A computed tomography scan — also called a CAT scan or CT scan — is performed by a 3-D X-ray machine. A computer integrates multiple images to make a cross-sectional picture, showing “slices” of the body. It uses the same basic technology as an X-ray machine.
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CT can be used anytime a detailed picture of the body is required. The most common use of CT in the toddler population is head CT to look at the brain, especially after a child has fallen and hit her head. Abdominal CT is also quite common to visualize organs such as the appendix, liver, and spleen when certain illnesses are suspected.
The scanner is shaped like a giant, narrow doughnut (it is not elongated like an MRI scanner) with multiple small X-ray cameras. The patient lies on a narrow table that slides into and out of the “doughnut.” This machine is not claustrophobic because it is not an enclosed space. Rather, only the part of the patient’s body being imaged is “inside” the machine.
Older CT scanners took a while to shoot the various X rays, but newer models take only seconds. This is important because a toddler used to require sedation to have a CT scan. Now, with the faster machines, most children can lie still for 30 to 120 seconds while the machine takes pictures, obviating the need for sedating medicines and pediatric anesthesia.
Occasionally, a child will need to be given a contrast material, a liquid that will show up on the CT scan. This liquid is called a contrast material because it helps show contrast between certain internal structures. Typically, an IV is placed in the child’s arm, and the liquid is injected into the vein before the pictures are taken. Some images require an oral contrast, and some even require a rectal contrast. Depending on the suspected problem, sometimes the CT scan is done immediately after the contrast material is given, while other times two to four hours must pass before the picture is taken. A radiologist — a doctor who specializes in medical imaging — will decide whether and what type of contrast to use, as well as how long to wait between giving the contrast and completing the CT scan.
It is possible to have an allergic reaction to the contrast material. Reactions can range from hives to allergic respiratory failure (anaphylaxis). Children with an iodine or shellfish allergy are at increased risk for a reaction to the contrast.
CT can image just about any part of the body. It is a painless procedure, but there are small risks of radiation exposure. One way to think about this is in terms of normal daily radiation exposure. A single chest X ray is equivalent to the amount of radiation a person is exposed to over two and a half days of just walking around on earth. A CT scan, which uses multiple X-ray images, ranges from 240 to 1,200 days of radiation exposure.