Scintillator (gamma camera)

Last revised by Francesco Sciacca on 9 Dec 2023

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.

A scintillator crystal may have properties of phosphorescence, where trapped charges within the crystal are released as light later (known as afterglow), which can negatively affect the gamma camera performance 3.

The scintillator consists of a large, single photoluminescent crystal - typically thallium-activated sodium iodide, NaI(Tl) 2 - 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 high atomic number of 53, and is able to absorb 90% of photons from Technetium-99m. However, it only able to absorb 30% of photons from iodine -131 2.

However, sodium iodide has low mechanical stability and is prone to shattering with changes in temperature. It also rapidly absorbs moisture and therefore is sealed in an airtight aluminum cylinder 2.

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.

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