The ECAT HRRT: an example of NEMA scatter estimation issues for LSO-based PET systems

The ECAT high-resolution research tomograph (HRRT) is a three-dimensional (3-D)-only dedicated brain positron emission tomograph with LSO and GSO scintillators. In this paper, the system has been looked at as an example of issues that need to be addressed when evaluating LSO-based system following the recent NEMA NU 2-2001 protocols. The LSO scintillators contain the isotope ¹⁷⁶Lu which is radioactive and creates a small amount of single counts and, depending on the low level discriminator threshold for validated singles, also true coincidence events from a cascade of gamma rays following the beta decay of ¹⁷⁶Lu into ¹⁷⁶Hf. The presence of intrinsic random and true coincidence events has an effect on the scatter fraction determination if using the guidelines according to NU 2-2001. We show here that these guidelines have to be changed in order to obtain accurate determination of the scatter fraction. In this paper, we have determined the scatter fraction for different low level discriminator settings. Since the scatter fraction determinations also comprise full count rate studies for the NEMA phantom, we have in addition extracted the sensitivity information for singles, true + scatter, and for the NEC data.