Scintillator (gamma camera)

The scintillator is the component of a gamma camera which receives the gamma rays emitted from a radionuclide in a nuclear medicine scan and converts it to visible light photons. It is located just behind the collimator device.

The scintillator consists of a large, single photoluminescent crystal (typically thallium-activated sodium iodide) spanning the dimensions of the field of view of the gamma camera.

When a gamma-ray strikes the crystal, photons of visible light are produced at a rate proportional to the energy of the gamma-ray. The light produced is detected by the photomultiplier tubes that sit behind the crystal, which convert it to an electrical signal.

Sodium iodide (NaI) is the most common crystal used in gamma cameras. It is favored due to the following properties:

• high atomic density
• high light output
• fast speed
• low cost

However, sodium iodide has low mechanical stability and is prone to shattering with changes in temperature. It also rapidly absorbs moisture and therefore is hermetically sealed in an aluminum cylinder.

The thickness of the crystal results in a trade-off between absorption and spatial resolution. Thicker crystals absorb a larger proportion of gamma rays but allow for greater scatter and therefore poorer spatial resolution.