In terms of depth, how deep are the crystals in a PET detector block?

In a PET detector block, the crystals are typically designed to be in the range of 10-20 mm in depth. This depth is optimal for capturing gamma photons emitted from positron annihilation events within the patient's body. The crystals are crucial because they convert the high-energy photons into visible light, which can then be detected by photomultiplier tubes or other light detection devices.

The chosen depth allows for sufficient interaction of the photons with the crystal material, ensuring effective detection and localization of the emitted radiation, which is essential for producing high-quality imaging results in positron emission tomography. This depth also balances issues like resolution and sensitivity, making it a standard in the design of PET detectors. The other ranges listed would either lead to insufficient photon interaction or would be unnecessarily deep, affecting the resolution and effectiveness of the imaging.