Gamma camera

Gamma cameras (also called scintillation cameras or Anger cameras) are the predominant nuclear medicine imaging machine currently in use. They permit the acquisition of planar images. They are also central to single photon emission computed tomography (SPECT).

Design

From the exterior to interior a camera is comprised of ¹:

• head cover: usually made from glass

• collimator: usually made from lead with fine holes and septa

• scintillator: single photoluminescent crystal, usually made from thallium-activated sodium iodide

• photomultiplier tubes

• preamplifiers

• electronics including: analog to digital converters, digital summing and positioning circuits and correction circuits

The camera itself is above to rotate to allow images to be obtained from different angles. Gamma cameras may be found in isolation (scintigraphy) or in combination with a CT scanner (single photon emission computed tomography).

Principle

Gamma radiation emitted from the radionuclide administered to the patient (most commonly Tc-99m) travels in all directions. A fraction of the radiation travels towards the gamma camera, of that an even smaller fraction travel at the correct angle to the septa of the collimator and is allowed to strike the crystal. When the gamma photon strikes the crystal a light photon is produced, this light photon is then converted into an electrical signal and amplified by the photomultiplier tube and is further amplified by the pre-amplifiers. The amount of light reaching the photomultiplier is proportional to the electrical signal produced ^1,2.

Analog to digital converters then convert the signal. Positioning circuits then determine the X-position and Y-position of the interaction of the gamma ray and the crystal. The summing circuit produces an energy (Z) signal created by adding all of the individual signals from the pre-amplifiers. Correction circuits then correct for errors in the positioning and energy of the interactions ¹. The Z-signal will pass through a pulse height analyzer (PHA) before sending to a computer ⁴. The X, Y and Z signals are then processed and displayed on a computer ^1,4.

History and etymology

The original gamma camera was developed in the 1950s by American engineer and physicist Hal Anger (1920-2005) ³.