Computed Tomography Colonography (CTC), often called a Virtual Colonoscopy, is a minimally invasive radiological procedure used to examine the large intestine. This screening test utilizes a specialized CT scanner and advanced computer software to create detailed, cross-sectional images of the colon and rectum. The primary goal of CTC is detecting precancerous polyps and early-stage colorectal cancer, offering an alternative to traditional optical colonoscopy. The procedure generates hundreds of digital images reconstructed into 2D and 3D views, giving radiologists a “virtual” tour of the colon’s interior. Unlike optical colonoscopy, CTC does not require the insertion of a long, flexible scope or typically require sedation.
Preparing for the Scan
Achieving a clean colon is necessary for an accurate CT Colonography, as residual stool can mimic a polyp and lead to a false positive result. Preparation involves a strict regimen beginning days before the scan, typically including a low-fiber diet and strong laxatives to cleanse the bowel. Patients must also drink large amounts of clear liquids to maintain hydration and flush out the digestive tract.
A crucial step in CTC preparation is “fecal tagging,” where the patient ingests a small amount of an oral contrast material, such as barium or iodine, one to two days before the examination. This contrast agent mixes with any residual stool and fluid, making it appear bright white on the CT images. This allows the radiologist to easily differentiate remaining fecal matter from actual soft-tissue polyps, which appear gray. The tagging process provides a safety net against misinterpreting small amounts of lingering material.
Executing the CT Scan
The procedure is relatively quick, typically lasting 10 to 15 minutes once the patient is positioned on the CT table. To achieve optimal visualization of the inner lining, the colon must be fully distended, or inflated, to smooth out the folds of the intestinal wall.
Distention is accomplished by inserting a small tube into the rectum, through which gas is gently introduced. Carbon dioxide (CO2) is the preferred gas for insufflation because it is absorbed quickly into the bloodstream and expelled by the lungs, reducing post-procedure discomfort. Once the colon is inflated, the patient is scanned while lying in two positions: supine (on the back) and prone (on the stomach).
Scanning in both positions is necessary because gravity causes any remaining fluid and mobile stool to shift. This allows the radiologist to view all surfaces of the colon wall and confirm that a potential lesion is fixed, not mobile fecal matter. The CT scanner utilizes a low-dose radiation technique to capture the images, minimizing the patient’s exposure while providing high-quality diagnostic pictures.
Indications for Use
CT Colonography is often selected as a screening method for individuals at average risk for colorectal cancer who cannot undergo or refuse a conventional optical colonoscopy. A primary indication is for patients with medical conditions that make sedation or anesthesia risky, such as severe cardiopulmonary disease or advanced frailty. Since CTC requires no sedation, patients can return to normal activities immediately after the test.
CTC is also frequently used when a standard optical colonoscopy is unsuccessful or incomplete due to anatomical complications or an obstructing mass. In these situations, CTC can successfully visualize the remaining section of the colon that the scope could not reach. However, CTC is generally not recommended for high-risk patients, such as those with a strong family history or inflammatory bowel disease, who are more likely to require immediate tissue biopsy or polyp removal.
Interpreting Findings and Follow-Up
The acquired images are analyzed by a radiologist specializing in CTC interpretation, often using 2D cross-sections and 3D renderings that simulate flying through the colon. The radiologist systematically examines the entire colon for irregularities, focusing on polyps, which are mound-like projections from the wall. CTC is highly accurate for detecting large polyps, generally those measuring 10 millimeters or greater, which carry the highest risk of becoming cancerous.
If the scan is negative (no significant polyps, usually 6 millimeters or larger, are found), the patient is typically recommended to repeat the CTC screening in five years. If the radiologist identifies a significant polyp (10 millimeters or larger, or three or more smaller polyps), the result is positive, requiring a follow-up optical colonoscopy. This subsequent procedure is necessary because CTC cannot remove tissue or polyps, while optical colonoscopy allows for therapeutic intervention and pathological analysis.