Time alignment of time of flight positron emission tomography using the background activity of LSO

Time alignment of a positron emission tomography scanner (PET) is an important calibration as the nature of the data collected in PET is inherently time-correlated. This calibration is a key concern in scanners which employ time-of-flight (TOF) capability to localize events along a particular segment of a line of response. Emission data collected with TOF information requires the time difference measured by the scanner to be tightly coupled to the spatial information such that the true origin of the annihilation event of the radioisotope is colocated in both the time and spatial domains. There are many techniques to time align a PET scanner but all involve an external source. Our research shows that the background radiation of LSO provides an intrinsic source of radiation that can be used to measure and correct the time offsets for detector pixels. The physical distances of the static pixel locations can be compared to the measured time of flight of background events generated by LSO beta decay and cascade gamma traversing the PET field of view. The resulting information can be used to correct for the time offset between a pixel and all other pixels that the originating pixel is in coincidence with. This method can enable system operators to perform continuous time alignment whenever the system is idle, and can reduce personnel radiation exposure by reducing the need to handle sources during the calibration process.