Self-Time Alignment Method for Patient-Based Quality Control

Abstract

The development of silicon photomultiplier (SiPM) based detectors resulted in significant improvement of time-offlight (TOF) resolution in PET. Higher standards in validating TOF information are desirable, since TOF reconstruction is sensitive to the misplacement of events based on time measurements. On photomultiplier (PMT) based scanners, time alignment estimation was stable over long periods of time, so frequent time calibration procedures were not necessary. New high performance SiPM PET scanners will require close monitoring in performance, at least over the initial phase of clinical use. This may result in a renewed interest in a self-time alignment (TA) method, primarily as a quality control (QC) procedure. In this work we use the idea behind the self TA procedure, where unknown object activity is estimated from non-TOF data. Modeled TOF data are compared against measured data in order to find individual detectors’ time offsets (TOs). TOs are estimated simultaneously by matching the TOF center of mass between modeled and measured TOF data. This method can be considered as a refinement of the currently used Siemens TA procedure. The method is generalized for continuous bed motion (CBM) acquisition, which is becoming standard on Siemens scanners. An additional advantage is that the CBM daily QC phantom can be axially shorter, since it is not required to cover all the field of view at once. The algorithm was tested on a Siemens Vision, next generation SiPM PET/CT scanner prototype, and mCT scanner. An image quality phantom and patient data were used for this investigation. The SiPM prototype TOF reconstructions showed that relatively small errors in time offsets can be detected in the final image. This leads to higher standards for newly designed scanners. The patient data reconstruction showed that undetected residual TOs can be corrected by use of the self TA procedure. The mCT scanner CBM patient data verified the method application for this scanning mode.