1. A flat panel radiation detector, comprising: a plurality of complementary metal-oxide-semiconductor (CMOS) tiles arranged to form a light sensing panel comprising a circular active area; and a non-imaging ring disposed on the light sensing panel, wherein the circular active area of the light sensing panel is defined by an inner boundary of the non-imaging ring, and wherein the non-imaging ring is disposed on the light sensing panel such that portions of outer CMOS tiles of the plurality of CMOS tiles are masked by the non-imaging ring so as to define the circular active area.