1. An apparatus comprising one or more detector modules, at least one of the one or more detector modules comprising a sparse detector, the sparse detector comprising: an array of scintillator crystals generating scintillation in response to radiation, wherein the array of scintillator crystals have a plurality of positions, a position is void or occupied by a scintillator crystal or a block of a light-transmitting material, the array of scintillator crystals are sparsely arranged in two directions of the array of scintillator crystals according to at least a sparsity rule that at least a portion of scintillator crystals of the array of scintillator crystals are spaced apart by one or more positions that are void or occupied by one or more blocks of a light-transmitting material, and the sparse arrangement of the scintillator crystals of the sparse detector exhibits a random pattern in the two directions; and an array of photodetector elements configured to generate electrical signals in response to the scintillation, wherein at least a portion of the array of photodetector elements are coupled to the array of scintillator crystals, and the electrical signals are output for generating imaging data.