Scatter Fraction, count rates, and Noise Equivalent Count Rate of an RPC TOF-PET system: Simulation study following the NEMA NU2-2001 standards

Scatter Fraction (SF) and Noise Equivalent Count Rate (NECR) of a 240 cm wide axial field-of-view Positron Emission Tomography (PET) system based on Resistive Plate Chamber (RPC) detectors with 300 ps Time of Flight (TOF) resolution were studied by detailed simulations using GEANT4 (version 9.2, patch 04), following the NEMA NU2-2001 protocol, both with the standard 70 cm length phantom and an axially extended one (180 cm length). Simulation-produced data were processed to account for detector readout with a τ_is = 0.2 μs non-paralyzable dead time for time signals followed by a τ_ps paralyzable dead time for position signals, for which pileup events can be rejected or accepted with a coarse position (1 cm σ Gaussian distribution and 3 cm bins in the axial and transaxial directions, respectively). Concerning NECR, the best coincidence trigger strategy consisted on performing a multiple time window coincidence sorter retaining both single coincidence pairs and all single pairs out of multiple coincidences, followed by acceptance of coincidences for which a direct TOF-reconstructed point falls inside a tight region surrounding the phantom. In these conditions, SF was 51.8% for the standard NEMA NU2-2001 phantom and 53.7% for the extended one, and independent of τ_ps. With a value τ_ps = 3.0 μs, and rejecting pileup position events, peak NECR was ~167 kcps at ~7.6 kBq/cm³ for standard NEMA NU2-2001 phantom and ~164 kcps at ~3.0 kBq/cm³ for the extended phantom. For an achievable value of τ_ps = 1.0 μs, NECR was ~349 kcps (~486 kcps) at ~7.6 kBq/cm³ (~16.8 kBq/cm³) for the standard NEMA NU2-2001 phantom, and ~323 kcps (~460 kcps) at ~2.9 kBq/cm³ (~6.5 kBq/cm³) for the extended one. In conclusion, present and previous works reveals that RPC TOF-PET is expected to outperform current PET - canners.