Developing an Energy Resolution Model for Monte Carlo-Based SPECT Simulation and Reconstruction Using a GE Discovery NM/CT 670 Pro System

Sammanfattning

Aim: In order to apply Monte Carlo (MC)-based algorithms for simulations of gamma camera acquisitions, the intrinsic characteristics of the system must be properly defined. This study aimed to determine the energy resolution of a GE Discovery NM/CT 670 Pro gamma camera system, and to develop a nonlinear model for integration in the MC-based program Sahlgrenska Academy Reconstruction Code (SARec). A part of the study also aimed to evaluate different measuring conditions, such as dead time, source geometry, and set-up geometry, and their effect on the energy resolution. Methods and materials: Measurements were performed using said gamma camera system. Spectra were retrieved by extracting listmode data. A MATLAB script was created to determine the full-width half-maximum (FWHM) of the recorded photopeaks using a Gaussian fitting technique. To determine optimal measurement conditions, variations in FWHM under different acquisition circumstances were studied. The effects of dead time were examined by measuring a 131I source at various distances and at different time points. The impact of source geometry and measurement set-up were evaluated using 177Lu, in a syringe and a vial. An evaluation of count statistics was performed by measuring a 241Am source as well as a 177Lu syringe using different acquisition times. To develop the energy resolution model, intrinsic measurements were performed using 241Am, 177Lu, 99mTc, 133Ba, and 131I. The recorded FWHM was fitted as a nonlinear function of photon energy, thus obtaining the energy resolution model. Results: Significant dead time effects arose at counts rates over 180 kcps. Set-up geometry did not affect the measured energy resolution. A small difference in FWHM was observed between the 177Lu syringe and vial (0.11 and 0.43 percentage points at 113 and 208 keV, respectively). Calculated FWHM values did not change significant after acquiring 1,500 kcounts in the photopeak window. The measured energy resolution was 11–8.3% at E = 59–364 keV. The resulting energy resolution relationship was "FWHM = 0.1239 + 0.5428" √(E+0.021E^2 ) with a R2 of 0.89. Conclusion: The evaluated measurement conditions did not affect the energy resolution, although a connection between low count statistics and variations in the calculated FWHM of the peak was observed. The established nonlinear relationship between photon energy and energy resolution provided a better fit to the measured energy resolution, compared to the linear model employed in SARec today, and would thus provide better perquisites for accurate MC-based simulation and reconstruction. However, validation of the model should be performed to evaluate performance.