A comparison of PET detector modules employing rectangular and round photomultiplier tubes

We have compared the high resolution BGO detector blocks from the EXACT HR PET system which use two dual-cathode rectangular photomultiplier (PM) tubes with a new block design, the EXACT HR PLUS, which uses four round PM tubes. Despite the lower coupling area between photocathode and scintillator, the HR PLUS block compares favorably with the HR block. The energy resolution averages 20% for the HR PLUS block and 23% for the HR block, with efficiency variations of 17% in both blocks. Additional measurements were carried out on the HR PLUS block to characterize depth of interaction effects and cross-talk between elements. Coincidence line spread function measurements had a FWHM of 3.0 mm in the axial direction and 2.9 mm in the transaxial direction. In light of these results, limitations of the BGO block design are discussed and some solutions proposed