A system is described that makes use of positron emitting isotopes for locating brain tumors. This system inherentiy provides more information about the distribution of radioactivity in the head in less time than existing scanners which use one or two detectors. A stationary circular array of 32 scintillation detectors scans a horizontal layer of the head from many directions simultaneously. The data, consisting of the number of counts in all possible coincidence pnirs, are coded and stored in the memory of a Two-Dimensional Pulse- Height Analyzer. A unique method of displaying and interpreting the data is described that enables rapid approximate analysis of complex source distribution patterns. (auth)