Related narrative: Virtual Colonoscopy
Computed tomographic (CT) colography or virtual colonoscopy is a new diagnostic method based on spiral computed axial tomography. The technique produces high-resolution, two-dimensional axial images that are made three dimensional offline thereby simulating conventional colonoscopy. This method is performed on an air-distended, clean colon in a single acquisition, with patients holding their breath only once. Less invasive, the procedure takes less time for the patient and does not require sedation, though patients usually experience abdominal discomfort from the air insufflation. Thorough bowel cleansing and distention is critical. It is a time-consuming process for the radiologist to interpret the data.
Sensitivity and Specificity
The validity of a screening test is its ability to provide a good preliminary indication of who has a disease and who does not. Calculations measuring validity result in two values: sensitivity and specificity. Sensitivity measures the proportion of people who test positive and who truly have the condition. Specificity measures the proportion of people who test negative and who are truly free of the condition. Ideally, a screening test would be 100% sensitive and 100% specific; that is, the screen would never result in false positive or false negative results. Frequently, a high sensitivity value comes at the sacrifice of specificity, and vice versa.
Conventional colonoscopy is usually used at the gold standard reference test when measuring the validity of virtual colonoscopy, but between 10-20% of polyps and up to 5% of cancers may be missed on conventional colonoscopy, affecting the validity value of virtual colonoscopy. Also, differences in reported validity can be explained by study design, patient selection, and the rapidly evolving technique. The learning curve for interpretation is reportedly steep and long.
In several studies, sensitivity for polyps:
Specificity has ranged from 62% - 95% depending on the polyp size.
Researchers who have conducted screening tests using virtual colonoscopy have concluded:
"Early performance of CT colography seems promising for detection of colorectal polyps 5 mm and larger."
"Flat lesions are impossible to see by this method."
"Shows potential as a diagnostic tool for colorectal neoplasia, it is currently not sufficiently sensitive for widespread use."
"VC does not yet appear to be suitable for colorectal cancer or polyp screening."
"Virtual colonoscopy is relatively simple and is less invasive than conventional colonoscopy."
Virtual colonoscopy is not sensitive for detecting small polyps. However, in general, there is some controversy over what should be done with polyps between 1-5 mm in diameter. The prevalence of polyps <10 mm in people >50 is 30-50%.
The screening guidelines include the option of a barium enema or conventional colonoscopy after a suggestive sigmoidoscopy. At this point, virtual colonoscopy is not practical as a screening test because of its cost and the time it requires a radiologist to interpret the data
Large polyps >10 mm especially if they are flat and located in the right colon are still missed regularly with virtual colonoscopy.
Virtual colonoscopy will benefit from the development of faster scanners with greater resolution that will enhance accuracy; refinements in software for processing data; development of image interpretation software that will enhance automation; and better methods for colonic distention and cleansing.
Fenlon HM, Nunes DP, Schroy PC, et al. A comparison of virtual and conventional colonoscopy for the detection of colorectal polyps. NEJM, 1999; 341(20):1496-1503.
Halligan S, Fenlon M. Science, medicine, and the future: Virtual colonoscopy. British Medical Journal, 1999; 319(7219):1249-1252.
Hara AK, Johnson CD, Reed JE, et al. Detection of colorectal polyps with CT colography: initial assessment of sensitivity and specificity. Radiology, 1997; 205(1):59-65.
Mendelson RM, Foster NM, Edwards JT, et al. Virtual colonoscopy compared with conventional colonoscopy: a developing technology. Medical Journal of Australia, 2000; 173(9):472-475.
Pescatore P, Glucker T, Delarive J, et al. Diagnostic accuracy and interobserver agreement of CT colonography (virtual colonoscopy). Gut, 2000; 47:126-130.
Spinzi G, Belloni G, Martegani A, et al. Computed tomographic colonography and conventional colonoscopy for colon diseases: a prospective, blinded study. American Journal of Gastroenterology, 2001; 96(2):394-400.