Positron Emission Tomography ... Health Article

Definition

The positron emission tomography (PET) unit is a device used to produce images of the body that reflect biochemical changes taking place in the body. Among the body imaging technologies used in medicine, the PET unit is characterized by its use of positron-emitting tracer substances. Because of its use of short-lived positron-emitting tracers, the PET unit can provide images of biochemical processes. This feature of PET technology distinguishes it from computer tomography (CT) and magnetic resonance imaging (MRI) technologies, which can provide only images of the structure of the body.

Purpose

The purpose of the PET unit is to provide images reflecting biochemical changes occurring within the body. The PET unit can also, when used in conjunction with mathematical models of organ systems, quantify biochemical activity (e.g. blood flow, metabolic activity in tissue).

Because the PET unit can provide information on biochemical function, it is particularly helpful in assessing tissue viability and biological processes related to tissue health. The PET unit is used for these purposes most often in the fields of neurology (study of the brain), cardiology (study of the heart), and oncology (cancer).

In neurology, the PET unit is used to diagnose and differentiate among different types of epilepsy, dementia, and cerebrovascular disease. Because the regions of the brain that are affected by these abnormalities have blood flow and glucose utilization patterns that are different from healthy parts of the brain, the PET unit can—by using tracers that follow cerebral blood flow, glucose pathways, and oxygen metabolism—identify areas of the brain that are affected. During epileptic seizures, blood flow and glucose use increase in the area of the epileptogenic focus (site originating the seizure). PET scans are used to identify these foci in patients with drug-resistant epilepsy so that surgical intervention can target these seizure-prone areas. In dementia, the PET unit is used to distinguish Alzheimer's disease from other types of dementia, because each type of dementia has a characteristic glucose utilization pattern in the brain. The PET unit is also used to evaluate and monitor treatments for stroke patients by measuring cerebral blood flow, glucose metabolism, and oxygen levels.

In cardiology, the PET unit is used to assess the metabolism and function of myocardial tissue. Blood flow and fatty acid metabolism are measured by the PET unit, and areas that are affected by coronary artery disease are easily identified. The state of the myocardial tissue, as reflected in the PET scan, also helps the cardiologist determine the best intervention, e.g. an angiogram rather than a heart bypass.

In oncology, the PET unit has had a long history of being used for the diagnosis and localization of brain tumors. Because tumors have greater blood flow directed to them than normal brain cells, the PET scan can identify where the tumor is localized by pinpointing the area with abnormally high blood flow. More broadly, the PET unit can be used in many parts of the body to grade the severity of tumors and identify metastatic processes. Moreover, because PET identifies variations in metabolic activity, the scans are particularly useful in assessing the effectiveness of radiological treatment of cancer; unlike other types of imaging, PET scans can distinguish between (non-viable) scar tissue caused by the radiological treatment of tumors, and viable tumor cells that might have been missed by the treatment.