Research with PET has added immeasurably to our current understanding of flow, oxygen utilization, and the metabolic changes that accompany disease and that change during brain stimulation and cognitive activation. Clinical uses include studies of Alzheimer's disease, Parkinson's disease, epilepsy, and coronary artery disease affecting heart muscle metabolism and flow. Even more promising with regard to widespread clinical utilization of PET are recent developments showing that PET can be used effectively to locate tumors and metastatic disease in the brain, breast, lung, lower gastrointestinal tract, and other sites. Early evidence also indicates that quantitative studies of tumor metabolism with PET can be used for non-invasive staging of the disease. Compared to other cross-sectional imaging techniques like MRI and CT, PET is distinguished by its immense sensitivity-its ability to quantitatively determine and display tracer concentrations in the nanomolar range.
PET imaging begins with the injection of a metabolically active tracera biological molecule that carries with it a positron-emitting isotope (for example, 11C, 13N, 15O, or 18F). Over a few minutes, the isotope accumulates in an area of the body for which the molecule has an affinity. For example, glucose labeled with 11C, or a glucose analog labeled with 18F, accumulates in the brain or in tumors, where glucose is used as the primary source of energy. The radioactive nuclei then decay by positron emission. The ejected positron combines with an electron almost instantaneously, and these two particles undergo the process of annihilation. The energy associated with the masses of the positron and electron is divided equally between two photons that fly away from one another at a 180ºangle. Each photon has an energy of 511 keV. These high-energy g-rays emerge from the body in opposite directions, to be detected by an array of detectors that surround the patient (Fig. 6.1). When two photons are recorded simultaneously by a pair of detectors, the annihilation event that gave rise to them must have occurred somewhere along the line connecting the detectors. Of course, if one of the photons is scattered, then the line of coincidence will be incorrect. After 100,000 or more annihilation events are detected, the distribution of the positron-emitting tracer is calculated by tomographic reconstruction procedures from