A device that detects and records the spatial distribution of radioactive compounds in the human body. It consists of a large scintillation crystal (or multiple crystals) of sodium iodide and a collection of photomultiplier tubes above the crystal, connected to it by a transparent material.

Radiation in the form of low-energy gamma radiation from an emitter causes a scintillation at a certain point P in the photomultipliers. This causes pulses in the photomultipliers, the size of the pulses depending on the position of both P and the tubes. All the pulses produced in the photomultiplier are analysed to enable the sizes and positions of the sources of radiation to be found. This information can be converted into spots on a cathode-ray tube.