Weill Cornell Imaging at NewYork-Presbyterian

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Computed Tomography (CT or CAT) Scan

What is a CT or CAT scan?

A CT or CAT scan is a diagnostic imaging procedure that uses a combination of x-rays and computer technology to produce cross-sectional images (often called slices), both horizontally and vertically, of the body. A CT scan shows detailed images of any part of the body, including the bones, muscles, fat, organs, and blood vessels. CT scans are more detailed than standard x-rays.

In standard x-rays, an invisible beam of energy is aimed directly at the body part being studied. A plate behind the body part captures the variations of the energy beam after it passes through skin, bone, muscle, and other tissue. While much information can be obtained from a regular x-ray, a lot of detail about internal organs and other structures is not available.

During CT scans, the x-ray beam moves in a circle around the body as oppose to a straight line during a general x-ray. This allows many different views of the same organ or structure, and provides much greater detail. The x-ray information is sent to a computer that interprets the x-ray data and displays it in 2-dimensional form on a monitor. Additional advanced technology and computer software makes three-dimensional (3-D) images possible.

CT scans may be done with or without contrast. "Contrast" refers to a substance taken by mouth or injected into an intravenous (IV) line that causes the particular organ or tissue under study to be seen more clearly. Contrast examinations may require you to fast for a certain period of time before the procedure. Your physician will notify you of this prior to the procedure.

CT scans may be performed to help diagnose tumors, investigate internal bleeding, or check for other internal injuries or damage.

You will need to let your physician know if you have ever had a reaction to any contrast dye or if you are allergic to iodine. If you have any medical conditions or recent illnesses, inform your physician. If you are pregnant or think you may be pregnant, you should notify your physician.

How is a CT or CAT scan performed?

CT scans can be performed on an outpatient basis, unless they are part of a patient's inpatient care. Although each facility may have specific protocols in place, generally, CT scans follow this process:

  1. When the patient arrives for the CT scan, he/she will be asked to remove any clothing, jewelry, or other objects that may interfere with the scan.
  2. If the patient will be having a procedure done with contrast, an intravenous (IV) line will be started in the hand or arm for injection of the contrast medication. For oral contrast, the patient will be given medication to swallow.
  3. The patient will lie on a scan table that slides into a large, circular opening of the scanning machine.
  4. The CT staff will be in another room where the scanner controls are located. However, the patient will be in constant sight of the staff through a window. Speakers inside the scanner will enable the staff to communicate with and hear the patient. The patient will have a call bell so that he/she can let the staff know if he/she has any problems during the procedure.
  5. As the scanner begins to rotate around the patient, x-rays will pass through the body for short amounts of time
  6. The x-rays absorbed by the body's tissues will be detected by the scanner and transmitted to the computer
  7. The computer will transform the information into an image to be interpreted by the radiologist.
  8. It is very important that the patient remain very still during the procedure. You may be asked to hold your breath at various times during the procedure.
  9. The technologist will be watching the patient at all times and will be in constant communication.
  10. The patient may be asked to wait for a short period of time while the radiologist examines the scans to make sure they are clear. If the scans are not clear enough to obtain adequate information, the patient may need to have additional scans performed.
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