Computed Tomography (CT) Scans

General information

Other names

Computed axial tomograph (CAT) scans.

Purpose

• To obtain a two-dimensional view of a cross section of the brain or other internal organ.
• To detect tumors, bleeding, and other abnormalities that may not show up on an ordinary X-ray.

How it works

• Multiple X-rays are taken as the CT X-ray tube revolves around the patient.
• A computer calculates the amount of X-ray penetration through the specific plane(s) of the body part(s) examined, and gives each a numeric value (density coefficient).
• This information is fed into a computer, which translates the values into different shades of gray.
• These images are displayed on a television monitor and photographed as a series of two-dimensional images depicting a cross section of the part under examination.

Preparation

• You will be shown the CT machine and asked to express any concerns.
• If you experience claustrophobia in small, enclosed spaces, you may be given a mild sedative to quell your anxiety.
• If an intravenous contrast agent is to be used, you will be asked to abstain from ingesting food and water for at least four hours beforehand.
• Before entering the unit, you will be asked to remove any jewelry or other objects that may interfere with clear X-ray images.

Test procedure

• You will be asked to lie on a narrow examination table, which slides into the scanner (see figure 3.1).

  
  FIGURE 3.1
  In CT scanning of the head, the X-ray tube revolves around the area being examined as the table slowly moves the patient across the area in question.

   

• As you lie as motionless as possible, the CT tube revolves slowly, taking multiple X-ray images, which are reconstructed into two-dimensional views of a cross section of the body.
• The table is then moved slightly to take another set of images through another plane of the body; typically, three to seven planes are imaged, but this varies according to the part of the body under examination.
• In some instances, the entire length of the body may be scanned; in others, only a relatively narrow section.

Variations

• CT scanning may be performed in conjunction with other imaging studies and diagnostic procedures, such as X-rays of the joint or spinal column (eg, arthrography or myelography). This allows more detailed images of the entire joint structure than can be obtained from X-rays alone.
• A relatively new innovation is spiral CT imaging, which allows for continuous scanning as the gantry table slides through the unit. This technique cuts the amount of time needed for whole-body scanning.

After the test

• You may be asked to wait while a radiologist quickly reviews the images to make sure that the part of the body under study has been adequately photographed.
• If necessary, you may be asked to return to the scanner for additional images. Otherwise, you will be able to resume normal activities.
• If an intravenous contrast medium was used, you will be instructed to drink extra fluids to speed its removal from the body.
• You should also watch for delayed allergic reactions, such as hives, a rash, itching, or perhaps a rapid heartbeat. Such symptoms usually appear within two to six hours; in severe cases, an antihistamine or steroid medication may be prescribed to ease discomfort.

Factors affecting results

• Obesity, movement during the examination, and the presence of metallic objects can interfere with obtaining clear images.
• In some cases, excessive gas or fecal material in the intestines can give misleading results in an abdominal CT scan

Interpretation

A radiologist will interpret the scans.

Advantages

• CT scanning provides a painless, noninvasive method of obtaining a detailed view of internal organs.
• In many instances, CT scanning eliminates the need for more invasive procedures

Disadvantages

• The test is costly and may not be available in small hospitals and rural areas.
• It is contraindicated during pregnancy, and may not be suitable for those who are very obese.