A pixelated
detector assembly comprising a stack of thin
detector crystals, each
detector crystal having a pair of planar surfaces bound by edges substantially thinner than the dimensions of the surfaces. The stack is disposed such that the
radiation to be detected is incident on one set of edges of the stack of detector crystals. The dimension of the planar surfaces in the general direction of incidence of the
radiation incidence is sufficient to ensure that substantially all of the
high energy photons to be detected are absorbed within the depth of the detector
assembly. Each of the detector crystals has a two-dimensional pixelated
anode array formed on one of its planar surfaces. A
cathode is formed on its opposite planar surface, preferably covering substantially all of the surface. The position of interaction of a
photon in the plane perpendicular to the direction of the incident
radiation, is determined by which of the detector crystals in the stack detects the absorption, and by which of the rows of pixelated anodes in that
crystal detects the absorption. The
depth of interaction of a
photon is determined by the location of the particular
anode pixel in the above-mentioned row of pixelated anodes where the
photon absorption is detected. The detector
assembly is thus able to detect the point of interaction of a photon in all three dimensions.