Optimizing Acquisition Parameters in TOF PET Scanners

Noise equivalent counts (NEC) have been used as a measure of PET image quality for many years. As a result, clinical patient dosage and scan times have been determined based upon this figure of merit. Since NEC represents a global measure of signal-to-noise ratio (SNRNEC) in non time-of-flight (Non TOF) PET, we used a lesion detectability metric (channelized hotelling observer signal-to-noise ratio, CHO SNR) to better assess the impact of patient dosage, scan time and time-of-flight (TOF) PET on image quality. Our results indicate that there is a linear relationship between SNR_NEC and CHO SNR for most singles rate or activity levels, with an increase in SNRnec due to either increased scan time or increased activity leading to a subsequent increase in CHO SNR. At very high rates, there is an indication that the rate of increase in CHO SNR is lower compared to the gain in SNRNEC as measured on the Gemini TF PET scanner. This reduced gain may be due to some degradation in timing and spatial resolution at high rates. Our results also show that the gain in CHO SNR for a small 20-cm diameter cylinder due to TOF is modest (14%). For a larger 35-cm diameter cylinder we see a greater increase in CHO SNR with TOF (gain of 42%), leading to the possibility of reducing scan time by half, or achieving better lesion detectability over a similar Non TOF scanner.