How to Perform Endoscopic Ultrasonography in the Esophagus and Mediastinum

Esophagus

Obtaining high-quality images of the esophageal wall is one of the more difficult tasks that an endosonographer will encounter. One has to deal with the “catch 22” that pits adequate coupling of the ultrasound signal to the esophageal wall against wall compression. This situation can lead to inaccurate assessment of invasion depth in patients with early esophageal cancer or to missing lesions completely in the case of varices. Numerous techniques can be used to overcome these conflicting goals.

In the case of a relatively advanced mass in the esophagus, minimal or no balloon inflation is sufficient to couple the ultrasound signal to the esophageal wall without causing compression that adversely affects staging accuracy. In this circumstance, the electronic radial instrument has an advantage over the mechanical radial device because of the absence of ringdown artifact and the superior near-field resolution of electronic array technology. Periesophageal structures (e.g., lymph nodes) are not affected by the amount of balloon inflation.

When compression of the esophageal wall needs to be avoided, several different techniques can be used. The simplest is to instill water into the gut lumen by pressing the air/water button to its first position. This maneuver sprays water across the endoscopic image lens. Remarkably, this does a very good job of filling the lumen with water while reducing the risk of aspiration. This technique can be used with the standard radial echoendoscope or when using a high-frequency catheter probe in conjunction with a single- or dual-channel forward-viewing endoscope. The images generated are often fleeting because of peristalsis and variability in water filling. As a result, the cine function on the console becomes important in that it allows one to freeze the image and then scroll through the stored images to save the best one. High-resolution esophageal images can be obtained only when the esophagus is in its relaxed state, and this occurs only periodically. Agents normally used to paralyze the stomach, duodenum, and colon have little to no effect on esophageal contractions.

A second method that can be used with a radial scanning echoendoscope is to instill water through the biopsy channel. If this technique is used, it is recommended that water be slowly siphoned into the esophagus rather than actively pumped or vigorously instilled by syringe. There is a very real risk of aspiration if high volumes are instilled over a short time, especially when topical pharyngeal anesthesia has also been applied.

Until the advent of the electronic radial echoendoscope, the device of choice for high-quality images of the esophageal wall was a high-frequency ultrasound probe. However, the newer electronic radial echoendoscopes have excellent near-field resolution and provide superb images without the need for significant balloon inflation. Nonetheless, if one wishes to stage early (T1m,sm) esophageal cancer (to determine the presence or absence of penetration through the muscularis mucosa), high-frequency catheter probes (20 to 30 MHz) would still be considered the instruments of choice.

When catheter probes are used for esophageal imaging, several techniques can be used. One method is to use a bare catheter and instill water through the air/water channel. A second method is to use an ultrasound catheter with an attachable balloon. This technique still risks compression of the esophageal wall layers with inflation of the balloon. However, because the focal length of the catheter is very short, only a small amount of balloon inflation is necessary, thereby minimizing this risk.

Another technique that has been described is to affix a transparent, low-compliance condom onto the end of a double-channel endoscope ( Fig. 6.1 ). The condom is taped onto the end of the endoscope such that approximately 2 to 3 cm of the condom protrude beyond the tip of the endoscope. This redundant portion of the condom is folded across the imaging lens as the endoscope is passed into the esophagus. During the intubation process, it is extremely important to avoid instilling air (a common habit) because this will inflate the condom and could compromise the patient’s airway. After entering the esophagus, the instrument is passed into the stomach lumen, and air is “bled” from the condom tip (instill water-aspirate; reinstill; reaspirate and repeat until all the air is gone). Once the condom has been bled, the endoscope is withdrawn to the level of the lesion, and the condom is filled with water. Because of the low compliance of the condom, it tends to elongate rather than compress the wall layers. The ultrasound catheter is then advanced into the lumen of the condom, and imaging proceeds ( Video 6.1 ). With this technique, the coupling of the ultrasound waves to the esophageal wall is virtually perfect. With the transparent condom, the lesion can be viewed endoscopically in real time, thus assuring that the catheter probe is positioned correctly. Because the water is completely contained within the condom, there is no risk of aspiration.

Video 6.1

Examination of the Esophagus Using a High-Frequency Catheter Probe Passed Via a Dual-Channel Gastroscope Using the Condom Technique (With Narration)

Whichever technique is used, the risk of aspiration should be minimized while good coupling of the ultrasound waves to the esophageal wall is achieved without inducing compression. These techniques are used for patients with early esophageal cancer, with Barrett esophagus with or without nodules, and with small submucosal lesions.

The other major problem with esophageal endoscopic ultrasonography (EUS) is tangential imaging. The esophagus is often perceived as a straight tube, but in most cases it has some tortuosity. The imaging section of an echoendoscope, as well as a catheter probe, is straight and rigid. Imaging a tortuous tube with a straight instrument creates tangential imaging. The endosonographer must be trained to recognize tangential imaging and must be aware of the maneuvers that will correct it. The consequence of unrecognized tangential imaging is overstaging malignant lesions or missing the layer of origin of a submucosal lesion. Tangential imaging is characterized by focal thickening of the esophageal wall associated with blurring and triangulation of the deep border of the esophageal wall ( Fig. 6.2 ). If one recognizes tangential imaging, the corrective action is usually to use all four directional dials (do not torque the scope shaft) to move the transducer in the direction where tangential imaging is seen. When the deep edge of the muscularis propria layer becomes smooth and the layer is seen sharply, tangential imaging has been corrected.

Mediastinum

Radial Echoendoscope

Examination of the mediastinum with a radial echoendoscope is relatively straightforward. The learning curve should be short (compared with EUS of the pancreas) because the EUS images correlate with a thoracic computed tomography (CT) scan. It is recommended that a systemic approach be applied to all EUS examinations and that images be presented with a standard orientation. This approach holds true for mediastinal imaging. To begin the mediastinal study, the echoendoscope tip is placed in the distal esophagus near the gastroesophageal junction. The aorta is a round, anechoic structure that is a constant anatomic finding throughout the examination until withdrawal proximal to the aortic arch. It is recommended that the endoscopic ultrasound image be presented on the monitor in an orientation that exactly matches a CT slice. To accomplish this, the aorta should be rotated (using the rotation function on the instrument panel, not by torquing the scope shaft) to the 5-o’clock position. This will present the spine at 7 o’clock ( Fig. 6.3 ), and the heart and respiratory tree will emerge in the 12-o’clock position.