Positron Emission Tomography (pet Scan)

If your doctor ever refers you for a PET scan, you will be introduced to a fairly new medical imaging technique. Since this emerging modality is so new, a lot of the general public is not aware of what a PET scan exactly is. This essay will help explain the concept of this modality and the characteristics of it that allow doctors to diagnose and manage the proper care for some of today's most devastating medical conditions known to man.

Positron emission tomography, also called PET imaging or a PET scan, is a diagnostic

examination that uses positively charged particles. The particles are radioactive positrons that detect changes in the body's metabolism and chemical activities. Positrons are tiny particles emitted from a radioactive substance administered to the patient which helps provides a color-coded image of the body's function, rather than its structure.

During a PET scan, a substance called a tracer that produces radioactive positrons either is injected into a vein or inhaled as a gas. The radioactive substance is produced in a machine called a cyclotron and attached to a natural body compound. The most common body compound that the positrons are attached to is glucose. Other natural body compounds commonly used to attach the radioactive substances are carbon, nitrogen, oxygen and ammonia. The compounds become altered to allow them to emit positrons. Once the tracer enters the body it will take approximately 30 to 90 minutes for the substance to travel to a specific target organ, such as the brain or heart. During this time, you will be asked to rest quietly and avoid significant movement or talking, which may alter the way the substance localizes in a certain tissue. There the tracer emits positrons, which collide with electrons to produce gamma rays. These gamma rays are detected by a ring-shaped PET scanner and analyzed by a computer to form an image of the target organ's metabolism or other functions. Different colors or degrees of brightness on a PET image represent different levels of tissue or organ function. For example, because healthy tissue uses glucose for energy, it accumulates some of the tagged glucose, which will show up on the PET images. However, cancerous tissue, which uses more glucose than normal tissue, will accumulate more of the substance and appear brighter than normal tissue on the PET images.

PET is usually done on an outpatient basis. Your doctor will give you detailed instructions on how to prepare for your examination. You should wear comfortable, loose-fitting clothes that do not contain any kind of metallic objects whatsoever. You should not eat for four hours before the scan and will be encouraged not to drink a lot of fluids after midnight. Your doctor will instruct you regarding the use of medications before the test. A PET scan is painless, except for a mild skin prick if the tracer is injected. Once the tracer is given, the PET scan must be done immediately because the positron-emitting tracers usually decay or lose their positrons rather quickly. In general, there are no restrictions on your daily routine after the test. However, drink plenty of fluids to flush the radioactive substance from your body.

A PET scanner resembles a CT or MRI machine in a way. The PET scanner will have it's own examination room. The machine itself has a hole in the middle and looks like a large doughnut. Within this machine are multiple rings of detectors that record the emission of energy from the radioactive substance in your body and permit an image of your body to be obtained. While lying on a cushioned examination table, you will be moved into the hole of the machine. During the scanning procedure, you must lie very still. Since the scanning table will glide you through the PET scanner, you do not need to move at all. An entire scan could take 30 minutes to 2 hours all depending on what that particular exam calls for. The images are displayed on the monitor of a nearby computer, which is similar in appearance to the personal computer you may have in your home.

PET is most often used by oncologists and physicians specializing in cancer treatment, neurologists and neurosurgeons specializing in treatment and surgery of the brain and nervous system, and cardiologists specializing in the treatment of the heart. However, as advances in PET technologies continue, this procedure is beginning to be used more widely in other areas.

PET scans are used most often to detect cancer. A PET scan can be particularly effective in identifying if you have specific cancer, if it has spread. These scans can be performed on the whole body. In addition, a scan can be valuable in evaluating the effectiveness of chemotherapy. It is an excellent test to monitor for recurrence of disease. PET shows whether or not a cancerous tumor is benign or malignant. No other imaging technique can do this. Also it allows for finding the extent of a cancer which is better known as staging. Early intervention is PET's most important benefit. The earlier the detection, the likelier the cure

PET is the most accurate test to reveal coronary artery disease and impaired blood flow or to rule out its presence. A PET scan may reveal early coronary artery disease and damaged or dead heart muscle due to a heart attack. It can also tell the difference between nonfunctioning dead heart muscle from poorly functioning heart muscle that would benefit from a procedure, such as heart surgery, angioplasty, coronary artery bypass surgery or an implant, to increase blood flow to heart muscle. Therefore, it can also be particularly important in people who have previously had heart attacks.

PET scans of the brain are used to evaluate patients who have memory disorders, suspected or proven brain tumors or seizure disorders. These scans can identify the exact site in the brain that seizure activity is taking place. This is especially important for children with uncontrollable seizures. Alzheimer's disease is becoming detectable with the use of a PET scan. A PET scan can detect mild physiological changes in the brain even when no signs or symptoms of Alzheimer's disease are evident

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