The Imaging of the peribronchial structures can be done from six stations. Although there is no sharp boundary between the six stations, the structures mentioned at each station are generally seen in the positioned mentioned in the table.
Endobronchial ultrasound (EBUS) visualizes structures within and adjacent to the airway (1). Most operators do not follow any standard positions of imaging during EBUS and use the computed tomography scan as a roadmap for imaging of the lymph nodes. A small window angle (50 to 75 degrees) in linear EBUS as compared with linear endoscopic US (130 to 180 degrees) makes visualization of the anatomic ultrasound landmarks difficult with EBUS. For better orientation, it is useful to recognize key anatomic landmarks and their relationship to the airways, apart from observing the position of the probe while performing EBUS. In this section we describe the mediastinal and parabronchial anatomy of different parts of the respiratory tract which is practically important during EBUS.
2. The ten commandments of imaging
2.1. Instrument and comparison of available scopes
Two types of scopes are available. Broadly the major difference is in the diameter and field of view. The diameter of pentax scope is larger than Olympus scope (7.4 mm vs 6.9 mm.) The field of vision and depth in imaging of the pentax scope is larger (field of vision 75° vs 50°, depth 5 cm vs approximately 2 cm)
2.2. Indication of EBUS
The main indications are evaluation of benign and malignant mediastinal lymphadenopathy. The role of linear EBUS for T staging of lung cancer is not yet defined. Limited assessment of M staging and loco regional spread of lung cancer is possible.
2.3. Alternative modalities of imaging and tissue sampling
Majority of lymph nodes can be seen by EBUS and EUS both while doing endoscopic imaging. The choice to do FNA by EBUS or EUSrest on the experience of the endosonographer. Some endosonographer are more experienced in EBUS while others are more experienced in EUS. However for persons who are experienced in both it may be difficult to choose between the two for FNAC if both are approachable.
2.4. Optimization of the imaging
The ultrasound waves are high frequency sound waves.
Ultrasound beam loses strength of the beam over time or distance travelled and the echoes that return from deeper structures are weak compared to those returning from the close structures. This phenomenon is called attenuation.
Attenuated waves need to be amplified before analysis. The echoes that come from deep within the body are more attenuated and need more amplification to make a smooth image. This is done by Time-Gain Compensation (TGC). The images thus amplified contain echoes of approximately equal strength from all the depths of tissue.
The waves make image. Images have resolution. The ability of the beam to differentiate two objects is called spatial resolution. Higher spatial resolution shows two points as separate while low spatial resolution shows them as a single blurred point.
Resolution has two main dimensions. Analysis along the length is called axial resolution, analysis along the breadth is called lateral resolution.
Axial resolution is determined by the frequency. Axial resolution is most important in determination of quality. High frequency shortens the pulse length and gives a better axial resolution. The frequency is inversely proportional to the penetration depth. At 5 M Hz ultrasound penetrates approximately 6 cm while at 10 M Hz the penetration field of ultrasound is about 3 cm.
Lateral resolution is determined by focal length. Lateral resolution differentiates between two points lying horizontal to the ultrasound beam. It is determined by the width of the Ultrasound beam. Changing focus of the beam to the level of investigation gives optimum lateral resolution at the point of focus.
Image depends on resolution. Changing frequency improves axial resolution while changing the focus improves lateral resolution.
So optimization of image is done by following techniques.
1. Choose the correct frequency2. Focus the area
2.5. Orientation of imaging
The cranial caudal convention for longitudinal abdominal imaging and linear EBUS varies throughout the world. Endosonographers and radiologists all over the world do not yet have a universal convention on the demonstration of images in linear EBUS so far as cranial and caudal is concerned. In UK, USA and France the patient’s cranial (head) and caudal (feet) are to the right and left of the screen respectively. In Japan and Germany the direction is reversed. We will follow the cranial to right and caudal to left convention for our presentation.
2.6. Providing anaesthesia
Some professionals prefer local anaethesia for EBUS. Others prefer general anaethesia.
2.7. Position of patient. Supine, left lateral or sitting. To each his own
Most of the conventional bronchoscopist use two hands to maneoure the scope. The EBUS operator who hold the echoendoscope in both hands cannot get used to manipulating the knobs of the ultrasound screen. The left lateral position of EBUS allows the operator to intermittently remove the right hand for manipulation of the screen.
2.8. Artifacts in imaging
There are two main artifacts during imaging from tracheobronchial tree. The artifacts of cartilage gradually disappear as the scope is advanced more distally into the trachea-bronchial tree. Mirror image artifact is created by vessels.
2.9. Air is the enemy water is the friend
There are two methods of establishing contact. Simple apposition of the probe against the wall may be enough to establish contact and use of balloon is not usually required with pentax scope. Generally the limited range of imaging 50° vs 75° requires contact with a balloon in Olympus EBUS scope.
2.10. There are three main movements of echoendoscope
2.10.1. In and out movement
In and out movements are done to position the scope at the desired cm landmark of imaging and is the key movement for changing the position.
2.10.2. Clockwise or anticlockwise rotation
In a linear EBUS scope where the imaging is in a longitudinal axis clockwise or anticlockwise rotation is the key movement for changing the view. Rotation changes the field totally in a linear scope only. Rotation alone brings no change in view in a radial EUS scope.
2.10.3. Angulations of scope, up or down
Angulations of scope, up or down is required for achieving close contact with the wall of trachea.
3. The CM landmarks in EBUS during imaging from mediastinum
While doing EBUS the operator generally enters the trachea approximately at 15 cm distance from the incisor. The trachea is about 10 cm long so the carina is generally reached when the scope lies at about 25 cm distance. The left bronchus is about 5 cm long and the lower end of left main stem bronchus is reached at an approximate distance of 30 cm. Similarly the 30 cm distance is the lower limit of reach in the right bronchus which includes 2.5 cm of right main stem bronchus and 2.5 cm of intermediate bronchus. The diameter of EBUS scope generally does not allow any further negotiation beyond the reach of intermediate bronchus on the right and left main stem bronchus on the left side. While doing imaging from the respiratory tract an additional cm landmark of importance is upper border of arch of aorta which lies at about 22cm distance. The lower border of arch of aorta lies at about 23 to 24 cm. The lower border of azygos vein and the upper border of left pulmonary artery lie approximately at 25 cm distance.
The trachea lies in front of esophagus which commences at the level of the cricoid cartilage at about 15 cm distance from incisor teeth. For the purpose of description, the esophagus can be divided into cervical (CE) from cricoid to 18 cm, upper thoracic [TE (U)] from 18-25 cm, till approximately the tracheal bifurcation), mid thoracic [TE (M)] from 25-32 cm, till approximately below the subcarinal area and lower thoracic [TE (L)] from 32-38 cm segments. While doing imaging from the trachea certain cm landmark of importance are upper border of arch of aorta (23cm), the lower end of trachea (25 cm), lower border of arch of aorta (25 cm), the lower border of azygos vein (25 cm), the upper border of left pulmonary artery (25 cm) and the upper border of left atrium (30 cm). While pushing the EBUS scope into esophagus the crux of diaphragm is seen at 40 cm.
4. Applied anatomy of mediastinum
4.1. The compartments of mediastinum
It is important to have a brief idea of mediastinum before discussing the mediastinal structures. For descriptive purposes, the mediastinum is arbitrarily subdivided by a transverse plane that passes through the sternal angle and the lower border of the fourth thoracic vertebra. The superior mediastinum is above this plane and is limited superiorly by the superior thoracic aperture; the inferior mediastinum is below the plane, and the diaphragm limits it inferiorly. The inferior mediastinum is further compartmentalized based on its relation to the pericardial sac: the sac and its contents compose the middle mediastinum; between the sac and the sternum is the anterior mediastinum; between the vertebral bodies and the pericardial sac is the posterior mediastinum. The contents of the posterior mediastinum include the esophagus; the descending thoracic aorta and its branches; the veins of the azygos system and the thoracic duct.
4.2. The trachea
The trachea is about 10 cm long and present from 15 to 25 cm distance from incisor. It is kept patent by a series of cartilages embedded transversely in its wall.The posterior wall of trachea is flat where the ends of cartilage bar are united by trachealis muscle and fibroelastic tissue. This surface is applied to esophagus. The cervical part of trachea is about 3-4 cm in length and is related to lobes of the thyroid gland before it enters superior mediastinum where it lies in midline. The division of the trachea into principal bronchi takes place behind the ascending aorta, to the right of, and below, the arch of the aorta, approximately at the level of sternal angle (level of fourth thoracic spine). The lower end of the trachea is displaced slightly to the right of the midline by the arch of the aorta, which occupies the angle between the trachea and the left bronchus.
On the right, the superior lobar bronchus branches off the principal bronchus before the later enters the hilum. The remaining main stem intermediate bronchus, gives off, more distally, the middle lobe bronchus, which runs forward and downward. The intermediate bronchus continues as inferior lobar bronchus.The left principal bronchus gives off the superior lobar bronchus as soon as it has entered the hilum, and the remaining main stem becomes the bronchus of the inferior lobe.
The figure 5 A shows six different positions (which will be discussed in later part of chapter) of imaging by Endobronchial ultrasound. 1 Imaging from the upper trachea, 2. Imaging from the lower trachea, 3. Imaging from the right main bronchus, 4. Imaging from Intermediate bronchus, 5. Imaging from upper part of left bronchus, 6. Imaging from the distal part of left bronchus. Fig. B. to F shows the relationship of parabonchial& vascular structures to trachea. Fig. B relationship of tracheobronchial tree to esophagus. Fig. C. relationship of tracheobronchial tree to azygos vein and superior vena cava. Fig. D. relationship of tracheobronchial tree to arch of aorta. Fig. E. relationship of tracheobronchial tree to pulmonary trunk and the two branches of pulmonary trunk. Fig. F. relationship of tracheobronchial tree to left atrium and the draining veins of the left atrium.
4.3. The right bronchus
The right bronchus passes behind the ascending aorta and the superior vena cava towards the root of lung. Two structures are related to anterior wall of right main bronchus, in the upper part the ascending aorta lies anteromedially and the superior vena cava is placed anterolaterally. The right pulmonary artery lies in close relationship with the intermediate bronchus. The right pulmonary artery initially lies first below and medial to intermediate bronchus, then in front of it and finally lies lateral and behind the intermediate bronchus. The azygos vein lies behind the right bronchus for some length and then arches over it to drain into superior vena cava.
4.4. The left bronchus
The left bronchus, in its course toward the hilus, passes through the loop formed by the arch of the aorta, emerging from behind the ascending aorta and passing downward and to the left in front of the descending aorta before it enters the lung. Two structures are placed in anterior wall of left bronchus. The arch of aorta lies close to upper part of left main bronchus and the left pulmonary artery ascends over its anterior surface just distal to the arch of the aorta.
4.5. The root of lung and pulmonary ligament
The roots of the lungs are posterior to the upper part of the pericardial sac. Most posterior in the upper part of each root is the bronchus, in front of it are the pulmonary artery and, in an even more anterior plane, the superior pulmonary vein. The arrangement of the pulmonary artery, pulmonary vein and bronchus in right and left lung root is slightly different. The left pulmonary artery is located above, rather than directly in front of, the left bronchus.
Usually, two bronchi the upper lobe bronchus and the bronchus intermedius are seen at the right hilum when the entire lung is removed. In the left hilum, only one bronchus is seen. The pulmonary ligament is the inferior redundant part of the pleura that surrounds the root of the lung and provides the dead space in which the root of the lung may move up and down during respiration.
4.7. The aorta
As soon as the ascending aorta emerges from the pericardial sac, it begins to arch backward, and the segment that runs in a nearly sagittal plane in the superior mediastinum is known as the arch of the aorta. The arch of aorta begins behind manubrium sterni and runs first upwards backwards to the left and in front of trachea. It is then directed backwards on the left side of trachea and continues downwards on the left side of T4 vertebra and at the lower border of T-4 continues as descending aorta. The left common carotid the left subclavian and the brachiocephalic arteries arise from the convexity of arch. They are crossed anteriorly by the left brachiocephalic vein just above the convexity of arch of aorta.
The space below the concavity of arch of aorta is sometimes called as subaortic tunnel. The bifurcation of pulmonary trunk the right pulmonary artery and the left bronchus lies in this inferior concavity. The trachea and esophagus fit into the slight concavity that faces to the right.
4.8. The esophagus
For most of the length the esophagus lies in the posterior mediastinum, with the descending thoracic aorta posteriorly to its left. On its right side, it is covered by mediastinal pleura. The right pulmonary artery, the left principal bronchus, the transverse and oblique sinuses of the pericardium and the left atrium lie anterior to the esophagus.
4.9. The pulmonary trunk, the right pulmonary artery and left pulmonary artery
The pulmonary trunk (length approximately 5 cm) lies a little to the left of midline in the chest and divides into left and right branches. The left pulmonary artery immediately leaves the pericardial sac and just outside the pericardial sac is connected to the arch of the aorta by the ligamentumarteriosum. The upper border of the left pulmonary artery lies at a little higher level than the upper border of right pulmonary artery. The RPA runs horizontally behind the ascending aorta and superior vena cava before it leaves the pericardial sac in the concavity of the arch of the aorta. The right pulmonary artery emerges from the sac posterior to the superior vena cava and crosses the intermediate bronchus distal to the origin of the superior lobe bronchus. Soon after leaving the pericardial sac both right and left pulmonary arteries arch over the respective principal bronchi as they enter the hila of the lungs. The relationship of these pulmonary artery to the bronchus is more or less fixed and it gives a fair idea of the position to the endosonographer. The left pulmonary artery crosses the left principal bronchus and at the hilum, is superior to it, whereas the right pulmonary artery crosses the intermediate bronchus, having given off a major branch to the upper lobe before it enters the hilum. The ascending aorta, SVC and upper right pulmonary vein lie anterior to RPA. The esophagus and right bronchus lie posterior to RPA. The left bronchus and descending aorta lie posterior to LPA.
4.10. The pulmonary veins and the left atrium
The left atrium has two portions. The posterior half of the chamber, into which the four pulmonary veins empty and the anterior half. There are four pulmonary veins, two each (superior and inferior), on the right and left sides. On the left side the superior and inferior veins drain the upper and lower lobes of the lung, respectively. The superior pulmonary vein is the most anterior structure, and the inferior pulmonary vein is the most inferior structure in the root of the lung. Thus on both sides, the principal order of the structures in an anteroposterior direction in root of lung is vein, artery, bronchus. The inferior pulmonary veins lie below the bronchus on both sides, and below them is the pulmonary ligament into which they can expand. The right inferior pulmonary vein crosses the esophagus and the right superior pulmonary vein crosses right pulmonary artery and right bronchus. On the right side the superior vein crosses behind the superior vena cava; and the inferior vein crosses behind the right atrium before they pierce the pericardial sac over the left atrium. The left inferior pulmonary vein cross the descending aorta.
Stations of EBUs Imaging
|Respiratory Landmark||Upper Trachea||Lower Trachea||Right Main Stem Bronchus||Right Intermediate Bronchus||Left Upper Bronchus||Left Lower Bronchus|
|Main Arteries||BT, LCC, LSCA||Arch of Aorta|
|Main Veins||LBCV, RBCV||AV, SVC||SVC||RPV||—||LPV|
|Group of Lymph Node||2R, 2L, 3a, 3p||4R, 4L, 5 & 6||7, 10R||7, 8, 11R||7, 10L||7, 8, 11L|
|Vertebral Landmark||T-3||T-4||T-5 Upper Border||T-5||T-6||T-6|
|Other Structures||Thymus||Thymus||—||Chambers of Heart||—||Chambers of Heart|
5. Technique of imaging by EBUS scope
This evaluation can be done in patients referred for EBUS on an outpatient basis, under conscious sedation using midazolam and oral xylocaine spray. A linear EBUS scope (Pentax EB-1970UK) with 100° field of view, 45° forward oblique angle, a window angle of 75° and a distal end optic width of 7.4 mm was used for the purpose of description in this chapter. Imaging was done after endoscopic visualization from intrathoracic part of trachea and bronchus from six positions. Clockwise or anticlockwise rotation was done after apposition against the wall to change the axis of imaging. Cranial side left and caudal side right imaging convention was followed.
5.1. Imaging from upper trachea
Imaging is done 5 cm above carina where a clockwise and anticlockwise rotation from anterior wall of trachea to 90° on either side will show the right and left lateral tracheal walls and 180° rotation will show the esophagus behind the posterior wall of trachea (Fig 11 & 12). The presence of air in trachea and cartilage in the wall create prominent artifacts during imaging (Fig. 13 & 14).
On clockwise rotation, the brachiocephalic veins are seen joining near the right anterolateral wall of trachea to form the superior vena cava. The lower border of left brachiocephalic vein forms the upper boundary of station 2R and superior vena cava forms the posterior boundary of station 3a.
On anti clockwise rotation, the upper margin of the arch of aorta is seen from which the origin of left common carotid artery (left posterior boundary of station 3a), left subclavian artery and brachiocephalic trunk can be seen (Fig 11). Brachiocephalic trunk crosses to the right side of trachea, the left common carotid artery runs close to left wall of trachea and the left subclavian artery goes close to apex of lung where it creates mirror image artifact (Fig. 15 & 16). The transverse plane through the superior border of aortic arch makes lower border of the station 2L on the left and a transverse plane through the lower border of brachiocephalic vein forms lower border of 2R lymph nodes on the right side (Figures 17 & 18).
5.2. Station 2: Imaging from lower trachea
A clockwise rotation from above carina shows the right anterolateral wall of the distal one third of trachea where the superior vena cava is seen. If clockwise rotation is continued, the azygos vein can be seen joining the superior vena cava in the right tracheobronchial corner as a kidney-shaped vessel. The inferior border of station 4R is formed by the inferior margin of the azygos vein (Fig. 19, 20 & 21).
An anti-clockwise rotation shows the pulsatile arch of aorta in the left anterolateral wall of distal one third of trachea, the superior margin of which forms the boundary between the station 4L and 2L lymph node stations. Determination of the lower boundary of station 4L and 5 is done from left bronchus after clear visualization of upper rim of left pulmonary artery.
Two transverse lines across the upper and lower border of arch are important in determining the area of lymph nodes of station 6 which are better seen by endoscopic ultrasound (Fig. 22).
5.3. Station 3: Imaging from right main bronchus
From the right main bronchus, the subcarinal space, the right pulmonary artery, right pulmonary vein, superior vena cava and ascending aorta are seen close to the anterior wall (Fig. 23, 24, 25& 26, 27& 28). Turning the scope anti-clockwise shows station 7 nodes of subcarinal area (Fig 29). Station 10R lymph nodes are seen close to the right main bronchus.
Fig. 23., 24., 25.& 26. Bronchoscopy showed extrinsic compression on the right bronchus. EBUS was done from the right bronchus. Four pulsations are seen. One by one placement of the pulse doppler is done over the pulsations. The pulse Doppler is placed on pulsation 1 in this figure and characterizes it as superior vena cava. The superior vena cava can be followed up by pulling up the scope into right lateral wall of trachea. The rest of the vessels are as follows–2: Right pulmonary vein, 3: Right pulmonary artery, 4: Ascending aorta. SVC = Superior vena cava
5.4. Station 4: Imaging from intermediate bronchus
Imaging from right intermediate bronchus the subcarinal area (station 7 & 8 nodes) left atrium mitral valve and left ventricle are seen anteriorly (Fig. 30 & 31). On pushing down endoscopic visualization of inferior border of right intermediate bronchus demarcates the lower boundary of station 7 on the right side. Station 11 lymph nodes are seen close to intermediate bronchus.
5.5. Station 5: Imaging from upper part of left main bronchus
The left pulmonary artery is seen in the anterior wall of bronchus. On anticlockwise rotation, the probe faces the posterior wall of left main bronchus where the descending aorta, esophagus and vertebral column are seen (Fig. 32, 33). A clockwise rotation shows station 7 lymph nodes (Fig. 34). Station 10 L lymph nodes are seen near the left main bronchus on anti-clockwise rotation (Fig. 35).
5.6. Station 6: Imaging from lower part of left main bronchus
With the scope in lower part of left bronchus just above the secondary carina the left atrium and station 8 lymph nodes can be identified anteriorly. It is often possible to get a four chambered view of heart with a 75° EBUS scope (Fig. 36). The lymph nodes of station 11L are seen below the secondary carina.
6. Comparison of EBUS and EUS
No standard comparison has been made till now and the choice of procedure (EBUS first or EUS first) is still under debate. The reasons to choose EUS FNA over EBUS FNA for lymph nodes are—
EUS scope has a large diameter and it is easier to pass the FNAC needle through the channel of the scope.
The field of vision is more with a EUS scope (130° to 180°) as compared to EBUS scope (50 to 70°).
The use of elevator allows more angles for the operator to enter the lymph node.
The use of right and left knobs allows more angles for entry into the lymph node.
The presence of cartilage during entry from trachea may be difficult
GV neck = great vessels of neck, AA = arch of aorta, Arc. AV = Arch of azygosvein,BT = Brachiocephalic Trunk, LCC = Left Common Carotid Artery, LSCA = Left Subclavian Artery, SVC = Superior Vena Cava, RPA = Right Pulmonary Artery, LPA = Left Pulmonary Artery, DA = Descending Aorta, LBCV = Left Brachiocephalic Vein, RBCV = Right Brachiocephalic Vein, AV = Azygos Vein, RPV = Right Pulmonary Vein, LPV = Left Pulmonary Vein, T = Thoracic Vertebra, R = Right, L = Left. Fig. 5 SVC = Superior vena cava, Arc. A = Arch of aorta, AV = Azygos vein, AA = Ascending aorta, RPA = Right pulmonary artery, DA = Descending aorta, LPA = Left pulmonary artery, PT = Pulmonary trunk, LB = Left bronchus, RB = Right bronchus, UT = Upper trachea, LT = Lower trachea, Es = esophagus. Tr. = Trachea, Es. = Esophagus, PT = Pulmonary trunk, DA = Descending aorta, Es. = Esophagus, Lig. art. = Ligamentumarteri-osum. SCA = Subcarinal area, TD = Thoracic duct, LA = Left atrium, MV = Mitral valve, LV = Left ventricle, RV = Right ventricle, RA = Right atrium, RPV = Right pulmonary vein, LPV = Left pulmonary vein,
RA = Reverberation artifact, SCA = sub carinal area, RMB = right main bronchus, LMB = left main bronchus, TA = Truncus anterior, AP window = aortopulmonary window