COLONIC POLYPS: US DETECTION

 

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

Direct display of colonic polyps relies on demonstration of a spherical or ovoid hypoechoic lesion arising within colonic lumen (Figure 4). The hallmark for sonographic identification, in our experience, is the presence of demonstrable vascularity within such a lesion on Doppler.

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).

Inflammatory polyps represent areas of elevated inflamed or normal mucosa. These may be sessile or pedunculated and are usually seen in patients with inflammatory disorders of the colon such as ulcerative colitis, Crohn's disease and dysenteric colitis. The term “pseudopolyp” is sometimes used to emphasise the non-neoplastic aetiology of these lesions [15]. Inflammatory polyps are similar to mucosa in echotexture, and thus recognised as extensions of the mucosa (Figure 10). Other changes indicating inflammatory bowel disease are usually present aiding differentiation.

Lipomas are also seen as intraluminal polypoid lesion, but can be recognised on ultrasound owing to their characteristic echogenic appearance.

The presence of malignancy may not be possible to estimate on ultrasound due to lack of reliable features. However, an apparent loss of wall stratification at the base of a large sessile polypoid lesion can suggest a carcinoma (Figure 11). The presence of a gas pocket retained within a large polyp indicates ulceration and is usually seen in malignant lesions (Figure 12). Detection of these features may warrant further CT investigation in addition to flexible sigmoidoscopy or colonoscopy.

 

Potential pitfalls

Several pitfalls that may lead to false-positive findings should be kept in mind when imaging the colon with ultrasound. These, in our experience, may be related to several factors, including a deficient scanning technique (such as tangential imaging), peculiar appearance of the colon owing to prominent convergent and bulbous haustral folds, presence of undigested food pieces and foreign bodies, and impacted diverticulum.

Tangential ultrasound imaging of the colonic wall in transverse plane, particularly in the areas of haustral folds, often results in false polypoid projections, which may be confusing (Figure 13a,b). This pitfall is avoided by multiplanar scanning of the colon with liberal transducer angulations that allow obtaining three-dimensional information.

Care should be taken not to confuse bulbous and complex haustral folds with polyps when the colon is imaged in longitudinal plane (Figure 13c). The linear, elongated nature of folds will aid differentiation in that circumstance. Another pitfall is to visualise a focal bulge in the colonic wall in cross-section that is present owing to prominent taeniae coli imaged during their contraction (Figure 14a).

Fragments of undigested food, pills or capsules and other foreign bodies may be occasionally encountered in the colon, but these are identified owing to their mobility, lack of vasculature and peculiar geometry, incompatible with that of polyps. The use of graded compression whereby the anterior and posterior walls of the bowel are opposed is helpful in displacing faeces and foreign bodies. A polyp may be reliably differentiated from faeces by the presence of continuation of mucosa and echogenic submucosa connecting the head of the polyp to the colonic wall (Figure 5d). The presence of a vascularised pedicle is also confirmatory of a polyp. In addition tiny cysts may be seen in the head of the polyp corresponding histologically to glands containing mucus [7].

Impacted diverticulae can be a potential source of confusion when they bulge prominently into the colonic lumen, but are readily recognised on ultrasound owing to the presence of trapped gas and inspissated stool (Figure 14b).

Fortunately, pitfalls leading to false-positives are usually avoided with careful technique. We did not encounter any false positive studies, which is in line with other observers reporting very high specificity of 99.4% for colonic polyp detection [7,8]. Of course, low sensitivity for detection of colonic polyps is a weakness of conventional ultrasound. Perhaps future advances in ultrasound imaging may someday permit this radiation-free non-invasive modality to play a much greater role in this area of colorectal imaging.

Conclusion

When integrated into routine scanning, brief sonographic examination of the accessible colon can reveal unsuspected large colonic polyps, which appear as spherical or ovoid, well-defined hypoechoic lesions within colonic lumen. Demonstration of vascularity within such lesions on Doppler is confirmatory. This may maximise the usefulness of conventional ultrasound and potentially help reduce the incidence of a common cancer since colonic polyps may harbour an early carcinoma or lead to malignancy.