1. An apparatus comprising: a sparse detector, the sparse detector comprising: an array of scintillator crystals generating scintillation in response to radiation, at least a portion of the scintillator crystals being spaced apart according to at least a sparsity rule; 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; 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.