Colorectal cancer is often preventable if the precursor adenoma is detected and removed. Although ultrasound is clearly not one of the widely accepted screening techniques, this non-invasive and radiation-free modality is also capable of detecting colonic polyps, both benign and malignant. Such colon lesions may be encountered when not expected, usually during general abdominal sonography. The discovery of large colonic polyps is important and can potentially help reduce the incidence of a common cancer, whereas detection of a malignant polyp at an early stage may result in a curative intervention. This pictorial review highlights our experience of sonographic detection of colonic polyps in 43 adult patients encountered at our institutions over a 2-year period. 4 out of 50 discovered polyps were found to be malignant lesions, 3 polyps were hyperplastic, 1 polyp was a hamartomatous polyp and the rest were benign adenomas. The smallest of the detected polyps was 1.3 cm in diameter, the largest one was 4.0 cm (mean 1.7 cm; median 1.6 cm). In each case, polyps were discovered during a routine abdominal or pelvic examination, particularly when scanning was supplemented by a brief focused sonographic inspection of the colon with a 6–10 MHz linear transducer.
In this paper, we illustrate the key sonographic features of different types of commonly encountered colonic polyps in the hope of encouraging more observers to detect these lesions, which may be subtle.
Colonic polyps: sonographic morphology
When visible, colonic polyps usually lend themselves well to evaluation. On close inspection with a 6–10 MHz linear array transducer, surprisingly detailed views of the polypoid lesions can be achieved (Figure 5). The polyp pedicle is visualised as a prolongation of the mucosa, with a submucosal muscularis layer connecting the head of the polyp to the colonic wall (Figure 5d). Feeding vessels may also be seen. Adenomatous polyps may have smooth, or slightly convoluted, or lobular surface contour.
Even when the colonic lumen contains some echogenic residue, the polyps may be conspicuous owing to their low reflectivity (Figure 6). Echogenic faecal residue may also be moved away from the polyp by gentle compression with the transducer, which will improve visualisation.
Sessile polyps may be recognised when the lesion lies closely related to the colonic wall. Differentiation between pedunculated, sessile and flat polyps, however, is difficult unless the vascular stalk is readily visualised. When vascularity within the lesion is uncertain on inspection with colour Doppler, spectral Doppler analysis can confirm the presence of true vasculature (Figure 7). When no vascularity within the lesion is identified, then constant position, shape and size of such a lesion throughout the examination may suggest a polyp (Figure 8). Clearly, the observed lesion is unlikely to represent a polyp if it is avascular on Doppler and has moved with the transducer compression or in the course of the examination.
Discovery of a colonic polyp in a young individual will usually call for a targeted work-up for a polyposis syndrome. Hamartomatous polyps, classically associated with Peutz–Jeghers polyposis syndrome, may occasionally occur as isolated lesions (Figure 9).