Optimizing targeted radionuclide therapy (TRT) requires patient-specific estimation of organ doses. The organ doses are estimated from quantitative nuclear medicine imaging studies, many of which involve planar wholebody scans. We have previously developed the Quantitative Planar (QPlanar) processing method and demonstrated its ability to provide more accurate activity estimates than conventional geometric-mean based planar (CPlanar) processing methods using physical phantom and simulation studies. The QPlanar method uses the maximum likelihood-expectation maximization (ML-EM) algorithm, 3D organ VOIs, and rigorous models of physical image degrading factors to estimate organ activities. However, the QPlanar method requires alignment between the 3D organ VOIs and the 2D planar projections and assumes uniform activity distribution in each VOI. This makes application to patients challenging. As a result, in this paper we propose an extended QPlanar (EQPlanar) method that provides independent-organ rigid registration and includes multiple background regions. We have validated this method using both Monte Carlo simulation and patient data. In the simulation study, we evaluated the precision and accuracy of the method in comparison to the original QPlanar method. For the patient studies, we compared organ activity estimates at 24 hours after injection with those from conventional geometric mean based planar quantification using a 24 hour post-injection quantitative SPECT reconstruction as the gold standard. We also compared the goodness of fit of the measured and estimated projections obtained from EQPlanar method to those from the original method at 4 other time points where gold standard data was not available. In the simulation study, more accurate activity estimates were provided by the EQPlanar method for all the organs at all the time points compared with the QPlanar method. Based on the patient data, we concluded that the EQPlanar method provided a substantial increase in accuracy of organ activity estimates from 24 hr planar images compared to the CPlanar using 24 hr SPECT as the golden standard. For other time points, where no golden standard is available, better agreement between estimated and measured projections was observed by using EQPlanar method compared to the QPlanar method. This phenomenon is consistent with the improvement in goodness of fit seen in both simulation data and 24 hr patient data. Therefore, this indicates the improved reliability of organ activity estimates obtained though EQPlanar method.
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