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Simultaneous acquisition SPECT brain imaging using a 99mTc labeled perfusion agent and an 123I labeled neurotransmitter agent has potential applications in diagnosis of neurodegenerative diseases. However, the close proximity of the photopeaks of the two isotopes results in significant crosstalk. We have previously optimized the acquisition parameters for 201Tl/99mTc dual isotope imaging based on an ideal observer signal-to-noise ratio (SNR). In this work, we determined the optimal energy window, i.e., the one providing the optimal tradeoff between minimizing the crosstalk and maximizing the detection efficiency for 99mTc/123I simultaneous acquisition brain SPECT. High-count listmode projections of an anthropomorphic striatal brain phantom were acquired. Projection images with different sets of energy window centers and widths were derived from the listmode data. Those projections were then weighted and summed appropriately to represent various 99mTc/123I activity concentration ratios. The values of a no-low-frequency ideal observer SNR were calculated for a lesion detection task using 99mTc only, 123I only and the simultaneous projection data. The maximum ideal observer SNR values were used as the metric to select the optimal acquisition parameters. For a 99mTc only acquisition, the optimal energy window was 30 keV wide centered at 144 keV, and for an 123I only acquisition the optimal window was 30 keV wide centered at 161 keV. For simultaneous acquisition, the 123I window tended to be higher and narrower and the 99mTc window tended to be lower and narrower than for the separate-isotope acquisition. Use of the projection images acquired with optimal parameters will provide improved data from which to perform simultaneous SPECT reconstructions.