False adrenergic transmitters and positron emission tomographic imaging of myocardial sympathetic innervation.

D uring the past 50 years, isotopic tracer methods have become standard techniques for analysis of dynamic biochemical and physiological processes in vitro and in vivo. When liquid scintillation spectrometry was introduced approximately 35 years ago, assay of /3 emitting isotopes was simplified. This resulted in widespread application of isotopic methods to study metabolism and disposition of physiologically important substances and drugs. The development of autoradiography made possible direct quantitative imaging of radioisotopically labeled compounds in tissue. With the advent of computer-assisted tomographic methods, it became possible to image quantitatively, in intact animals and in humans, isotopes that emit high-energy, penetrating radiations. The spatial resolution that can be obtained when imaging positron-emitting isotopes (e.g., carbon-11, oxygen-15, nitrogen-13, and fluorine-18) is superior to other types of radioisotopes. Problems of production and rapid incorporation of these relatively short-lived positron-emitting isotopes into appropriately selected radiopharmaceuticals are being solved, and an increasing number of processes are being studied with positron emission tomography (PET). In this issue of Circulation, Schwaiger et al report the use of PET imaging of myocardial