PET/CT FUSION TECHNOLOGY: — Combination of Imaging Shows More than Either Method Alone

Computed tomography (CT),— often called CAT scan, is a familiar and widely used medical imaging procedure used to show detailed images of body structures including bone and soft tissue. Positron emission tomography (PET), a newer type of imaging, provides information about how well the body's organs and soft tissues are functioning. When these procedures are used together and the findings are integrated, the combination is often referred to as PET/CT fusion or hybrid imaging.

Tumors are more effectively treated when they are detected at an earlier stage. PET by itself is able to detect very small tumors but cannot locate them precisely—that function is left to the CT part of the fusion examination. By defining the extent or "stage" of a patient's disease, PET-CT imaging helps to determine the type and intensity of treatment needed. During the treatment phase, fusion imaging can help direct radiation therapy precisely to a tumor site. Finally, after the diagnostic and treatment stages, PET/CT fusion imaging can be used to monitor patients and show how well a tumor is responding to treatment. In patients who are not responding well to a particular treatment, this allows and early change in treatment to one that may be more effective.

How It Works

CT uses x-rays passed through the body at many angles to make cross-sectional images, or slices, of the part of the body being examined. In many cases the patient receives contrast material intravenously to obtain even clearer images of any abnormality that may be present. For PET, the patient receives an intravenous (IV) injection of a natural body chemical such as glucose (or sugar) combined with radioactive fluorine. The radioactive substance, or tracer, emits tiny particles called positrons that react with electrons within cells in the body to produce gamma rays. The emerging gamma waves are detected by the PET scanner, which uses a computer program to locate any area where the body tissues show increased uptake of the injected sugar. Because malignant tumors take up more glucose than most other tissues, a specific tumor site can be located and the tumor accurately measured.

The Experience of PET/CT Fusion Imaging

The PET/CT scanner is an open doughnut-shaped unit in which patients very seldom feel claustrophobic, or closed in. The patient can see what is happening, and the technologist has good access to the patient. Most prescribed medications may be taken on the test day, but with water only. Patients are asked not to eat or drink anything except water four to six hours before the exam. The day before, patients eat high-protein, low-carbohydrate meals and avoid eating candy, gum, mints and other things that contain sugar. Two or more glasses of water are taken an hour before testing. Exercise is avoided for 24 hours.

The imaging technologist places an intravenous (IV) line in the patient's arm to inject the radiopharmaceutical glucose needed for PET. In addition, contrast material may be given by mouth or by IV to make certain body structures or tumors stand out more clearly. After 45 to 60 minutes of resting still while the injected glucose spreads throughout the body, the patient will lie on a table and pass through the gantry (the doughnut-shaped ring) of the PET/CT unit. The scan itself takes about 25 minutes, and the entire procedure about two hours.

Benefits of PET/CT Fusion Imaging

• Using these imaging methods together in a single examination provides highly defined three-dimensional (3-D) images of virtually any part of the body.
• The PET component can help to detect changes in the body's chemistry that may take place before there are detectable anatomical changes.
• If a biopsy is needed, the exam can show exactly where to take a tissue sample.
• In patients treated for cancer with chemotherapy, PET/CT fusion imaging can accurately show how well the tumor is responding.
• When PET or CT scans are done alone, some patients will have uncertain findings—not definitely normal or abnormal. The combined PET/CT exam results in significantly fewer uncertain findings.
• Fusion imaging is convenient, as the results generally are available on the same day.
• Fusion scanning is a non-invasive and safe examination. It most often is done as an outpatient procedure, allowing the patient to leave immediately afterwards.
• The radioactive tracer is short-lived and causes no known side effects.

Common Uses of PET/CT Fusion Imaging

Cancer detection, treatment and monitoring: Fusion imaging is able to precisely define the location and size of a tumor and indicate spread of disease or metastasis that may be missed by CT or ultrasound alone. Many types of cancer can be detected by PET/CT imaging, including tumors of the lung, brain, breast, intestinal tract, abdominal organs and thyroid gland. Recurrence of a tumor in its original site may also be identified. The method is particularly helpful when evaluating structures such as the head and neck region. PET/CT can accurately locate lymph nodes, often the site of spreading cancer. PET by itself has proven to be helpful in monitoring how patients respond to cancer treatment, and the fusion study may enhance this role.

Cardiology: Following a heart attack, PET/CT fusion imaging can show blood flow to the heart muscle and how much of the heart wall has been damaged. The examination also can help determine the status of patients before coronary bypass surgery and those who have received a heart transplant. Imaging of the heart has improved with the recent introduction of CT scanning devices with many detectors (multidetector CT). These units can show motion of the heart as well as calcium deposits in the coronary arteries.

Future Uses: Researchers are conducting studies to determine whether PET/CT imaging can help evaluate inflammation and infection or determine the origin of fever when no cause is apparent. In the area of neurological disease, the examination has detected small lesions in the brain that take up an abnormal amount of glucose. It can also be employed to determine the recurrence of brain tumors, locate causes of seizures and aid in diagnosing causes of mental disturbances in patients, such as those with suspected Alzheimer's Disease. Other indications will very likely emerge as fusion imaging comes into more frequent use.