Wai Kit Lee
Colorectal cancer is common, over 13 000 cases were diagnosed in Australia in 2005.
The pathogenesis of colorectal cancer has been well investigated and usually occurs in a
predictable sequence progressing from dysplasia, to carcinoma in situ before becoming
an invasive malignancy. The symptoms and signs of colorectal polyps and masses are
often nonspecific, however, given that polyps are easily cured with polypectomy, it
is vital to have an accurate and acceptable diagnostic test. Traditional tests include
conventional (optical) colonoscopy and double contrast barium enema. Computed
tomographic (CT) colonography is a newer, minimally invasive method for examining
the colon for colorectal polyps.
To inform general practitioners about CT colonography, its evidence, indications,
controversies and extracolonic ancillary findings.
The evidence supporting CT colonography is discussed along with how it is performed,
as well as a discussion of the factors unique to it, such as extracolonic findings and
How is CT colonography performed?
Computed tomographic (CT) colonography is a low radiation dose CT scan performed in supine and prone positions following a full colonic preparation, and then followed by colonic insufflation with carbon dioxide via a rectal catheter with no need for sedation. Supine and prone positions are required to move any residual colonic fluid that may obscure polyps and ensure that each colonic segment is adequately distended. Faecal tagging can be used. This involves patients drinking 150 mL of barium liquid with meals starting 48 hours before the examination and allows easy differentiation of residual faecal material from polyps. Faecal tagging is omitted in patients with an incomplete optical colonoscopy as it allows a same day study to be performed and avoids patients having to repeat bowel preparation. The images can be reviewed in any plane or reconstructed into a ‘virtual colonoscopy’ allowing a colonic ‘fly through’ simulation of optical colonoscopy (OC). Researchers are actively exploring computer bowel cleansing so that CT colonography can be performed without requiring a bowel preparation, however, this has not yet eventuated.
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