What is computed tomography (CT) scanning?

Indications and Procedures

Computed tomography (CT) scanning collects X-ray data and uses a computer to produce three-dimensional images, called tomograms, of body cross sections, or slices. The noninvasiveness of CT scanning yields easy and safe body part analysis based on varying tissue opacity to X-rays. Bone absorbs X-rays well and appears white. Air absorbs them poorly, so the lungs are dark. Fat, blood, and muscle absorb X-rays to varying extents, yielding different shades of gray. Tumors and blood clots, for example, appear as areas of abnormal shades in normal tissue.

CT scanning is used to analyze disorders of the brain (brain CT) and most body parts (body CT), yielding tomograms that are hundreds of times more definitive than conventional X-rays. For example, conventional abdominal X-rays show bones and faintly outline the liver, kidneys, and stomach. Tomograms clearly depict all abdominal organs and large blood vessels.

Physicians call CT scanning the most valuable diagnostic method because, without it, the symptoms that patients describe may not be identified clearly as minor, serious, or life-threatening. For example, a subjective description of repeated headache does not reveal whether the cause is tension, stroke, or brain cancer. Before CT scanning, an accurate diagnosis often required complex or dangerous identification methods.

A CT patient changes into a hospital gown, removes any metal possessions, and lays on a table that can be raised, lowered, or tilted. During a scan, the patient enters a doughnut-shaped scanner that holds an X-ray source, detectors, and computer hookups. In brain CT, the patient’s head is in the scanner. Some CT patients have experienced claustrophobia, which can be prevented with faster scanner speeds and less-enclosed scanners. A patient who must stay still for an extended time may be given a sedative. If small anomalies are foreseen, then contrast materials are given before or during the procedure. These materials include barium salts and iodine, X-ray blockers that allow better visualization of specific tissues. Subjects may take the materials orally, by enema, or intravenously.

The CT scanner generates a continuous, narrow X-ray beam while moving in a circle around the patient’s head or body. The beam is monitored by X-ray detectors sited around the aperture through which the patient passes. Slices are produced as the scanner circles the head or body. Between slices, the table moves through the scanner. Slices become tomograms seen on a cathode-ray tube (CRT) and are stored in a computer. The procedure used to take twenty to forty minutes in a standard scanner. However, in the newer spiral CT, which is now standard in most hospitals, a patient is scanned rapidly as the X-ray tube rotates in a spiral. There are no gaps, as with slices, and tissue-volume tomograms are produced. A simple spiral scan is completed while the patient holds his or her breath, aiding the detection of small lesions and decreasing scan artifacts. Spiral CT, twenty times faster than standard CT, is useful in all patients, from restless children to the critically ill.

Uses and Complications

CT scanning detects organ abnormalities, and a major use is in diagnosing and treating brain disease. Even the earliest scanners could distinguish tumors from clots, aiding in the diagnosis of cancer, stroke, and certain birth defects. Furthermore, brain CT saves lives as physicians avoid risky methods requiring opening of the brain for pretreatment diagnosis. In addition, postsurgical scans can find recurrences or metastases.

Body CT allows for better damage appraisal of broken bones than does conventional X-ray analysis. Another use of body spiral CT is in the diagnosis of pulmonary embolism; it is safer than using pulmonary angiography, which maneuvers a catheter from the heart to the pulmonary artery. CT scans can also guide surgery, biopsy, and abscess drainage and can help fine-tune radiation therapy. Speed and excellent soft tissue elucidation make CT scanning invaluable for trauma detection in emergency rooms.

There are few side effects to CT scanning. Preparation for a scan may be mildly uncomfortable, but it is rarely dangerous. Before body CT, subjects often fast, take enemas to clear the bowels, and receive contrast materials through enemas or IVs. If contrast materials are used, then physicians must be told of allergies, especially to iodine. Contrast materials—enhancers of specific tissue CT—may cause hot flashes. Barium enemas for lower gastrointestinal tract scans cause full feelings and urges to defecate.

Perspective and Prospects

British engineer Godfrey Hounsfield and American physicist Allan Cormack won the 1979 Nobel Prize in Physiology or Medicine for the theory and development of computed tomography. CT scanning was first used in 1972, after Hounsfeld made a brain scanner holding an X-ray generator, a scanner rotated around a circular chamber, a computer, and a CRT. The patient laid on a gurney, head in the scanner, and emitter detectors rotated 1 degree at a time for 180 degrees. At each position, 160 readings entered the computer, so 28,800 readings were processed.

CT scanning is essential to radiology, which began in 1885 after Wilhelm Conrad Röntgen discovered X-rays. The rays soon became medical aids, and for years broad X-ray beams were sent through body parts to exit onto film, yielding conventional X-ray images. Bones absorb X-rays well, appearing white, and conventional images can show bone fractures and give some soft tissue data. However, soft tissue evaluation is poor, the tissues superimpose, and estimating their condition is difficult. CT scans allow convenient, noninvasive analysis.

CT scans and stereotaxic neurosurgery, later joined, have improved diagnosis and treatment. For example, the implantation of electrodes in a brain can be monitored using CT, enhancing accuracy. Similar techniques are used in breast biopsy. Current progress in CT scans includes thinner slices, spiral scans, and fast-operating standard scanners. Because complex scans expose patients to more radiation than do conventional X-rays, fast scans are preferred to minimize patient risk.


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