You will be asked to lie on a narrow table that slides into the center of the scanner.
Once you are inside the scanner, the machine's x-ray beam rotates around you. (Modern "spiral" scanners can perform the exam in one continuous motion.)
Small detectors inside the scanner measure the amount of x-rays that make it through the part of the body being studied. A computer takes this information and uses it to create several individual images, called slices. These images can be stored, viewed on a monitor, or printed on film. Three-dimensional models of organs can be created by stacking the individual 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 of time.
Generally, complete scans take only a few minutes. The newest multidetector scanners can image your entire body, head to toe, in less than 30 seconds.
Certain CT exams require a special dye, called contrast, to be delivered into the body before the test starts. Contrast can highlight specific areas inside the body, which creates a clearer image. If your doctor requests a CT scan with contrast, you will be given it intravenously (by IV) through a vein in your arm or hand.
How to prepare for the test
Some people have allergies to IV contrast and may need to take medications before their test in order to safely receive this substance.
Contrast can be given several ways, and depends on the type of CT being performed.
It may be delivered through a vein (IV) in your hand or forearm.
It may be given through the rectum using an enema.
You might drink the contrast before your scan. When you actually drink the contrast depends on the type of exam being done. The contrast liquid may taste chalky, although some are flavored to make it taste a little better. The contrast eventually passes out of your body through your stools.
If contrast is used, you may also be asked not to eat or drink anything for 4-6 hours before the test.
If you weigh more than 300 pounds, have your doctor contact the scanner operator before the exam. CT scanners have a weight limit. Too much weight can cause damage to the scanner's working parts.
Since x-rays have difficulty passing through metal, you will be asked to remove jewelry and wear a hospital gown during the study.
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 sensations are normal and usually go away within a few seconds.
Why the test is performed
CT rapidly creates detailed pictures of the body, including the brain, chest, spine, and abdomen. The test may be used to better view the structures inside the chest.
A thoracic CT may be done:
After a chest injury
When a tumor or mass (clump of cells) is suspected
To determine the size, shape, and position of organs in the chest and upper abdomen
To look for bleeding or fluid collections in the lungs or other areas
What abnormal results mean
Thoracic CT may show many disorders of the heart, lungs, or chest area, including:
CT scans and other x-rays are strictly monitored and controlled to make sure they use the least amount of radiation. CT scans do create low levels of ionizing radiation, which has the potential to cause cancer and other defects. However, the risk associated with any individual scan is small. The risk increases as numerous additional studies are performed.
In some cases, a CT scan may still be done if the benefits greatly out weigh the risks. For example, it can be more risky not to have the exam, especially if your health care provider thinks you might have cancer.
An abdominal CT scan is usually not recommended for pregnant women, because it may harm the unborn child. Women who are or may be pregnant should speak with their health care provider to determine if ultrasound can be used instead.
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, sneezing, vomiting, itching, or hives may occur. Rarely, 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 the operator can hear you at all times.
In people with kidney problems, the dye may have toxic effects on the kidneys. In these situations, special steps may be taken to make the CT scan safer.
A CT scan is one of the best ways of looking at soft tissues such as the heart and lungs.
Aziz ZA, Hansell DM. Techniques in thoracic imaging. In: Adam A, Dixon AK, eds. Grainger & Allison's Diagnostic Radiology: A Textbook of Medical Imaging. 5th ed. New York, NY: Churchill Livingstone; 2008:chap 11.
Stark P. Imaging in pulmonary disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 84.
David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.