A positron emission tomography (PET) scanner, including a first detector portion arranged circumferentially around a patient pallet, the first detector portion having a predetermined axial extent and transaxially subtending less than 360 degrees with respect to a central axis of the scanner defined by the first detector portion, wherein the first detector portion includes a plurality of first detector elements; and a second detector portion arranged separately from and opposing the first detector section, the second detector portion including a plurality of second detector elements, the second detector elements being of a different type than the first detector elements, wherein each of the first detector elements includes photomultiplier tubes (PMTs); and each of the second detector elements includes photosensors of a different type from the PMTs of the first detector elements.