An important diagnostic procedure in the treatment of cancer or other diseases is the detection and location of radioactive material that has been introduced into the body of the patient. In this proposal, the objective is to construct (and subsequently clinically test) an instrument that covers an area of 25 cm x 25 cm with a resolution of 3 mm, not only in the X- and Y-direction, but also in the Z-direction. The sensitivity of the instrument will be better than those of the presently available commercial detectors for a source in the center of the body. Furthermore, the proposed instrument has these capabilities for the important radionuclides of oxygen-15, nitrogen-13, carbon-11, fluorine-18, and gallium-68, as well as for the many other isotopes decaying by positron emission. The availability of an instrument with an improvement of 3 in spatial resolution over the usual 10 mm of present instruments, along with a gain in sensitivity, plus "focusing" capability in the Z-direction, will have a tremendous impact on the effectiveness of diagnostic nuclear medicine.