Evaluation of the depth of interaction (DOI) for the High Resolution Research Tomograph (HRRT) - a comparison between scanners with and without DOI

The High Resolution Research Tomograph (HRRT) is the first clinical LSO scanner with depth of interaction (DOI) capability. At the moment two HRRT's exist: one is located at the Max-Planck-Institute in Cologne and is equipped with two 7.5 mm thick crystal layers and DOI (HRRT-D); the second HRRT is located at the VUme in Amsterdam with a single crystal layer (HRRT-S). This constellation allows evaluation of scanner characteristics with and without DOI. The effects of DOI on resolution, sensitivity, scatter fraction (SF) and noise equivalent count (NEC) rates were studied. These characteristics were measured according to the NEMA NU2-1994 or 2001 standards. Emission scans were acquired in 64 bit list mode. For HRRT-D emission data were sorted into files with coincidences between the layers nearest to the center of the scanner only and into files with all layer combinations providing DOI. Resolution data were analyzed with different data compressions (span 3 and 9) and for FORE+2D-HOSP and 3D-OSEM. Different scatter fractions between both scanners were observed for lower level discriminator values (lids) of higher energy. NEC rates were about 3 times higher for HRRT-D. Best resolution values of 2.0 mm FWHM at 0 cm radial offset from the center and 2.5 mm FWHM at 10 cm radial offset were found using span 3 and 3D-OSEM for HRRT-S. Equal values were found for HRRT-D using DOI. For span 9 data and FORE+2D-HOSP the axial resolution decreased at off center locations to 4 mm FWHM or more. The higher NECR of HRRT-D compared with HRRT-S is explained by the higher sensitivity, obtained with the additional crystal layer. Differences in scatter fractions might be explained by the difference in crystal energy spectra of both scanners. Comparison of resolution data of both scanners showed that high spatial resolution was preserved with DOI.