Author: Julia Brosch-Lenz, Munir Ghesani, Francesc Massanes, Michael Morris, Babak Saboury, Eliot Siegel, Alexander Hans Vija 👨🔬
Affiliation: Institute of Nuclear Medicine, Siemens Medical Solutions USA Inc., Molecular Imaging 🌍
Purpose: Quantitative imaging of the radiopharmaceutical distribution is crucial for treatment evaluation and dosimetry. However, as patient numbers for Lutetium-177-(177Lu)-labelled therapies continue to grow, imaging duration becomes a significant challenge, and concerns about quantitative reproducibility increase. This study explores a “triple”-peak 177Lu-SPECT reconstruction that include the 54-65 keV X-ray peaks and the 113kev photopeak in addition to the 208keV photopeak to increase image counts.
Methods: Four timepoint 177Lu-PSMA-617 SPECT images were obtained for five patients at a Siemens Symbia Pro.specta™ X3 SPECT/CT with 5/8” crystal and MELP collimator between day 0 and 8 post-therapy. The data was acquired in three bed positions (head-thigh) over 360° with 60 projections per detector with 5s/projection. Besides xSPECT QuantTM reconstruction using the 208keV photopeak (“SinglePeak”), a quantitative prototype reconstruction used a combination of 208keV, 113keV, and 57keV windows (“TriplePeak”) with adapted energy window-based scatter correction and custom calibration. Count statistics, quantitation, and noise characteristics as assessed by coefficient of variation (CoV) were evaluated between SinglePeak and TriplePeak reconstructions. Time-activity-curves were fitted in MIRDfit and absorbed doses were calculated using MIRDcalc.
Results: The scatter-corrected counts ratio in the total field-of-view between TriplePeak and SinglePeak was 2.2. The CoVs of SinglePeak vs. TriplePeak for kidneys, liver, spleen, and lesions across patients and timepoints were 40±21% vs. 39±21%, 54±32% vs. 53±32%, 64±49% vs. 62±49%, and 25±5% vs. 26±5%. Percentage differences in activity between reconstructions were 11±5%, 10±1%, 10±6%, and 8±7% for kidneys, liver, spleen, and lesions. Percentage differences in absorbed dose were 8±4%, 10±1%, 9±2%, and 7±4%.
Conclusion: This study demonstrated an increase in counts for TriplePeak compared to SinglePeak reconstruction that warrants short 177Lu-SPECT acquisition times with improved signal-to-noise level. Small impact on image quantification and dosimetry was found. Next steps will focus on optimization of reconstruction methodology and scatter correction including simulations and more patients.