How to Perform Anorectal Endoscopic Ultrasonography

The Perianal Area

Examination of the perianal area is simplicity itself. No special patient preparation is required. The patient is told that any discomfort will be similar to having a finger in the anus and that the procedure will likely be less uncomfortable than digital rectal examination by a doctor. To the patient, the rigid probe is potentially a frightening piece of equipment, so it is worth mentioning that only the distal few centimeters will enter the anus (as opposed to rectal endosonography, in which insertion is obviously deeper). Some endosonographers place all patients in the left lateral position, whereas others prefer female patients to be in the prone position for examination. Placing women in the left lateral position can potentially distort anterior perineal anatomic features, with the result that the asymmetric images obtained will be difficult to interpret, especially with respect to perineal scarring. ¹

Appropriate equipment is essential for successful anal endoscopic ultrasonography (EUS). The standard (most commonly described in the literature) is the Bruel-Kjaer mechanical radial rigid probe. In the early days of EUS, when the principal instrument was the mechanical radial echoendoscope, examiners attempted to use this scope for anal EUS examination. However, the near-field imaging was poor, and the anal sphincters were often obscured by the ringdown artifact. Consequently, Olympus designed and marketed a rigid rectal probe compatible with its mechanical radial processor. However, with the introduction of electronic radial echoendoscopes, a flexible instrument is now available that can deliver high-quality images of anal anatomy and has rendered the dedicated rigid probe obsolete.

The rigid probe is prepared as necessary for the transducer being used. Some systems, for example, require the transducer head to be filled with degassed water to achieve acoustic coupling. This is accomplished by injection using a syringe through a side port. The probe must be maneuvered during filling so that all air is expelled through a pinhole located at the tip of the cone.

Whether or not water filling is required, the rigid probe tip is lubricated with ultrasound jelly and then is covered with a condom, which is itself lubricated to facilitate insertion. The probe is then inserted into the anus, and image acquisition is started by the operator. The probe is inserted so that its tip lies just in the distal rectum. The probe is then withdrawn gently to examine the anal sphincters. As for all ultrasound examinations, the clinical findings are generally based on the image displayed on the monitor screen in real time (with the exception of three-dimensional acquisition, in which case the examination in its entirety can be replayed later). However, still images are usually required, and it is convenient to obtain these at three levels: the proximal, middle, and distal anal canal. These three anatomic levels are imaged at standard magnification, and the examination is then repeated at a higher magnification so that six images are obtained, three at each magnification. The probe is oriented so that anterior (i.e., the 12-o’clock position) is uppermost and is then withdrawn. The examination is normally very quick, perhaps only a minute or so for the experienced operator who is familiar with normal and abnormal anatomy, especially when the sphincters are normal. The technique for imaging does not vary whether a rigid probe or an electronic radial flexible probe is used.

The Rectum

EUS of the rectum is mainly performed to examine suspicious rectal polyps or to stage rectal cancer. From country to country, huge differences exist in the use of EUS for this indication. Patients should be prepared with an enema or complete bowel preparation to evacuate all stool from the area to be investigated. For the start of the examination, the patient is usually placed in the left lateral position. The position may be changed during the examination. For noncircumferential masses or laterally spreading polyps, the patient should be positioned so that the mass or polyp is in the dependent position to allow easy submersion in water. This is also an easy way to determine which wall of the rectum is involved (anterior, posterior, left, or right). Sedation is not usually necessary because the rectosigmoid junction is not passed with the instrument.

The examination is usually begun with a therapeutic endoscope with a built-in washing function. This equipment allows inspection of the mass and provides an opportunity to clear any residual stool that could degrade imaging. It also allows filling of the rectum to indicate position of the patient that will optimize water filling.

There is no standard advice for the equipment to be used. For staging of tumors located very distally in the rectum, rigid radial scanning probes are often used. An alternative is a radial scanning echoendoscope, as used in the upper gastrointestinal tract. The advantage of echoendoscopes is that they can be advanced higher up into the rectum with help of the (oblique-viewing) optics. Linear echoendoscopes can also be used, with the advantage of enabling the examiner to perform EUS fine-needle aspiration (FNA) biopsy of extrarectal abnormalities such as lymph nodes or suspected tumor recurrences after surgery. The linear probes sometimes offer a further advantage because the tumor and mural layers can be followed in the same image. This sometimes makes it easier to determine the exact involvement of the deeper layers. Finally, mini-probes can be used in patients with superficial lesions. With 12-MHz mini-probes, a penetration depth of 2 cm is generally possible.

Using a balloon around the tip of the rigid probe or echoendoscope removes the air and allows for good acoustic coupling between probe and tumor. Filling of the rectum with water is sometimes helpful, especially in the case of smaller lesions that would otherwise be compressed with a balloon. Complete filling of the rectum with water is usually not possible and should not be attempted because it is much easier to change the patient’s position. When the bowel has been prepared with an enema, care should be taken not to fill the colon extensively with water, because this may mobilize stool located in the proximal colon.

Usually, the instrument is positioned proximal to the tumor, the balloon is slowly inflated, and the lumen is filled with water ( Video 17.1 ). From this position, the transducer should be positioned in the center of the colon to achieve perpendicular imaging of the rectal wall layers ( Fig. 17.1 ). One should then look for the perirectal anatomic features. The universal landmark is the urinary bladder. Once the bladder has been identified, the image should be mechanically rotated so the bladder is located at the 12-o’clock position ( Fig. 17.2 ). The instrument should be withdrawn slowly, with the transducer kept in the middle of the colon. The left-right and up-down dials should be used to adjust the transducer to maintain its position in the middle of the colon. The examiner must not torque the instrument because this will cause tangential imaging and potentially lead to inaccurate assessment of the depth of tumor penetration. When withdrawing the probe in the male, the seminal vesicles will be seen as echo-poor, elongated structures at the 12-o’clock position (see Fig. 17.2 ). Further withdrawal will bring the prostate into view. The prostate is seen as a hypoechoic, bean-shaped structure at the 12-o’clock position ( Fig. 17.3 ). In female patients, withdrawal of the scope from the bladder first reveals the uterus ( Fig. 17.4A ), which is a rounded, hypoechoic structure at the 12-o’clock position. Then the vagina is seen as an elongated oval, hypoechoic structure with a characteristic hyperechoic band in the center that represents air (see Fig. 17.4B ). It is important to recognize perirectal structures because invasion into any of them represents T4 disease. In addition, one must distinguish these structures, especially the seminal vesicles, from lymph nodes.