3. Energy Resolution
It is not unusual in a typical patient study for there to be more Compton-scattered than unscattered γ rays striking the detector (see Fig. 11-9). Because the Compton-scattered photons have lower energy, it is possible to discriminate against them using pulse-height analysis. The energy resolution of the detector determines the efficiency with which this can be accomplished. Good energy resolution is perhaps the most important performance feature of the camera system for this purpose. Energy resolution, like intrinsic spatial resolution, depends largely on statistical fluctuations in the number of light photons collected from a scintillation event (Chapter 10, Section B.7). Thus good light collection efficiency is a prerequisite for good energy resolution. As well, because the number of light photons released in a scintillation event increases almost linearly with γ-ray energy, E, (Fig. 10-11), energy resolution improves approximately in proportion to 1/ E (Fig. 10-13).