A fully integrated pulse charge generator embedded in a 64-channel readout ASIC dedicated to a PET/CT detector module

The LabPET^TM II is a new Positron Emission Tomography/Computed Tomography (PET/CT) scanner dedicated to small animals imaging under development at the Université de Sherbrooke. Its design requires nearly 37 000 detectors spread over a ring of 15 cm in diameter and 12 cm of axial length. This new scanner will break the submillimetric spatial resolution barrier in PET mode thanks to a new double 4 × 8 array of 1.1 × 1.1 mm² avalanche photodiode (APD) coupled to an 8 × 8 LYSO crystal matrix. In order to readout individually each pixel of this detector block, a 64-channel Application Specified Integrated Circuit (ASIC) was developed based on Time-over-Threshold (ToT) technique. The ToT computations allow the extraction of both the energy and time of occurrence of PET signals. There are also adjustable gains located in each individual analog front-end chain that enables the detection of low energy X-ray photons (~ 42 keV) required for CT imaging mode. As a result of the complexity of this fully mixed-signal chip, calibrating the scanner and testing every single channel can be a cumbersome job on PCB using an external charge injector equipment. To facilitate those operations, a fully integrated pulse charge generator (PCG) was designed for the LabPET™ II ASIC. The PCG injects a 3-bit adjustable amount of charge in each channel in order to verify their impulse response and to calculate intrinsic energy and time resolution for both PET and CT modes. It can also be used to evaluate the analog front-end electronics noise, by knowing the electronic gain of each channel individually. The PCG can inject a charge that ranges from 35 fC to 56 fC by steps of 2.6 fC in PET mode and from 2.3 fC to 5.1 fC in steps of 0.3 fC at a rate of 1 kHz.