This is an abbreviated version of the complete article.*
Computed tomography (CT) scanners use x rays to obtain cross-sectional views of the body.
CT scans help physicians diagnose diseases or conditions and guide instruments, such as needles or catheters (tubes), during procedures.
The CT scanner consists of a gantry, a large donut-shaped device; a control console; and a computer that displays images on a monitor.
Compared with conventional x rays, CT scans show regions of the body in much more detail and allow a different visualization of the body.
A CT scan, or CAT scan, is a common term for computerized axial tomography, a painless diagnostic imaging test that displays two-dimensional images of internal structures of the body on a computer screen. This test often takes less than 30 minutes to perform. Patients can receive a CT scan on an outpatient basis or as part of an inpatient hospital stay.
CT scans can be taken of many sections of the body, including the abdomen, chest, and brain. The Images come from the reflection of x rays off tissues of varying densities. Sometimes a contrast dye is given to a patient intravenously, rectally, or orally to make hollow or fluid-filled structures such as blood vessels more visible. The use of contrast material during CT scanning doubles procedure time.
CT scanning is an improvement over conventional x ray imaging because it captures higher-resolution images.
A version of CT scanning called spiral or helical scanning, also known as 3D computerized imaging, can capture three-dimensional images. During spiral scanning, the patient continually moves forward on a table as the x ray source rotates around his or her body.
Typically, patients are asked to avoid solid foods for about 4 hours before the procedure, but they can have clear liquids, such as juice or clear broth. Before the test, the patient changes into a hospital gown and removes any jewelry or other objects that may interfere with x rays.
Because CT scanning uses x rays, it exposes the body to radiation. These radiation doses are very small, but they can damage cells, which can increase a person's risk for cancer.
Patients that may be unsuited for a CT scan include patients who:
Are pregnant (fetuses are vulnerable to x ray radiation);
Have an allergy to contrast dye (rare);
Have unstable vital signs; and
WHAT TO EXPECT
The patient lies still on a table, which slides into the gantry, the donut-shaped device that houses the scanning equipment.
An x ray tube slides around the gantry, passing narrow beams of x rays in an arc over the body. These beams reflect onto an x ray detector positioned opposite the x ray tube. After one arc, the scanning table moves forward a certain distance and the tube transmits another arc of x rays.
The x ray detector transmits the x ray energy to a computer, which transforms the information about the reflected energy into an image.
Medical technicians operate the scanning machine from another room and constantly monitor the patient. Through speakers in the CT scan room, they remind the patient to stay still and to hold his or her breath during the x ray scans, because motion blurs the images.
When the patient receives the results depends on how long it takes for an appropriate radiologist to become available to interpret the scans.
Patients can resume normal activities immediately.
If patients have ingested a contrast agent, they are asked to drink fluids following the test for hydration and to speed its excretion.
Medical Review Date: June 23, 2004
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