Evaluation of a 25–511keV list mode readout system for a large field-of-view gamma camera

We evaluate the performance of a large field-of-view gamma camera (9.5mm thick NaI(Tl) crystal) connected to a new 80-channel list-mode read out system. The final goal of this setup is its use for simultaneous sub-mm imaging of positron emitters (511keV) and single photon emitting tracers (25–400keV) in a small animal scanner. All of the 55 photomultiplier tubes (PMT) in our test gamma camera have been plugged into the channels of the readout system which gives at the output the digitized charge-integrated values of the PMT signals for the gamma ray events in the scintillation crystal. In addition to that, we developed data processing and calibration methods to obtain corrected images from the detector. Only those PMT values above a pedestal value are written to disk in order to minimize the size of list files without losing relevant information. In an offline analysis we estimate the position and energy of the gamma interactions by using a charge centroid finding algorithm in combination with energy, linearity and uniformity corrections obtained from calibration data. We evaluated the performance closely following NEMA NU 1–2007 standard over energies ranging from 27 keV (I-125) up to 511 keV (Na-22). We found that the spatial resolution varies from 7.1 mm (27keV) to 3.0 mm (511 keV). The energy resolution ranges from 22.7% (27keV) to 9.7% (511keV). The differential spatial linearity was found to be better than 0.3 mm over the useful field of view (UFOV) for all energies. The integral and differential uniformity were found to be better than 2.8% and 1.8% over the whole range of energies. Our readout system and software are showing a good performance over an unprecedented wide range of energies with calibration data taken only for 27, 140 and 511 keV. With more sophisticated statistical positioning algorithms operating on the individual PMT signals we believe that the UFOV can eventually be enlarged with the same detector hardware and that both spatial and energy resolution can be slightly improved.