Perirectal fat invasion is diagnosed sonographically by the presence of irregularity of the outer hypere-choic layer that corresponds to the perirectal fat interface (Fig. 14). These findings should be associated with disruption of the hyperechoic layer corresponding to the submucosa and thickening of the hypoechoic layer representing the muscolaris propria. Contiguous organs are not involved. About 10% of such tumours are unfortunately accompanied by a narrowing of the lumen and angulation and it may be difficult or impossible to advance the probe proximal to the tumour (Fig. 15). To perform a complete staging by ERUS, a residual lumen of 2 cm is necessary because only those structures seen at a 90° angle to the probe can be assessed correctly. Under these cir-
Fig. 15. Three-dimensional ERUS showing a tumour that narrows the rectal lumen
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