CT colonography
CT colonography - 6mm polyp (arrow) in axial view
CT colonography provides a non-invasive method of detection of colonic disease and has a potential role in the early detection of colon cancer. Computer modelling of data aquired by axial CT scanning provides a simulated representation of the mucosal surface of the colon- " virtual colonoscopy" - in addition to multiplanar visualisation of the distended colon in the axial ,coronal and sagittal planes.
The particular advantage of CT is its ability to view structures beyond the mucosa - bowel wall and mesentery - as well as the entire contents of the abdomen and pelvis , allowing detection of what may be clinically occult but significant disease in other organs.
CT without prior bowel preparation
Many colonic diseases may be identified on abdomino-pelvic thin- section multislice CT without the need for prior bowel preparation and cleansing and in many instances an unprepared colon is preferable. Acute colitis, acute diverticulitis and appendicitis are better assessed in their non-distended bowel. Most tumours > 2cm can be identified. The use of "stool tagging", impregnating faecal content with the prior administration of positive contrast (barium or gastrografin ) can enhance the detection of small tumours, even in the unprepared colon , which can be a significant advantage in the frail and elderly.
CT colonography (CT pneumocolon)
The symptomatic or asymptomatic patient where the goal is the detection of colon cancer or potentially pre-malignant polyps, full CT colonography provides a viable and attractive alternative to colonoscopy, with comparable sensitivity and specificity for lesions of greater than 1cm in size.
The procedure requires full bowel purgation followed by per-rectal insufflation of CO2 (carbon dioxide) to fully distend the colon. No sedation is required. Scans are obtained in the supine and prone position after relaxation of the colon with Buscopan. Intravenous contrast is usually given - particularly where there are symptoms, as this can enhance both the sensitivity and specificity of the examination.
Advantages:
. Speed
. Patient preference
. No need for sedation- an advantage where there are co-morbid conditions which may increase the risk of colonoscopy
. Complete examination - useful where colonoscopy fails to reach the caecum ( a same day service is offered following incomplete colonoscopy to avoid re-preparation of the patient.)
. Visualisation of the colon proximal to an obstructing distal tumour
. Abitity to see disease outside the bowel
Disadvantages :
. Radiation - less of a problem in the population over 50 years , but to be considered where multiple examinations are contemplated. Average radiation dose is approx 0.44 rem (equivalent to 2 adbominal films)
. Inability to biopsy or remove polyps
. Lower sensitivity for flat or small lesions
Most published studies demonstrate sensitivity for over 90% for the detection of polyps > 1cm . The detection of smaller polyps between between 6-9mm is less (50-70%) although histological analysis of these smaller polyps suggests that many are non-adenomatous in nature.