The patient is the source of gamma rays after injection of radiopharmaceuticals.
The collimator (usually made of lead) filters out the gamma rays that do not align with the hole direction so as to improve the spatial localisation of the radiation source. The collimator should be as close to the patient as possible to maximise signal-to-noise ratio (SNR).
Thalium-activated sodium iodide (NAI (TI)) scintillation crystal converts gamma rays into light photons. The crystal is usually rectangular, measuring 60x50cm in cross-sectional area and 6 to 13mm in thickness. Thin NAI crystal is instrumental in improving the spatial resolution of gamma camera.
The light guide allows good optical contact between the scintillation crystal and photomultiplier tubes (PM tubes). Some systems use optical grease (thinner than light guide) to further improve the spatial resolution.
PM tube is a vacuum tube that can converts small amount of light into large number of photoelectrons through the use of dynodes as electron multiplier. The electical signal is then feed into preamplifier, analogue-to-digital (ADC) convertor, and logic circuits that correct discrepancies of X and Y positions of the radiation source. These signals are then feed into the computer for further processing.
Meanwhile, the Z-signal which contains the information on the energy of the radiation source is feed through the pulse height analyser (PHA). PHA selects a narrow full width a half maximum (FWHM) to increase the chance of selecting an unscattered gamma photon and reduce noise from scattered low-energy gamma photons. The narrower the FWHM, the higher the energy resolution. Gamma camera usually have energy resolution of 9%. The Z-signal is also feed into the computer for further processing.