Anal Ultrasonography

Law and Bartram1 described the technique of anal ultrasonography in 1989, and in the past 14 years, the technique has achieved widespread popularity. In recent years, endoanal ultrasonography has emerged as a popular diagnostic modality for anal sphincter mapping and imaging. The procedure is performed as an outpatient procedure, is relatively quick, is virtually painless, and therefore is preferred by patients over other more invasive tests, such as needle electromyography. Anal ultrasonography can provide a detailed image of the anal sphincter muscula-ture.2 The internal anal sphincter (IAS) appears endosono-graphically as a hypoechoic circular band, which is seen most prominently at the level of the mid anal canal. The range of thickness of this muscle is 1.5 to 4 mm for males and 2 to 4 mm for females. There does not seem to be a relationship between IAS thickness and body weight, gender, or height, but there is a direct correlation for thickness with advanced age.3 The external anal sphincter (EAS) appears as a thicker circular mixed echogenic band outside of the hypoechoic internal sphincter. Other structures of the pelvic floor, such as the puborectalis, urethral sphincter, vagina, and outlines of the bony pelvic and ischiorectal fossae, are also demonstrated on endoanal ultrasonography.4

There is no special preparation necessary for anal ultra-sonography. The patient is positioned in the left lateral decubitus position. A digital rectal examination is performed to evaluate for anal lesions, stenosis, and tone. We use a Bruel & Kjaer scanner with a 10-MHz transducer that has a sonolucent hard plastic cover filled with degassed water. A condom filled with ultrasound gel is placed over the probe and is lubricated with gel. This provides for effective acoustic coupling in the anal canal. Transducers vary in frequency; a probe with a higher frequency provides clearer sonographic images (resolution) but has a shorter focal range (penetration). A 7.0-MHz probe has a focal length of 2 to 5 cm, whereas a 10.0-MHz probe has a focal length of 1 to 4 cm.

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