The Gamma Camera

A gamma camera detects the gamma photons emitted by a tracer inside the patient and forms an image of where the tracer is.Its main parts, in the order the photons meet them, are:• Collimator: a thick sheet of lead with many long, thin parallel tubes (holes). Only gamma photons travelling along the tubes pass through; photons arriving at an angle are absorbed by the lead walls. This makes each point on the image correspond to the point in the body directly below it, giving a sharp image.
• Scintillator: a large crystal of sodium iodide. Each gamma photon that is absorbed produces thousands of visible light photons.
• Photomultiplier tubes: a hexagonal array of tubes behind the crystal. Each converts the light into an electrical pulse and amplifies it greatly.
• Computer and display: the tube nearest the flash gives the largest pulse, so the computer compares the sizes of the pulses from nearby tubes to locate where each gamma photon struck. Thousands of such events build up the image.The brightness at each point shows how much tracer is in the tissue below. The collimator throws away most gamma photons, so the scan takes several minutes to collect enough counts for a clear image.