A comparison of count rate performance for 15O-water blood flow studies in the CTI HR+ and Accel tomographs in 3D mode

The count rate performance of two commercial PET tomographs, the CTI HR+ (BGO) and Accel (LSO), is compared from simulations of blood flow studies. Data were derived from in vivo studies of blood flow (¹⁵O-water) in the brain, heart and abdomen using the EXACT3D tomograph (6 rings of HR+ detector blocks). Noise-equivalent count (NEC) rate curves were simulated from these data for The HR+ and Accel using (a) standard pulse integration times and coincidence windows, (b) measured sensitivities and scatter fractions, and (c) adjustment of singles rates to reflect the different axial fields-of-view. The original administered doses for the EXACT3D studies were 163 MBq (brain), 92.5 MBq (heart) and 189 MBq (abdomen). These doses are constrained by the sensitivity of the EXACT3D (at least twice as high as standard commercial tomographs) and the count rate limit of the acquisition system. If the original (in vivo) EXACT3D doses are doubled to compensate for the lower sensitivity, the simulations show that peak NEC for the HR+ is still higher than or very close to that for the Accel in the three types of study. The superior count rate performance of the Accel gives improved NECs when dose is further increased. Thus the use of LSO detectors does not automatically confer advantages in count rate performance as this depends on the specific tomograph construction and the administered dose. These results reflect the lower intrinsic sensitivity of the Accel and its similar scatter fraction to the HR+ despite the higher light output.