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FIGURE 4. (A) Photograph and 
-camera image of 4-quadrant bar phantom, with schematic diagram illustrating setup for assessment of spatial resolution using such phantom. (Though seldom used in practice, sheet source that may be filled with different radionuclides and used to evaluate extrinsic uniformity is also shown in photograph.) (B) Energy spectrum for 662-keV -ray emitted by 137Cs, illustrating definition of energy resolution as percentage FWHM of photopeak energy, E.
