Energy Scale-Factor Estimation for Use in Photomultiplier Tube Energy Calibration Using C-SPECT

An array of photomultiplier tubes (PMTs) provides energy readout for gamma cameras, leading to event selection and positioning. However, operational and environmental changes, such as temperature, can cause PMTs to “drift” away from their nominal energy readouts and, therefore, require a correction procedure to return to their reference energies. We present two methods for determining the energy-scale change of each PMT using data collected on C-SPECT, a dedicated cardiac single-photon emission computational tomography (SPECT) scanner. A scan of a vertical line source of 99mTc provides the data from which we produce an energy histogram for each of the 130 PMTs. Each energy histogram is composed of events passing an energy-fraction selection to give events closest to the PMT center. We consider energy fractions ranging from 0.25% to 5.00%. For our analysis, we use bootstrapping to create data realizations as well as emulating energy-scale changes (simultaneously and independently for all PMTs) in the data. Using the average relative error as a measurement of the accuracy and the standard deviation from taking bootstrapped replicates of our data as a measurement of our precision, we determine the energy-scaling to within -0.05% $\pm ~0.03$ % (mean and standard deviation, respectively).