A PET detector with depth-of-interaction determination

Photon penetration into the detectors of circular tomographs blurs PET images near the edge of the field of view. To reduce this error, a 30 mm long Bi 12 GeO 20 crystal was divided into two distinct regions by painting a black band around it and collecting light with photomultipliers at its back and top-front. The location of the band was chosen so that the count rates from each region were equal. The response of the top-front photomultiplier for an event behind the band was only 6% of that for an event at the crystal front. Similar results were obtained when plastic light guides coupled the top-front of the crystal to the photocathode. Selecting a lower-level discriminator for this photocathode to distinguish front from rear events was also easily done, except when very narrow light guides were used.