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What is Computed Axial Tomography?

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  • Written By: Caitlin Kenney
  • Edited By: Bronwyn Harris
  • Last Modified Date: 04 December 2016
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Computed axial tomography, also called computed tomography, CAT scan, or CT scan, is a medical diagnostic technology that makes images of the internal structures of the body. These scans use X-ray technology run through a computer to image a section of the body. X-rays, a type of electromagnetic radiation with short wavelengths, are beamed through the targeted section of the body from several different angles. The focus point comes out as a clear, three-dimensional or cross-sectional image and the other tissues in the area are blurred, making it easier for the physician to differentiate the targeted tissues. Computed axial tomography is commonly used to diagnose conditions or damage in the soft tissues or bones in the head, chest, joints, colon, lungs, and heart.

A normal X-ray scan beams X-rays through a part of the body from one source and receives the penetrating rays on a screen on the other side of the body. These rays are absorbed in varying degrees in the body depending on the densities of the organs inside, the screen catches more or less X-rays. This leaves energized particles on the screen that document the tissues inside, with denser tissues showing up whiter, less dense tissues showing up grayer, and the background appearing black. This picture is then run through a computer to create a two-dimensional image of the body.

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A computed axial tomography scan works in a similar way, but rotates the X-ray source around the targeted tissue to get many cross-sectional X-ray images of the focal point. These images are then compiled to create a 3D picture of the tissue in question. One can imagine, however, that an image compiled of all of these flat scans taken from so many different angles would be complex and difficult to differentiate. To solve this problem, CAT scan machines revolve the beams around the body so that the targeted tissue is always in focus, but the surrounding tissues become blurred.

A modern X-ray machine for CT scans usually looks like a large box or circle with a donut hole in the center. The patient is placed on a table which is moved into the machine so that the area to be examined is inside the hole. The radiation technologist, a professional trained in radiology, will ask the patient to remain still while the exam is taking place to ensure the clearest results. The procedure usually takes thirty minutes to an hour and a half to complete, though it can be more or less time consuming depending on the patient. In some cases, a contrast dye may be injected into the patient prior to the scan to allow for better viewing of the blood vessels.

Computed axial tomography can be used on many sections of the body. In the head, physicians use CT scans to diagnose tumors, deformities, aneurysms and associated conditions, infections, bone density issues in the skull, and to check for hemorrhaging or bone fractures after an injury. Chest X-rays can help diagnose aortic dissection, or a fissure in the aorta, pulmonary embolisms, or the blockage of the pulmonary artery, pneumonia, lung cancer, and other lung abnormalities. A special computed axial tomography technique also allows images to be taken of the heart in a procedure called a cardiac CT scan. These typically seek to exclude coronary artery disease as a cause of chest symptoms.

CAT scans are also performed on joints to examine fractures and on the abdomen and pelvis to look for infections, kidney stones, tumors, bowel obstructions, and other abnormalities. These examinations should not cause any pain, though some patients may be uncomfortable staying still for the length of the procedure. The amount of radiation typically used in computed axial tomography does not pose a large risk to adults, but may increase the likelihood of developing cancer in children. Lower settings have been developed for use on children, though it is recommended that they receive magnetic resonance imaging or an ultrasound instead of a CT scan when possible. Pregnant women should avoid exposure to radiation unless it is absolutely necessary.

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