloads reached
\n\t
IntechOpen publishes 1996 Nobel Prize in Chemistry winner Harold W. Kroto’s “Strategies to Successfully Cross-Link Carbon Nanotubes”. Find it here.
\n\t
IntechOpen and TBI collaborate on a project to explore the changing needs of researchers and the evolving ways that they discover, publish and exchange information. The result is the survey “Author Attitudes Towards Open Access Publishing: A Market Research Program”.
\n\t
IntechOpen hosts SHOW - Share Open Access Worldwide; a series of lectures, debates, round-tables and events to bring people together in discussion of open source principles, intellectual property, content licensing innovations, remixed and shared culture and free knowledge.
\n
\n\n
2012
\n\n
\n\t
Publishing milestone: 10 million downloads reached
\n\t
IntechOpen holds Interact2012, a free series of workshops held by figureheads of the scientific community including Professor Hiroshi Ishiguro, director of the Intelligent Robotics Laboratory, who took the audience through some of the most impressive human-robot interactions observed in his lab.
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2013
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IntechOpen joins the Committee on Publication Ethics (COPE) as part of a commitment to guaranteeing the highest standards of publishing.
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2014
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IntechOpen turns 10, with more than 30 million downloads to date.
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IntechOpen appoints its first Regional Representatives - members of the team situated around the world dedicated to increasing the visibility of our authors’ published work within their local scientific communities.
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2015
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Downloads milestone: More than 70 million downloads reached, more than doubling since the previous year.
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Publishing milestone: IntechOpen publishes its 2,500th book and 40,000th Open Access chapter, reaching 20,000 citations in Thomson Reuters ISI Web of Science.
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40 IntechOpen authors are included in the top one per cent of the world’s most-cited researchers.
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Thomson Reuters’ ISI Web of Science Book Citation Index begins indexing IntechOpen’s books in its database.
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2016
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IntechOpen is identified as a world leader in Simba Information’s Open Access Book Publishing 2016-2020 report and forecast. IntechOpen came in as the world’s largest Open Access book publisher by title count.
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2017
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Downloads milestone: IntechOpen reaches more than 100 million downloads
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Publishing milestone: IntechOpen publishes its 3,000th Open Access book, making it the largest Open Access book collection in the world
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