Integrated front-end electronics for Silicon PhotoMultiplier readout in medical imaging applications

The 4D-MPET project aims to design a positron emission tomography detection module capable of working inside a magnetic resonant imaging system. The proposed detector will feature a threedimensional architecture based on two tiles of silicon photomultipliers coupled to a single LYSO scintillator on both its faces. Silicon photomultipliers are magnetic-field compatible photo-detectors with a very small size enabling novel detector geometries that allow the measurement of the depth of interaction. Furthermore they can be fabricated using standard silicon technology, have a large gain in the order of 10E6 and are very fast thus allowing evaluating the time of flight. Based on custom integrated circuits, the readout electronics include an innovative current mode front-end coupled to a novel time to digital converter. The former, implemented in AMS 0.35 micron SiGe-BiCMOS technology, features a very low input impedance (17 Ohm) current buffer and a large bandwidth (1 GHz), which lead to a time resolution of 100 ps FWHM. The time to digital converter exploits the combination of a submicron technology (UMC 65 nm LLLVT) together with a systolic topology so as to work at a high frequency of 2.5 GHz. This yields to a nominal time resolution of 29 ps ( whereas the photon energy is evaluated with a bin size of 400 ps by using a time over threshold technique. Finally, the depth of interaction measurement is performed by an external FPGA with a simulated spatial resolution of 1.3 mm FWHM along the z coordinate.