MED
In the literature: open
Joint depth-of-interaction and timing measurement
In plain words
Where inside the crystal the gamma ray stops affects both how long the light takes to arrive and the image sharpness at the edges of the scanner. Measuring both at once is unsolved.
Precise statement
Design a detector measuring depth of interaction to better than 2 mm in crystals 20 mm deep while keeping coincidence time resolution below 100 ps, with the light-transport delay correlation between the two measurements corrected rather than averaged. An answer is a demonstrated detector with both figures met simultaneously.
What would settle it
Bench characterization of a detector reporting both resolutions from the same events.