CT Colonography (CTC)
What is CT colonography?
- This is a non invasive means of imaging the colon. CTC 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 breath hold.
- CTC uses the patient data acquired from a multislice CT scanner and combines with computer software that post processes the date to generate both 2D and 3D images of the colon for analysis. More recent software additionally allows a 3-D display with a fly-through view as well as a 360 degree virtual dissection view. There is no consensus on whether a primary review of 3-D images or 2-D-images should be preferred. Studies using the 3-D review with correlation with 2-D images had a higher sensitivity than studies using a primary 2-D approach.
- The total radiation exposure is in the range of 8.8 and 10.2 mSv (approximately similar to barium enema)
- In 1999, it was proposed that the term virtual colonoscopy be replaced by CTC and it is by this term that the technique is now known.
Discuss the need for CTC?
A non invasive way of imaging colon is helpful because:
- Colonoscopy is invasive and not without risks.
- Patient acceptability for colonoscopy is low
- 63.5- 84% of persons undergoing screening colonoscopy do not have neoplasia and therefore would not have required colonoscopy.
Thus a screening method with a high sensitivity for colorectal lesions that could preselect people with neoplasia who would then undergo colonoscopy for removal of these lesions would seem ideal. CT colonography (CTC) is considered an ideal pre-test by some experts.
Discuss the efficacy of CTC?
- Studies have consistently shown that CTC is superior to double-contrast barium enema (DCBE) in the detection of colorectal lesions
- The sensitivity is similar to colonoscopy for colorectal lesions of size 10 mm or more. The sensitivity is variable for lesions less than 10 mm in size.
- A recent meta-analysis of CTC performance showed an overall per-polyp sensitivity of 66%. Sensitivity for polyps >=10 mm was 76% with a lower sensitivity of 59% for polyps 6-9 mm. However, there was a wide variation between different studies. Overall specificity was 83%, with an increase to 92% for polyps >=10 mm.
How is CTC performed?
- Colon is cleansed using bowel preps similar to colonoscopy
- Distension of the colon is necessary to be able to assess the colon surface. This is achieved by insufflating room air or CO2 into the colon. The insufflation can be performed manually, be patient controlled or automated.
- After distension of the colon a CT scan of the abdomen is performed both in the supine and in the prone position (each lasting approximately 13 seconds). 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.
- Buscopan may be used to reduce peristalsis during imaging.
What is faecal tagging?
- The imaging characteristic of stool is well established (central air inclusion, shift with dual positioning). However, problems still occur if the stool presents as an immobile structure or a polyp with abutting stool or frankly as a polyp. This may lead to a false positive diagnosis. Faecal tagging offers a solution to this problem.
- Because stool has the same attenuation as the colon mucosa labelling the stool with a contrast agent is required (so-called faecal tagging). This is achieved by oral intake of barium or oral contrast (sodium/meglumine diatrizoate) mostly starting 48 h before CTC. The orally ingested contrast impregnates the residual stool and so appears as hyperdense on CT. This hyperdense stool is in strong contrast to the soft tissue density of the normal colonic structures and tumoural lesions.
- Faecal tagging helps in using a reduced bowel preparation (or even no prep).
- Faecal tagging does not seem to be necessary in the case of a complete bowel preparation and scanning in the prone and supine positions.
What are the contraindications for CTC?
Absolute contraindications: Physical weight restrictions (for scanners), acute abdomen, acute diverticulitis, recent pelvic or abdominal surgery, toxic megacolon,
Relative: Pregnancy, hip joint prosthesis (patients with metallic hip have significant artefact in the pelvis with limited evaluation of colonic segments in this region), claustrophobia, incompetent ileocaecal valve (as colonic distension will be suboptimal).
What are the complications of CTC?
CTC is safe but not completely without risk. Two large retrospective reports from the UK and Israel including nearly 29 000 mostly symptomatic patients reported a perforation rate of 0.06% and 0.08%, respectively.
Other complications occasionally reported consisted of nausea and vomiting
What are the disadvantages of CTC?
- Impaired ability to detect flat neoplastic lesions- detection of flat lesions is especially important as they are more likely to contain advanced histology. The accuracy of CTC for flat lesions is unknown.
- Nearly all of the published studies CTC were performed by highly experienced academic radiologists. It is currently unknown if similar results will be achieved outside such centres.
- Radiation exposure: The exact excess cancer risk due to the exposure to radiation by CTC is unknown. It is estimated that 1.5-2.0% of all cancers in the US might be attributable to the radiation from CT studies in general. The risk depends on age and dose of radiation used. For a 50-year-old individual one CT colonography scan was estimated to increase the life-time risk of cancer by 0.13-0.15% compared to 0.07 for a 70-year-old.
- According to mathematical models CTC currently is less cost-effective than colonoscopy. CTC will further lose effectiveness if the referral rate for colonoscopy is too high (for e.g. for polyps less than 10 mm in size).
- CTC is not suitable for mucosal diseases such as inflammatory bowel disease or angiodysplasia
What are the benefits of CTC?
- Non-invasive
- No sedation
- Short procedure
- Better tolerated by patients than barium enema. There is less movement required during the examination and no messy barium or post procedure constipation.
- Extra colonic structures are routinely assessed during CTC.
What are the current indications for CTC?
Current evidence does not seem to allow recommending widespread CTC screening outside of studies or specialised centres. CTC is the method of choice for evaluating the colon proximal of obstructing lesions and after incomplete colonoscopy.
CTC can also be used for imaging the large bowel in patients thought to be unfit for colonoscopy. In these patients prep less or limited bowel prep may be used with faecal tagging.
