[Menu] [Previous] [Next]

Overview of PET

Positron emission tomography (PET) has enhanced our understanding of the biochemical basis of normal and abnormal functions within the body, and permitted biochemical examination of patients as part of their clinical care. These capabilities are important because:

  1. The basis of all tissue function is chemical.
  2. Diseases result from errors introduced into its chemical systems by viruses, bacteria, genetic abnormalities, drugs, environmental factors, aging, and behavior.
  3. The most selective, specific, and appropriate therapy is one chosen from a diagnostic measure of the basic chemical abnormality.
  4. Detection of chemical abnormalities provides the earliest identification of disease, even in the presymptomatic stages before the disease process has exhausted the chemical reserves or overridden the compensatory mechanisms of the brain.
  5. Assessment of restoration of chemical function provides an objective means for determining the efficacy of therapeutic interventions in the individual patient.
  6. The best way to judge whether tissue is normal is by determining its biochemical function.
Another principle relates to the value of examining these biochemical processes with an imaging technology. Because in most cases the location and extent of a disease is unknown, the first objective is an efficient means of searching throughout the body to determine its location. Imaging is an extremely efficient process for accomplishing this aim, because data are presented in pictorial form to the most efficient human sensory system for search, identification, and interpretation‹the visual system. Recognition depends upon the type of information in the image, both in terms of interpreting what it means and how sensitive it is to identifying the presence of disease.

PET provides the means for imaging the rates of biologic processes in vivo. Imaging is accomplished through the integration of two technologies, the tracer kinetic assay method and computed tomography (CT). The tracer kinetic assay method employs a radiolabeled biologically active compound (tracer) and a mathematical model that describes the kinetics of the tracer as it participates in a biological process. The model permits the calculation of the rate of the process. The tissue tracer concentration measurement required by the tracer kinetic model is provided by the PET scanner, with the final result being a three-dimensional (3-D) image of the anatomic distribution of the biological process under study.

Radiolabeled tracers and the tracer kinetic method are employed throughout the biological sciences to measure such processes as blood flow, membrane transport, metabolism, synthesis, and ligand-receptor Interactions; for mapping axonal projection fields through anterograde and retrograde diffusion; measurement of cell birth dates; marker assays using recombinant DNA techniques; radioimmunoassays; and the study of drug Interactions with chemical systems of the body. The tracer technique continues to be one of the most sensitive and widely used methodologies for performing assays of biological systems. PET allows the transfer of the tracer assay methodology to the living subject, particularly humans. PET builds a bridge of communication and investigation between the basic and clinical sciences, based upon a commonality of methods used and problems studied.

The transfer of tracer methods from the basic biological sciences to humans with PET is made possible by the unique nature of the radioisotopes used in PET to label compounds: 11C, 13N, 150, and 18F. These are the only radioactive forms of the natural elements (l8F is used as a substitute for hydrogen) that emit radiation that will pass through the body for external detection. Natural substrates, substrate analogs, and drugs can be labeled with these radio isotopes without altering their chemical or biological properties. This allows the methods, knowledge, and interpretation of results from tracer kinetic assays used in the basic biological sciences to be applied to humans by the quantitative measurement abilities of the PET scanner.

[Excerpted from: Phelps, M.E., Positron Emission Tomography. In: Mazziotta, J. and Gilman, S., Eds., Clinical Brain Imaging: Principles and Applications, 1992, F.A. Davis Company, pp71-107]
[MENU]