1. An apparatus, comprising: processing circuitry configured to obtain projection data representing an intensity of radiation detected at a plurality of detectors, wherein the plurality of detectors includes first detectors and second detectors, the projection data includes a plurality of datasets including a first dataset and a second dataset respectively corresponding to the first detectors and the second detectors, and the first detectors having a different detector configuration than the second detectors, wherein the respective detector configurations of the first detectors and second detectors are respectively determined by a detector type and a projection geometry and of pixel elements arranged within the respective first detectors and second detectors, and reconstruct a combined image using the plurality of datasets, wherein each dataset of the plurality of datasets corresponds to a respective system-matrix equation representing respective projection geometries corresponding to the plurality of datasets, and the combined image is reconstructed using system matrices of the plurality of datasets, at least one modified-dual variable, and using a splitting-based subproblem method.