Sonographic features of colorectal cancer reflect its macroscopic pathology. Colorectal carcinoma classically appears as segmental thickening or as a bulky mass of the colonic wall with heterogeneous low echogenicity (Lim 1996).

Segmental wall thickening is usually short and asymmetric, eccentric or circumferential, with irregular contours (Fig. 2). This type of cancer is more frequently found in the left colon, where it determines narrowing of the lumen up to colonic obstruction. In this context, the lumen usually does not contain feces, but a small amount of gas, which may be also observed in the proximal pre-stenotic tract of the colon. Indeed, pre-stenotic tract may contain a large amount of fecal and gaseous content that may make the evaluation of the colonic walls more difficult (Fig. 3).

The mass involving the colonic wall is more frequently found in the right colon. It may be variable in size, hypoechoic, and with irregular contours (Fig. 4). Often high echoes denoting intraluminal gas and feces may be visible within the mass, so that it appears as a “pseudokidney” (Fig. 5).

The wall thickening with abrupt segmental loss of wall layer stratification and disappearance of haustra coli may be features of malignancy (Kuzmich et al. 2009) (Fig. 6; Video). Colonic wall may also show increased vascularity at color Doppler (Fig. 7). However, these signs are not specific, as they can also be observed in inflammatory diseases or colonic ischaemia (Lim 1996; Shirahama et al. 1994). Adjacent regional roundish hypoechoic lymphadenopathies should also be searched for because their presence may reinforce the suspicion of malignancy. Sonography of the liver should be performed as well to assess the presence of metastasis when a colorectal cancer is detected or suspected.

However, it should be kept in mind that the absence of these findings at sonography does not exclude the diagnosis of colonic carcinoma. Small cancers and polyps can be overlooked, and the suboptimal diagnostic quality of the scan—due to bowel gas or obesity—can lead to false-negative results. On account of these limitations sonography is not an effective technique of screening for colonic cancer.

Several studies have evaluated the accuracy of transabdominal ultrasound in detecting colorectal cancer. Most of these studies showed that ultrasound detects accurately advanced colorectal tumors, included those causing colonic obstruction, but misses small cancers and polypoid lesions (Price and Metreweli 1988; Rutgeerts et al. 1991; Richardson et al. 1998; Grunshaw et al. 2000; Martínez-Ares et al. 2005; Gluvic et al. 2008). This has also been shown using transabdominal hydrocolonic sonography, namely the assessment of the colonic walls after bowel preparation with laxatives and following instillation of water into the colon (see “Oral Contrast-Enhanced Bowel Ultrasound”). Eleven studies have assessed the accuracy of hydrocolonic sonography in detecting colorectal cancer showing a mean sensitivity of 90 % (range 0–97 %). High sensitivity has also been reported in assessing the T stage of colorectal cancer (mean, 85.2 %), but more than 80 % of these lesions were in an advanced stage (T3 or T4). On the contrary, the sensitivity of this technique in assessing the N stage of the tumour and polyps was much more disappointing (Maconi et al. 2009).