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CT   

Overview | Risks

Alternative names:

Computed axial tomography (CAT) scan

Definition:

Tomography is a method of body imaging in which the X-ray source and/or detection device (e.g., film) rotate around the patient. In computed tomography (CT), a thin X-ray beam rotates as small detectors measure the amount of X-rays which make it through the patient or particular area of interest. By use of a complex algorithm, a computer analyzes the data to construct a cross-sectional (axial) image. These images can be stored, viewed on a monitor, or printed on film. In addition, three-dimensional models of organs can be created by stacking the individual images, or "slices".

How the Test is Performed

The patient will be asked to lie on a narrow table (gantry) that slides into the center of the scanner. Depending on the study being performed, the patient may need to lie on his/her stomach, back, or side. If contrast media (dye) is to be administered, an IV will be placed in a small vein of a hand or arm.

Much like standard photographic cameras, subject motion causes blurred images in CT. Therefore, the technologist operating the scanner and supervising the patient will give instructions through an intercom when to hold one's breath and not move.

As the exam takes place, the gantry will advance small intervals through the scanner. Modern "spiral" scanners can perform the examination in one continuous motion of the gantry.  Generally, complete scans will only take a few minutes, however, additional contrast-enhanced or higher-resolution scans will add to the scan time. The newest multi-detector scanners can image the entire body, head to toe, in under 30 seconds.

How to Prepare for the Test

The patient may be asked to drink oral contrast either immediately prior to, or 4 to 6 hours before, the CT scan. The contrast may be composed of non-reactive (inert) chalky-tasting barium sulfate which will eventually pass in the stools or absorbable clear gastrograffin solution. The health care provider may also advise fasting (no solids or liquids) for 4 to 6 hours if contrast dye is to be used.

The CT scanner has a weight limit to prevent damage to the mechanized gantry. Have the health care provider contact the scanner operator if you weigh more than 300 pounds.

Since metal is very, very dense, the X-ray beam has difficulty passing through it and results in errors in the involved constructed slices (artifact). Therefore, the patient will be asked to remove jewelry and wear a hospital gown during the study.

How the Test will Feel

The X-rays are painless. The primary discomfort may be from the need to lie still on the table.

If intravenous contrast dye is given, the patient may initially feel a slight burning sensation within the injected arm, a metallic taste in the mouth, and a warm flushing of the body. These sensations are normal and usually reside within a few seconds.

Why the Test is Performed

CT provides rapid, detailed cross-sectional imaging of the patient which can then be reconstructed into three-dimensional models, as needed. Intravenous contrast enhanced scans allow for evaluation of vascular structures and further evaulation of masses and tumors.

CT is often utilized in the trauma setting to evaluate the brain, chest, and abdomen. As well, CT can be used to guide interventional procedures, such as biopsies and placement of drainage tubes.

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