1. An apparatus 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, and the array of scintillator crystals are arranged according to 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 an array of photodetector elements configured to generate an electrical signal in response to the scintillation, wherein at least a portion of the array of photodetectors are coupled to the array of scintillator crystals; and a processor configured to: obtain imaging data from the sparse detector; generate a plurality of virtual scintillator units according to the sparsity rule related to the sparse detector, wherein a virtual scintillator unit corresponds to two or more neighboring positions of the array of scintillator crystals; calculate efficiency of a line of response based on the plurality of virtual scintillator units; and generate, based on the efficiency of the line of response and the imaging data, an image.