Transrectal ultrasonography of anorectal diseases: advantages and disadvantages

Abstract

Transrectal ultrasonography (TRUS) has been widely accepted as a popular imaging modality for evaluating the lower rectum, anal sphincters, and pelvic floor in patients with various anorectal diseases. It provides excellent visualization of the layers of the rectal wall and of the anatomy of the anal canal. TRUS is an accurate tool for the staging of primary rectal cancer, especially for early stages. Although magnetic resonance imaging is a modality complementary to TRUS with advantages for evaluating the mesorectum, external sphincter, and deep pelvic inflammation, three-dimensional ultrasonography improves the detection and characterization of perianal fistulas and therefore plays a crucial role in optimal treatment planning. The operator should be familiar with the anatomy of the rectum and pelvic structures relevant to the preoperative evaluation of rectal cancer and other anal canal diseases, and should have technical proficiency in the use of TRUS combined with an awareness of its limitations compared to magnetic resonance imaging.

Keywords: Anal canal; Fistula; Rectal neoplasms; Ultrasonography.

Fig. 1.. Optimal transrectal ultrasonography scan.

The axial view clearly shows the layers of the rectal wall with proper rectal distension after cleansing with an enema. The transducer is placed in the center of the rectal lumen.

Fig. 2.. Three-dimensional (3D) transrectal ultrasonography images from data transportation.

After 3D acquisition, the axial (upper left and lower middle) images are immediately reconstructed to correspond to any axis that the operator wants, such as coronal (upper middle) and sagittal (lower right) views. The upper right image shows an orthogonal view. In addition, the 3D dataset can be manipulated to render images with enhanced surface features (surface render mode, lower left) and depth features (opacity, thickness, luminance, and filter settings), facilitating the further identification of surrounding tissues.

Fig. 3.. Normal rectal wall layers shown with ultrasonography.

The innermost hyperechoic layer indicates the interface of the balloon and the mucosal surface of the rectal wall. The inner hypoechoic layer represents the mucosa and muscularis mucosa, followed by a thicker hyperechoic submucosa layer. The next hypoechoic layer shows the muscularis propria and the outermost hyperechoic layer corresponds to the perirectal fatty tissue.

Fig. 4.. Normal ultrasonogram of the anal canal.

A. The anal canal is usually divided into three levels for examination. In the upper anal canal, the puborectalis muscle is seen as a U-shaped echogenic band. In the middle anal canal, the internal anal sphincter is most clearly seen as a thickened hypoechoic layer. In the lower anal canal, the echogenic external anal sphincter is seen together with the termination of the internal anal sphincter. B. On coronal reformatted three-dimensional ultrasonogram, a hypoechoic longitudinal layer indicates the internal sphincter, terminating at the lower anal canal. The external anal sphincter is represented by a hyperechoic layer running through the outer aspect of the internal anal sphincter.

Fig. 5.. Perianal anatomic areas on three-dimensional (3D) transrectal ultrasonography.

Intersphincteric, ischioanal, and supralevator spaces are defined by the muscular landmarks on the coronal view.

Fig. 6.. T1 rectal cancer.

Axial transrectal ultrasonography shows that the hypoechoic tumor (arrows) is confined to the first inner three layers and that the hyperechoic submucosa layer is slightly thinned.

Fig. 7.. T2 rectal cancer.

The hyperechoic submucosa layer is disrupted by a thickening of the muscularis propria (arrows), which indicates T2 rectal cancer.

Fig. 8.. T3 rectal cancer.

A. An axial three-dimensional ultrasonogram shows that the hypoechoic mass (arrows) extends over the muscularis propria into the perirectal fat. B. An oblique sagittal reformatted image shows the hypoechoic T3 tumor (arrows) extending into the perirectal fat tissue.

Fig. 9.. Extension of perianal inflammation (cryptoglandular hypothesis).

If an abscess develops in a superficial gland, the rupture of the abscess extends into the intersphincteric space forming a fistular tract that reaches the skin. Alternatively, a pelvic infection may pass through the external sphincter and enter the ischioanal fossa. The infection sometimes extends to the low perianal space and the supralevator space.

Fig. 10.. Intersphincteric perianal fistula.

A. The hypoechoic tract (arrow) is seen between the internal (IAS) and external anal sphincters (EAS) in the axial image. B, C. Threedimensional reconstructed sagittal (B) and coronal (C) images better represent the exact location of the fistula (arrows).

Fig. 11.. Extrasphincteric perianal fistula.

A. An axial three-dimensional ultrasonogram shows that the hypoechoic tract (arrow) lies along the intersphincteric space, extending into the external anal sphincter (EAS). B. The hypoechoic tract (arrows) passes from the intersphincteric space through the external sphincter into the ischiorectal fossa on coronal image.

Fig. 12.. Perianal abscess.

At the level of the middle anal canal, a perianal abscess is visible in the 6 o’clock direction. It contains a hyperechoic focus with an acoustic shadow (arrow) that is presumably a gas bubble. The coronal image reflects the overall shape of the abscess (arrows). The sagittal three-dimensional image displays the perianal abscess (arrow) with a gas bubble.

Fig. 13.. Anal cancer.

Using three-dimensional images, the tumor location, shape, and size can easily be defined. The anal cancer appears as a lesion (arrows) that penetrates into the perianal fat interruption of the internal anal sphincter.

Fig. 14.. Sphincter trauma.

In the 6-8 o’ clock direction, the external and internal anal sphincters show discontinuation and thinning at the level of the lower anal canal (arrows). Axial transrectal ultrasonography shows localized hypoechoic scar tissue in the external sphincter at the seven o’clock position. Sagittal and coronal threedimensional images also display abrupt interruptions (arrows) of the sphincters.