Calibration of Dual-Ended Readout of Axially Oriented 100-mm-Long LYSO Crystals for Use in a Compact PET System

We are investigating the use of 100-mm-long axially oriented scintillator crystals read out at either end by position-sensitive photomultiplier tubes (PSPMTs) for application to a small-animal positron emission tomography (PET) system. A system design of this type requires a robust and efficient calibration of the detector response in the light-sharing direction in order to accurately decode the location of photon interaction. In this paper, we evaluate methods for determining the axial positioning calibration of dual-ended readout of lutetium yttrium oxyorthosilicate (LYSO) crystals wrapped in either Teflon or 3M Enhanced Specular Reflector (ESR). We compare a collimated irradiation method to a uniform irradiation method recently proposed by Shao. In the collimated irradiation method, an electronically collimated beam of 511-keV photons aimed perpendicular to the long axis of the crystal is used to irradiate the crystal at nine locations in steps of 1 cm. The ratio of the two PMT signals at each irradiation position is used to give the axial-positioning calibration relating ratio signal to position. In the method by Shao, the crystal was irradiated using either a uniform Ge line source, a Na point source, or the intrinsic background activity in the LYSO. To check the validity of both calibration methods for unbalanced detectors, three voltage variations ranging from 750-800 V were used for the two PMTs. For cases with balanced detector gains, the calibration curves obtained using uniform irradiation and Shao's method agreed very well with the reference curve obtained from the collimated irradiation measurements. For the case with detectors that had unbalanced gain, agreement between uniform irradiation and collimated irradiation methods was only achieved if the trigger threshold energies of the two PMTs were adjusted to give similar single-event rates in both PMTs. These findings suggest that Shao's calibration method using uniform irradiation will provide a simple, accurate, and rapid method for calibration of 100-mm-long dual-ended readout detectors.