What is a CT scan?
Computed tomography (also known as CT or CAT scanning) uses special equipment to obtain multiple cross-sectional images of the organs and tissues. CT produces images that are far more detailed than a conventional x-ray. CT is especially useful because it can simultaneously show many different types of tissue, including the lungs, heart, bones, soft tissues, muscle and blood vessels. Modern CT scans utilize a method called spiral (or helical) CT that captures images of the chest from many angles and, with the aid of a computer, processes the images to create cross-sectional pictures or "slices" of the area of interest. The images can then be printed out or examined on a monitor.

CT scanning is considered to be a "patient-friendly" procedure because it involves relatively low radiation exposure and is not invasive. Contrast material is sometimes injected through a vein to make the blood vessels and soft-tissue masses stand out. During the CT examination patients usually lie flat on their back, but occasionally they may be asked to lie on their side or on their stomach. Patients are periodically asked to hold their breath. No preparation is required prior to the exam. Patients generally do not require a sedative to remain calm.

What are some common uses of the procedure?

ABDOMEN AND PELVIS

Because it is a non-invasive procedure that provides detailed, cross-sectional views of all types of tissue, CT is becoming the preferred method for diagnosing many diseases of the bowel and colon, including diverticulitis and appendicitis, and for visualizing the liver, spleen, pancreas and kidneys. In cases of acute abdominal distress CT can quickly identify the source of pain. When pain is caused by infection and inflammation, the speed, ease and accuracy of a CT examination can reduce the risk of serious complications caused by a burst appendix or ruptured diverticulum and the subsequent spread of infection. In cases where bowel obstruction is suspected, CT may be the best imaging test.

CT is often the preferred method for diagnosing many different cancers, including colon cancer, since the image allows a physician to confirm the presence of a tumor and to measure its size, precise location and the extent of the tumor's involvement with nearby tissue. CT examinations of the lower GI tract can be used to plan and properly administer radiation treatments for tumors and to guide biopsies and other minimally invasive procedures. Many dedicated shock-trauma centers have a CT scanner in the trauma department. CT can also play a significant role in the detection, diagnosis and treatment of vascular disorders that can lead to stroke, gangrene or kidney failure.

CHEST

CT of the chest is used to take a closer look at findings detected on conventional chest x-rays or may be used to investigate and try to explain clinical signs or symptoms of disease of the chest. The CT examination may provide more specific information regarding the nature and extent of the findings or, in some cases, determine that the chest is normal.

CT may be used to detect and evaluate the extent of tumors that arise in the lung and mediastinum, or tumors that have spread there from other parts of the body. CT is routinely used to assess whether tumors are responding to treatment.

You may have heard that, in recent years, some people have chosen to have a chest CT scan to screen for lung cancer. This makes the most sense for those who are former or current cigarette smokers, as they are at much greater risk of cancer than are nonsmokers. The best hope of curing lung cancer is to find it as early as possible, making it easier to treat. CT is able to detect even very small abnormalities that could be early lung cancer, which would not be visible on a conventional chest x-ray. A special low-dose CT technique is used for lung cancer screening. CT of the chest is not able to detect every cancer.

Chest CT also can demonstrate other lung disorders, such as old or new pneumonia, tuberculosis, emphysema, bronchiectasis, and diffuse interstitial lung disease. When the clinical findings and regular chest x-ray are inconclusive, CT may clarify the situation. Inflammation or other diseases of the pleura, the membrane covering the lungs, can be seen in CT images. High-resolution CT (HRCT) may be used for further evaluation. This uses thinner slices with possible expiration and prone views.

A CT angiogram (CTA) may be performed to evaluate the blood vessels (arteries and veins) in the chest. This involves injecting the iodine into a vein a little faster, and also, more numerous and thinner slices are obtained through the chest in order to see the arteries to better advantage.

SINUS

The paranasal sinuses are hollow, air-filled spaces located within the bones of the face and surrounding the nasal cavity. The nasal cavity is a system of air channels connecting the nose with the back of the throat. There are four pairs of sinuses, each connected to the nasal cavity by small openings. The paranasal sinuses are lined with a special tissue called a nasal mucous membrane that secretes mucus to keep the nose and sinuses moist and thus to humidify the air passing toward the lungs. Normally, air passes in and out of the sinuses and mucous drains from the sinuses into the nose.

Computed tomography (CT) can provide important information about sinus and nasal structure. CT is able to detect sinuses that are filled with fluid or sinus membranes that are thickened. It can assist your physician in diagnosing sinusitis, which can be due to either infection or inflammation in one or more of the sinuses. This condition may be acute (sudden and short), chronic (ongoing and long-term) or recurring (repeated). Among the causes of sinusitis are allergies, upper respiratory infection and an obstruction in the nose. When the nasal mucous membrane swells, the opening from the sinuses to the nasal cavity can become blocked. Fluids that would normally drain into the nose become trapped in the sinuses and infection can set in. Pressure, pain, headache, fever and tenderness in the sinus area can result.

CT may also be used to define the anatomy before surgery or to give further information about certain tumors of the nasal cavity and sinuses.

HEAD

• Detection of bleeding, brain damage and skull fractures in patients with head injuries.
• Detecting a blood clot or bleeding within the brain shortly after a patient exhibits symptoms of a stroke.
• Detection of stroke, especially with a new technique called Perfusion CT.
• Evaluation of the extent of bone and soft tissue damage in patients with facial trauma, and planning surgical reconstruction.
• Detection of bleeding in a patient with a sudden severe headache who may have a ruptured or leaking aneurysm.
• Detection of most brain tumors.
• Diagnosing diseases of the temporal bone on the side of the skull, which may be causing hearing problems.
• Detection of enlarged brain cavities (ventricles) in patients with hydrocephalus.
• Determining whether inflammation or other changes are present in the paranasal sinuses.
• Planning radiation therapy for cancer of the brain or other tissues.
• Guiding the passage of a needle used to obtain a tissue sample (biopsy) from the brain.
• Non-invasive assessment of aneurysms or arteriovenous malformations through a technique called CT angiography.
• Detecting diseases or malformations of the skull.
• Three-dimensional imaging of the skull and brain structures.

How should I prepare for the CAT scan?
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 be asked to remove hairpins, jewelry, eyeglasses, hearing aids and any removable dental work that could obscure the images. You also may be asked to refrain from eating or drinking anything for an hour or longer before the exam. Women should always inform their doctor or x-ray technologist if there is any possibility that they are pregnant. Please contact the office for proper prep.

What does the equipment look like?
The CT scanner is a large, square machine with a hole in the center, something like a doughnut. The patient lies still on a table that can move up or down and slide into and out from the center of the hole. Within the machine an x-ray tube on a rotating gantry moves around the patient's body to produce the images, making clicking and whirring noises as the arm moves. Though the technologist will be able to see and speak to you, you will be alone in the room during the exam.

How does the procedure work?
In many ways CT scanning works very much like other x-ray examinations. Very small, controlled amounts of x-ray radiation are passed through the body while different tissues absorb the radiation at different rates. With plain radiology, an image of the inside of the body is captured when special film is exposed to the absorbed x-rays. With CT, the film is replaced by an array of detectors that measure the x-ray profile.

Inside the CT scanner is a rotating gantry that has an x-ray tube mounted on one side and an arc-shaped detector mounted on the opposite side. During each full rotation, as the fan-shaped x-ray beam is emitted through the patient's body, an image of a thin section is acquired. The detector records about 1,000 images—or profiles—of the expanded x-ray beam with each rotation. The profiles are then reconstructed by a dedicated computer into two-dimensional images of the sections that were scanned. Multiple computers are typically used to control the entire CT system.

You might think of it as looking into a loaf of bread by cutting the bread into thin slices. When the image slices are reassembled by computer the result is a very detailed, multidimensional view of the body's interior.

With spiral—or helical—CT, refinements in detector technology support faster, higher-quality image acquisition with less radiation exposure. The current spiral CT scans are called multidetector CT and are most commonly two- to 16-slice systems. The multidetector CT also allows applications like CT angiography to be more successful.

With conventional CT, small lesions may frequently go undetected when a patient breathes differently on consecutive scans, as a lesion may be missed by unequal spacing between scans. The speed of spiral scanning and a single breath hold increase the rate of lesion detection.

How is the CAT scan performed?
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. 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.

A CT examination of the gastrointestinal tract requires the use of a contrast material (this may be barium, iodine or in some cases water) to enhance the visibility of certain tissues. The contrast material may be swallowed. Under certain circumstances, as when evaluating blood vessels, you will have contrast material injected into a vein shortly before scanning begins. If this is necessary, you will be asked whether you have any allergies to medications or iodine (which is a part of many contrast materials) and whether you have a history of asthma, diabetes, a heart disorder, multiple myeloma or kidney disease. These conditions may indicate a greater risk of an adverse reaction to contrast material. Kidney disease makes it harder to eliminate contrast material from the bloodstream.

A CT examination usually takes 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.

What will I experience during the procedure?
CT scanning causes no pain, and with spiral CT the need to lie still for any length of time is reduced. For examinations of the abdomen and lower gastrointestinal tract 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. Many patients also receive iodine intravenously (injected into a vein) to help evaluate blood vessels and organs such as the liver, kidneys and pancreas.

You will be alone in the room during the scan; however, the technologist can see, hear and speak with you at all times. With pediatric patients, a parent may be allowed in the room to alleviate fear but will be required to wear a lead apron to prevent radiation exposure.

Who interprets the results and how do I get them?
radiologist, who is a physician experienced in CT and other radiologic examinations, will analyze the images and send a signed report with his or her interpretation to the patient's primary care physician. The physician's office will inform the patient on how to obtain their results. New technology also allows for distribution of diagnostic reports and referral images over the Internet at some facilities.

Copyright © 2005
Radiological Society of North America, Inc. (RSNA)

Copyrighted by either the Radiological Society of North America (RSNA), 820 Jorie Boulevard, Oak Brook, IL 60523-2251 or the American College of Radiology (ACR), 1891 Preston White Drive, Reston, VA 20191-4397.