Head CT is done in the hospital or radiology center.
You lie on a narrow table that slides into the center of the CT scanner.
While inside the scanner, the machine's x-ray beam rotates around you.
A computer creates separate images of the body area, called slices. These images can be:
Viewed on a monitor
Printed on film
Three-dimensional models of the head area can be created by stacking the slices together.
You must be still during the exam, because movement causes blurred images. You may be told to hold your breath for short periods.
Complete scans usually take only 30 seconds to a few minutes.
How to Prepare for the Test
Certain CT exams require a special dye, called contrast. It is delivered into the body before the test starts. Contrast helps certain areas show up better on 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 doctor know if you have ever had a reaction to contrast. You may need to take medicines before the test in order to safely receive it.
Before receiving the contrast, tell your health care provider if you take the diabetes medicine metformin (Glucophage). You may need to take extra precautions. Also let your provider know if you have any kidney function problems as the IV contrast can worsen this problem.
If you weigh more than 300 pounds, find out if the CT machine has a weight limit. Some machines do.
You will be asked to remove jewelry and wear a hospital gown during the study.
How the Test will Feel
The x-rays produced by the CT scan are painless. Some people may have discomfort from lying on the hard table.
Contrast given through a vein may cause a:
Slight burning feeling
Metallic taste in the mouth
Warm flushing of the body
This is normal and usually goes away within a few seconds.
Why the Test is Performed
A head CT scan is recommended to help diagnose or monitor the following conditions:
CT scans use 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 small. You and your doctor 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 doctor 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. If a person with an iodine allergy is given this type of contrast, nausea or vomiting, sneezing, itching, or hives may occur.
If you absolutely must be given such contrast, your doctor may give you antihistamines (such as Benadryl) or steroids before the test to prevent an allergic reaction.
The kidneys help remove iodine from the body. Those with kidney disease or diabetes may need to receive extra fluids after the test to help flush the iodine out of the body.
In rare cases, the dye may cause a life-threatening allergic response called anaphylaxis. If you have any trouble breathing during the test, you should notify the scanner operator immediately. Scanners come with an intercom and speakers, so someone can hear you at all times.
A CT scan can reduce or avoid the need for invasive procedures to diagnose problems in the skull. This is one of the safest ways to study the head and neck.
Other tests that may be done instead of a head CT scan include:
Positron emission tomography (PET) scan of the head
Broder J, Preston R. Imaging of the head and brain. In: Broder J, ed. Diagnostic Imaging for the Emergency Physician. Philadelphia, PA: Elsevier Saunders; 2011:chap 1.
Shaw AS, Prokop M. Computed tomography. In: Adam A, Dixon AK, Gillard JH, Schaefer-Prokop CM, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 6th ed. Philadelphia, PA: Elsevier Chuchill Livingstone; 2014:chap 4.
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.