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

The ECAT HRRT (high resolution research tomograph) is a 3D 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 2001 protocols. The HRRT´s imaging performance has previously been tested with phantoms and FDG scans performed in animal and human brains showing significantly improved spatial resolution, below 2.5 mm for animal studies and below 3 mm for brain studies. 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 in low count rate situations, for ex. when the scatter fraction is determined (NEMA 2-2001 protocol). We show here how these difficulties can be overcome and indicate ways to improve the scatter fraction estimate.