PET Scintillators

Scintillators that have a relatively short absorption length even at 511 keV are needed in PET devices, since high sensitivity is a requirement in them. Shorter crystal needles (~2 cm) need to be used in order to reduce parallax error (if the angle of incidence at the crystal needle is not 90°, it is difficult to determine which crystal needle was actually stricken).

The light yield of the crystal has to be high so that its signal-to-noise ratio and energy resolution is good. As far as the emission spectra are concerned, there are practically no restrictions because the PMTs can cover a wide spectral range. If one intends to use semiconductor detectors to convert light into electrical signals, they have to choose a scintillator material the emission spectrum of which is limited to wavelengths longer than 400-500 nm.

Because we detect in coincidence, fall time has to be shorter than 100 ns, this is a fundamental requirement. Rise time is also an important parameter in TOF-PET devices.

The background radiation of the crystal can be handled with energy windowing, but the hygroscopicity of the crystal can cause problems, since the crystal is pixelated. Because of the isolation from humidity the fill factor decreases, thus sensitivity drops as well.

The PET device is basically expensive, so the relatively high price of the crystals is not a problem. The small size of the crystals is especially favourable as far as manufacturing is concerned, growing single crystals in this size range can easily be done.

Since crystal needles have to be produced for many pixels, good workability and cuttability is a basic requirement.
The possible materials used as PET scintillators are listed in the figure below. Crystals with a short absorption length such as BGO (bismuth germinate), LSO (lutetium oxyorthosilicate) or LYSO (lutetium-yttrium oxyorthosilicate) are the most popularly used in PET devices.

 

 

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