CT in Radiology
Computed Tomography (also known as CAT Scan, Computed Axial Tomography) is a technical advance over standard x-rays. A x-ray source and detector are moved about a focal plane in the body and a computer is used to generate an accurate cross-sectional image of the body. This powerful technique is used to evaluate pathology throughout the body. Administration of intravenous iodinated contrast aids in the evaluation of solid organ disease, for example metastatic lesions within the liver. Iodinated contrast may also be injected into the joints in the detection of ligament tears or other joint abnormalities.
What Does the Equipment Look Like?
The CT scanner is a large, square machine with a hole in the center, something like a doughnut. The patient lies still on a table that can move up or down, and slide into and out from the center of the hole. Within the machine, an x-ray tube on a rotating gantry (or frame) moves around the patient’s body to produce the images, making clicking and whirring noises as the arm moves. Though the technologist will be able to see and speak to you, you and the technologist will separated by a wall with a window.
How Does the Procedure Work?
Unlike conventional x-rays, which produce pictures of the shadows cast by body structures of different density, CT scanning uses x-rays in a much different way. In CT, numerous x-ray beams are passed through the body at different angles, and special sensors measure the amount of radiation absorbed by different tissues (and lesions such as a tumor). As you lie still, the scanner parts revolve around you (although you cannot see this happen), emitting and recording x-ray beams from as many as a thousand points on the circle. A special computer program then uses the differences in x-ray absorption to form cross-sectional images, or “slices,” of the head and brain. These slices are called tomograms, hence the name “computed tomography.”
How Should I Prepare for the Procedure?
You should wear comfortable, loose-fitting clothing for your CT exam. Anything that might interfere with imaging of the head—such as earrings, eyeglasses, dentures, dental implants, or hairpins—should be removed.
No special preparation is needed for a CT scan unless you are to receive a contrast material—a substance that highlights the organs and blood vessels and makes abnormalities easier to see. If the physician believes that an intravenous (IV) injection of a contrast material will be helpful, you will be asked in advance whether you have had allergies in the past or have ever had a serious reaction to medication. Many contrast materials contain iodine, which can cause such a reaction in persons who are allergic. If you have known allergies to other medications it may raise the possibility that you might have an allergic reaction to the contrast material. The radiologist also should know if you have asthma, multiple myeloma, or any disorder of the heart, kidneys, or thyroid gland, or if you have diabetes—particularly if you are taking Glucophage. Typically you will be asked to sign an “informed consent” form before having CT with injection of a contrast material.
Women should always inform their doctor or x-ray technologist if there is any possibility that they are pregnant. In some cases an alternate study will be performed to reduce or eliminate the radiation exposure to the fetus.