A CAT scan (computed axial tomography) is a noninvasive or minimally invasive test that uses a rotating x-ray device to create computerized, detailed, cross-sectional images of body parts. It may be performed alone or with the use of a dye (contrast medium).
CAT scans, also called CT scans, are used to diagnose a number of injuries, conditions and diseases, including internal injuries, heart conditions, cancer and many causes of joint pain.
CAT scans are generally performed by a radiology technician in a hospital or outpatient clinic. During the test, the patient lies down on a scanning table, which slides slowly into a short, open-ended tunnel. If a contrast medium is used, it can be administered by injection, by mouth or through an enema. The test is painless except for brief discomfort that may be felt if an injection is used.
The length of a CAT scan varies from a few minutes to an hour, depending on the type of equipment used and the condition and body part for which it is used. Patients usually have to remain still for most of the time during a CAT scan. Patients may usually return to normal activities after the test.
About CAT scan
A CAT scan (computed axial tomography) is an imaging test that uses multiple x-ray images, taken from different angles, to create three-dimensional images of body structures. It is generally noninvasive and painless but is considered minimally invasive if a dye (contrast medium) is injected into a vein. CAT scans often employ digital x-rays to produce images on a computer screen.
CAT scans, also known as CT scans, are used to diagnose numerous injuries, conditions and diseases. They can also be used to determine the progress of a previously diagnosed disease. A CAT scan can be used in diagnosing conditions including:
Cancer and benign tumors
Cardiovascular conditions, including heart disease, stroke and blood clots
Brain problems such as head injury or epilepsy
Aspects of trauma pain, including fractures and internal injuries or bleeding
Muscle and bone disorders, such as osteoporosis
Sources of neck pain, such as whiplash
Sources of back pain, including herniated disc, sciatica and spinal stenosis
Sources of chest pain, such as sarcoidosis
Sources of abdominal pain or pelvic pain, such as appendicitis, kidney stones or colon obstruction
Sources of leg pain, such as shin splints
Causes of shoulder pain, arm pain, knee pain and other joint pain
Arthritis, including osteoarthritis, ankylosing spondylitis, juvenile arthritis and juvenile rheumatoid arthritis
Infectious diseases including encephalitis, meningitis and Lyme disease
Pinched nerves
Gait disturbances
Causes of dental pain and orofacial pain
CAT scans may be used to rule out conditions, such as a brain tumor in someone suffering from headaches. They scans may also be used to plan radiation treatment for tumors, to guide biopsies and to plan surgery.
Of all the imaging tests that are available, the CAT scan is often best able to produce images of bone, blood vessels and soft tissues. It is generally the test of choice when examining patients who have been injured (e.g., as the result of a car collision or sports injury).
A CAT scan can provide detailed images of nearly every part of the body, including:
The chest (thoracic CAT scan), including the heart, aorta and lungs
The abdomen, including the appendix, liver, kidneys, spleen, pancreas and other abdominal organs
The head, including the brain, vessels of the brain, eyes, inner ear and sinuses
The back, including the spinal column
The neck, including the cervical spine and blood vessels of the neck
The shoulders, including the rotator cuff
The pelvis and hips, including the bladder and gastrointestinal tract
The skeletal system, including bones of the hands, feet, ankles, legs, arms and jaw
An increasing number of people choose to have annual preventive CAT scans. These full-body scans are designed to detect certain diseases in early stages, but the value of these tests is highly controversial among medical experts. There is no validity yet in doing these kinds of scans, which are offered by nonmedical personnel.
Critics contend that the scans will detect ultimately benign conditions and require a series of follow-up tests resulting in a loss of time and money. Another concern is that individuals may think a preventive scan is a suitable alternative to regular physical examinations with a primary-care physician. Researchers have cautioned about an increased risk of cancer in patients exposed to radiation from annual CAT scans, such as people with cystic fibrosis. However, annual CAT scans may be recommended for some patients, such as thoracic scans for smokers or others at increased risk for lung cancer.
Because a CAT scan involves x-rays, pregnant women are advised not to have abdominal or chest CAT scans. If the use of contrast medium is recommended during the test, then individuals should report any history of blood-clotting disorders or allergic reactions to any of the following:
Iodine
Shellfish (e.g., crab, shrimp)
Strawberries
Unlike some other types of imaging tests, such as MRI (magnetic resonance imaging), people with pacemakers or defibrillators (devices implanted in the chest to regulate the heart) can get CAT scans without experiencing difficulties.
Types and differences of CAT scan
Many medical facilities are using a type of CAT scan (computed axial tomography) called a multislice CAT scan. Providing up to 64 image slices in one test, they reveal internal structures in greater detail than earlier CAT scans could.
In the 1990s, a form of CAT scan was developed called a spiral CAT scan (also called a helical CAT scan or volumetric CT). This 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 one minute. Although some facilities still use earlier-generation CAT scan machines, spiral CAT scans are expected to replace them soon.
Spiral CAT scans have several advantages over earlier machines. For example, because the machine rotates in a continual spiral path it can be used to create three-dimensional images of body areas. The test can also be performed in less time. This reduces distortion caused by movement as a patient breathes, which is inevitable during longer scans. A spiral CAT scan can often be performed while the patient holds a single breath, which limits movement and delivers a sharper picture.
The spiral CAT scan also produces results much faster. Results may even be available before the patient leaves the testing site. In addition, the test’s high level of sensitivity makes it a particularly useful tool in diagnosing small cancerous tumors and blood clots in the lungs. Spiral CT angiography provides a noninvasive alternative to conventional cardiac angiography, an x-ray procedure that requires a catheter to be inserted into a blood vessel and threaded up to the heart.
A new technique known as virtual endoscopy uses a small lighted tube (endoscope) during a spiral CAT scan to allow physicians to see inside organs or other body structures without surgery or special instruments. A virtual colonoscopy (also known as a CAT colonography) produces images of the colon and is used to diagnose colon and bowel disease. It can be performed with a CAT scan or an MRI.
Another type of CAT scan that is sometimes used is a combined PET/CAT scan. This combines CAT scans with another imaging procedure called positron emission tomography (PET) scan. The PET scan creates color pictures of chemical changes that occur in tissues, a process known as metabolic activity. PET scans are most often used to detect cancer or evaluate signs of coronary artery disease. Cancerous tumors are more active than normal tissues, so they appear as different colors on the PET scan. This technology, however, is available at only a limited number of facilities.
Ultrafast computed tomography is a technique used in cardiology. Ultrafast CT, also called electron-beam CT, can help detect calcium deposits in the coronary arteries and other signs of heart disease, but this innovation is still being refined.
There is also a nuclear medicine technique called a SPECT scan (single-photon emission computed tomography).
Before, during and after the CAT scan
Immediately before the test, patients may be asked to change into a hospital gown. All metallic objects, such as watches and jewelry, should be removed so they do not interfere with test results. Infants or children receiving the test may be given a sedative to ensure they are relaxed and still during the test.
If the CAT (computed axial tomography) scan is being performed without the use of a dye (contrast medium), no special preparation is needed. If a contrast medium will be used, patients are instructed not to eat or drink for about four to six hours before the test. A contrast medium contains an iodine liquid that makes certain organs and tissues in the body more visible on a CAT scan. This highlights the structure of a specific organ or vessel to better show the presence of disease or injury.
Contrast media may be administered orally (by mouth), through an enema or by injection into a vein. Injection of the dye may produce a warm, flushed feeling for several minutes or a metallic taste. Both sensations are normal and disappear quickly.
CAT scans are generally performed by a radiology technician in a hospital or outpatient clinic. During the test, the patient is asked to lie down on a scanning table, which slides slowly into a short, open-ended tunnel. The table can be raised, lowered or tilted. Straps and pillows may be used to keep the patient in a fixed position. During a head CAT scan, the table may be fitted with a special cradle that holds the head.
Parents with infants or children receiving a CAT scan should be permitted to remain with the child during the test. Parents accompanying children may be asked to wear a lead apron to prevent exposure to radiation.
Speakers and a microphone are present to allow the patient to communicate with the technician conducting the test. The patient will be asked to lie still, and may be asked to hold the breath for short periods of time, as the machine takes the CAT scans. At a few places patients can bring their own recorded music and listen to that during the CAT scan.
In a conventional CAT scan, the x-ray sensing unit and detectors rotate around the body part 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 type of CAT scan performed, the procedure may take anywhere between 10 minutes to an hour.
If a sedative is given, patients are generally advised not to drive themselves from the testing site. Otherwise, patients can generally return to their regular daily routines. If contrast media were administered through an injection and the injection site remains sore, patients should notify their physician.
Potential risks with CAT scan
Radiation levels used with CAT scans (computed axial tomography) are higher than those used with x-rays. Although some patients are concerned about radiation exposure from a CAT scan, they should weigh the potential risks against the potentially life-saving benefits of the information obtained from the test.
In rare cases, the dye (contrast medium) used during a CAT scan may cause an allergic reaction in the form of mild itching or hives (small, raised, reddened areas of skin). In more severe reactions, shortness of breath and swelling of the throat or other body areas may occur. Patients who notice any of these symptoms developing during the procedure should immediately alert the technician performing the test.
Kidney function tests are performed before giving dye to patients. In rare cases, dye is contraindicated if kidney function is severely compromised.
Patients should also inform their physician of the following:
Pregnancy. Another diagnostic test will likely be substituted to reduce the risk of exposing the fetus to radiation.
Asthma or allergies. Contrast media can produce allergic reactions in some people, especially those with a history of asthma or allergies.
Other medical conditions. Certain medical conditions also may increase the likelihood of side effects to contrast media. These include:
Diabetes
Heart disease
Kidney problems
Thyroid conditions
In some situations CAT scans may not be very reliable (e.g., back pain). Some patients with back pain may have normal results from a CAT scan, and there are patients whose CAT scan shows changes of the spine but who have no back pain at all.
Questions for your doctor regarding CAT scan
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about CAT scans:
Why is a CAT scan being recommended for me?
Which of my conditions or suspected conditions can be diagnosed with a CAT scan?
What information about me will a CAT scan provide you?
Will getting a CAT scan cause me discomfort?
How should I prepare for a CAT scan?
Will my CAT scan involve a contrast medium?
Why is it important for me to remain still during a CAT scan?
Why should I get a CAT scan over another type of test, such as an MRI?
Where and when is my test scheduled?
When will I get the results, and who will explain them to me?
What do my test results show?
If my CAT scan shows I need treatment, what are my options, and which do you recommend?
Should I get an annual preventive CAT scan, or do you advise against this?
