You will be asked to lie on a narrow table that slides into the center of the CT scanner. You may lie on your back, or you may lie face-down with your chin raised.
Once you are inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam without stopping.)
A computer creates separate images of the body area. These are called slices. The images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of the body area can be created by stacking the slices together.
You need to stay still during the exam, because movement causes blurred images. You may be told to hold your breath for short periods of time. Straps and pillows may be used to keep you still during the procedure.
The actual scan should take about 30 seconds. The entire process should take 15 minutes.
How to Prepare for the Test
For some tests, you will need to have a special dye, called contrast, to be delivered into the body before the test starts. Contrast helps certain areas show up better on the x-rays.
Contrast can be given through a vein (IV) in your hand or forearm. If contrast is used, you may also be asked not to eat or drink anything for 4 to 6 hours before the test.
Let your health care provider know if you have ever had a reaction to contrast. You may need to take medicines before the test in order to safely receive this substance.
Let your provider know if you have kidney problems. Contrast may not be able to be used if this is the case.
Before receiving the contrast, tell your provider if you take the diabetes medicine metformin (Glucophage). You may need to take extra steps to prepare.
If you weigh more than 300 pounds, find out if the CT machine has a weight limit. Too much weight can cause damage to the scanner's working parts.
You will be asked to remove jewelry and wear a hospital gown during the scan.
How the Test will Feel
Some people may have discomfort from lying on the hard table.
Contrast given through an IV may cause a slight burning sensation, a metallic taste in the mouth, and a warm flushing of the body. These feelings are normal. They will go away within a few seconds.
Why the Test is Performed
CT rapidly creates detailed pictures of the sinuses. The test may diagnose or detect:
CT scans expose you to more radiation than regular x-rays. Having many x-rays or CT scans over time may increase your risk for cancer. However, the risk from any one scan is very small. You and your provider should weigh this risk against the benefits of getting a correct diagnosis for a medical problem.
Some people have allergies to contrast dye. Let your provider know if you have ever had an allergic reaction to injected contrast dye.
The most common type of contrast given into a vein contains iodine. A person with an iodine allergy may have nausea or vomiting, sneezing, itching, or hives if given this type of contrast.
If contrast is needed, you may be given antihistamines (such as Benadryl) or steroids before the test.
The kidneys help remove iodine out of the body. Those with kidney disease or diabetes may need to get extra fluids after the test to help flush the iodine out of the body.
Rarely, the dye may cause a life-threatening allergic response called anaphylaxis. If you have any trouble breathing during the test, let the scanner operator know right away. Scanners have an intercom and speakers, so the operator can hear you at all times.
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Jason Levy, MD, Northside Radiology Associates, Atlanta, GA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.