Radio-isotopes and regional blood flow.

not previously been satisfactory clinical methods for tissue flow measurement. Recent techniques with radio-isotopes which can be used in man are therefore of interest, and this editorial reviews generally the theory underlying their use and their application in the past few years. The great advantage of radio-isotopes is that they may be measured easily in the body using an external detector, and the main development of recent years has been the increasing use of highly diffusible gamma emitters for such measurement. Blood flow can be determined by observing the rate of disappearance of isotope from the site at which it is injected, and it is customary to replot such disappearance slopes logarithmically. The decay constant k is obtained from the half-disappearance time, and, using the Fick principle, flow is given by F= kA, where A is the partition coefficient between the tissue and blood. This method of residue detection assumes that a constant ratio exists between specific activity in the tissue and the capillary bed, that equilibration is rapid, and that concentration gradients within the tissue do not occur. If the tissue is evenly perfused, the disappearance slope follows a single exponential curve, a simple state that applies to the heart and testis. Most tissues, however, including brain, kidney, liver, and skin, are unevenly perfused, and compartmental analysis is then applied to the decay slopes on semilogarithmic paper. By a process of deconvolution or 'curve peeling', various compartments are obtained whose flow rates and