Introduction:
CT (computed tomography) colonography, also called virtual colonoscopy (VC), is a diagnostic imaging technique for the colon of the large intestine. A colon is the longest part of the large intestine, which is a tube-like structure having a small intestine at one side and the on its other end the anus. This scan needs the colon to be distended with an enema (a procedure used to cleanse the bowel) which helps to see the polyps clearly during the scan.
What Are the Uses of CT Colonography?
CT colonography is commonly used in the following conditions:
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Diagnosis of colorectal cancer.
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Detect the polyps (abnormal growth from the lining of the colon) in the colon.
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Staging of colon cancer.
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Screening of colon cancer (usually recommended at the age of 45).
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Patients with a large tumor (cancer) block the way for the camera of a traditional colonoscopy (a procedure that uses a flexible tube with a camera at the end to visualize the colon).
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Weak elderly patients.
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The patients who are allergic to general anesthesia.
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For patients who have an incomplete optical colonoscopy, the CT colonography can be accomplished at the exact time as if the colon is cleaned.
How Do Patients Prepare for CT Colonography?
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For the CT colonography procedure, the health provider may ask the patient to sign a consent form before the scan.
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Medical conditions and regular medicines of the patients should be informed to the doctor.
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The patient should inform them if they are pregnant or breastfeeding.
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Patients are instructed to drink only clear liquid for about two to three days before the procedure, such as water, clear tea or coffee, gelatin, filtered fruit juices, and fat-free broth.
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Patients should not eat or drink anything 12 hours before the scan.
What Are the Different Types of CT Colonography?
Based on their techniques, CT colonography is classified into:
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Water Enema Method: Before the water enema method, patients are instructed to take four to six doses of laxatives (enema), such as polyethylene glycol in powder form dissolved in 500ml of water per dose before the procedure. During the water enema method of CT colonography, the patient's bowel is filled with water and a contrast agent (through intravenous injection). This procedure requires two CT imaging.
For the first imaging, patients are asked to lie in the left lateral position on the procedure table slightly elevated. Then a flexible tube is inserted into the rectum, which is connected to the bag that contains 2 liters of warm water, which enters the colon by gravity (retrograde colonic distension). Then, the patient is asked to change their position to their right side, which helps in even water distribution. The first CT image is obtained in a supine position.
Second CT imaging is obtained after 70 to 80 minutes of the intravenous injection of iodine-based contrast materials (120 mL at a rate of 3 mL/sec). Finally, the enema is evacuated by lowering the bag. Chest examination is included if the tumor is found during the scan. For uncontrolled patients, the gas enema method is preferred.
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Gas Enema Method: A day before the gas enema method, the patient's bowel is prepared by swallowing barium contrast for fecal tagging (to differentiate feces from polyps) and iodine-based contrast (a special dye) for liquid residue tagging. The gas enema method of colonography is also called virtual colonoscopy. In this method, a Co2 insufflator is used to control the pressure of Co2 (carbon dioxide) that can cause perforation of the colon when passing through the rectal tube. Co2 gas is absorbed quickly by the bowel mucosa, and it does not cause pain during distension of the colon. At the end of the procedure, Co2 can be deflated easily. It should be distended up to the cecum (a part of the large intestine).
CT Imaging of the colon is done at two positions: a prone position and a supine position (to mobilize the residues). This position can be substituted with right and left lateral positions.
How Is CT Colonography Taken?
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For a CT colonography scan, patients are advised to be in a sterilized gown and placed on their backs on the narrow table in the scanner.
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During CT colonography, the doctor passes a small, thin, flexible tube into the patient's rectum, which is about two inches. Carbon dioxide gas from the electric pump (gas enema method) or liquid enema and water (water enema method) is used to inflate (expand) the colon, which prevents the hiding of the polyp.
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The technician may use a pillow or straps to hold them in a required position. They utilize a remote control from another area to change the table into the computed tomography machine. The table gradually drives for scanning. Patients may move via the machine multiple times. Patients may be asked to hold their breath for some time to acquire more satisfactory images. A thin beam of X-ray circles almost a portion of the body. It gives a sequence of illustrations from diverse angles. A computer utilizes these illustrations to make cross-sectional images known as slices that are 2D illustrations. This procedure is reprised to bear multiple slices. The computer positions the slices to obtain thorough (3D) colon illustrations of the colon. Any movement involving breathing and body motions can produce artifacts in the pictures. This process typically carries around 15 minutes to finish.
After finishing the scan, patients may be asked to wait until the technician reviews the images to make sure the images are clear enough to read by the doctor. Patients can continue their regular diet and activities after the scan. Breastfeeding mothers are instructed to avoid feeding for about 24 hours after the scan if they receive the iodinated contrast materials. The contrast material may pass through the breast milk.
What Are the Limitations of CT Colonography?
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Intestinal obstruction (blockage).
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Bowel incontinence (a condition that causes fecal leakage due to improper bowel movements).
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Allergic reactions to contrast material.
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For renal failure patients, the use of contrast materials is not recommended. In such cases, the water enema method should be performed only with rectal water filling.
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Acute abdomen syndrome (sudden abdominal pain associated with other symptoms such as nausea, vomiting, and diarrhea).
What Are the Risks Associated With CT Colonography?
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Iodine-based contrast materials used in CT colonography may cause allergic reactions such as headaches, itching, and hives (skin rashes). Some of the severe symptoms of contrast materials are difficulty in breathing, rapid heart rate, low blood pressure, and swelling of the throat or other body parts.
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CT colonography scan produces ionizing radiation. Ionizing radiation may damage the DNA and lead to cancer. But the hazards are very few. The hazards of fatal cancer in computed tomography scans are around one in 2000. The hazard might rise with every scan in a lifetime.
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There might be a risk of bowel perforation when the colon is inflated with air.
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CT colonography in pregnant women causes birth defects in the fetus in the womb.
Conclusion:
CT colonography provides detailed information on the structure and abnormalities of the colon by enhancing the structures using high technologies when compared to traditional colonoscopy. CT colonoscopy can be done quickly and is cost-effective. The major advantage of the gas enema method is that it does not require an injection of a contrast agent. CT colonography is a minimally invasive procedure that is required when the traditional colonoscopy cannot reach the entire length of the colon.
