PET is a new imaging technique that uses radio-labelled molecules to view molecular interactions of biological processes in vivo. PET represents a merging of biology with medicine to form the technology referred to as biological or molecular imaging. This merging is occurring at all levels, from imaging of molecules themselves to imaging of viruses, bacteria, cells, organ systems and whole organisms. The organism’s may range from the most simple systems to humans, but in each case the objective is to determine the mechanisms of organised system function. This system may be the entire body or an organ such as the liver or brain in which a collection of cells function as an integrated system. Molecular imaging techniques such as PET can thus reveal whole systems as well as examine the different molecular mechanisms. PET scan is currently not available anywhere in India. However, it is expected that these machine would be installed in a few centres in the next 1-2 years.

This article focuses on PET, its principles, current uses and its potential uses.

PET is an imaging technology that uses positron-labelled molecules in very low amounts to image and measure biological processes with minimal disturbance to the biological processes. The images of the body are obtained by injecting or inhaling radioactively-labelled chemical substances which are equivalent or closely analogous to naturally occurring substances in the body. The most commonly used compound is Fluorodeoxyglucose (FDG) a normal molecular glucose, the basic energy fuel of cells. FDG is labelled with positron emitting radioisotope like 15O, 13N, 11C,18F. After injecting, FDG gets distributed throughout the body by way of bloodstream and enters organs, where it traces the transport and phosphorylation of glucose.

The fluorine atom in FDG, suffers a radioactive decay, emitting a positron (an electron with a positive electrical charge). When a positron collides with an electron, annhilation occurs, liberating energy in the form of two beams of gamma rays in opposite directions. The two photons produced from positron annihilation, are detected when these 2 photons strike opposing detectors. Detectors or scanners are arranged either in dual-head configuration or around entire circumference of the organ. Modern dual-head and circumferential PET scanners can form more than 50 tomographic image planes simultaneously. PET images can be for selected organs or for the entire body.

Alzheimer’s Disease

PET can provide an early and accurate diagnosis of Alzheimer’s disease. In clinical studies, PET could detect Alzheimer’s disease with an accuracy of greater than 90% and also at least 2.5y earlier than other sophisticated clinical diagnostic methods. This has important implications, since Alzheimers can then be diagnosed at an early and more treatable stage of the disease; it can be differentiated from other dementias; the disease can be biologically staged and response to therapy can also be assessed. Additionally, the drug can also be labelled and its occupancy at the target site assessed and compared with the clinical response, which may help in optimising the dosages. Many other diseases can exist in the body in a silent, asymptomatic phase for a considerable time e.g. cancers, Parkinson’s disease and Huntington’s disease. PET studies have shown that these metabolic abnormalities can be detected approximately 5 to 7 years before the expression of clinical symptoms.

Cardiac Disease

Accurate detection of coronary artery disease and characterization of cardiac tissue viability can allow more effective use of existing therapies. PET can determine the viability and therefore reversibility of the effects of coronary artery disease by identifying patients who retained glucose metabolism in the affected myocardial areas. This is based on the biochemical principle that glucose is a protective substrate for generating ATP in oxygen-limited states to maintain the viability of tissue despite limitation or loss of the local cardiac function.

It is well known that neoplastic degeneration is associated with increases in glycolysis because of a progressive loss of the tricarboxylic acid cycle (TCA). Thus, glucose consumption increases as neoplastic degeneration progresses. Thus high levels of signal in FDG imaging delineate neoplasms from surrounding tissue and detect small lesions. The detection of abnormal tumor metabolism before abnormal change allows for accurate diagnosis of primary and recurrent tumors, accurate determination of tumor extent after diagnosis and prediction and assessment of treatment response.

Attempts are being made to develop a new class of technology that fuses PET and Computer Tomography (CT) giving rise to the following advantages: Improvement of PET image quality through fast, accurate, low-noise attenuation correction by CT; identification and definition of biological abnormalities by PET, with display of the surrounding anatomy by CT for improved localization, planning of surgery biopsy radiation therapy with CT, and acquisition of CT based diagnostic information.

PET studies with radiolabeled drugs have been used to provide new information on drug uptake, distribution, and kinetics and their influence on the relationship to the biochemical, behavioural, therapeutic and toxic effects of drugs. PET provides a new perspective on drug research by virtue of its ability to directly assess both pharmacokinetic and pharmacodynamic events in humans and in animals. Moreover, the new generation of high-resolution, small-animal cameras (MicroPET) hold the promise of introducing imaging in the early stages of drug development and make it possible to carry out longitudinal studies in animals and to study genetically altered animals. Thus PET can contribute significantly to the process of drug development through understanding of the molecular mechanisms underlying drug action and simultaneously determining effective drug doses for clinical trials. It can also, determine the duration of drug action and examine potential drug interactions. MicroPET and PET can permit movement between animal models and patients with common methods of assessment.