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.
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