In 2-D PET scanners employing septa, scattered radiation is reduced by the septa, placing less importance on good energy resolution. Additionally, the reduced sensitivity in 2-D limits the maximum countrates encountered in clinical FDG studies. In contrast, 3-D PET scanners rely on good energy resolution to reduce the scattered radiation and also must deal with countrates, which are typically 5 times higher than in 2-D mode. To achieve good energy resolution, 3 factors must be considered: 1) choice of a scintillator with good intrinsic energy resolution, 2) choice of a crystal dimension which transmits a uniform amount of light to the PMT in order to avoid light loss along the length of the crystal and 3) choice of a crystal-to-PMT coupling which collects a uniform amount of light from all crystals. As PET scanners are being designed using new, faster scintillators for 3-D imaging, the appropriate trade-off between energy resolution and countrate capability must be found to give the best overall system performance. An example of a fully 3-D PET scanner is the Allegro (ADAC Laboratories), which uses GSO as the detector material. Given the right choice of material and design parameters, good quality, high contrast images can be obtained in 3-D in a relatively short time.