Ultrasonography of the Anal Region

Endosonography of anal region does not require any preliminary preparation, nor the use of contrast media; a laxative enema prior to the procedure is enough. Sedation is not required and the patient is examined in the left lateral position. Prior to the endoanal ultrasound examination, careful inspection of the perianal area followed by a digital examination is essential, and complements the endoanal ultrasound examination, allowing to rule out stenoses and obtain a sphincter relaxation. To evaluate the anorectal segment we usually employ a Brϋel-Kjær Kawk (B&K Medical Systems) 2012 EXL scanner with 10 MHz type 2050 endoanal transducer, which can offer three-dimensional image reconstruction. A translucent plastic cap is placed over the transducer, and filled with water to provide the acoustic medium for transmission of sound waves. The plastic cap may be preferred over a water-filled latex balloon to cover the transducer, as it allows uniform imaging of the anal canal and eliminates the risk of balloon entrapment in the rotating probe. The endoanal US probe is conventionally held parallel (co-axial) to the lumen of the anal canal: the anatomic reconstruction obtained with the patient in left lateral decubitus shows the anterior aspect of the anal canal on the upper part of the screen and the posterior aspect on the lower part of the screen. 

The different anatomic layers of the normal rectal wall are reconstructed by the US probe as alternated concentric hyper- and hypoechoic bands (Fig. 1)

Fig. 1: the anatomic diagram of normal rectal wall and the corresponding ultrasonographic bands.

The rectal wall is usually reconstructed in 5 layers (Fig. 2), the first white circle being the interface between the balloon or cap and the rectal mucosa. The second layer, black and hypoechoic, is the mucosa and superficial aspect of the submucosa. Then, the third layer, white and hyperechoic, is the interface between submucosa and muscolaris propria; a further black hypoechoic layer is the muscle layer; last, a white hyperechoic external band represents the interface with the perirectal fat. 

Fig. 2: The 5-layered EUS model of the rectal wall, showing the alternation of hyperechoic and hypoechoic bands. 

In the lower portion of the rectum it is possible to identify different perivisceral structures, including the prostate in males.

The probe is lubricated with a water-soluble jelly and gently inserted through the anus. Endoanal ultrasonography very nicely demonstrates the normal anal canal. Three levels of the anal canal are imaged, and each has specific landmarks that are seen endosonographically. These three levels are: the upper anal canal, the mid-anal canal, and the distal anal canal (Fig. 3-4). 

Lo strato aggregato mucoso-sottomucoso tipicamente viene rappresentato, ecograficamente, come una banda chiara iperecoica adiacente al trasduttore. Lo sfintere anale interno (IAS) appare come un cerchio scuro, ipoecoico, che presenta uno spessore variabile fra 2 e 4 mm. Lo sfintere anale esterno (EAS) è un muscolo striato che appare ecograficamente come una banda ecogenicamente mista, situato esteriormente allo sfintere interno (ecograficamente ipoecoico). I margini esterni dello sfintere anale striato, confinanti col grasso perirettale, non vengono identificati in modo ben definito. Il canale anale viene tipicamente suddiviso in superiore, medio ed inferiore, in rapporto ai punti di repere anatomici (Margine Anale, Linea Pettinata, Muscolo Pubo-rettale)(Fig. 3-4)

Fig. 3: the anal canal is subdivided into upper, mid and distal. 

Fig. 4 A-B: endosonographic appearance (A) of the anal sphincter complex (B): 1= external subcutaneous sphincter; 2 and 3 = superficial external sphincter; 4 = deep external sphincter; 5 = pubo-rectalis muscle; 6 = internal anal sphincter.

The transducer is advanced to the upper anal canal, which is recognized by the U-shaped puborectalis. This structure is the landmark for the upper anal canal. The puborectalis is a horseshoe-shaped, mixed echogenicity structure (figure 5). The upper portion of the internal sphincter can usually be seen in the upper anal canal, but can be variable. It is seen as a densely hypoechoic ring inside the puborectalis.

Fig. 5 A-B: endoanal ultrasound at the level of the upper anal canal (A female; B male). The puborectalis encircles the rectum posteriorly. 

Once the upper anal canal is identified and visualized, the probe is slowly withdrawn until the mid-anal canal is visualized. The mid-anal canal is distinguished by the concentric appearance of the internal and the external anal sphincters. At the level of the mid-anal canal, the internal sphincter defines this level, as the internal sphincter is seen as a prominent, circular, hypoechoic structure. The internal sphincter is a black, hypoechoic ring, approximately 2-4 mm in thickness. The high water content of the internal sphincter is the reason why it appears hypoechoic. Between the transducer and the internal sphincter is a hyperechoic ring, which represents the subepithelial tissues, including the submucosa and hemorrhoidal plexus. The external sphincter is a broad, mixed echogenicity, cylindrical band, approximately 5-8 mm in thickness. The striations of the muscle fibers can be seen, particularly with higher frequency transducers. The thickness of the total anal sphincter mechanism anteriorly measures 10-15 mm in females. In some instances the longitudinal muscle can be visualized as a thin, hyperechoic band surrounded by a thin, hypoechoic band between the internal and the external sphincter (Fig. 6 A-B). 

Fig. 6 A-B: normal endosonographic appearance of the mid-anal canal in females (A) and males (B). hyperechoic layer: interface between the balloon and mucosa; 2: hypoechoic layer: mucosa; 3: hyperchoic band: submucosa; 4: hypoechoic band: internal sphincter; 5: hyperechoic band: complex longitudinal muscle; 6: mixed echogenicity band between two arrows: external anal sphincter.

The perineal body thickness in female patients can be measured by placing the examiner's index finger into the vagina while scanning the sphincter at the mid-anal canal level. The phalanx of the finger will show up as a semi-circular hyperechoic shadow, and the distance from the luminal aspect of the internal sphincter to the digital reflection on the scanner represents the perineal body thickness. This can be measured and recorded.

The distal anal canal is the last segment of the anal canal to be visualized. This is identified by gradually withdrawing the probe until the internal anal sphincter is no longer visible, as the probe extends distal to the intersphincteric groove. The external sphincter can still be seen as a mixed echogenicity structure, which is elliptical in shape, with the anterior and posterior raphae giving this appearance (Fig. 7 A-B).

Fig. 7 A-B: subcutaneous portion of the external anal sphincter (A: male; B: female).

Three-dimensional endosonographic reconstruction (3D US) shows that the anterior portion of the external anal sphincter is usually shorter in females compared to males (Fig. 8 A-B).

Fig. 8 A-B: the 3-D US reconstruction demonstrates that the anterior portion of the external anal sphincter is longer in males (A) than in females (B).

Transverse and longitudinal US scans of the puborectalis, the internal and external sphincters can be obtained with 5-6 MHz linear probes. The examination is performed with the patient in lithotomy position. Transverse sections are obtained positioning the probe on the perineal body, between the anus and the vulva. The probe is progressively inclined to achieve concentric images of the muscle layers of the anal sphincter complex. Modifying the inclination of the probe and the pressure exerted on the perineum, the whole length of the anal canal can be assessed. Once the sphincter is visualized, the scanning plane can be rotated by 90 degrees to obtain a longitudinal section to visualize the whole anal canal and the puborectalis sling encircling the rectum at the level of the anorectal junction. To determine movement of the puborectal sling, the first caliper is fixed on the anterior border of the puborectalis, with the patient in the resting position. Holding the probe in a constant position, the patient is then asked to either strain or squeeze. The image is then frozen and the second caliper placed on the anterior border of the puborectalis in its new position. The distance between the two calipers can then be measured.

On a longitudinal plane, the internal anal sphincter appears as the prolongation of the circular muscle layer of the rectum. The external sphincter is visible on both transverse and longitudinal scans, and appears as a hyperechoic band immediately external to the internal anal sphincter (Fig. 9 A-B).

Fig. 9 A-B: perineal ultrasound scans of the anal sphincter (transverse scan, anterior portion above); A = US image of the upper anal canal; B = US image of the mid-anal canal. The internal sphincter appears as a hypoechoic ring (B). Mucosa and submucosa cannot be distinguished and are viewed immediately internal to the internal sphincter. The external anal sphincter appears as a hyperechoic ring at the level of the mid-anal canal (B). The puborectalis (PB) appears as a U-shaped sling at the level of the distal anal canal or anorectal junction  (A).

In the longitudinal scans the muscular sling posterior to the rectum has slight echogenicity. However, the movements of the sling in relation to the distal rectum and anal canal can be clearly appreciated during the “squeezing” contraction of the muscle (Fig. 10 A-C).

Fig. 10: perineal ultrasound; longitudinal scans. A = shortening of the puborectalis (PR) with narrowing of the anorectal angle (ARA – white arrows) during contraction; B = appearance of the PR at rest with a 90° ARA (white arrows); C = relaxation of the PR with posterior dislodgement during squeezing and opening of the ARA (white arrows). [R = Rectocele; Dis* = Distal].