Evaluation of modified window-based scatter compensation in quantitative 177Lu-SPECT for a ring-configured CZT SPECT-CT.

Abstract

Background: The aim was to evaluate bias and precision for ¹⁷⁷Lu-activity-quantification using window-based scatter compensation in a ring-configured CZT gamma camera (StarGuide™, GE HealthCare). This paper extends a previous study by applying modified scatter compensation (MSC) methods introduced with the StarGuidePlus upgrade. The modified methods account for contamination of scatter windows by primary photons due to tailing. Whilst the original compensation addressed tailing in the 208 keV peak only, the upgrade extends the concept to the 113 keV peak and enables the estimated primary signal in the scatter windows to be included to the measured projections (MSC + P).

Methods: Listmode-data for a NEMA phantom and anthropomorphic phantom were reframed to accommodate the updated settings for MSC and MSC + P. Reconstruction was performed using OS-EM (two to 30 iterations, 10 subsets) with compensation for attenuation, scatter (DEW/TEW, MSC and MSC + P), spatial resolution, and penetration at 208 keV. Volumes-of-interest following the manufacturer-specified sphere sizes were defined and activity-concentration was quantified for each sphere and the total phantom activity across six (10 min) time-frames. Bias was evaluated with mean relative error and precision with coefficient of variation (CV).

Results: Imaging at 208 keV generally results in similar activity concentrations and total activity estimates for MSC and MSC + P (mean relative error: - 24.8 to - 23.4%, CV: 0.6 and 0.7% for the larges sphere). For 113 keV, a slightly better precision is obtained for MSC and MSC + P (mean relative error: - 15.4 and - 21.5%, CV: 0.6 and 0.7% for the largest sphere in the NEMA phantom) but a larger bias compared with TEW (mean relative error - 7.1%, CV 0.8%). The major difference is for total activity where the MSC and MSC + P decrease bias compared to TEW (5 and 4% versus 21% for 113 keV in the NEMA phantom).

Conclusions: The improvement in activity-concentration accuracy in regard to bias and precision is modest in small-volume high activity-concentration regions. However, the findings suggest potential value in preservation of total activity for both MSC and MSC + P.