CT Colonography (Virtual Colonoscopy) is a non-invasive medical imaging test used for routine colorectal cancer screening. It serves as an alternative method for examining the large intestine, or colon, for signs of cancer and polyps. The procedure uses specialized X-ray equipment and computer technology to provide a detailed view of the colon’s interior without inserting a long, flexible camera through the rectum.
Defining CT Colonography
CT Colonography utilizes a standard Computed Tomography (CT) scanner to take hundreds of cross-sectional images of the abdomen and pelvis. The CT scanner employs X-rays to capture data, which sophisticated computer software then processes. This process generates detailed, two-dimensional and three-dimensional (3D) reconstructions of the entire large intestine.
The “virtual” aspect comes from these computer-generated images, which mimic an internal viewing experience. These 3D models allow a radiologist to electronically navigate the inside of the colon to search for abnormalities like polyps or masses. This visualization is achieved without physically inserting a long scope, which is the defining characteristic of traditional colonoscopy.
Patient Preparation and Procedure Steps
Effective CT Colonography relies on two essential preparatory steps to ensure a clear view of the colon wall.
Bowel Cleansing
The first step involves a modified bowel cleansing regimen, typically requiring a low-fiber diet for one to three days before the exam, followed by a laxative preparation to clear the colon of stool. This cleaning is necessary because residual fecal matter can mimic a polyp on the scan, leading to an inaccurate result.
Fecal Tagging
The second preparation step is known as fecal tagging. This involves drinking a specialized contrast agent, often iodine- or barium-based, mixed with clear liquids before the procedure. This agent coats any remaining stool or fluid, causing it to appear bright white on the CT images. The computer software can then easily distinguish this “tagged” material from the soft tissue of the colon wall and polyps, increasing the accuracy of the scan.
During the actual exam, the patient lies on the CT table. A small, flexible tube is inserted into the rectum to gently insufflate the colon with gas, typically carbon dioxide. Carbon dioxide is better absorbed by the body than room air and causes less cramping. Insufflation is necessary to fully distend the colon, ensuring the walls are stretched and flattened so polyps are not hidden in folds or collapsed segments.
The CT scanning process itself is very quick, usually taking less than 15 minutes to complete. Scans are performed with the patient in at least two positions, first lying on their back (supine) and then on their stomach (prone). Scanning in both positions allows gravity to shift any remaining fluid or stool, ensuring that the entire colon surface is visualized and preventing potential lesions from being obscured.
Comparing Virtual and Traditional Colonoscopy
A primary difference is the need for sedation. CT Colonography requires none, allowing patients to drive themselves home immediately and resume normal activities. Traditional colonoscopy typically involves sedation, necessitating a recovery period and a chaperone.
Another distinction relates to intervention. Traditional colonoscopy is both diagnostic and therapeutic, allowing the physician to remove polyps during the procedure. CT Colonography is purely diagnostic; while it accurately detects polyps six millimeters or larger, a subsequent optical colonoscopy is required for removal.
Both procedures carry a small risk of colon perforation, though the risk is considered lower for CT Colonography since it avoids maneuvering a long scope. CT Colonography does expose the patient to a low dose of ionizing radiation, which traditional colonoscopy does not.
Understanding the Results and Follow-Up
The radiologist interprets the CT Colonography images by reviewing the two-dimensional cross-sections and the three-dimensional “fly-through” reconstructions. Specialized computer-aided detection (CAD) software is often used to help identify small polyps, particularly those six to nine millimeters in size. The radiologist’s report classifies findings based on size and characteristics.
If the CT Colonography is negative (no clinically significant polyps or masses are identified), the patient is generally advised to return to a standard screening schedule, typically a repeat CT Colonography in five years.
If the scan is positive for a clinically significant lesion, such as a polyp 10 millimeters or larger, a mandatory follow-up optical colonoscopy is required. This is necessary to confirm the finding, obtain tissue samples for biopsy, and perform a polypectomy to remove the growth. For smaller polyps (six to nine millimeters), follow-up recommendations vary, sometimes involving a repeat CT Colonography in three years or a referral for optical colonoscopy.