New perspectives in nuclear imaging

In nuclear medicine, biodistribution is studied by quantitative nuclear imaging; chemistry is translated into radio-pharmacology; physics into instrumentation; and biology into physiology. Nuclear medicine can be thought of as applied physiology and physiological chemistry. The modern nuclear chemist is concerned with molecules, with relating chemical structure to biological distributions, as for iminodiacetate compounds. Using mixed ligand analysis, the hepatobiliary agent, HIDA, developed by Loberg, proved an anionic bis-complex with a charge of minus one. Further studies elsewhere showed that the co-ordination number of technetium was 5 and that an oxo-oxygen was in the apical position. From a series of analogues the amount of the complexes excreted in the bile was found to be directly proportional to the natural log of the molecular weight to charge ratio. The trend is towards the synthesis of labelled compounds whose biodistribution depends on their chemical interaction with structurally specific binding sites, i.e. receptors, enzymes or binding sites of active transport systems. Other examples include amino acids for pancreatic imaging; deoxyglucose for studies of regional brain and heart metabolism; fatty acids for studies of the heart; steroid hormones for breast tumours; and muscarinic compounds for study of the cholinergic system of the heart. Most of these compounds are labelled with 11C, 18F or 13N, available only from cyclotrons, but the extension to the more widely available 123I and sup(99m)Tc is also proceeding rapidly, stimulated chiefly by the success with the positron-emitting compounds. Advances in nuclear imaging include the development of both positron and single photon emission tomography. Biodistribution studies in man permit measurement of regional as well as global function. Improved perception is being augmented by improved quantification and automation