A computed tomography scan (CT scan or CAT scan) uses computer-controlled X-rays to take rapid, multiple three dimensionalimages or "slices" of a body area. An x-ray unit rotates around your body. 

A CT scan only takes seconds to perform.

CT scans include miminal risks which are generally outweighed by the benefits, but should still be considered.

• A CT Scan includes radiation exposure which can potentially be harmful over a lifetime. CT scans provide 100 - 500 times higher exposure to radiation than an ordinary x-ray. Experts disagree about the actual health risk from exposure to low levels of radiation from medical imaging such as a CT scan.  
• If the scan is with contrast, contrast agents add other risks, including possible harm to the kidneys and allergic reactions.  
• Unnecessary scans are also a waste of money.

Unnecessary scans can be avoided by taking the following steps:

• If you recently underwent the same test (even in another facility) give the doctor a copy of that test or provide information about how to get a copy of the test (for example, with the name of the facility and the doctor who ordered the test.)  
• Ask:  is this scan really necessary?  
  • If you think a CT scan is being pushed too strongly, you can ask a nother doctor for a second opinion. 
  • If you want to research this question on your own before consulting with your doctor, the American College of Radiology (ACR) has developed appropriateness criteria by condition and/or procedure. See: www.acr.org/ac
• If the test is necessary, ask: Are there alternatives that do not include radiation that would be just as effective such as a MRI or an ultrasound?
• Ask whether the doctor has a financial interest in the scanning facility. At least one study found that, on average, doctors who own CT scanners ordered 3 times more tests than doctors who did not have a financial state in a CT scanner.
• Ask for a copy of a CT Scan. 
  • Copies are usually free if requested at the time of the scan. Having a copy will help you avoid unnecessary duplicate scans if you later get treated at a different hospital.  
  • Keep the disk with you in your Medical Binder. Take your binder with you each time you see a doctor. There is no way to know up front what information will be helpful in a particular meeting.
  •  If the facility refuses to give you a copy, ask to speak with an administrator. You own your medical records and are entitled to a copy.

A CT scan  should only be undertaken in an accredited facility under the supervision of qualified people.

The other sections of this article provide the following information:

• CT Scans Compared to X Rays
• How To Vet A CT Scan Facility
• What It Is Like To Undergo A CT Scan

An  X-ray and a CT scan show different types of information. Although an experienced radiologist can get a sense for the approximate three-dimensional location of a tumor from an X-ray, in general, a plain X-ray is two-dimensional.

An X-ray looks all the way through a body without being able to tell how deep anything is. If a tumor is present, an x-ray can see it. Information is collected on a flat piece of film.

A CT scan is three-dimensional. It looks at several three-dimensional slices of a body (like slices of bread). In addition to seeing if a tumor is present, a CT scan can see roughly how deep it is in the body. A CT scan can be three dimensional because the information about how much of the X-rays are passing through a body is collected on a computer - not just on a flat piece of film.

The data from a CT scan can be enhanced to be more vivid than a plain X-ray.

For both plain X-rays and CT scans, the patient may be given a contrast agent by injection or orally through a drink. The contrast agent shows more clearly the boundaries between organs, or between organs and tumors.

In order to get the best results, you need qualified people performing the test on up-to-date, well maintained equipment.

To help assure the appropriate result, take the following steps when setting a facility for the scan:

Step 1: Check facility accreditation

• The facility should be accredited by the ACR (American College of Radiology). ACR sets minimum standards for the quality of the equipment and personnel training. If your doctor does not know whether a facility is accredited by the ACT, you can ask when you call for an appointment. 
• To find an accredited facility: Go to www.acr.org.  Then click on http://www.acr.org/Quality-Safety/Accreditation/Accredited-Facility-Searc h

Step 2. Check the people who will perform the scan. Ask:

• Who will be performing the exam?  
• What is her or her education? Experience?
• Is he or she board certified? (For information about board certification, click here.)

A CT scan is painless - and very fast.

When you are ready, you will be asked to remove all jewelry and to change into a hospital gown.

A technician will escort you into the room with the scanner. He or she will then help you lie down on the scanning "couch" which looks like a narrow, unpadded, examining table.

After positioning you,and telling you not to move, the technician will leave the room.

The examining table will then move through the donut shaped scanner. The technician will tell you when to hold your breath. The scanner then takes a series of pictures of the area to be studied. 

The technician sees the images immediately and checks them for readability.

NOTE: Do not expect to be told the results by the technician. A radiologist will write a report of his or her findings from the images and send them on to your doctor who will tell you the restuls.