Energy resolution is the precision with which the system can measure the energy of incident photons. For a source of 511 keV photons the ideal system would demonstrate a well-defined peak equivalent to 511 keV. BGO has low light yield (six light photons per keV absorbed) and this introduces statistical uncertainty in determining the exact amount of energy deposited. There are two possible ways to define the energy resolution for a PET scanner: the single event energy resolution, or the "coincidence" (i.e., both events) energy resolution.
Energy resolution is usually measured by stepping a narrow energy window, or a single lower-level discriminator, in small increments over the energy range of interest while a source is irradiating the detector(s). The count rate in each narrow window is then plotted to give the full spectrum. The data in Fig. 3.17 show the system energy resolution for single photons for a BGO tomograph for three different source geometries. An increase is seen in lower energy events in the scattering medium compared with the scatter-free air measurement.
Energy resolution is a straightforward measurement for single events, but less so for coincidence events. A method often used in coincidence measurements is to step a small window in tandem over the energy range. However, this is not the situation that is encountered in practice as it shows the spectrum when both events fall within the narrow energy band. It is more useful is to examine the result when the window for one coincidence of the pair is set to accept a wide range of energies (e.g., 100-850 keV) while the other coincidence channel is narrow and stepped in small increments over the energy range. This allows detection of, for example, a 511 keV event and a 300 keV event as a coincidence (as happens in practice). This is the method used in Fig. 3.18. It demonstrates energy resolution for a line source of 68Ge/68Ga in air of approximately 20% at 511 keV for a BGO scanner, similar to that obtained for the single photon counting spectrum.
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