What is a characteristic of the crystals used in PET detector blocks?

The crystals used in PET detector blocks play a critical role in the detection of gamma photons resulting from positron annihilation events. These crystals are designed to be highly effective in converting the gamma rays into visible light, which can then be detected and processed by photomultiplier tubes or other types of sensors.

The option stating that the crystals are typically 4 mm on each side correctly reflects the common dimensions of the scintillation crystals used in PET imaging. This size strikes an optimal balance between sufficient resolution for capturing detailed information about the distribution of the radiotracer within the body and maintaining a manageable form factor for effective handling and integration into detector arrays.

Choosing a size of around 4 mm enhances the detection efficiency while ensuring good spatial resolution, which is crucial for generating high-quality PET images. Smaller crystals might not capture enough of the emitted light, while much larger crystals could lead to increased scattering and reduced image clarity.

Overall, the characteristics of these crystals are pivotal in determining the performance of PET imaging systems in terms of spatial resolution, sensitivity, and overall image quality.