Simplified FPGA-based data acquisition system for PET

A data acquisition system for a single-ring, high-resolution, small-animal PET scanner is described. This single-ring test system is part of the development of a multiring scanner. The system was designed to be as simple (short development cycle, and low cost) and as flexible as possible while maintaining high performance standards necessary for high-resolution quantitative studies. Unlike the multiring system electronics under development, the test system does not use a clock, but identifies coincidences by overlapping logic signals. The ring consists of up to 18 multicrystal MiCE modules, each of which is coupled to a resistive summing board that produces two X and two Y position signals. These are sent to the inputs of an analog multiplexer circuit. Total energy signals are also produced and used as inputs to NIM discriminators. A field programmable gate array (FPGA) is used for all system logic functions. This allows software development, testing, implementation, and modification of logic circuitry without hardware modification. Inputs to the FPGA are the outputs of the 18 discriminators. The FPGA logic produces and locks an event signal whenever two modules along allowed lines of response trigger within a preset resolving time. The FPGA also produces detector codes identifying the detectors involved in the event. These are recorded and also sent to the multiplexer to switch the position signals from the modules involved in the event to a buffered CAMAC ADC system for digitization. A "not busy" signal from the ADC is used to reset the FPGA releasing the locked signals allowing further event acquisition.