Introduction

Computed Tomography (CT or CAT) scans take the idea of conventional X-ray imaging to a new level. Instead of finding the outline of bones and organs, a CT machine forms a three-dimensional computer model of patients' insides. Doctors can use CT scans to examine the body one narrow slice at a time to pinpoint specific areas.

The Basic Idea

CT scan machines produce X-rays, a powerful form of electromagnetic energy. X-ray photons are basically the same thing as visible light photons, but they have much more energy. This higher energy level allows X-ray beams to pass straight through most of the soft material in the human body.

A conventional X-ray image is basically a shadow: You shine a light on one side of the body, and a piece of film on the other side registers the silhouette of the bones. In a CT scan machine, the X-ray beam moves all around the patient, scanning from hundreds of different angles. The computer takes all this information and puts together a 3-D image of the body.

Scanning Procedure

The CT machine looks like a giant donut tipped on its side. The patient lies down on a platform, which slowly moves through the hole in the machine. The X-ray tube is mounted on a movable ring around the edges of the hole. The ring also supports an array of X-ray detectors directly opposite the X-ray tube. A motor turns the ring so that the X-ray tube and the X-ray detectors revolve around the body (in an alternative design, the tube remains stationary and the X-ray beam is bounced off a revolving reflector). Each full revolution scans a narrow, horizontal "slice" of the body. The control system moves the platform farther into the hole so the tube and detectors can scan the next slice.

Photo courtesy Department of Defense
Doctors usually operate CT scan machines from a
separate room so they aren't repeatedly exposed to radiation.

In this way, the machine records X-ray slices across the body in a spiral motion. The computer varies the intensity of the X-rays in order to scan each type of tissue with the optimum power. After the patient passes through the machine, the computer combines all the information from each scan to form a detailed image of the body. It's not usually necessary to scan the entire body, of course. More often, doctors will scan only a small section.

Since they examine the body slice by slice from all angles, CT scans are much more comprehensive than conventional X-rays. Today, doctors use CT scans to diagnose and treat a wide variety of ailments, including head trauma, cancer and osteoporosis. They are an invaluable tool in modern medicine.

What are some common uses of the procedure?

Because it provides detailed, cross-sectional views of all types of tissue, CT is one of the best tools for studying the chest and abdomen. It is often the preferred method for diagnosing many different cancers, including lung, liver and pancreatic cancer, since the image allows a physician to confirm the presence of a tumor and measure its size, precise location and the extent of the tumor's involvement with other nearby tissue. CT examinations are often used to plan and properly administer radiation treatments for tumors, to guide biopsies and other minimally invasive procedures and to plan surgery. CT can clearly show even very small bones, as well as surrounding tissues such as muscle and blood vessels. This makes it invaluable in diagnosing and treating spinal problems and injuries to the hands, feet and other skeletal structures. CT images can also be used to measure bone mineral density for the detection of osteoporosis. In cases of trauma, CT can quickly identify injuries to the liver, spleen, kidneys or other internal organs. Many dedicated shock-trauma centers have a CT scanner in the emergency room. CT can also play a significant role in the detection, diagnosis and treatment of vascular diseases that can lead to stroke, kidney failure, or even death.

How is the CT scan performed?

A scanned liver slice
Photo courtesy NASA

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.

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 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 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.

Commonly, a contrast material is injected into a vein to better define the blood vessels and kidneys, and to accentuate the appearance between normal and abnormal tissue in organs like the liver and spleen. Some people report feeling a flush of heat and sometimes a metallic taste in the back of the mouth. These sensations usually disappear within a minute or two. Some people experience a mild itching sensation. If it persists or is accompanied by hives (small bumps on the skin), the itch can be treated easily with medication. In very rare cases, a patient may become short of breath or experience swelling in the throat or other parts of the body. These can be indications of a more serious reaction to the contrast material that should be treated promptly, so tell the technologist immediately if you experience these symptoms. Fortunately, with the safety of the newest contrast materials, these adverse effects are very rare.

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

Benefits

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.

Risks

CT does involve exposure to radiation in the form of x-rays, but the benefit of an accurate diagnosis far outweighs the risk.

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.