Signal-to-Noise Gain at Variable Randoms Ratio in TOF PET

Time of Flight (TOF) Positron Emission Tomography (PET) produces an improvement in signal-to-noise ratio (SNR) over conventional non-TOF reconstruction. This improvement increases with the size of the patient and with better coincidence time resolution of the system. As a first approximation in TOF PET reconstruction, this gain in SNR is independent of the scatter and randoms components of the total signal. On the other hand, recently a model has been proposed in which the SNR gain of TOF PET vs. conventional PET is a growing function of the randoms ratio (number of random coincidences divided by number of trues plus scattered coincidences). This would provide an additional advantage of TOF PET reconstruction over conventional PET, particularly in the presence of a large randoms ratio. In this work we show measurements with a NEMA image quality phantom on a Siemens Biograph mCT TOF PET/CT scanner, which confirm the prediction of the model for filtered back projection reconstruction as well as for iterative reconstruction algorithms. The implication is that not only is Time of Flight (TOF) reconstruction more advantageous for large patients, but also that the improvement is higher for PET scans performed at higher count rates, compared to a conventional non-TOF reconstruction.