Computed Tomography
Introduction
Computed tomography is a sophisticated diagnostic imaging procedure capable of depicting anatomy at different levels within the body. This ability, known as cross-sectional imaging, is possible because the x-ray source rotates around the patient during a CT scan, encircling
the patient's body and capturing anatomical detail from many angles. Each rotation of the x-ray beam produces a single cross-sectional "slice" of anatomy, like the slices in a loaf of bread. Computed tomography allows physicians to see a single slice of the body, just as if you were taking a slice of bread out of a loaf. Using this technology, physicians can view the inside of anatomic structures, a feat not possible with general radiography.
Computed tomography scans, also called CT scans, are used for many types of diagnostic procedures. They may be used to examine the head to check for bleeding, tumors, blood clots or signs of stroke. In other parts of the body, CT may be used to distinguish whether a growth is solid or fluid-filled, detect ruptured disks in the spine, determine an organ's size and shape, and evaluate many types of disease processes. Information supplied by CT scans also can be used to determine the stage of some types of cancer, helping physicians decide how to treat the disease. Computed tomography also is used to help plan radiation therapy. The scans help the oncology team target treatment to the cancer site while protecting surrounding healthy tissue. In addition, CT may be used to guide biopsies (the collection of a sample of tissue to aid in diagnosis).
Patient Preparation
Your personal physician or the radiology facility where you are scheduled to have your CT procedure will give you detailed instructions describing how to prepare for your examination. You will be asked whether there is a chance that you might be pregnant. If you are pregnant, your health care provider will help you weigh the benefits of having a CT scan vs. the risks. In addition, if a contrast agent will be used during your examination, you will be asked if you have any allergies. You may also be asked about your medical history and your health in general.
Before your examination, a CT technologist will explain the procedure to you and answer any questions you might have. A CT technologist, also known as a radiologic technologist, is a skilled medical professional who has received specialized education in the areas of anatomy, patient positioning, patient care, radiation safety, imaging techniques and CT procedures.
During the Examination
Examination time can range from 10 minutes to more than an hour, depending upon the part of the body being examined and whether or not a contrast agent is used. For a head scan, you will be asked to remove eyeglasses, dentures and barrettes or hairpins. For a body scan, you will be asked to put on a hospital gown and to remove all jewelry, because metal can interfere with the imaging. You will be provided a secure place to store these items during your examination.
The CT technologist will position you on the scanning table. If you are undergoing a head scan, the technologist will place your head in a cradle to help prevent movement. For head scans and scans of other parts of the body, you will be secured onto the table with a safety strap. Even the slightest movement can blur the image, so it's important to hold still during the scan.
You may be given a contrast agent to drink before the examination begins, or it may be administered through an injection into a vein. The contrast agent helps visualize tissues in the area being studied. You may feel nauseous, flushed or headachy after the contrast is administered; these are normal reactions. However, if you feel itchy or short of breath, you may be having an allergic reaction to the contrast agent and you should tell the technologist immediately.
The technologist will guide the scanning table into the CT unit, which is a square or rectangular machine with a large circular hole in the center. The CT technologist will not be in the room during the scan, but will be able to observe you through a window from an adjacent room or through a video system and will be able to hear you and talk to you through a two-way microphone system.
During the scan, the x-ray tube within the CT unit will rotate around you, taking x-ray pictures of one very thin slice of tissue after another. As the x-ray tube rotates, you will hear a whirring sound. The table that you are on will move slightly to reposition you for each scan, but it moves so slowly that you might not even notice it.
The technologist will tell you when each scan sequence is beginning and how long it will last. You should remain as still as possible throughout the sequence, and for certain scans you may be asked to hold your breath for a few seconds.
The x-ray unit that rotates around your body is linked to a computer that processes each scan in a matter of seconds. The final scans, called "CT images," are sent to a monitor that the CT technologist observes throughout the procedure. The scans then can be output on film or recorded on tape or diskette.
When the exam is complete, your CT scans will be given to a radiologist - a physician who specializes in the diagnostic interpretation of medical images.
Post-examination Information
After your films have been reviewed by a radiologist, your personal physician will receive a report of the findings. Your physician then will advise you of the results and discuss what further procedures, if any, are needed.
The radiation you are exposed to during a CT scan is only slightly higher than from a regular x-ray, and it passes through you immediately. You are not "radioactive" following a CT procedure, and it is not necessary to take any special precautions following your examination.
If a contrast agent was administered, you may experience nausea, headache or dizziness following your examination. It's important to increase your water consumption in the days following the examination. If these symptoms persist, contact your physician.
Please remember that the material presented here is for informational purposes only. If you have specific questions about a medical imaging procedure, contact your physician or the radiology department of the institution where your test will be performed.