Figure 6. Radial blurring. The ring geometry of block detectors results in radial blurring due to nonuniform penetration of the detector elements by the coincidence photons. In more peripheral regions of the gantry (a, c), the annihilation photons may penetrate more than a single detector due to their oblique paths, thereby resulting in blurring of the final image. The degree of blurring is variable and depends on the obliquity of the paths of the photons through the detectors. Radial blurring is typically worse at the periphery of an image.