CT Scan

A CT scan (CAT scan or a computed tomography) is an x-ray procedure that allows a doctor to see the inside of a patient’s body without invasive surgery. The computed tomography technique utilizes computer technology to take several 2D cross-sectional “slices” of a patient’s inner organs and compose from them a final, 3D representation. The medical imaging technique is used very often in medical diagnoses and is easily one of the most common medical procedures available to patients.

Although it is similar to magnetic resonance imaging (MRI), CT scan is much more common in the United States. CT scanning is often the first step to diagnosing problems in the gastrointestinal (GI) tract, as it is used to examine the bowels for tumors, inflammation and other signs of colorectal disorder.

CT scanning is often the first step to diagnosing problems in the gastrointestinal (GI) tract, as it is used to examine the bowels for tumors, inflammation and other signs of colorectal disorder.

Usually, CT scans are ordered when traditional X-rays fail to provide enough details. On a CT scan, a doctor is able to see internal organs, bones, tissue and even blood vessels. The test is often used by gastroenterologists and radiologists to diagnose gastrointestinal cancer located in the bowels.

CT Scan Preparation and Procedure

During the procedure, patients are asked to remain completely still, sometimes holding their breath, while lying on a table connected to a large, cylindrical scanner. Cross-sectional images are taken once while the patient lies face up and again once while the patient is face down. The cross-sectional images are put together using advanced computer software, creating a 3D image that medical experts can study on a computer monitor.

During the procedure, patients are asked to remain completely still, sometimes holding their breath, while lying on a table connected to a large, cylindrical scanner. Cross-sectional images are taken once while the patient lies face up and again once while the patient is face down

The lab technician or nurse will probably give you a hospital gown to wear while you are being scanned, since metal objects like jewelry or glasses may distort the final CT image. Female patients who wear hair pins and other accessories should also be careful not to enter the CT scanner without removing these items first. Sometimes, older patients are asked to remove hearing aids and dentures.

Hours before the procedure, patients should refrain from eating or drinking. In some cases, your physician will instruct you to swallow a contrast material (sometimes the material is also inserted via the rectum or injecting using an intravenous line or IV) to assist in the image capturing—and foreign foods and liquids will mess up the solution’s effectiveness. As always, patients who may be pregnant or are experiencing any other medical conditions should be open with their doctors about their conditions to avoid any complications of the procedure.

Advantages and Disadvantages of CT Scan

Unlike other imaging techniques, such as MRI, that uses magnetization and radio frequency to capture images, CT scanning exposes patients to some ionizing radiation.

Advantages of CT revolve around the fact that it is quick and non-invasive. A doctor does not have to resort to open surgery to diagnose the patient’s abdominal pain, for example. However, the CT technology is not widely available yet, and the radiation exposure makes some patients think twice about choosing it over traditional methods. Furthermore, some smaller colorectal tumors and polyps cannot be detected using a CT scan, in which case a doctor must perform a conventional colonoscopy.  Additionally, CT colonography requires specialized centers and requires the same bowel preparation as conventional colonoscopy.  Radiologists reading a CT colonography will not usually report polyps less than 5mm in diameter, and cannot discern between stool, foreign bodies and polyps. 

The American Society for Gastrointestinal Endoscopy (ASGE) has released guidelines for screening of colon cancer.  CT colonography can be an alternative to colonoscopy in patients who cannot or choose not to undergo the risks of conventional colonoscopy, and should be performed at least every 5 years (10 years for colonoscopy).  If polyps are detected the usual recommendation is to procede with regular colonoscopy for further evaluation.  Those with signs of iron deficiency anemia, changes in bowel habits, weight loss or blood in their stool should undergo colonoscopy instead of CT colonography.

Reviewed 12/12/2011 by David M. Nolan, M.D.
Diplomate of the American Board of Internal Medicine, 2011
Currently a Fellow of Gastroenterology, at UCI 2011-2014