The emission of a single gamma ray is a very small-scale nuclear phenomenon. It is the role of the gamma-camera head to amplify this microscopic radiation into an electric signal that can be detected and measured. By exploiting a large number of readings of these electric signals, one can determine the map of the radioactive nuclei responsible for the emission of gamma rays.
The gamma-camera detection head consists of:
- a collimator
- a scintillating crystal
- an array of photomultiplier tubes
- an electronic system for detection and measurement of gamma energies and impacts
The collimator is a thick plate of lead or tungsten riddled with a large number of very thin parallel channels. The gamma rays which can pass through it are those whose direction is perpendicular to the surface of the lead plate and the scintillating crystal. The channel axes point towards the body part under examination, and the lead or tungsten stops all gamma photons travelling at an oblique angle. Other collimators can be designed using different techniques: a pinhole collimator is used for thyroid scintigraphy scans, whereas fan-shaped collimators are used for imaging of the brain.
The detection element at the heart of a gamma camera is a large rectangular crystal of sodium iodide doped with thallium: NaI (Tl). The crystal has the ability to stop incoming gamma rays and convert part of the deposited energy into scintillations.
Behind the crystal, an array of small photomultipliers converts photons of light into electric signals. From the hits in a set of photomultipliers, one can determine the energy of the incoming gamma rays as well as the approximate position of their impacts on the crystal. The gamma rays whose energies do not fall within a certain range of the energy of the radioactive sample (a spectroscopic window) are discarded, and do not contribute to the final picture.
The gamma camera is positioned in such a way as to ensure that it selects the gamma photons being emitted by the organ under diagnostics.