CT Colonography

CT colonography (CTC) is a relatively new method of investigating the large bowel. It is made possible by the availability of multislice CT scanners that are able to scan the entire abdomen and pelvis with fine slices in a single breathhold. Equally important is the increased speed of computers to postprocess this large data set and display it in a useful and interactive format.

Method

Bowel preparation as for barium enema.

Buscopan IV given to some patients, but not routinely.

Insufflation of carbon dioxide into the colon.

CT scans of the abdomen in the prone and supine positions, each lasting approximately 13 seconds (a lower radiation dose technique is used than for standard abdominal scanning). The supine and prone positions are to distinguish mobile faecal material from fixed polyps and also to shift residual fluid within the bowel lumen, which can obscure pathology.

Interpretation on workstation.

Risks

The risks for bowel preparation and use of Buscopan are the same as for barium enema or colonoscopy.

Radiation dose is approximately that of a conventional barium enema.

Benefits

No sedation.

Brief examination.

Better tolerated by patients than barium enema. There is less movement required during the examination and no messy barium or post procedure constipation.

The use of CO2 significantly reduces bloating and spasm after the examination.  This is because CO2 is much more soluble than room air.

Extra colonic structures are routinely assessed during CTC.

Role of CTC

1	.  Diagnosis/exclusion of colorectal neoplasia in symptomatic patients. Probably about as accurate as barium enema for this indication. Patient selection: patients with high probability of neoplasia generally undergo colonoscopy, while those with a low probability could be assessed with CT colonography. Especially useful in elderly/frail patients. NB: Not suitable for mucosal diseases such as inflammatory bowel disease or angiodysplasia.
2	.  Incomplete colonoscopy. This is a not infrequent, current use of CTC. This occurs when the colonoscopist is unable to intubate the entire colon, either due to technical difficulties or an obstructing lesion. CTC is carried out immediately after the colonoscopy.
3	.  Screening for colorectal neoplasia. At present high-risk subjects undergo screening colonoscopy. For average risk subjects, there is no consensus on the best method of screening. CTC may become useful in this role.

CTC has a reported sensitivity of 73 - 91% for polyps > 10 mm but is less sensitive for polyps < 5 mm. However it is probably not important to detect these small polyps in the screening situation, as approximately 50% are hyperplastic polyps and the lead-time of adenomas of this size turning into carcinomas is long enough to be detected in the subsequent screening round.

CTC is available at St. George's Radiology and Cashmere Radiology.  Please feel free to speak with any of the radiologists if you have any questions about this procedure or suitability for individual patients.

 

Dr Scott Wells
Radiologist
CHRISTCHURCH RADIOLOGY GROUP