The gamma camera is the major imaging device used in diagnostic medicine. It usesrays from sources such as technetium – 99 inside the patient. Inside the camera is a single large crystal of sodium iodide with about 0.5 % thallium iodide, 25 – 40 cm in diameter and about 10 mm thick. This material scintillates, producing flashes of visible light when illuminated byrays.

The %gamma rays pass up through the collimator, consisting of a honeycomb of cylindrical tubes in a lead plate which detects photons from one direction only. The photons hit the scintillator crystal, producing flashes of light. This light is detected by the photomultiplier tubes, and the output from the tubes is processed to produce an image.