In nuclear medicine, PMTs are arrayed (usually in a honeycomb pattern) over a large (40cm or so) flat crystal. When the tracer isotope in the body emits a particle that interacts with the crystal, the photons from that interaction reach the PMTs at slightly different times. After some non-hairy math you can resolve a location where the tracer substance is, roughly, in the body.
The less-crisp images from this process earn the nickname of "unclear medicine", but for lung scans and the like there is no substitute.