Abstract: In a PET system having a box-like configuration where four detector panels enclose a field of view, improved photon efficiency is provided by arranging the panels such that a side face of each panel makes contact with a front face of another panel. This arrangement allows for photon efficiency to be improved by “filling in the corners” of the system and/or by adjusting panel positions to conform the field of view to the imaging target along two dimensions. Such adjustment of the field of view does not require altering the size of the detector panels. Furthermore, photon efficiency in embodiments of the invention can be considerably better than the photon efficiency of conventional cylindrical PET arrangements (e.g., as on FIG. 1). Elimination of the wedge-shaped inter-module gaps of the cylindrical geometry can significantly increase efficiency, because Compton scattering into these gaps can be a significant loss mechanism.