Helical computed tomography (CT) is used to generate high-resolution, two-dimensional axial images of the abdomen and pelvis. Three-dimensional images of the colon simulating those obtained with conventional colonoscopy can be reconstructed from the data obtained. Basic imaging principles: cleansing the patient’s colon using a standard barium enema or colonoscopy bowel preparation, colonic insufflation with room-air and thin-section, low dose CT of the abdomen and pelvis followed by off-line computer manipulation of the CT dataset to facilitate inspection of the colonic wall.

Bowel Preparation

Virtual colonoscopy requires a well prepared, cleansed colon to achieve appropriate sensitivity and specificity. Retained stool can simulate polyps decreasing specificity while retained fluid can obscure polyps resulting in decreased sensitivity. The standard colonoscopy preparation (polyethylene glycol electrolyte solution, Go-Lytely; Braintree Laboratories, Braintree, Mass) tends to result in more retained fluid and several investigators prefer a common barium enema preparation E-Z-EM, Inc. , C.B.Fleet, which appears to result in less retained fluid. At this time, a full colon cleansing preparation is required to achieve acceptable results. Orally administered barium contrast products to mark or tag fecal residue are under investigation. The incentive behind the development of such products is to minimize bowel preparation requirements and increase patient compliance. The efficacy of these agents remains is currently being assessed. E-Z-EM also makes a low residue diet kit to take the day prior to exam, nutritious enough for patients to adhere to the bowel prep regimen which is essential for good results.

CT scan acquisition:

Typical CT imaging parameters (4) include a collimation of 2mm or less, 75-110 mA, 110 kVp, and a 512 x 512 matrix. A single acquisition of the abdomen and pelvis is obtained with the patient breath holding for about 15 seconds.  Following the supine scan, the CT is routinely repeated with the patient prone. The use of both supine and prone CT datasets helps differentiate mobile stool from fixed pathology such as cancers and polyps, allows more even distension of the colon, and improves visualization of segments of colon obscured by intraluminal fluid (2,5) .

Radiation dose

The amount of radiation used in CT for virtual colonoscopy can be substantially lower than the amount used for routine abdominal and pelvic CT. Less radiation can be used for virtual colonoscopy examinations than for many other procedures using CT, because there is high contrast between the colon wall and potential polyps and the gas that is pumped inside it. The surface of the colon wall stands out more clearly with a lower dose of radiation. The newer CT scanners make the examination possible with an even lower dose of radiation. Most CT manufacturers are installing automated features on the scanners that allow the dose to be decreased when scanning relatively thinner areas of patient anatomy. In fact, as performed today, the effective dose that a patient receives from a virtual colonoscopy examination is lower than that from a conventional double-contrast barium enema.
The dose is approximately 20% lower than the typical dose for double-contrast barium enema, another conventional radiological test to screen for colon cancer (4.53 mGy for men and 7.45 mGy for women).