An iterative energy-centroid method for recalibration of PMT gain in PET or gamma camera

In gamma and positron emission tomography cameras, photomultiplier tubes (PMT) are still the most popular photosensors. Gain uniformity of the PMTs in these instruments is very important for preventing degradation in image resolution and distortion. When an Anger decoding detector head is exposed to a uniform flood source without lead collimation, the energy spectrum acquired by any single PMT has no photopeak for conducting recalibration. A method using the centroid of a wide energy spectrum even without photopeak as a gain-tuning index is presented in this paper. With a uniform flood illumination, without collimation, the gain equalization of PMTs within an array can be tuned to its initial level by an iterative centroid match. If an electrically adjustable gain control circuitry such as tunable PMT high-voltage or variable-gain amplifier is applied, the recalibration can be done automatically with this method. Our experiments demonstrated a 2.5%-3% gain accuracy that is enough to eliminate distortion in total energy and two-dimensional decoding. Counting efficiency increases significantly, and the calibration or system-tuning time will be shortened because of efficiency improvement.