Eighty channel multiplexed list mode data acquisition system for a 25–511 keV gamma camera

We have developed an 80 channel readout system for gamma cameras which sends PMT charges in list mode format via a USB bus to the acquisition computer. This is expected to allow for more accurate offline position and energy estimation with e.g. maximum likelihood methods. The goal is to apply this to cameras that can image gamma rays from 25 up to 511keV. The design of each channel includes amplification, pulse-shaping, trigger discrimination, gated integration, sample & hold as well as analog multiplexed ADCs to reduce cost and power consumption. A single FPGA (Field Programmable Gate Array) manages the whole acquisition process: triggering, gating, holding and multiplexing, event counting, event timing, data assembly storage and communication. In addition it rejects pulses suffering from tail pile-up. The channels are grouped in five sections, 16 channels each, around the central FPGA. Each section is connected to a 12-bits 40 Ms/s pipelined ADCs and a common USB2 commercial board, which sends the PMT signal charges to the computer together with 8 words of 12 bits for counting and general purpose functions (176 Bytes/event). Using a clinical gamma camera (EPIC, ADAC) with 55 photomultiplier tubes (PMT) of 2 and 3 inches (R1306/R1307) attached to a NaI(Tl) scintillator crystal, a sustained rate of 200k event/s (35 MB/s) with a dead time of 2.4 μs was observed when acquiring with all channels simultaneously. The signal input range is up to 115 pC with a noise level of 53 fC. The system can be easily adapted to a particular photon detector or to the decay time of a particular scintillator. By irradiating our gamma camera with different isotopes we found that the signal response is 96% linear for a wide range: 25-511 keV, which is very interesting for several novel clinical and pre-clinical applications.