The problem of radioisotope imag ing in nuclear medicine is to develop an image containing a maximum of in-formation formed by fly-rays which in turn have been "focussed" by only ap-erture limitation. Gamma-emitting radio-labeled medicinals are designed to be concentrated by certain tissues. Abnormal regions may be indicated by either enhanced or reduced concentra tion of the labeled compound. The distribution of the radionuclides, as revealed by their T-rays (e.g., 140 KeV in the case of 99mTc), may then be the basis for diagnosis not possible with X-rays or other nonsurgical techniques. Because the radiation exposure of the patient must be minimized, the images formed are limited by quantum statistics, and indeed the practical detectors are those which respond to single γ-ray quanta. The image quality available with current techniques is modest by the usual standards of X-ray or light imaging; a 25 cm diameter image field seldom contains more than 1000 resolved image elements. Indeed the total information content is often carried by a total of only 100,000 photons. Examples of isotope images useful for diagnosis are given in Figure 1: (a) a brain containing a tumor, (b) a frame of a cerebral blood flow study, and (c) a pair of adrenal glands. In view of the unique nature of the information gained, even this very modest image quality is invaluable in diagnosis. An illustration of this value is the fact that about 2,000,000 radionuclide diagnoses are now undertaken each year in the U.S.