Development of a Digital Baseline Restorer for high-resolution PET detectors

For a data processing circuit using Anger positioning method on a high-resolution PET detector, it is difficult to measure precisely the position of the interaction, because a signal baseline fluctuates by causes such as temperature dependence of the detector sensitivity and the circuit drift, pulse pileup at high count rates, and scintillator afterglow. Particularly in the case of the number of scintillator elements much greater than the number of photo detector elements, both high signal precision and stability are required because the positioning error is caused by a slight baseline change. Usually AC-coupled circuit is utilized to reduce the influence of the baseline change, but may not work at high count rates. Then we adopted a DC coupling and have developed a DBLR (Digital Baseline Restorer) method to correct the baseline shift in real time. The amount of baseline shift is periodically estimated during the acquisition based on the shrinkage of a 2D position histogram map of the detector. The ratios of the total count of the outermost region and the inner region for each direction of the 2D map are fed back to correct the position calculation on FPGA. We evaluated the proposed method with a four-layer DOI (depth of interaction) detector that consists of 4,096 LGSO crystal elements and a 64-channel PS-PMT (position sensitive photomultiplier tube). The DBLR can effectively correct the baseline shift and reduce the map shrinkage to less than 0.3% even at a high count rate of 243 kcps.