Endosonographic Anatomy of the Anal Canal

Knowledge of the anal canal and pelvic floor anatomy is required in order to better understand how

Fig. 8. Normal ultrasonographic six-layer structure of the mid anal canal in a male. Axial image (a), schematic representation (b)

perianal sepsis develops and to more accurately classify fistulae and abscesses. On ultrasound, six hypoe-choic and hyperechoic layers can be seen in the normal anal canal [14]. The ultrasonographer must have a clear understanding of what each of these six lines represent anatomically. From inner to outer, the first hyperechoic layer corresponds to the interface of the plastic cone with the anal mucosal surface, the second hypoechoic layer to the mucosa, the third hyper-echoic layer to the subepithelial tissues, the fourth hypoechoic layer to the internal anal sphincter (IAS), the fifth hyperechoic layer to the longitudinal muscle (LM) and the sixth mixed echogenic layer to the external anal sphincter (EAS) (Fig. 8). Three-dimensional endorectal ultrasound offers a valuable supplement to conventional ultrasound. The six layers of

Anal Ultrasound
Fig. 9. Normal ultrasound anatomy of the sphincter complex in three-dimensional image

the anal canal are clearly illustrated in the coronal plane as well as in the transaxial and the longitudinal image planes (Fig. 9).

The hypoechoic layer that represents the IAS can be traced superiorly into the circular muscle of the rectum. Its thickness varies from 1.5 mm to 4 mm (mean 3.0±0.5 mm) and increases with age owing the presence of more fibrous tissue as the absolute amount of muscle decreases. The longitudinal muscle is 2.5±0.6 mm in males and 2.9±0.6 mm in females. This muscle is moderately echogenic, which is surprising, as it is mainly smooth muscle; however, an increased fibrous stroma may account for this. The average thickness of the EAS is 8.6±1.1 mm in males and 7.7±1.1 mm in females. Thickness of the IAS and EAS should be measured at the 3 o'clock and 6 o'clock positions in the midlevel of the anal canal.

Ultrasound imaging of the anus can be divided into three levels: deep, mid and superficial [14]. The level refers to the following anatomical structures: (1) deep: the sling of the puborectalis and the deep part of the EAS; (2) mid: the anococcygeal ligament, superficial part of the EAS, IAS and perineal body; and (3) superficial: the subcutaneous part of the EAS. The first ultrasound image recorded is normally at puborectalis level, where the perineal body is also seen in females. This image is normally documented and labeled HIGH (Fig. 10). In a normal patient, moving the probe a few millimetres in the distal direction will show an intact anterior EAS forming just below the superficial transverse perineal muscles. This image is a mid-canal projection where the IAS, conjoining longitudinal muscle and the superficial EAS all are identified. This image will be labeled

MID (Fig. 11). When the probe is pulled further out, the image of the IAS will disappear and only the subepithelium and the subcutaneous segment of the LM + EAS will be seen. This last image will be labeled LOW (Fig. 12). The anterior part of the EAS differs between genders. In males, it is symmetrical at all levels; in females, it is shorter anteriorly and there is no evidence of anterior ring high in the canal (Fig. 13). In examining a female subject, the ultrasonographic differences between the natural gaps (hypoechoic areas with smooth, regular edges) and sphincter ruptures (mixed echogenicity, due to scarring, with irregular edges) occurring at the upper anterior part of the anal canal must be kept in mind [15].

Normal External Rectum
Fig. 10. Normal ultrasound anatomy of the deep level of the anal canal demonstrating the puborectalis. Anteriorly, a thin arc of muscle from the deeper part of the sphincter may be seen in males (a) whereas the deep part of the external sphincter is not recognisable in females (b)
Endosonography Rectal Crohn

Fig. 11. Bulbospongiosus muscle, transverse perineal muscles and external anal sphincter meet in the perineal body. In males (a), a plane of fat persists between the transverse perinei and the external sphincter whereas in females (b), the transverse perinei fuse with the external sphincter. Schematic representation (c)

Crohn Disease Anatomy
Fig. 12. Image at the superficial level demonstrating the subcutaneous external anal sphincter. The internal sphincter is absent at this level

Fig. 11. Bulbospongiosus muscle, transverse perineal muscles and external anal sphincter meet in the perineal body. In males (a), a plane of fat persists between the transverse perinei and the external sphincter whereas in females (b), the transverse perinei fuse with the external sphincter. Schematic representation (c)

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