Preparing for a CT colonography (also called a virtual colonoscopy) takes about two days and involves three main steps: adjusting your diet, taking laxatives to empty your colon, and drinking oral contrast agents that help the scanner distinguish leftover stool from actual polyps. The prep is similar to what you’d do before a traditional colonoscopy, though the volume of laxatives can sometimes be smaller.
Why the Prep Matters
A CT colonography uses a CT scanner to create detailed 3D images of the inside of your colon. For those images to be useful, your colon needs to be as clean as possible. Any remaining stool can look like a polyp on the scan, potentially leading to a false alarm or a missed finding. The entire prep is designed around two goals: clearing your colon of solid waste and coating whatever small amount remains with a contrast agent so the radiologist can digitally subtract it from the images.
One Week Before: Dietary Changes
About seven days before the scan, you’ll need to cut out foods that are difficult to digest or leave visible residue in the colon. That means avoiding seeds (poppy seeds, tomatoes, cucumbers, watermelon), nuts, popcorn, celery, grapes, green peas, beans, and seaweed. You can still eat most other foods during this week, including cooked or canned vegetables, but skip raw fruits and vegetables, whole kernel corn, potato skins, and whole grains like oatmeal, brown rice, quinoa, and wheat bread.
The Day Before: Clear Liquids and Laxatives
Starting at midnight the night before (or the morning of the day before, depending on your facility’s protocol), you switch to a clear liquid diet. Allowed liquids include water, black coffee, tea without milk, apple juice, white grape juice, white cranberry juice, clear broth, ginger ale, and gelatin. Avoid anything red, purple, blue, green, or orange, and skip all dairy products and alcohol.
The laxative regimen typically begins in the afternoon. A common protocol uses two medications together:
- A stimulant laxative (bisacodyl): You swallow two 5-milligram tablets with water around 4 p.m. These stimulate the muscles of your colon to push contents through.
- An osmotic laxative (MiraLAX): You dissolve an entire 238-gram bottle of powder into 64 ounces of a clear liquid, such as a sports drink, which helps replace electrolytes. Starting about 15 minutes after the bisacodyl, you drink one 8-ounce glass every 15 minutes until you’ve finished four glasses. Some protocols call for a second round later that evening or the following morning.
If you have diabetes, use sugar-free versions of clear liquids and sports drinks. Sports drinks are a particularly good choice for anyone doing this prep because the laxatives cause significant fluid loss, and the electrolytes in the drink help offset that.
Oral Contrast Agents for Fecal Tagging
This step is unique to CT colonography and is not part of a standard colonoscopy prep. Along with your laxatives, you’ll drink two types of contrast liquid: a small bottle of liquid barium sulfate and a small bottle of an iodine-based solution. These coat any residual stool and fluid left in the colon, making them appear bright white on the CT images. Radiologists can then digitally “subtract” those bright areas, leaving a cleaner view of the colon wall.
The barium can be stored in the refrigerator or at room temperature. The iodine solution should be kept at room temperature and out of direct sunlight. Your facility will provide specific timing for when to drink each one, usually staggered throughout the day before the scan alongside your laxative doses.
What to Expect During the Prep
The laxatives will cause frequent, watery diarrhea. Plan to stay home and near a bathroom for most of the day before your scan. This is the most uncomfortable part of the process for most people. Nausea, bloating, and cramping are also possible. Staying well hydrated is essential, both for comfort and safety, since the diarrhea pulls a significant amount of water and electrolytes from your body. Keep sipping clear fluids throughout the day, even beyond the required MiraLAX mixture.
If you experience side effects severe enough that you cannot finish the prep, contact the facility that ordered your scan. An incomplete prep can mean unusable images and a rescheduled exam.
The Morning of the Scan
Most facilities ask you to stop drinking all liquids two to four hours before your appointment. Some protocols include a final dose of bisacodyl early that morning, especially if your scan is scheduled for later in the day. Do not eat any solid food. Your facility will give you specific timing based on your appointment.
The scan itself is quick, usually 10 to 15 minutes. A small, flexible tube is inserted into the rectum to gently inflate the colon with air or carbon dioxide, and you’ll lie on your back and then your stomach while the scanner takes images. There is no sedation, which means you can drive yourself home and eat normally afterward.
Adjustments for Kidney Disease and Older Adults
If you have chronic kidney disease, certain laxatives pose real risks. Sodium phosphate-based laxatives in particular can cause dangerous spikes in blood phosphate levels and should be avoided entirely if your kidney function is below a certain threshold. Polyethylene glycol-based preps (like MiraLAX) are generally considered safer for people with kidney concerns.
Older adults, especially those over 55 or taking blood pressure medications like ACE inhibitors or diuretics, face a higher risk of dehydration and electrolyte imbalances during any bowel prep. Drinking electrolyte-containing fluids rather than plain water helps reduce the chance of a dangerous drop in sodium levels. If you take daily medications, ask your prescribing provider which ones to take or skip on prep day, since some pills may not absorb properly during active diarrhea.
How This Compares to Traditional Colonoscopy Prep
The basic structure is the same: dietary restrictions, laxatives, and a day of clear liquids. The key differences are the addition of oral contrast agents for fecal tagging (which traditional colonoscopy doesn’t require) and the possibility of a lower-volume laxative regimen. Some CT colonography protocols use a reduced amount of cleansing liquid compared to a full colonoscopy prep, though this varies by facility. The tradeoff is that a traditional colonoscopy allows the doctor to remove polyps during the same procedure, while a CT colonography is diagnostic only. If polyps are found, you would still need a conventional colonoscopy to have them removed.