Colorectal cancer screening detects growths called polyps that can develop into cancer over time. CT Colonography, commonly known as Virtual Colonoscopy, is a modern technique that offers a non-invasive way to examine the large intestine. It serves as an important option for individuals seeking effective cancer screening by creating a detailed map of the colon.
Understanding CT Colonography
CT Colonography is a specialized medical imaging procedure that uses a Computed Tomography (CT) scanner to generate detailed images of the entire colon and rectum. The CT scanner captures a series of thin-slice, two-dimensional cross-sectional X-ray images of the abdomen and pelvis. These images are sent to a computer workstation for reconstruction.
The process creates a three-dimensional (3D) digital model of the large intestine’s interior surface. Radiologists can digitally navigate or “fly through” this reconstructed model, which is why the term “virtual” is used. This technique is primarily indicated for screening average-risk individuals for polyps and early-stage colorectal cancer. It is also an alternative when a traditional optical colonoscopy is incomplete due to an obstruction or anatomical difficulty.
Preparing for the Procedure
The success of CT Colonography relies on a thorough cleaning of the colon before the scan. This preparation ensures that no retained fecal matter obscures the view of the colonic walls or is mistaken for a polyp. Preparation typically begins one to three days before the appointment with dietary modifications.
The patient starts with a low-fiber diet and progresses to a clear liquid diet the day before the exam, avoiding all solid foods. To facilitate a clear colon, the patient must also take laxatives, often a polyethylene glycol (PEG) solution, which induces frequent bowel movements.
A necessary step is the ingestion of an oral contrast agent, sometimes referred to as “fecal tagging.” These iodine or barium-based liquids, such as Gastrografin or Omnipaque, are consumed in the hours leading up to the procedure. The contrast coats any residual stool, allowing the radiologist to easily distinguish the tagged fecal material from a true polyp on the final images.
The Imaging Process
The imaging process begins with colon distension. A small, flexible tube is gently inserted into the rectum, and air or carbon dioxide (CO2) is introduced into the colon. This gas inflates the large intestine, temporarily stretching its walls to flatten folds and wrinkles, providing the radiologist with an unobstructed view of the lining.
Carbon dioxide is preferred over room air because the body absorbs it more rapidly after the procedure, reducing post-procedure bloating and cramping. The patient is positioned on the CT scanner table, usually lying on their back (supine) for the first scan. A second scan is then performed with the patient lying on their stomach (prone).
Scanning in two different positions causes any remaining fluid or tagged stool to shift, ensuring polyps are not obscured by pooled liquid. The CT scan acquisition is fast, often taking less than 15 minutes, and the entire procedure requires no sedation. Although the insufflation can cause temporary fullness or mild abdominal discomfort, the patient can resume normal activities immediately afterward.
Comparing Virtual and Optical Colonoscopy
CT Colonography and traditional optical colonoscopy are both effective screening methods, but they differ significantly in execution. Optical colonoscopy, considered the standard, involves inserting a flexible tube with a camera into the colon. This procedure requires intravenous sedation and typically lasts 30 to 60 minutes. Optical colonoscopy also carries a higher risk of perforation, estimated at about 1 in 1,000 procedures.
CT Colonography’s advantage is its non-invasive nature, as it does not require sedation. Patients can drive themselves home and return to work immediately afterward. It also has a much lower risk of complications, with perforation rates estimated at about 1 in 22,000. Additionally, the CT scan can sometimes image the entire colon even if a blockage prevents the passage of a flexible scope.
The limitation of the virtual procedure is its inability to treat findings during the exam. If the CT Colonography detects a polyp, especially one 6 millimeters or larger, the patient must undergo a subsequent optical colonoscopy for removal and biopsy.
The CT scan exposes the patient to a low dose of radiation, which is inherent to the technology. A negative result on a virtual colonoscopy suggests a follow-up screening interval of five years. In contrast, a negative optical colonoscopy allows for a ten-year interval before the next screening is recommended.