Ultrasound Anatomy

On ultrasound the normal rectal wall is 2-3 mm thick and is composed of a five-layer structure [2] (Fig. 6). The first hyperechoic layer corresponds to the interface of the balloon with the rectal mucosal surface, the second hypoechoic layer to the mucosa and mus-colaris mucosa, the third hyperechoic layer to the submucosa, the fourth hypoechoic layer to the mus-colaris propria and the fifth hyperechoic layer to the

Fig. 6. Bidimensional ultrasonographic five-layer structure of the normal rectal wall

serosa or represents the interface of the rectum with the perirectal fat. Good visualisation depends on maintaining the probe in the centre lumen of the rectum and having adequate distension of the water-filled balloon with good acoustic contact with rectal wall.

The ultrasonographer must have a clear understanding of what each of these five lines represent anatomically. When staging a rectal cancer, various levels of the tumour must be optimally imaged and the integrity of the lines carefully assessed. Attention must be focused on the third hyperechoic layer. Once it has been ascertained that the middle hyperechoic line is broken, then an invasive lesion is recognised and attention is then turned to the thickness of the muscolaris propria and the integrity of the outer hyperechoic line to see if the perirectal fat is invaded. The fibrofatty tissue surrounding the rectum contains blood vessels, nerves and lymphatics and has an inhomogeneous echo pattern. Very small, 2-3-mm, round to oval hypoechoic lymph nodes may be seen and must be distinguished from blood vessels, which are also circular hypoechoic areas, but when followed longitudinally, they seem to extend further than the corresponding diameter and can often be seen to branch and to elongate in a longitudinal fashion, confirming that this is a blood vessel and not a node. Anteriorly the bladder, seminal vesicles and prostate can be identified in the male and the uterus, cervix and vagina in the female.

Three-dimensional ERUS offers a valuable supplement to conventional ultrasound. The 5 layers of the rectal wall are clearly illustrated in the coronal plane as well as in the transaxial and the longitudinal image planes (Fig. 7).

Endoluminal ultrasound defines anatomy of the anal canal and pelvic floor. Five and possibly six hypoechoic and hyperechoic layers can be seen [2].

Fig. 7. Three-dimensional ultrasonographic five-layer structure of the normal rectal wall

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 hyperechoic layer to the subep-ithelial 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). The hypoechoic layer that represents the IAS can be traced superiorly into the circular muscle of the rectum. Its thickness varies from 1.5 to 4 mm (mean 3.5±0.5 mm) and increases with age owing to the presence of more fibrous tissue as the absolute amount of muscle decreases. The LM 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, respectively. The thickness of the IAS and the EAS should be measured at the 3 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 portions [2]. The level refers to the following anatomical structures: (1) deep: the sling of the puborectalis and the deep part of the external sphincter; (2) mid: the anococcygeal ligament, superficial part of the external sphincter, internal sphincter and perineal body and (3) superficial: the subcutaneous part of the external sphincter. 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 labelled high. 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 LM and the superficial EAS all are identified. This image will be labelled mid. 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 labelled low.

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