CT colonography - pitfalls
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The interpretation of CT colonography can sometimes be difficult because of pitfalls, which may be a source of false negative and false positive findings. When suboptimal CT colonography techniques are applied, the number and severity of interpretive pitfalls can rapidly multiply. However, when proven state of the art techniques are consistently applied, there are a few potential pitfalls that may be encountered at interpretation.
Flat polyps coated with contrast material can be mistaken for tagged stool plaques. In 2D images, the soft tissue component of a flat polyp underlying the adherent coating of contrast material is much less evident than that of a sessile or pedunculated polyp and can easily go unnoticed 1.
Solutions: contrast enhancement may be used as a problem solving tool when a potential flat lesion is found and there are doubts.
Gas bubbles are common and mimic a polypoid lesion or diverticulum. In 3D images, they appear as round polyps and multiple adjacent bullae may simulate a villous polypoid lesion in 3D images 2. In 2D images, they are seen as a scalloped indentation of the top of a fluid level, because the bubble wall is not easily seen in 2D images.
Solutions: when bubbles are present with no surrounding fluid, they may be best resolved on 2D images with wider colon or bone window settings 3.
Sticky solid colonic residue
Frequently the colonic residue remains in the colon adherent to the wall resulting in false-positive interpretation with a pseudopolypoid lesion. The residue is sometimes easy to characterize because of air inclusions and a positional shift between the supine and prone acquisition. However, in case of sticky solid colonic residue, there is no positional shift .
Solution: Good preparation, which should include a combination of a low-residue diet (no vegetables or fruit during the three days before the examination), and fecal/fluid tagging the day before CT colonography. Fecal tagging consists of drinking positive contrast (e.g. Gastrografin) the day before CT colonography (suggested amount 180 mL of contrast with some water). This allows for differentiation between tagged food residue and a true non-tagged polyp 3.
Visualization of the rectum is hampered by the rectal catheter needed to insufflate the colon with gas. Also, the balloon may compress luminal lesions against the rectal wall reducing their perception 1.
Solution: Meticulous inspection, using 3D endoluminal and 2D views would be of value. This can be done by turning the virtual camera in a retrograde direction to inspect the “pericatheter” segment. Also, it is advised to deflate the balloon immediately before starting the prone acquisition to avoid compression of lesions by the balloon of the catheter 1.
The contraction of folds in case of spasm, the folds have thickened appearance 1.
Solution: Optimal colonic distension is crucial to flatten the semilunar folds. This can be obtained by combining smooth muscle relaxation, colonic insufflation with a CO2 injector and dual positioning (i.e., acquisition in the supine and prone position) 1.
The pericolonic structures at the inner part of the flexure are compressed and cause a thickening of the fold that may mimic tumoral lesions 4.
Solution: The thickening frequently presents with a lipomatous density representing the pericolonic structures. In 3D, this thickening is regular and smooth. Also, the thickened fold frequently has a different aspect between the supine and prone acquisition.
Enlarged ileocecal valve
Solution: It is mandatory to define the structure of the ileocecal valve by defining both lips of the valve (particularly on coronal reformatted images). Moreover, there is a change in aspect between supine and prone acquisition.
The appendicular base may have an impression on the cecal tip. Also, the appendiceal stump can protrude into the cecal lumen giving the appearance of a small pseudopolypoidal lesion 3.
Solution: This can be confirmed by localizing the appendix on the coronal and sagittal reformats. Also, the appendiceal orifice can be confirmed on the endoluminal 3D views. Any history of appendectomy can be obtained from the patient before the examination.