Design of an 81-Channel Read-out System for a Hybrid PMT/SIPM Modular Gamma-ray Camera

AdaptiSPECT-C is a dedicated human brain SPECT system currently under development. The system utilizes hybrid PMT/SiPM modular gamma-ray scintillation cameras with 81 photosensor channels for each camera. Here we present the read-out system design for the modular cameras. A front-end board inside the camera enclosure performs the current-to-voltage conversion for the photosensor signals. The back-end board receives the 81 analog signals and estimates time of interaction and amount of light acquired by each photosensor. The energy estimation is performed by sigma-delta modulation (SDM). A non-uniform 2-bit SDM is utilized for triggering and timing. The SDM approach utilized in the front-end board reduces the complexity of the system compared to conventional ADC methods, while maintaining system performance. The digital side of the SDMs is implemented on the FPGA of a system-on-chip, which also contains an ARM-based processor. A direct memory access (DMA) controller implemented on the FPGA writes the data to on-board memory. An embedded lightweight TCP/IP stack reads the data from memory and streams it to the system computer. The back-end board also monitors and controls camera temperature, and supplies power to the camera.