A CAT (computed axial tomography) scan or CT scan is a noninvasive imaging  test that uses a rotating x-ray device to create detailed cross-sectional images (or slices) of different body parts. It may be performed alone or with the use of a special dye (contrast medium).

CAT scans are used to diagnose a number of injuries, conditions and diseases, including those that affect the digestive system. Some of these conditions include appendicitis, pancreatitis, gallstones and other disorders affecting the pancreas, liver, colon and rectum.

Patients are urged to follow all preparatory steps recommended by their physician prior to a CAT scan. The test is generally performed by a radiology technician in a hospital or outpatient clinic.  

If a contrast medium will be used, it can be administered orally (by mouth), through an enema or intravenously (injected into a vein) before the scan. CAT scans are typically painless. However, patients may experience a stinging sensation or slight discomfort if a contrast medium is administered intravenously. During the test, the patient lies down on a scanning table, which slides slowly into a short, open-ended tunnel.

The length of a CAT scan varies from a few minutes to an hour, depending on the type of equipment used and the requests of the physician. Patients may usually resume normal activities after the test.

In the 1990s, a new form of CAT scan was developed called a spiral CAT scan (also called a helical CAT scan or volumetric CT). These CAT scans have several advantages over earlier types. They can be used to create three-dimensional images of body areas, and can be performed in less time than earlier tests. Another type of CAT scan that is sometimes used is a combined PET/CAT scan. This combines CAT scan with another imaging procedure called positron emission tomography (PET) scan.

There are some risks associated with CAT scans. Radiation levels used with CAT scans are higher than those used with x-rays. In rare cases, the contrast medium used during a CAT scan may cause an allergic reaction. In addition, CAT scan of the chest or abdomen is not typically recommended for women who are or might be pregnant.

Unlike other types of imaging tests, such as magnetic resonance imaging (MRI), people with pacemakers or defibrillators (devices implanted in the chest to regulate the heart) can receive CAT scans without experiencing difficulties.

A CAT (computed axial tomography) scan, or CT scan, is a noninvasive imaging test that uses multiple x-ray images, taken from different angles, to create three-dimensional images of body structures. It may be performed alone or with the use of a special dye (contrast medium). CAT scans often employ digital x-rays to produce images on a computer screen.

CAT scans are used to diagnose numerous injuries, conditions and diseases, including those that affect the digestive system. In particular, CAT scans are used to examine damage or inflammation to organs such as the pancreas, liver, rectum and colon.

They can also be used to determine the progress of a previously diagnosed disease. Problems that may be detected with a CAT scan include:

• Internal injuries or bleeding
• Muscle and bone disorders, such as osteoporosis
• Tumors, infections or blood clots
• Diseases, such as cancer or cirrhosis
  

  

CAT 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 currently available, the CAT scan is best able to produce images of bone, blood vessels and soft tissues. Therefore, 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 abdomen, including the liver, kidney, spleen, pancreas and other abdominal organs
  
  
• The rectum
  
  
• The chest (thoracic CAT scan), including the heart, aorta and lungs
  
  
• The head, including the brain, vessels of the brain, eyes, inner ear and sinuses
  
  
• The back, including the entire spinal column
  
  
• The neck, including the shoulders, cervical spine and blood vessels of the neck
  
  
• The pelvis and hips, including the bladder and gastrointestinal tract
  
  
• The skeletal system, including bones of the hands, wrists, feet, ankles, legs, arms and jaw

An increasing number of people in the United States are choosing to have preventive CAT scans performed on an annual basis. These full-body CAT scans are designed to detect certain diseases in very early stages, but the value of these tests is considered highly controversial among medical experts.

Critics contend that the scans will mostly detect conditions that are ultimately benign. Another concern is that individuals may think a preventive scan is a suitable alternative to regular physical examinations with a primary care physician.

It has been suggested that preventive CAT scans be reserved for higher risk individuals, as opposed to healthy, asymptomatic (without symptoms) people younger than age 40 without a family history of heart disease, cancer or other diseases.

Because a CAT scan involves x-rays, pregnant women should not have abdominal or chest CAT scans. If the use of a special dye (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 or shrimp)
• Strawberries
  

Research indicates that milk may be nearly as effective as some contrast agents (e.g., barium) for abdominal CAT scans. In the future, this may prove a less expensive, safer and more palatable alternative for some patients.

CAT (computed axial tomography) scans can be used to identify the causes of various digestive symptoms including abdominal pain, difficulty swallowing (dysphagia) and vomiting. They may be used to diagnose the following conditions that affect the digestive system:

• Appendicitis. A condition in which a small, finger-shaped pouch attached to the large intestinelarge intestine called the appendix becomes blocked or inflamed. The cross-sectional images that result from a CAT acan can help distinguish whether appendicitis or another abdominal condition causing similar symptoms is present.
  
  
• Cirrhosis of the liver. Scarring of the liver caused by disease. CAT scans can reveal damage to or inflammation of the liver.
  
  
• Colon disorders. The colon is the part of the large intestine that extends from the cecum to the rectum. CAT scans can help reveal the location of obstruction inside the colon. In addition, CAT scan technology can be used to perform a virtual colonoscopy, in which two- and three-dimensional images of the colon are created.
  
  
• Crohn's disease. A type of inflammatory bowel disease (IBD) that causes inflammation anywhere in the intestines. A CAT scan can be used to help detect an abscess (localized collection of pus in the body), obstruction or fistula (abnormal passage from a hollow organ to the body surface, or from one organ to another) in the abdomen of patients with this condition.
  
  
• Diverticulitis.  Occurs when diverticula or small, abnormal sacs in the large intestine become inflamed and infected.
  
  
• Gallstones. These form when bile stored in the gallbladder hardens into pieces of stone-like material. Gallstones can cause health problems such as pancreatitis and may need to be surgically removed. Ultrasound is more commonly performed to diagnose gallstones, but a CAT scan may be performed for potential problems in the pancreas due to gallstones.
  
  

  

• Internal fistula. An abnormal passageway that develops between two areas of the digestive tract. It can develop between two areas of intestine or between an intestine and a separate organ.
  
  
• Intestinal ischemia. Reduction in blood flow to the colon or small intestine. CAT scans can help diagnose this condition, particularly when an obstruction of a vein is the suspected cause of the condition.
  
  

  

• Intussusception. A condition in which one section of the small bowel or colon tunnels into an adjoining section.
  
  
• Pancreatitis. Inflammation of the pancreas, a gland that secretes digestive enzymes into the small intestine. These enzymes help digest fats, proteins and carbohydrates. Pancreatitis can be acute (short-term) or chronic (long-term) in duration. CAT scans can reveal damage to and inflammation of the pancreas. These tests can also reveal the presence of pseudocysts, which are accumulations of fluid and tissue debris.
  
  

  
  

• Volvulus. A condition in which a portion of the intestine twists around itself. It occurs most often in the colon, but also can occur in the stomach or the small bowel. CAT scans can reveal the degree of twisting and failure of normal rotation (malrotation) that is present, as well as whether a hole (perforation) has appeared in the colon.
  
  
• Cancer. CAT scans are often used to search for tumors and other abnormalities that may indicate cancer of the digestive tract, such as cancer of the pancreas, liver, colon or rectum.

In the 1990s, a new form of CAT (computed axial tomography) scan was developed called a spiral CAT scan (also called a helical CAT scan or 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 scan machines. 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 traditional CAT scans. 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 than traditional CAT scans. This reduces distortion caused by movement as a patient breathes, which is inevitable during longer scans. A helical CAT scan can often be performed while the patient holds a single breath, which limits patient movement and delivers a sharper picture.

The helical 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.

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 can be used to diagnose colon and bowel disease. It can be performed with a CAT scan or magnetic resonance imaging (MRI).

CAT (computed axial tomography) scans are generally performed by a radiology technician in a hospital or outpatient clinic. Patients are urged follow all preparatory steps recommended by their physician prior to the test. 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 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 mediums may be administered orally (by mouth), through an enema or intravenously (injected into a vein). Although CAT scans are typically painless, patients may experience a stinging sensation or slight discomfort if a contrast medium is administered intravenously. Additionally, injection of the dye may produce a warm, flushed feeling for several minutes or a metallic taste in the mouth. Both sensations are normal and disappear quickly.

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 CAT scan of the head, the table may be fitted with a special cradle that holds the head.

Parents of infants or children receiving a CAT scan are generally 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. Patients will be asked to lie very still, and may be asked to hold their breath for short periods of time as the machine captures the images.

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 resume their regular daily routines immediately following the test. If contrast mediums were administered through an injection and the injection site remains sore, patients should notify their physician.

Radiation levels used with CAT (computed axial tomography) scans 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 special 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.

Patients should also inform the physician of the following before the test:

• Pregnancy. Another exam will likely be substituted to reduce the risk of exposing the fetus to radiation.
  
  
• Asthma or allergies. Contrast mediums can produce allergic reactions in some people, especially those with a history of asthma or certain allergies (e.g., iodine, shellfish, strawberries).
  
  
• Other medical conditions. Certain medical conditions also may increase the likelihood of an allergic reaction to contrast mediums. These include:
  
• • Diabetes
    
  • Heart disease
    
  • Kidney problems
    
  • Thyroid conditions

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 CAT (computed axial tomography) scan-related questions:

1. Why do you recommend that I have a CAT scan?
   
   
2. Why is a CAT scan better for me than another type of test, such as magnetic resonance imaging (MRI)?
   
   
3. What type of CAT scan will I have?
   
   
4. Can you explain the procedure to me in detail?
   
   
5. What information will my CAT scan provide?
   
   
6. Will the CAT scan cause me any pain or discomfort?
   
   
7. How should I prepare for my CAT scan?
   
   
8. Will I need to have a contrast medium injected into me prior to the CAT scan?
   
   
9. Why is it important for me to remain still during the CAT scan?
   
   
10. What follow-up tests will I have to have after the CAT scan?