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SPECT Scintillators

SPECT devices belong to the ‘cheaper’ group of nuclear medicine devices, so price is a factor that has to be considered during their construction.

The SPECT devices (gamma cameras) usually have NaI scintillators. Their absorption length (the inverse of the attenuation factor) is 4 mm at 140 keV – the gamma energy of 99mTc generally used in SPECT –, so a 1 cm thick scintillator is perfectly adequate as it absorbs the majority of the incoming photons.
Due to the relatively low gamma energy (and to other effects listed later) a crystal with high light yield is needed, and NaI meets this requirement. In case of gamma cameras spatial resolution is limited primarily by the collimator (and because of the spatial blurring of the photomultipliers).

The emission spectrum of NaI covers the range of 300-500 nm. It is cheap to produce PMTs that operate in this spectral region. There are no big requirements for the gamma cameras used in SPECT devices as far as fall time (the time interval during which the intensity of the light emitted by the crystal drops to 1/e of its initial value) is concerned, because detecting in coincidence is not necessary. The density of the incoming photons is around 10000 cps (counts per second), so a few hundred ns long fall time is perfectly satisfactory.

The NaI crystal is a hygroscopic material but fortunately this is not a considerable problem, since there are large contiguous crystals in SPECT devices (and in planar gamma cameras), which can be easily protected from humidity. However, due to the large size of the crystal it has to be mechanically resistant.

Facts: in a NaI crystal 38000 photons/MeV are generated (because the average energy of the photons is ~3 eV, the efficiency of light utilization is ~12%), the density of the crystal is 3.7 g/cm3, the fall time is 230 ns, the peak of the emission spectrum is at 415 nm. In experimental SPECT devices CsI scintillators with photodiode readout are used, which have a better light yield; in spite of this, NaI scintillators are widespread.

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