About ten years aga devices and equipment were developed for producing quantitative images in vivo of the distribution of substanees with positron emitting radionuclide label of very short half-life (down to only several minutes). The positron-emitting isotopes can be the most important bio elements and are, therefore, suitable for the labeling of important physio logie substrates like sugars, amino acids ete. and their synthetic analogues. Their imaging, after distribution in vivo, is being performed by PET scan ning systems. It soon became apparent that positron-emission tomography could be...