A CAT (computed axial tomography) scan is a painless test that uses multiple x-ray images, taken from different angles, to create three-dimensional images of body structures. It is sometimes called a CT scan. Increasingly, CAT scans use digital x-rays to produce their images on a computer screen. A cardiologist may order the test to get a clear picture of the heart, the aorta (the main artery carrying oxygen-rich blood from the heart to the rest of the body), the lungs and the coronary arteries.
In recent years, CAT technology has rapidly advanced, and researchers have developed new uses for the technology. For example, many facilities now use a type of CAT scan called a helical CAT scan, which uses special slip-ring technology to create a continuously moving x-ray tube. Another type of CAT scan is the multislice CAT scan, which creates up to 64 images and allows physicians to look more closely at intricate heart structures.
If the test requires an injection of a special dye (contrast medium) or a sedative, then patients are generally advised not to eat or drink for four to six hours before the test. Otherwise, no special preparation is required. The length of a CAT scan depends upon the needs of the physician and the type of scanner being used. In newer equipment, a CAT scan may take less than one minute (sometimes seconds). Other CAT scans may take as long as an hour.
About CAT scans
A CAT (computed axial tomography) scan or CT scan is a noninvasive or minimally invasive test that uses multiple x-ray images, taken from different angles, to create three-dimensional images of body structures. Increasingly, CAT scans use digital x-rays to produce their images on a computer screen.
Of all imaging tests that are available, the CAT scan is best able to produce images of bone, blood and soft tissues. It is often the test of choice when testing patients who have been injured (e.g., as the result of a car collision or sports injury).
A chest or thoracic CAT scan can give clear pictures of the heart and lungs. A cardiologist may order a chest CAT scan after an abnormality has been seen on a chest x-ray to assess for:
Damage to, or disease in, the heart, which could be a sign of many different cardiac problems.
Abnormalities in the aorta (the main artery carrying oxygen-rich blood from the heart to the rest of the body), which could be a sign of aortic dissection.
Fluid in the lungs (pulmonary edema), which could be a sign of heart failure. The CAT scan may also reveal signs of chronic obstructive pulmonary disease.
A CAT scan may also be on the head to determine the course of emergency treatment for a stroke. The head CAT scan allows the physician to determine whether a stroke is due to a cerebral hemorrhage (which will be seen immediately on the CAT scan) or cerebral ischemia (which can take up to 48 hours to be seen on the CAT scan). It may also help the physician locate the exact position of the damage.
In the late 1990s, a new form of CAT scan was developed called helical CAT scanning, also called spiral and volumetric CT. This new method uses special slip-ring technology to create a continuously moving x-ray tube, which eliminates the bulky, time-consuming cables of earlier CAT scanning machines. As a result, the entire scan can be taken in a minute. Although there are still earlier-generation CAT scan machines, helical CAT scans soon are expected to entirely replace them.
Helical CAT scans have several advantages over the earlier version. Because the test is performed so quickly, it reduces distortion caused by inevitable movement during the longer scans as the patient breathes. A helical CAT scan can frequently be taken during one breath-hold, which gives a much sharper picture.
In cardiology, helical CAT scans have been helpful in the detection of aortic dissections and pulmonary embolisms in most areas of the pulmonary artery leading to the lungs. They have also been used in the diagnosis of abdominal aortic aneurysms and problems with the arteries leading to the kidneys (renal arteries) or the head carotid arteries. Helical CAT scans are enabling emergency room physicians to more quickly evaluate acute stroke (e.g., ischemic, hemorrhagic) with increased accuracy in identifying the site, nature and extent of blockages.
Many medical facilities have recently started using a new type of CAT scan known as a multislice CAT scan. These scans are capable of providing up to 64 image slices in a single test. This type of CAT scan can show the heart with a greater degree of clarity, making it possible to visualize and evaluate smaller heart structures such as the coronary arteries. These multislice scans are becoming increasing popular in medical facilities because of their improved accuracy.
Other types of CT scans that may be used to help diagnose heart conditions include:
Coronary multidetector computed tomography (MCT). This scan is sometimes used for patients who are admitted to hospital for chest pain. It is used to look at the patient’s arteries to determine whether there are signs of plaque buildup or stenosis (narrowing of the arteries), which may indicate a heart attack or unstable angina. The scan takes as little as 15 seconds to perform and may reduce the need for more invasive diagnostic tests, such as coronary angiograms, in some patients.
Electron-beam computed tomography (EBCT). This type of CT scan is used to look at aspects of the heart such as congenital defects and the efficiency of the heart muscle. It can also be used to look at calcium deposits in the coronary arteries, which is directly related to the occurrence of atherosclerosis.
Cardiac Positron Emission Tomography (PET). This is a minimally invasive type of CT scan involving a radioactive substance, which is injected into the patient. This substance allows the imaging machine to view how the heart is working and whether parts of it may be damaged.
Before, during and after the test
No special preparation is required for a CAT scan, unless the physician wishes to use an injection of a special dye contrast medium and/or a sedative before the test. If an injection will be given, then patients are generally asked not to eat or drink for about four to six hours before the test.
Immediately before the test, patients will change into a hospital gown, remove any watches or jewelry and sign a consent form. The dye will be administered, if necessary, into the patient’s vein. Injection of the dye may produce a warm, flushed feeling for several minutes. This is normal and will soon pass.
The patient will be asked to lie down on a scanning table, which slides slowly into a short, open-ended tunnel. Speakers and a microphone are present to allow the patient to communicate with the technician who is conducting the test. The patient will be asked to lie very still, and may be asked to hold their breath for short periods of time, as the machine takes the CAT scans.
In a conventional CAT scan, the x-ray “sensing unit” and detectors rotates around the body part that is being scanned, while the table moves by a fraction of an inch between scans. Patients are urged to speak with the technician if they are feeling uncomfortable about the sounds made by the machine or any other aspect of the test. Depending on the kind of CAT scan, it may take anywhere between one minute to an hour.
If a sedative was given, then patients are generally advised not to drive themselves from the testing site. Otherwise, patients can return to their regular daily routines. If they were given an injection and the injection site remains sore, they are encouraged to call their physician.
A computer is used to assemble the cross-sectional CAT scans into one three-dimensional image. A radiologist will interpret each of the final three-dimensional images and then send them to the patient’s cardiologist, who will go over the results with the patient at a follow-up appointment. Additional tests or a treatment plan will be designed accordingly.
Questions for your doctor
Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to CAT scans:
What, specifically, would this test be checking for in my case?
What type of results would you expect? What type of results would you consider abnormal?
Where would the test be conducted?
Are there any risks involved with this type of test that I should be aware of?
Can this test alone provide you with a diagnosis, or will other tests need to be performed as well?
Do I have any conditions or characteristics that may influence the results of this test in any way?
How will I be informed of the test results? What if I have questions about the results?
How long will it take before I find out the test results?
Can this test still be performed if I am pregnant? Could pregnancy influence the results?
