Monitoring Energy Calibration Drift Using the Scintillator Background Radiation

Scintillating materials commonly used in nuclear medicine can contain traces of isotopes that naturally emit gamma or beta radiation. Examples of these are ¹³⁸La contained in LaBr₃ and other Lanthanum based scintillators, and ¹⁷⁶Lu contained in LSO, LYSO, LuYAP and other Lutetium based scintillators. In particular,¹⁷⁶Lu decays into ¹⁷⁶Hf and emits a beta particle with maximum energy 589 keV, and a cascade of gamma rays of energies 307 keV, 202 keV and 88 keV. We propose to use the background radiation for monitoring of detector calibration drift and for self-calibration of detectors in complex detector systems. A calibration drift due to random or systematic changes in photomultiplier tube (PMT) gain was studied in a Siemens PET scanner, based on LSO blocks. Both a conventional radioactive source (⁶⁸Ge, 511 keV photons from electron-positron annihilation) and the LSO background radiation were used for calibration. The difference in the calibration peak shift at 511 keV estimated with the two methods was less than 10%.