Development of a daily quality check procedure for the High-Resolution Research Tomograph (HRRT) using natural LSO background radioactivity

A daily quality check plays an important role for clinical medical systems like a positron emission tomograph (PET). To guarantee image quality a constant monitoring of the scanner integrity is essential. On present PET scanners, the built-in transmission sources are being used for the measurement of a uniform blank scan and the computation of detector-block efficiencies. These efficiencies can be compared to reference data and deviations indicate changes of the scanner condition. On the High-Resolution Research Tomograph (ECAT HRRT) the design of the transmission source makes this method impractical. For that reason we investigated the use of the natural background radioactivity from the new scintillator material LSO as a uniform source. A procedure was developed to measure detector-block sensitivities and energy spectra directly. An initial quality check scan is the basis for comparison of daily scans, so that gain shifts of photomultipliers and specific hardware defects can be detected. With an improved list mode based data acquisition it was possible to reduce the acquisition time to around 3 h despite the low LSO count rate, so that this routine is as practical as previous procedures in clinical routine.