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.