Multiplexed Eighty Channel PMT List-Mode Data Acquisition System for 27-511 keV Gamma Ray Imaging

We have developed an 80-channel readout system for gamma cameras which acquires PMT signals and stores them in list-mode format on a computer. The system combines low noise and a large signal range to handle PMT signals from a wide range of SPECT and PET tracers simultaneously (27 to 511 keV). All channels of the read-out system have a gated integrator to determine the charge per event. Each 16 channels are multiplexed to a 12-bit 40 MS/s pipelined ADCs. Using multiplexing only 5 ADCs are needed which share a commercial USB 2.0 interface board to send the event data to the computer. The signal input range is up to 120 pC with a noise level of 57 fC. The integral non-linearity is better than 1% for all channels, while the channel response has a gain non-uniformity less than 2.4% for the different channels. The crosstalk between channels is better than 64 dB. The dead time is 2.5 μs, including a 760 ns integration period. The system can handle a sustained event rate of 155 kcps on the USB 2.0 bus. Connected to a clinical gamma camera we show that the isotopes 125 I (27-35 keV), 99m Tc (140.5 keV) and 22 Na (511 keV) can all be imaged without electronic adjustments thus enabling, for instance, simultaneous pinhole PET/SPECT. With this PMT list-mode acquisition system, advanced offline event positioning and energy estimation with e.g. maximum likelihood methods are enabled.