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Abstract BackgroundSingle-photon emission computed tomography (SPECT) provides a mechanism to perform absorbed-dose quantification tasks for$$\alpha$$ $\alpha$-particle radiopharmaceutical therapies ($$\alpha$$ $\alpha$-RPTs). However, quantitative SPECT for$$\alpha$$ $\alpha$-RPT is challenging due to the low number of detected counts, the complex emission spectrum, and other image-degrading artifacts. Towards addressing these challenges, we propose a low-count quantitative SPECT reconstruction method for isotopes with multiple emission peaks. MethodsGiven the low-count setting, it is important that the reconstruction method extracts the maximal possible information from each detected photon. Processing data over multiple energy windows and in list-mode (LM) format provide mechanisms to achieve that objective. Towards this goal, we propose a list-mode multi energy window (LM-MEW) ordered-subsets expectation–maximization-based SPECT reconstruction method that uses data from multiple energy windows in LM format and include the energy attribute of each detected photon. For computational efficiency, we developed a multi-GPU-based implementation of this method. The method was evaluated using 2-D SPECT simulation studies in a single-scatter setting conducted in the context of imaging [$$^{223}$$ ${}^{223}$Ra]RaCl$${_2}$$ ${}_2$, an FDA-approved RPT for metastatic prostate cancer. ResultsThe proposed method yielded improved performance on the task of estimating activity uptake within known regions of interest in comparison to approaches that use a single energy window or use binned data. The improved performance was observed in terms of both accuracy and precision and for different sizes of the region of interest. ConclusionsResults of our studies show that the use of multiple energy windows and processing data in LM format with the proposed LM-MEW method led to improved quantification performance in low-count SPECT of isotopes with multiple emission peaks. These results motivate further development and validation of the LM-MEW method for such imaging applications, including for$$\alpha$$ $\alpha$-RPT SPECT.