Improved walk-correction method for timing measurements in PET detector

For Time-of-Flight (TOF) PET detectors the timing resolution is one of the critical parameters for improving image quality. One of the effects degrading the timing resolution is the statistical fluctuation of the number of scintillation photons. The timing degradation is introduced by leading edge discriminator (LED) because the time of crossing the comparator threshold is signal-dependent. It is still possible to correct for this effect by measuring the total number of optical photons and this method is known as walk correction. When the threshold of LED is set very low to achieve the best timing resolution, the energy walk depends on the emitted light in the first ns of scintillation, but not the total integrated number of photons. There is a correlation between the integrated and initial numbers of photons, but the latter number is more significant for energy walk at low threshold. We describe an improved walk correction approach by measuring the rise time at the beginning of the pulse and then compensating the delay variation based on this value, thus improving the timing resolution with a minor increase of the complexity of readout electronics.