Improvement of the depth of interaction detector for PET on full energy pulse height uniformity

As part of the next generation PET project, uniformity of full energy pulse height for all crystal elements was improved for the development of the depth of interaction (DOI) detector constructed of three-dimensional crystal arrays. In our previous report, we found a DOI detector constructed of four stages of a 2 by 2 GSO crystal array provides poor uniformity of the energy pulse height distribution. The upper stage crystal elements which are separated from the photocathode of a PMT have a tendency to have a lower energy pulse height. For example, the ratio of the full energy peak of the top stage crystal to the bottom stage was about 0.3. We designed a new DOI detector to overcome this problem. By optimizing reflector arrangement, condition of crystal surface, and optical coupling between crystal elements, we got an energy pulse height in the upper stage which was almost the same height as for the bottom stage crystals. There was also good separation between each area corresponding to crystal elements on two-dimensional histograms calculated by an Anger-type position calculation. The uniform full energy pulse height of every stage crystal simplifies the electrical circuits, and may give a great advantage in getting an accurate scatter correction. It also improves energy and timing resolution.