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What is a CT Scan?

Computed Tomography Explained

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Updated February 04, 2013

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

What is a CT Scan?

X-ray computed tomography, better known as a CT scan or a CAT scan, is a non-invasive diagnostic medical test. The CT scan was first introduced in the 1970s, and currently tens of millions of scans are performed each year.

The CT scan is now a very common test due to its non-invasive nature. No incisions are made, and nothing is inserted into the body. The test is performed without touching the body or using X-rays to produce an image of the body's structures.

Unlike a traditional X-ray that produces one image of the body, the CT scan produces a series of images of the body that allow the radiologist reading the scan to see the body as a series of very thin slices, allowing for the diagnosis of many conditions.

How Is a CT Done?

A CT scanner consists of a moving table (on which the patient lies for the procedure), a large doughnut-shaped scanner and a control room. The patient lies down on the table, which can then be moved into place inside the hole of the scanner. The patient lies still while the bed slowly changes positions, sliding in or out of the scanner so the necessary body parts can be scanned without the patient moving.

The process is controlled by a specially-trained radiology technologist, who mans the scanner from another room. The length of time the scan requires varies based on the type of scan that is needed; however, most scans are completed in ten to fifteen minutes. The newest CT scanners are so fast that it takes longer to position the patient on the table than it takes to complete the scan.

Most patients are able to tolerate the CT scan process without difficulty, as the scanner is open on both sides, and the process is not painful. Lying completely still is the most challenging part of the test for many patients, as the table is hard and not very comfortable. But since the CT scan is less enclosed than most MRI machines, it doesn't require that you remain motionless for nearly as long as an MRI. Some patients may be asked to hold their breaths during the scan; 30 seconds is typically the longest a patient would be expected to do so.

Some scanners have a weight limit: patients over 300 pounds may not be able to use a standard scanner and may require a more specialized scanner.

CT With or Without IV Contrast

CT scans are performed with or without contrast: a special dye that helps improve the images produced by the CT scanner. For some studies, a contrast agent is given via IV, which then circulates through the bloodstream.

In the past, an allergic reaction to IV contrast was fairly common. Modern IV contrast causes far fewer allergic reactions; people who are allergic to shellfish or iodine are the most likely to experience issues. Some patients experience a warm sensation at the site of the IV or experience nausea, but most patients have no issues with the contrast dye.

Other Types of Contrast

In addition to IV contrast, there are other types of contrast, such as barium sulfate (also known as a barium swallow), which is taken as a drink prior to the procedure. The drink is chalky and may be flavored to make it more tolerable.

Contrast can also be given as an enema. Whether it's administered orally or rectally, it passes out of the body in bowel movements in the days following the test.

Risks of CT Scans

  • Contrast-induced nephropathy is a condition in which the kidneys are harmed by the IV contrast medication. This is most common in people who already have issues with their kidneys, such as renal insufficiency, or in diabetics who have sustained kidney damage due to their disease. For these patients, medications may be given prior to the study to minimize the risk of kidney damage or kidney failure.
  • An allergic reaction to contrast is far less common today that it was in the early days of CT scans. If you've ever had an allergic reaction to contrast, shellfish or iodine, be sure to notify your physician.
  • Radiation exposure is a risk of CT scans, but the chances of problems due to the small radiation level of the scanner per scan is minimal.
  • Fetal harm is a known risk of the CT scan. While more pregnant women are having CT scans than ever before, there are significant risks to the fetus associated with scans that should be discussed with a physician prior to the study. Scans are typically avoided unless the mother's health is at serious risk, and a pregnancy test may be performed in women of child bearing age prior to testing.

Sources

CT Scan. Medline Plus. Accessed 2013. http://www.nlm.nih.gov/medlineplus/ency/article/003330.htm

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