Summary of key articles demonstrating photoacoustic imaging of the eye.
Abstract
Photoacoustic imaging (PAI) is a novel, hybrid, non-ionizing, and non-invasive imaging technology with high-resolution, high sensitivity, high-contrast, and high depth of penetration. Hence, it has particularly useful applications in eye investigations. It can provide both anatomic and functional ocular characterizations. Many eye diseases, including macular degeneration and diabetic retinopathy, involve abnormalities in the vasculature, and thus the ability of PAI to affectively visualize the vasculature can be incredibly helpful to evaluate normal and disease states of the eye. In future research, PAI of the eye can be dramatically improved in terms of its resolution, use of contrast agents for molecular imaging, safety evaluations to develop a clinically approved system, and integration with existing fundus imaging modalities. Multimodality ocular imaging platforms have also been successfully developed by a combination of photoacoustic microscopy (PAM) with other optical imaging such as optical coherence tomography (OCT), scanning laser ophthalmoscopy (SLO), and fluorescence microscopy (FM). The multimodal images can accurately be acquired from a single imaging system and co-registered on the same image plane, enabling improved evaluation of eye disease states. In this book chapter, the potential application of photoacoustic imaging of the eye in both research and clinical diagnosis are comprehensively discussed as a powerful medical screening technique for visualization of various ocular diseases.
Keywords
- photoacoustic imaging
- photoacoustic ophthalmoscopy
- photoacoustic microscopy
- multimodal imaging
1. Introduction
The eye is unique organ in that it is optically transparent to visible light in normal health and thus offers optical access to internal structures. The refractive system of the normal eye includes the cornea, the fluid-filled anterior chamber, crystalline lens, and gel-like vitreous. These optically transparent structures have low absorption and light scattering to visible light, which plays a pivotal role in the entry of light and the formation of visual pathways. In the eye, the major endogenous light-absorbing materials are hemoglobin and melanin. Hemoglobin is present in blood vessels in the iris, choroid, ciliary body, retina, conjunctiva, and pathologic neovascularization. Melanin is present inside the uvea (iris, choroid and ciliary body), retinal pigment epithelium (RPE) and pigmented tumors.
Photoacoustic imaging (PAI), which converts light energy into sound waves, has been demonstrated to image hemoglobin [1, 2] and melanin [3, 4]. Thus eyes are an ideal system for PAI. PAI can provide an ocular vascular image based on the inherent optical absorbance of hemoglobin. Many eye diseases, including corneal neovascularization, macular degeneration, diabetic retinopathy, sickle cell retinopathy, and retinal vein occlusions, involve abnormalities in the vasculature. Thus the unique ability of PAI to provide detailed vascular morphologic information can assist ophthalmic diagnosis.
Currently several imaging modalities have been used to diagnose and monitor ocular diseases. Well-established imaging instruments include color fundus photography, ultrasound (US) imaging, fluorescein angiography (FA), indocyanine green angiography (ICGA), scanning laser ophthalmoscopy (SLO), optical coherence tomography (OCT) and OCT angiography (OCTA). Each imaging modality has advantages and disadvantages. OCT has a limited depth of penetration and has difficulty visualizing deep structures like the choroid and sclera. FA and ICGA require the use of an exogenous contrast agent that may cause adverse reactions and cannot specify the depth of the vessels. OCTA is unable to show leakage, provides limited view of microaneurysms, has a limited depth of penetration, and has a restricted field of view often with motion artifacts.
PAI is a new, emerging, non-ionizing and non-invasive imaging technology. PAI is based on the optical absorption contrast so that can provide anatomical information and functional analysis for the eye as well as with a high depth of penetration. In 2010, photoacoustic imaging was first described in the eye of a living animal [4]. Since then, there is a huge advance in photoacoustic imaging of the eye. Furthermore, the application of multimodality imaging systems including integrated photoacoustic microscopy (PAM) with other imaging modalities and the use of contrast agents for molecular imaging have undergone significant development. This chapter focuses on recent advances and applications of photoacoustic imaging of the eyes.
2. Photoacoustic imaging of the eye
Photoacoustic imaging systems can be divided into several categories which include: optical-resolution photoacoustic microscopy (OR-PAM), acoustic-resolution photoacoustic microscopy (AR-PAM), photoacoustic computed tomography (PACT), photoacoustic tomography (PAT) [5, 6]. Ocular PAI systems are primarily OR-PAM and AR-PAM systems due to the importance of high resolution imaging in the eye [7, 8, 9]. AR-PAM is used to image deep-tissues, where an illumination laser is diffusively delivered to the tissue and a focused acoustic detector is used to detect the induced PA signals. The lateral resolution of AR-PAM is determined by the acoustic focus spot, and the axial resolution is determined by the acoustic center frequency and bandwidth [5, 8]. OR-PAM has a tighter optical focus than AR-PAM and can achieve micrometer-level lateral resolution. The lateral resolution of OR-PAM is determined by the optical focal spot, and the axial resolution is still determined by the acoustic parameters [5, 8].
PAI can also be classified into mechanical-scanning systems and optical-scanning imaging systems [7]. In the mechanical-scanning mode, both the optical excitation and ultrasound transducer are simultaneously scanned over a planar surface. Each step of the scan can detect and generate a PA signal. During imaging, a water tank is necessary to maintain ultrasound coupling [6]. The mechanical-scanning can be used to visualize almost all the eye tissues, including the cornea, iris, lens, retina, choroid, sclera, and blood vessels [4]. In the optical-scanning mode, the focused optical illumination is raster-scanned using galvanometers, while the ultrasound transducer is kept stationary [10]. Figure 1a shows the schematic diagram of the optical-scanning in a multimodality system. Figure 1b illustrates the physical setup [11]. Compared with mechanical-scanning, optical-scanning can provide higher scanning speed, and it is suitable for chorioretinal microvasculature imaging, and compatibility with OCT and SLO [9, 12].
Currently, many applications of ocular PAI have been reported. This chapter summarizes those applications by the anatomic portion of the eye imaged and functional information.
2.1 PA imaging of the anterior segment vasculature of the eye
2.1.1 Normal iris vasculature
Anterior segment PA imaging of a normal eye receives signal from hemoglobin in the red blood cells of the iris microvasculature. Both AR-PAM and OR-PAM have been reported for iris vascular imaging. de la Zerda et al. [4] have used AR-PAM to qualitatively assess the rabbit eye
2.1.2 Corneal neovascularization
Normal corneal tissue is avascular and optically transparent. Corneal neovascularization is a pathologic condition that can lead to reduced vision characterized by the presence of blood vessels in the cornea. This pathophysiology has been observed in humans with inflammatory or autoimmune responses.
Liu et al. [16] have used a mechanical scanning OR-PAM system to image corneal neovascularization of alkali-burn-injured mouse eyes
Several of the above studies of the anterior segment vasculature images have utilized mechanical scanning OR-PAM system. However, mechanical scanning OR-PAM has limitations: the imaging speed is slow and careful coupling medium is required. Recently, a non-interferometric photoacoustic remote sensing microscopy system has been developed with a high scan rate and an improved signal-to-noise (SNR).
2.2 PA imaging of the posterior segment vasculature of the eye
2.2.1 Retinal vasculature
Posterior segment PAI of a normal eye images red blood cells in the retinal and choroidal microvasculature. To obtain the images, both AR-PAM and OR-PAM systems have been used, along with mechanical scanning and optical scanning systems.
Using an AR-PAM system, de la Zerda et al. [4] have used a pulsed laser (wavelength 740 nm) combined with a 25 MHz central frequency transducer to mechanically scan a living rabbit eye. They have obtained the blood distribution of the posterior eye. But owing to the system’s lateral resolution of 200 μm, limited features of the retinal or choroidal vasculature are visualized. Furthermore, it took 90 min to acquire the full 3-D image.
Jiao et al. [12] have developed an optical-scanning OR-PAM system for
Before 2017, most reported PAM imaging of the posterior segment used small animals, like mice and rats. Mice have an axial length of ~3 mm and rats ~6 mm, which is much smaller that the human axial length of ~23 mm. In order to improve clinical translation of the technology, Tian et al. [19] have developed a novel PAM system to visualize rabbit chorioretinal vasculature. Rabbit eyes have an axial length of 18.1 mm [23], which is almost 80% of the axial length of human eyes. They have used a 570 nm pulsed laser. The excited PA signals have been captured by a custom-built needle-shaped ultrasonic transducer (27 MHz; bandwidth: 60%) placed in contact with the conjunctiva by ultrasound gel. The laser pulse energy on the rabbit cornea was 80 nJ or half the ANSI safety limit [24]. The lateral and axial resolution of the PAM system were 4.1 and 37 μm, respectively. This PAM system obtain high-resolution images of New Zealand rabbit retinal and choroidal vasculature (Figure 4d–f).
2.2.2 Choroidal vasculature
Wei et al. [25] have first applied an OCT-guided PAOM system to image the choroidal vessels in albino rats. The laser output optical wavelength was 578 nm, and the laser pulse energy was set below 40 nJ. The induced PA waves were detected by a custom-built, unfocused needle ultrasonic detector (35 MHz; bandwidth: 50%). The lateral resolution of PAOM was around 20 μm and the axial resolution of PAOM was 23 μm. Based on the OCT cross-sectional images, the distance from the retinal vessel layer to the choroid was found to be 200 μm [26]. Thus, the retinal and choroidal vessels were well resolved along the axial direction. Figure 5a–c show PAOM images of segmented retinal vessels and choroidal vessels. Song et al. [20, 27] have used an integrated PAOM and OCT system to detect
2.2.3 Retinal neovascularization
Retinal neovascularization (RNV) is the growth of abnormal new retinal blood vessels and represents a major cause of vision loss and blindness. RNV is a common complication of several retinal diseases, including proliferative diabetic retinopathy (PDR), retinal vein occlusions (RVO), sickle cell retinopathy, and retinopathy of prematurity (ROP).
Zhang et al. [11] have demonstrated an integrated PAM, OCT, and fluorescence microscopy (FM) multimodality system to evaluate RNV
2.2.4 Choroidal neovascularization
Choroidal neovascularization (CNV) is characterized by the abnormal growth of new vessels originating from the choroidal vasculature and their subsequent growth under the RPE, subretinal space, or a combination of both [29]. CNV most commonly occurs in neovascular age-related macular degeneration (AMD), which is a major cause of vision loss. Dai et al. have used PAM system to investigate laser-induced rat CNV evolution. For the PAM system, the laser output wavelength was 532 nm, and the pulse energy was 60 nJ. The induced PA signals from the retina were detected by a customized ultrasonic transducer (30 MHz; bandwidth: 15 MHz). the axial and lateral resolution of PAM were 50 and 20 μm, respectively. The new capillaries growing from the choroid up to subretinal space can be distinguished as shown in Figure 7.
2.3 PA imaging in retinal metabolism
Retinal oxygen metabolic rate (rMRO2) is an essential parameter in the retina and can help further understanding of some blinding diseases, such as diabetic retinopathy [31, 32] and glaucoma [33, 34]. The precise measurement of rMRO2 can be critical in investigating these blinding diseases. Song et al. [35] and Liu et al. [36] have successfully determined rMRO2 in rats by integrating PAOM and OCT. Obtaining rMRO2 measurements required measuring retinal blood flow and sO2 together. They have quantified retinal blood flow by Doppler SD-OCT, and retinal sO2 by PAOM at three wavelengths (570, 578, and 588 nm). Owing to the distinct light absorption spectrum between HbO2 and HbR, multi-wavelength imaging can assess the sO2 in retinal vessels. They have calculated total retinal blood flow as 7.43 ± 0.51 and 7.38 ± 0.78 μL/min within the venous and arterial systems, respectively. The sO2 value in arterial and venous blood were 93.0 ± 3.5 and 77.3 ± 9.1%, respectively. In PAOM system, they used a tunable laser (output laser wavelength: 570, 578, and 588 nm), the laser energy was 40 nJ/pause. Acoustic waves were detected by an unfocused small-footprint ultrasonic transducer (40 MHz; bandwidth: 30 MHz). The lateral resolution was 20 μm, and the axial resolution was 23 μm. The study measured rMRO2 in normal small animals. Hariri et al. [37] have demonstrated the use of PA ocular imaging (PAOI) in measuring chorioretinal oxygen saturation (CR-sO2) gradients in New Zealand white rabbits with ocular ischemia model. The PAOI signal showed a sixfold decrease in CR-sO2 after significant elevation of IOP during ischemia. In the PAOI, they used a tunable laser (680–970 nm), 750 and 850 nm were used to differentiate the HbO2 and HbR. A linear array ultrasound transducer with a center frequency of 15 MHz was used to detect acoustic signals. The lateral and axial resolution were 580 and 290 μm, respectively. One limitation of the study was balancing special resolution and penetration depth. Although they could achieve much deeper penetration, they could not discriminate between the retina and the choroid, nor could they distinguish individual vessels.
2.4 PA imaging of melanin of the eye
Melanin is naturally present in the eye within the iris, ciliary body, pigmented choroid, and RPE [38]. The RPE is a single layer of epithelial cells beneath the neurosensory retina and tightly adherent to the underlying choroid. The RPE plays a crucial role in the overall health of the retina: nourishing photoreceptors and disposing of retinal waste and metabolic end products. The melanin concentration in RPE can decrease over time due to light exposure and oxidative stress [39]. RPE melanin decrease is a sign of ocular senescence and is a risk factor and a pathognomonic sign of AMD [40, 41]. To further study melanin, PAI has been used to specifically detect and quantify melanin [42].
In 2010, Silverman et al. [43] for the first time have demonstrated that PAI can acquire images of melanin in the iris in
The above papers are qualitative studies of eye melanin by PAI. PAI also has the capability of providing a quantitative reading of melanin concentration in the eye. Shu et al. [49] have performed a Monte Carlo (MC) stimulation to investigate light propagation and energy deposition in the eye and tested the feasibility of using PAOM to quantify the RPE melanin concentration. In this study, they used PA signals from retinal blood vessels as references to measure RPE melanin variation. However, the main challenge in accurate quantification of RPE melanin concentration is to distinguish the RPE from underlying pigmented choroid. The RPE is a 10 μm thick cell monolayer and is tightly attached to the choroid, and thus a higher axial resolution of PAOM was necessary. Shu et al. [45] then have used a micro-ring resonator (MRR) detector in their PAOM system to increase the axial resolution (<10 μm). They obtained images where the RPE and choroid can be distinguished in
PAI can obtain volumetric images of melanin in the eye and can quantify melanin in the RPE and the choroid. However, the technique has yet to be explored in eye disease models and the human eye, so it is unknown that how it can be used in eye research and clinic in the future.
2.5 Multimodal PA imaging system
PA imaging can obtain structural and functional imaging of the eye. PAI can also be combined with other ocular imaging devices, such as OCT, SLO, AF, AO, FA, and fundus photography. These combined multimodal platforms can compensate the weaknesses of each system and provide more comprehensive information of the eye. That would be very beneficial for investigating ocular pathology and detecting disease. For instance, with correcting wave-front errors with AO, the lateral resolution of PAM can be improved [44]. With PAM and AF combined, information on the distribution and concentration of retinal melanin and lipofuscin can be obtained
3. Limitations and future directions
3.1 Innovative laser sources for PA imaging
PA imaging can achieve anatomic information with monochromatic pulsed laser illumination. However, to acquire ocular functional information using PAI, multi-wavelength laser illuminations is required. An optical parametric oscillator (OPO) (for example, the Ekspla NT-242, pulse repetition rate 1 kHz, duration 3–6 ns, tunable wavelength range 405–2600 nm) can be used as a suitable laser source to obtain the above information. However, in multimodality systems, such as PA imaging and OCT, the two modalities can require different types of illuminations and thus separate light sources. Liu et al. [48] have used a single ultrafast laser source (pulse repetition rate 10 kHz, pulse duration 3 ns, center wavelength 800 nm; bandwidth 30 nm) to acquire melanin-specific images of
3.2 Non-contact detection of PA signal
In reported studies to date, researchers either placed a water tank on the cornea or directly coupled an ultrasonic needle transducer with the eyelid or sclera using ultrasonic gel or balanced salt solution (BBS). Given the sensitivity of the human eye, this is not ideal for clinical application on patients. Non-contact remote sensing has been described where an interferometer is used to sense the surface displacement induced by PA ultrasound signals [53, 54]. Discovering a non-contact ultrasonic detection with both stability and sensitivity is a promising area of active investigation.
3.3 Field of view of PA imaging
The field of view (FOV) of PA imaging is limited by the ultrasound transducer. The typical scan area of reported studies has been shown in Table 1. For most studies, the image area was no more than 3 × 3 mm. The human retina area is about 10 cm2, so increasing the field of view would be beneficial, possibly through novel ultrasound transducer configurations or array patterns.
PA system | Applications | Publication | Scanning pattern | Laser illumination | US | Speed and size | Resolution | Animal |
---|---|---|---|---|---|---|---|---|
AR-PAM | whole eye | de la Zerda [4] | Wide field illumination | W: 700/740 nm E: 0.5 mJ/cm2 | CF: 15/25 MHz B: 60% | 90 min for 12 × 8 mm | L: 200/240 μm A: 83/50 μm | rabbit |
OR-PAM | Iris vasculature and sO2 | Hu [13] | Mechanical scanning | W: 570/578 nm E: 40 nJ | CF:75 MHz | 120 min for 2 × 2 mm | L: 5 μm A: 15 μm | Swiss Webster mouse |
OR-PAM | Iris vasculature | Rao [15] | Hybrid-scanning | W: 532 nm E: 60 nJ | B: 25 MHz | 128 s for 4 × 4 mm | L: 3.5 μm A: <31 μm | Albino mouse |
OR-PAM | Iris vasculature | Zhao [14, 55] | Mechanical scanning | W: 532 nm E: 11 mJ/cm2 | CF: 75 MHz | 0.5 frame/s | L: 5.43 μm | SD rat |
OR-PAM | Corneal NV; iris vasculature | Liu [16] | Mechanical scanning | W: 532 nm E: 80 nJ | CF:10 MHz B: 80% | 20 min for 3 × 3 mm | L: 2.76 μm A: 50 μm | C57BL6/J mouse |
OR-PAM | Anterior vasculature | Jeon [17] | Mechanical scanning | W: 532 nm | CF: 50 MHz B: 50 MHz | 3 × 3 mm | L: 3 μm | BALB/c mouse |
OR-PAM | Corneal NV; blood flow and sO2 | Kelly-Goss [18] | Mechanical scanning | W: 532/559 nm E: 50/30 nJ | CF:35 MHz B: 70% | 30 min for entire cornea | 2.7 μm | Transgenic Tie2-GFP mouse |
PAOM+SD-OCT | Retinal vasculature; RPE | Jiao [12] | Optical scanning | W: 532 nm E: 40 nJ | CF: 30 MHz B: 50% | 2.7 s for 2 × 2 mm | L: 20 μm A: 23 μm | Long Evans rat |
PAOM+SD-OCT | Retinal and choroidal vasculature | Wei [25] | Optical scanning | W: 578 nm E: 40 nJ | CF: 35 MHz B: 50% | 2.7 s for 2 × 2 mm | L: 20 μm A: 23 μm | SD rat |
PAOM+SD-OCT + AF-SLO + FA | Retinal and choroidal vasculature; RPE | Song [20] | Optical scanning | W: 532 nm E: 40 nJ | CF: 40 MHz B: 15 MHz | 2.7 s for 2 × 2 mm | L: 20 μm A: 23 μm | Albino /pigmented rats |
Fundus camera +PAOM | Retinal and choroidal vasculature | Liu [22] | Optical scanning | W: 532 nm E: 60 nJ | CF: 35 MHz B: 50% | 2.7 s for 2 × 2 mm | L: 20 μm A: 23 μm | Albino /pigmented rats |
PAOM+SD-OCT | Retinal and choroidal vasculature; RPE | Song [27] | Optical scanning | W: 532 nm E: 40 nJ | CF: 40 MHz B: 16 MHz | 2.7 s for 2 × 2 mm | L: 20 μm A: 23 μm | Albino /pigmented rats |
PAM+OCT | Retinal and choroidal vasculature | Tian [19] | Optical scanning | W: 570 nm E: 80 nJ | CF: 27 MHz B: 60% | 65 s for 3 × 3 mm | L: 4.1 μm A: 37 μm | Albino rabbit |
PAM+OCT + FM | Retinal vasculature and RNV | Zhang [11] | Optical scanning | W: 532 nm E: 80 nJ | CF: 27 MHz B: 60% | 65 s for 3 × 3 mm | L: 4.1 μm A: 37 μm | Albino/pigmented rabbits |
PAM+OCT | RVO and RNV | Nguyen [28] | Optical scanning | Multi-wavelength E: <80 nJ | CF: 27 MHz B: 60% | 65 s for 3 × 3 mm | L: 4.1 μm A: 37 μm | Albino rabbit |
PAM+OCT | CNV; retinal structure; RPE | Dai [30] | Optical scanning | W: 532 nm E: 60 nJ | CF: 40 MHz B: 16 MHz | 19.5 frame/s | L: 20 μm A: 50 μm | Brown Norway rat |
PAOM+OCT | Retinal rMRO2; sO2; blood flow | Song [35], Liu [36] | Optical scanning | W:570/578/588 nm E: 40 nJ | CF:30 MHz B: 15 MHz | 2.7 s for 2 × 2 mm | L: 20 μm A: 23 μm | SD rat |
OR-PAM | Chorioretinal sO2 | Hariri [37] | Optical scanning | W: 750/850 nm E: 8 ± 0.5 mJ | CF: 15 MHz | 0.8 frame/s | L: 580 μm A: 290 μm | Albino rabbit |
AO-PAM | Ciliary body, RPE melanin | Jiang [44] | Optical scanning | W:532 nm E: 40 nJ | CF:30 MHz B:50% | 2.7 s for 2 × 2 mm | L: 2.5 μm A: 23 μm | Pig |
PAOM+AF | RPE melanin; lipofuscin | Zhang [46] | Optical scanning | W: 532 nm E: 40 nJ | CF: 30 MHz B: 50% | 2.7 s for 2 × 2 mm | L: 4.5 μm A: 23 μm | Albino/pigmented rats |
PAOM+SD-OCT | RPE melanin | Liu [47] | Optical scanning | W: 1064/532 nm E: 20 nJ | CF:30 MHz B: 50% | 2.7 s for 2 × 2 mm | L: 20 μm A: 23 μm | SD/Long Evans rats |
Broadband PAM | RPE melanin concentration | Shu [45] | Optical scanning | W: 532 nm | MRR B: 280/27 MHz | 22 s per image | A: <10 μm | Porcine/human RCCs |
3.4 Speed of PA image acquisition
Several current PA imaging systems can acquire high-resolution images in <1 min. The imaging speed is limited by the laser pulse repetition rate. The image speed can be reduced until it depends on the ultrasound propagation time from the posterior eye. The advantage of improving imaging speed is that it will reduce motion artifacts and increase patient comfort. Ideally a clinical PA system would acquire images in less than a second to a couple seconds.
3.5 Lateral and axial resolution of PA
The lateral resolution of PA imaging is determined by the smallest achievable spot size of the illuminating light in the retina. Tian and Zhang et al. described the lateral resolution of their PAM system as 4.1 μm [11, 19]. AO allows one to correct for ocular aberrations and has been applied to ophthalmic imaging to improve resolution. A feasibility study was conducted on the integration of AO with PAM for
The axial resolution of PA imaging is determined by the bandwidth of the ultrasound transducer. The larger ultrasonic bandwidth will allow higher axial resolution, but with less sensitivity. Therefore, a balance is required between axial resolution and sensitivity.
3.6 Real-time PA imaging
Currently many PA images require post-image processing to display images. Further investigation of an advanced ophthalmic PA imaging system is to obtain real-time imaging.
3.7 Imaging the choroidal vasculature in pigmented animals
PA imaging is often limited to albino animals when imaging the choroid due to the high PA signal of the RPE melanin in pigmented animals. Further methods need to be developed and refined to penetrate the RPE layer and allow improved choroidal visualization. NIR light has been proposed as a solution, but further testing is needed.
3.8 Imaging ocular disease animal models
PA imaging has been demonstrated in some animal eye models to date, including mouse corneal neovascularization model, rat CNV model, and rabbit retinal neovascularization model. PA imaging needs to be further explored in more eye disease models to evaluate how the information provided by PA imaging will be used clinically in the future.
3.9 Exogenous contrast agents and theranostics
To improve PA image sensitivity and specificity, exogenous contrast agents can be utilized. PA imaging with exogenous contrast agents also allows one to extend the imaging scope to molecular imaging [56, 57]. Exogenous contrast agents for PA imaging include organic and inorganic agents. However, each agent has advantages and limitations. Organic agents (e.g., ICG) can have a limited level of contrast enhancement but a more rapid path to clinical translation [58]. Inorganic agents (e.g., gold nanoparticle (AuNP)) can offer higher contrast but a less rapid path to clinical translation due to less long-term evidence of biosafety [59, 60]. Thus, exploring suitable exogenous contrast agents with safety and high contrast can be used for PA imaging will be meaningful in future research. In addition, theranostic agents that can be used both for diagnostic imaging and therapy, should be further refined and developed to allow for targeted therapy at the time of imaging.
3.10 Safety evaluations and clinical approval
Before PA imaging can be applied to human imaging, a thorough evaluation of laser safety is necessary. Although the reported systems compare their laser fluence to the ANSI laser safety regulations, [24] one must monitor the long-term effect of both single and multiple imaging sessions on the structure and function of the retina. In additional, regulatory approval should be sought for a clinical system so that PA can be applied to patients and diseases.
4. Conclusion
The chapter introduces the applications, recent developments, and future directions of PA imaging in the eye. It has been demonstrated that PA imaging can provide both anatomic and functional information of eye with high-resolution, high sensitivity, high-contrast, and high depth of penetration. This chapter describes the ocular structure of PA imaging including normal vasculature of the iris, retina, and choroid, neovascularization in cornea, retina, and choroid, and melanin of the RPE. This chapter summarized PA imaging to quantify the functional information of measuring the vascular sO2 and quantifying the absolute melanin concentrations. Limitations and future directions of PA imaging of the eye are also discussed.
Acknowledgments
This research was supported by a grant from the National Eye Institute 1K08EY027458 (YMP), Fight for Sight-International Retinal Research Foundation FFSGIA16002 (YMP), unrestricted departmental support from Research to Prevent Blindness, and the University of Michigan Department of Ophthalmology and Visual Sciences, and China Scholarship Council No.201806370270.
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