CT scanning is painless, noninvasive, and accurate. CT examinations are fast and simple.
Computed tomography (CT) is a diagnostic procedure that uses special x-ray equipment to obtain cross-sectional pictures of areas inside the body. A computer then assembles these pictures into detailed images of organs, bones, and other tissues. CT imaging is particularly useful because it can show several types of tissue-lung, bone, soft tissue, and blood vessels-with great clarity. Using specialized equipment and expertise to create and interpret CT scans of the body, radiologists can more easily diagnose problems such as cancers, cardiovascular disease, infectious disease, trauma, and musculoskeletal disorders. A CT scan of the body is a patient-friendly exam that involves little radiation exposure. During a CT scan, the person lies very still on a table. The table slowly passes through the center of a large x-ray machine. The person might hear loud clicking or whirring sounds during the procedure. People may be asked to hold their breath at times, because breathing or other movement can result in blurred pictures. In some cases, a contrast agent, or “dye,” may be given by mouth, injected into a vein, or given by enema before the CT scan is done. The contrast dye can highlight specific areas inside the body, resulting in a clearer picture.
Advantages of computed tomography
- Conventional radiographs depict a three dimensional object as a two dimensional image. Their main limitation is that overlying tissues are superimposed on the image.
- Computed tomography overcomes this problem by scanning thin slices of the body with a narrow x ray beam which rotates around the body, producing an image of each slice as a cross section of the body and showing each of the tissues in a 10 mm slice.
- Another limitation of the conventional radiograph is its inability to distinguish between two tissues with similar density, such as soft tissue and fluid.
- Computed tomography can differentiate between tissues of similar density because of the narrow x ray beam and the use of “windowing”.
Benefits vs Risks
- Unlike other imaging methods, CT scanning offers detailed views of many types of tissue, including the lungs, bones, soft tissues, and blood vessels.
- CT scanning is painless, noninvasive, and accurate. CT examinations are fast and simple. For example, in trauma cases, they can reveal internal injuries and bleeding quickly enough to help save lives.
- Diagnosis made with the assistance of CT can eliminate the need for invasive exploratory surgery and surgical biopsy. CT scanning can identify both normal and abnormal structures, making it a useful tool to guide radiotherapy, needle biopsies, and other minimally invasive procedures.
- CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems
- CT does involve exposure to radiation in the form of x-rays, but the benefit of an accurate diagnosis far outweighs the risk. The effective radiation dose from this procedure is about 10 mSv, which is about the same as the average person receives from background radiation in 3 years. See the Safety page for more information about radiation dose.
- Special care is taken during x-ray examinations to ensure maximum safety for the patient by shielding the abdomen and pelvis with a lead apron, with the exception of those examinations in which the abdomen and pelvis are being imaged. Women should always inform their doctor or x-ray technologist if there is any possibility that they are pregnant.
- Nursing mothers should wait for 24 hours after contrast injection before resuming breast feeding.
- The risk of serious allergic reaction to iodine-containing contrast material is rare, and radiology departments are well equipped to deal with them.
The technologist begins by positioning the patient on the CT table. The patient’s body may be supported by pillows to help hold it still and in the proper position during the scan. As the study proceeds, the table will move slowly into the CT scanner “doughnut.” Depending on the area of the body being examined, the increments of movement may be so small that they are almost undetectable, or large enough that the patient feels the sensation of motion. You should wear comfortable, loose-fitting clothing for your CT exam. Metal objects can affect the image, so avoid clothing with zippers and snaps. You may also be asked to remove hairpins, jewelry, eyeglasses, hearing aids, and any removable dental work, depending on the part of the body that is being scanned. You may be asked not to eat or drink anything for one or more hours before the exam. Women should always inform their doctor or x-ray technologist if there is any possibility that they are pregnant.
A CT examination often requires the use of different contrast materials to enhance the visibility of certain tissues or blood vessels. The contrast material may be injected through an IV directly into the blood stream, swallowed, or administered by enema, depending on the type of examination. Before administering the contrast material, the radiologist or technologist will ask whether the patient has any allergies, especially to medications or iodine, and whether the patient has a history of diabetes, asthma, a heart condition, kidney problems, or thyroid conditions. These conditions may indicate a higher risk of reaction to the contrast material or potential problems eliminating the material from the patient’s system after the exam. A CT examination usually takes from five minutes to half an hour. When the exam is over, the patient may be asked to wait until the images are examined to determine if more images are needed.
CT scanning causes no pain, and with spiral CT, the need to lie still for any length of time is reduced. For different parts of the body, the patient preparation will be different. You may be asked to swallow either water or a positive contrast material, a liquid that allows the radiologist to better see the stomach, small bowel, and colon. Some patients find the taste of the contrast material mildly unpleasant, but most can easily tolerate it. Your exam may require the administration of the material by enema if the colon is the focus of the study. You will experience a sense of abdominal fullness and may feel an increasing need to expel the liquid. Be patient; the mild discomfort will not last long.
You will be alone in the room during the scan; however, the technologist can see, hear, and speak with you at all times.
Some people may be concerned about the amount of radiation they receive during a CT scan. It is true that the radiation exposure from a CT scan can be slightly higher than from a regular x-ray. However, not having the procedure can be more risky than having it, especially if cancer is suspected. People considering CT must weigh the risks and benefits.