The High Resolution Research Tomograph (HRRT) is the first PET scanner with depth of interaction (DOI) capability. Presently three different versions have been developed: one equipped with two 7.5 mm thick crystal layers (HRRT-DC), the second with only a single 7.5 mm crystal layer (HRRT-S) and the latest HRRT with two 10mm thick crystal layers (HRRT-D). In this study the performance of the new HRRT-D was assessed and compared with the other two HRRTs. The characteristics were measured according to the NEMA NU2 standards. Similar scatter fractions between all three scanners were observed. NEC rates of the HRRT-D were about 8 and 3 times higher than those of HRRT-S and HRRT-DC, respectively. However, spatial resolution of the new HRRT-D is somewhat lower than that of HRRT-DC and HRRT-S. Use of thicker crystals in the new HRRT-D improved the sensitivity and NECR performance significantly at the cost of only a small deterioration of the spatial resolution compared with the other HRRT designs.