Roadmap to fully-digital PET/CT scanners

The ever-increasing needs of molecular imaging now require significant upgrade of conventional PET and CT scanners. Upcoming research protocols ask for low doses, submillimeter resolution, high sensitivity and multimodality. Current scanner technologies are mainly based on analog ASICs having a long design-cycle which hinders rapid scanner improvements and can hardly keep up with the new requirements of biomedical research. With new high-speed processors and configurable electronics, combined with early digitization of the signals from detectors, digital signal processing can flexibly and concurrently deal with many of those requirements. The present paper highlights past, present and foreseen developments in PET/CT signal processing. In particular, different model fits, filtered interpolation and neural networks are compared for timestamping and pulse shape discrimination. Recursive (ARMAX, AR...) and non recursive (Wiener, Fast Fourier transforms, Wavelets...) filtering are compared for crystal identification. Advanced pile-up correction, baseline restoration and energy measurement in photon-counting CT are also discussed. Finally, new techniques dealing with realtime event processing for Compton-scatter LOR computation and alternate random estimation will be briefly introduced. Pros and cons of each method are discussed and the best methods identified for a roadmap to fully digital PET/CT scanning is presented.