Computed tomography colonography



Computed tomography colonography (CTC) is a minimally invasive method of examining the colon and rectum. 



It requires bowel preparation and the oral administration of fecal tagging agents prior to the insertion of a rectal tube, which is used to inflate the colon with carbon dioxide. 



A low-dose CT scan is then performed



It is performed in two positions: supine scan and then either a prone or lateral decubitus study. 



Imaging software enabls the colon to be examined in both a multi-planar two-dimensional and a three-dimensional 


virtual colonoscopy mod.



CTC is well tolerated, does not require sedatioN.



Associated with a perforation rate of 0.04%, the vast majority of which are asymptomatic and managed conservatively.



CT colonography can be performed immediately following a simple polypectomy but should be delayed in patients who have undergone complex endoscopic intervention as this increases the risk of perforation. Likewise, 



CTC should be avoided after complex endoscopy, and in   patients with active colitis or obstructing strictures.



In a study with over 1200 patients comparing same-day CTC with colonoscopy (OC), CTC had a sensitivity of 94% for the detection of polyps over 10mm, performing as well as OC.



The high sensitivity of CTC for the detection of colorectal cancer (CRC) has been confirmed in meta-analyses involving 49 studies and 11,151 patients.



The sensitivity of CTC for the detection of polyps 6-9 mm: a meta-analysis reporting a sensitivity of 59% for these lesions.



Radiologists do not report polyps that are less than 6mm, as the overwhelming majority of these do not harbour advanced histology.



The interval cancer rates following a negative CTC are low and in one study involving 1050 patients with a negative CTC and follow-up average of 4.7 years found one interval cancer.



Another study with 1429 patients with negative CTC and mean follow-up of 5.7 years found two interval cancers, one occurring 5 years post CTC and the other 10 years post initial CTC.



The inherently high contrast between the gas containing gut lumen and soft tissue colonic wall allows for a low dose CT to be performed without reducing the sensitivity of the examination. 



Typical radiation doses for CTC are 5mSv or less.



Modern  methods is allowing the dose to fall as low as 1 mSv which is less than half of the annual natural background radiation dose. 



Modelling studies suggest CTC every 5 years between the ages of 50 and 80 years, would prevent between 24 and 35 CRCs for every radiation-induced malignancy.



The radiation dose of CTC is significantly lower than the dose acquired during inferior tests such as barium enema.



CTC examines the colonic mucosa, the contents of the abdominal and pelvic cavities, and the spine and lung bases. 



Extracolonic findings are frequently encountered.



Bowel preparation is required prior to the examination.


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