Virtual colonoscopy is a new modality for evaluating the colon first described by Vining in 1994. Using sophisticated post-processing computer software to generate images acquired from conventional spiral CT scan images, virtual colonoscopy allows the operator to navigate through the colon in any direction, providing visualization of the entire colonic mucosa. As such, it holds great promise in being a non-invasive method for examination of the colon.

How then, does virtual colonoscopy compare with standard optical colonoscopy, which remains the gold standard test?

Indications
The indications for both virtual and standard colonoscopies are the same. The common reasons are listed below:

-	Screening exam for anyone above age 50
-	Presence of occult blood in the stool
-	Per-rectal bleeding
-	Change in bowel habits
-	Chronic abdominal pain
-	Iron deficiency anaemia
-	Significant, unexplained weight loss
-	Family history of colon cancer
-	Personal history of colonic polyps or cancer
-	Surveillance in individuals with inflammatory bowel disease

Bowel preparation
Like standard colonoscopy, thorough bowel preparation is required to ensure that the colon is as clean as possible since retained stools can simulate polyps. In addition, retained luminal fluid may obscure small lesions. In contrast, the endoscopist may be able to remove most if not all of these retained fluids via the suction channel during standard colonoscopy.

Procedure
The cleansed colon must be distended with air throughout its entire length via the anus, which may be uncomfortable. In standard colonoscopy, only the segment of colon being visualized needs to be distended. While patients are not routinely sedated during virtual colonoscopy, intravenous sedation and analgesia are given during standard colonoscopy to minimize patient discomfort.

While the actual time taken for image acquisition may be short, the whole process may take up to 30 minutes as the patient typically needs to be in both supine and prone positions to allow for redistribution of colonic fluid and gas to detect small polyps. The process may be longer if stool or bowel spasm is present. Standard colonoscopy can be completed in less than 10 minutes.

Virtual colonoscopy, however accurate, remains purely a diagnostic tool.

In contrast, polyps can be completely removed during standard colonoscopy. For larger polyps as well as other conditions including tumours and colitis, biopsies obtained during the procedure provide valuable histological diagnosis to guide further management.

Test characteristics
There is an increasing number of studies comparing virtual and standard colonoscopies. So far, the published results have been highly variable. A recent meta-analysis of 33 studies with a total of 6393 patients shows the sensitivity of virtual colonoscopy ranging from 48 percent for polyps < 6mm to 85 percent for polyps > 9mm. Specificity was in the range of 92 to 97 percent. The authors concluded that while virtual colonoscopy was highly specific, the marked variation in sensitivity remains an issue which must be resolved before its role can be defined.

Safety
Standard colonoscopy is generally a safe procedure and complications are rare. The two most important complications are that of bleeding and colonic perforation.

Bleeding can occur following biopsies or removal of polyps. However, it is usually minimal and self-limiting.

Perforation of the colonic wall is a serious complication. Fortunately, it is uncommon with an incidence of 1 in 1000 and is usually associated with therapeutic procedures during colonoscopy such as polpectomy. It should, however, be borne in mind that the incidence is much lower with experienced endoscopists. Most iatrogenic perforation will require surgical intervention.

While bleeding is virtually unheard of in virtual colonoscopy, there have been at least 2 reports of colonic perforation from air insufflation. One patient had an obstructing sigmoid lesion while the other had ulcerative colitis.

Cost
Currently, there is no difference in the cost between virtual colonoscopy and standard colonoscopy in Singapore, i.e. about $900-1000 per test.

Conclusion
While virtual colonoscopy is an exciting new development, its exact clinical role remains to be determined. Currently, it is a reasonable option in selected settings, for example, intolerance and technical difficulties during standard colonoscopy, or when one encounters an obstructing colorectal cancer.

Dr Ong Kong Wee, Registrar
Dr Ho Kok Sun, Consultant

Dept of Colorectal Surgery
Singapore General Hospital