A simple and improved digital timing method for positron emission tomography

A simple and improved digital timing method was developed for positron emission tomography (PET). This method, the so-called 'initial rise interpolation method', is based on an important characteristic of gamma signals: a properly pre-amplified and sampled gamma signal pulse can be characterized to arrive with an initial rise from the baseline and then reach a maximum rise. The pulse arrival time was obtained by calculating the intersection of the initial rise line with the baseline for each gamma signal pulse. Using an 8-channel 100 MHz free-running ADC and FPGA combined data acquisition (DAQ) card, three digital timing methods were employed to measure the coincidence timing resolution of two types of recently developed 3 mm×3 mm PET sensors (a fast and a slow GAPD). The results showed that the initial rise interpolation method provides the best timing resolution for both types of GAPDs: 0.7 ns FWHM for fast GAPD and 1.5 ns FWHM for slow GAPD (digital CFD: 1.5 and 2.2 ns FWHM; maximum rise interpolation: 1.8 and 2.7 ns FWHM). By implementing the initial rise interpolation method into the FPGA of DAQ card, PET images of two 18F line sources were acquired successfully using a pair of 4×4 GAPD-LYSO array detectors (single pixel size: 3 mm×3 mm). The acquired image spatial resolution was a 3.1 mm FWHM. Furthermore, the simulation was performed to evaluate the effects of the pulse rise time, pulse amplitude and front-end noise level on the timing resolution estimated using the three digital timing methods. In accordance with the measurement results, the simulation results also showed that the initial rise interpolation method provided the best timing resolution. These results show that this simple and improved digital timing method is reliable and useful for the development of high performance PET.