Quantitative SPECT Imaging of the alpha-emitter Th-227 with Ra-223 Crosstalk Correction

578 Objectives: Th-227 and its daughter Ra-223 are important novel nuclides for alpha immunotherapy. In order to calculate the dose received by the patient, accurate quantification of the activity is essential. Quantitative SPECT imaging of Th-227 is challenging because of the low yield of photons, complicated emission spectrum and its decay to Ra-223. The aim of this study was to develop a quantitative Th-227 SPECT reconstruction method that accounts for Ra-223 contribution to Th-227 imaging to provide accurate estimates of absorbed doses in the different organs. Materials & Methods: We incorporated a Ra-223 estimation model into an iterative reconstruction algorithm of Th-227. The method has been previously developed and evaluated for quantitative I-131 SPECT reconstruction with model-based downscatter compensation. The Th-227 projection image, contaminated by Ra-223 emissions, was first reconstructed using the OS-EM algorithm with compensations for attenuation (A), scatter (S), and full collimator-detector response (D), including geometric and septal penetration and scatter effects. These compensations were based on the assumption that all photons were from Th-227. The reconstructed image was then used as the input activity distribution to obtain Ra-223 projections, assuming that the distribution came from Ra-223, and scaled according to Th-227/Ra-223 parent-daughter decay scheme. The estimated Ra-223 projection was then used as an additional crosstalk correction in a second reconstruction to obtain a Th-227 image. However, because the first reconstructed image was not the true Ra-223 activity distribution, we repeated the estimation process several times until the change in the Ra-223 estimate was small. The converged Ra-223 estimate was then used in a final reconstruction to obtain the Th-227 image. We evaluated the method using simulated and experimental data. The simulation study was for 6 spheres with an activity concentration of 2.5 kBq/cm3 and sizes ranging from 1 to 28 cm3 in a water-filled cylinder with no background activity using the SIMIND MC simulation program. We generated 20 noise realizations for the cases when the ratios of Ra-223 to Th-227 were 0 and 1, which models the cases where Th-227 was pure and 42 days post purification, respectively. In addition, we conducted a phantom experiment by imaging a 3-cm diameter sphere, filled with a solution containing 2 micro-curies of Thorium-227, placed in a water-filled cylinder using a Siemens Symbia Intevo dual-head SPECT/CT system, equipped with a 9.5 mm NaI(Tl) crystal and a medium energy collimator, for 128 minutes. Projection images were acquired immediately after Th-227 was purified and on days 14 and 20. We used an acquisition energy window of 200-350keV. Results & Discussion: Simulation — We reconstructed the noisy projections and evaluated the quantitative accuracy of the estimated activities in the spheres for the cases of imaging pure thorium and when Ra-223 was present. The errors of the reconstructed Th-227 activities in the largest sphere were -1.8% and -2.7%, for the two cases, respectively. For the smallest sphere, the errors were -21% and -18% for the two cases, resulting largely from partial volume effects. Experiment — We evaluated the quantitative accuracy of the estimated activities in the sphere for the different imaging scenarios. The activities were underestimated by 9±2.3%, 11±1.9% and 12±2.1%, for the images acquired at days 0, 14 and 20, respectively. We also observed very good agreement between the Monte Carlo simulated and experimental Thorium-227 projections. Conclusion: We have developed and evaluated a quantitative reconstruction method for Th-227 SPECT that accounts for Th-227 decay to Ra-223. For better quantitative results, iterative based reconstruction methods with many updates and partial-volume compensation are needed. These results suggest that clinical quantitative Th-227 SPECT imaging is feasible and can account for Ra-223.