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Home Page > Procedures > What is CT? |
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Computed Tomography Alternative Names CAT scan, computed axial tomography, computed tomography imaging. What is CT? Computed tomography (CT) is a computer-aided X-ray technique. X-rays consist of electromagnetic waves of energy. They penetrate the body to varying extents depending on the density of the structures being viewed. The result is a black-and-white image of an interior portion of the body. CT combines the use of a digital computer with a rotating X-ray device to create detailed cross-sectional images (also known as slices or sections) of different organs and body parts such as the lungs, liver, kidneys, pancreas, pelvis, extremities, brain, spine, and blood vessels. CT scan produces sequential views of the body that can be thought of as slices of bread. How does CT work? How is the image formed? A CT scanner consists of a highly sensitive X-ray beam that is focused on a specific plane of the body. The scanner is operated by rotating an X-ray tube around the patient's body. A fanlike beam of X-rays is sent out through the part of the body being examined (eg, the skull in brain imaging). As this beam passes through the body, different tissues of the body absorb different amounts of the X-ray beam, causing it to change. A series of detectors on the opposite side of the body picks up the altered beam and measures how the different tissues change it. The detectors then feed this information into a computer. The X-ray tube and detectors are constantly rotating around the body; for every 360° rotation, one cross-sectional image is created. During the examination, hundreds of images are obtained, and the computer later combines these to make the final image. Each original image represents an area of the body that is only a few millimeters wide. The computer then analyzes the information on the basis of differences in tissue density, and it produces a reconstructed image of a thin cross-section, or slice, of the body. The analyzed data is then fed into a cathode-ray tube (the device responsible for producing pictures on your TV) and a picture of the X-rayed, cross-section of the body is produced. On CT scans, bone appears white, gases and most liquids appear black, and other tissue can have varying shades of gray depending on density. How is the patient prepared for the study? What should he or she tell the doctor? The specific preparation varies with the area of the body being studied. In general, preparation for CT may include the following. If a contrast material (a special X-ray dye) is being used and/or if the use of sedatives is anticipated, the patient may need to fast for 4-6 hours before the procedure. In general, patients may take prescription medications with a small amount of water. However, they should check with the radiologist first. The patient may be asked to sign a consent form. All jewelry, dentures, glasses, and metal objects that may interfere with the examination must be removed before the patient enters the CT unit. Women are asked if they are pregnant. If a female patient is or believes that she might be pregnant, she should tell the technologist before the examination. The patient undergoing the examination should tell the physician or technologist if he or she has had any of the following: asthma, life-threatening event in the past, or any allergies to medicines, especially iodinated contrast material. Patients should check with their healthcare provider, hospital radiology department, or radiologist to see if any other preparation is needed. How is the test performed? The scanner looks like a short doughnut-shaped device attached to a table. The patient lies on a narrow table that slides through an opening in the machine called the gantry. While the patient is in the gantry, an X-ray tube rotates around the patient, creating computer-generated X-ray images. X-rays are painless. The primary discomfort may be caused by the need to lie still on the table. During the test, it is very important for the patient to remain still because motion can result in blurring images. If a patient is restless or anxious, sedatives may be given. If the doctor orders a contrast-enhanced examination, the patient may experience some discomfort when the intravenous line is inserted. The contrast agent may be given orally, intravenously, or both. For instance, if increased contrast between different tissues or organs is needed, the dye is administered as an injection into a vein. An oral contrast agent may be required for CT examinations of the abdomen and pelvis. The contrast material allows the radiologist to more clearly see a certain area or structure of the body. The moveable table raises and lowers the patient and moves the patient in and out of the scanner. The table advances slightly (by 1/4 to 1/2 inch) between each scanning session to properly align the patient for the next image. The portion of the body that is being studied is moved into the gantry, and this is the only portion of that body that is exposed to the X-ray beam. A technologist specially trained in the use of CT equipment performs the examination by operating a computer in an adjoining room. The technologist will tell the patient when to hold his or her breath and when to breathe normally. A sound system is built into the machine to allow the technologist to communicate with the patient during the test. With brain CT, the patient's head is placed in a holder with a strap across the forehead. How long does the examination take? Each exposure takes only a few seconds, and the entire examination may last anywhere from a few minutes to an hour, depending on the area of the body being studied and the technique used. For example, head CT examinations usually take 5-20 minutes. What conditions are studied with CT? A CT scan can decrease or eliminate the need for invasive procedures to diagnose problems in the brain, spine, and neck. A CT scan is recommended in the following situations:
CT scans are also used to view the facial bones, the jaw, the temporal bones, and the sinus cavities. CT imaging of the head and brain can be use to detect tumors, show blood clots and blood vessel defects, show enlarged ventricles (caused by a build up of cerebrospinal fluid), and image other abnormalities such as those of the nerves or muscles of the eye. CT is the method of choice for imaging acute trauma. CT examinations are fast and simple, and they provide the physician with a quick overview of possibly life-threatening pathology. CT also helps the treatment team to rapidly determine if surgery is needed. With the advent of spiral (or helical) CT, the continuous acquisition of complete CT volumes can be used for the diagnosis of blood vessels. Techniques for this include CT angiography (CTA). For instance, the carotid arteries and the intracranial vasculature can be imaged with CTA by using only an intravenous injection of contrast material and not an invasive catheter, as in previous techniques. Are there any risks involved? Patients may have some concern regarding exposure to ionizing radiation. Well-qualified radiologists are trained in radiation protection, and they develop protocols to minimize the dose of radiation without compromising the quality of the images. Nonetheless, CT scanning does pose minimal risks associated with X-ray exposure, although it is significantly less than the risk of ordinary radiography. The exposure is monitored and regulated to minimize the amount of radiation needed to produce the image. Patients should consult their healthcare provider about the risks if multiple CT scans are needed over a period of time. During pregnancy, abdominal CT scanning is usually not recommended unless the patient has an emergency situation, because the radiation exposure may pose a risk to the developing fetus. The true incidence of adverse effects after the administration of intravascular contrast material is not known precisely; however, most adverse effects are mild to moderate (eg, hives, headache, warm sensations). Adverse effects range from hives to more severe allergic reactions. The latter may be of increased incidence in patients with food allergies, asthma, or other allergic conditions. The patient should alert the technologist of any past allergic reactions before the examination. The injection may cause a cool or warm sensation, as well as a metallic taste in the mouth that lasts for a few seconds. When a contrast medium is given, it is eliminated through the patient's urine and is not visible. These effects usually do not require treatment, and they occur in only 5-12% of all patients receiving ionic, high-osmolality contrast media. Serious reactions are rare and occur in only 0.1-0.2% of patients receiving high-osmolality contrast media and in only 0.016% of those receiving low-osmolality contrast media. These effects occur in the first few minutes after the agent is given, and the patient should immediately report any unusual sensation to the technologist or physician.
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