A gamma-ray detector includes a plurality of modular one-dimensional arrays of monolithic detector sub-modules. Each monolithic detector sub-module includes a scintillator layer, a light-spreading layer, and a photodetector layer. The photodetector layer comprises a two-dimensional array of photodetectors that are arranged in columns and rows. A common printed circuit board is electrically coupled to the two-dimensional array of photodetectors of the plurality of modular one-dimensional arrays of monolithic detector sub-modules of a corresponding modular one-dimensional array. The two-dimensional array of photodetectors can be electrically coupled in a split-row configuration or in a checkerboard configuration. The two-dimensional array of photodetectors can also have a differential readout.