Calibration of dual-ended readout of axially oriented 100 mm long LYSO crystals for use in a compact PET system

Abstract

We are investigating using dual-ended readout of axially-oriented long thin scintillator crystals in detectors for a compact geometry animal PET system. In this work we determine the axial-positioning calibration of a 2 × 2 × 100 mm3 LYSO crystal wrapped in either Teflon or 3M Enhanced Specular Reflector (ESR) readout at either end by position sensitive photomultiplier tubes (PSPMTs). We compare two methods: the first using a 9-step collimated irradiation method and the second using the method proposed by Shao [Shao et al., Med. Phys. 35(12)5829–5840, 2008], which uses uniform irradiation. In the 9-step 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, we investigated two variations. In the first, the crystal was irradiated using a uniform Ge-68 line-source at 25 cm and in the second, with a 22Na point source located at 10 cm from the crystal. To check the validity of both the calibration methods for unbalanced detectors, three voltage variations ranging from 750–800 V were used for two PMTs resulting in a higher gain for one detector than the other. LYSO intrinsic activity was also used for the calibration of balanced and unbalanced detectors. It was found in both the methods that the ratio signal for the case of the Teflon wrapped crystal is not linearly dependent with irradiation position, rather following a 3rd order polynomial shape. For the case of ESR the relation was linear. Both the linear and nonlinear axial-positioning functions determined from the nine-step irradiation method show excellent agreement with those determined using Shao's method for a 100 mm long crystal. This method can thus be used to provide a simple axial-positioning calibration with a single measurement both for linear and nonlinear response of 100 mm long crystals. The method will significantly simplify the calibration of these dual-ended readout detectors, especially for the cases of a large number of crystals in a 2D crystal array in a detector. In the case of unbalanced detectors, the calibration is sensitive to gain variation; however, the spatial resolution remains the same (results not shown). The 9-step results reasonably agree with the uniform irradiation of 22Na for central 75% of crystal region. The LYSO intrinsic activity source uniform irradiation results have good agreement with those of 22Na for the three voltage variations. LYSO intrinsic activity can thus be used instead of an external source for the calibration of our proposed complete PET system.