B.PET DETECTOR AND SCANNER DESIGNSAs discussed in Section A.6, detection efficiency (Equation 18-10) is an important parameter in PET scanner sensitivity and performance. Sodium iodide detectors, which are the “workhorse” for many nuclear medicine applications, also have been used for PET scanners. Indeed, as discussed in Section B.3, it is possible to use appropriately modified dual-headed SPECT systems for PET imaging. However, because of the relatively high energy of the 511-keV annihilation photons, sodium iodide generally is not the detector material of choice for PET imaging. For these reasons, most PET scanners use denser higher-Z scintillation detectors arranged in rings or banks of discrete elements around the scanned object. These systems not only provide a high detection efficiency but they allow the simultaneous collection of data for all projection angles with a completely stationary set of detectors. In this section, we discuss the design of modern PET detector systems and scanners. Reference 2 is a useful review describing emerging detector technologies for PET.