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We determined theoretical bounds on performance in estimating 99mTc and 123I dopamine transporter and receptor striatal activity concentrations under both sequential and simultaneous imaging conditions. Anatomically realistic brain projection datasets, representing pre-synaptic transporter and post-synaptic dopamine receptor studies using 99mTc-Trodat and 123I-IBZM, respectively, were generated by Monte Carlo (MC) simulation. Both simultaneous and sequential acquisitions were considered; twice as many total counts were simulated for each isotope for simultaneous imaging. For each condition, signal-to-noise ratios (SNR) were calculated as the ratio of the true striatal activity to the square root of the Cramer-Rao lower bound (CRB) on precision of activity concentration estimation for both known and unknown striatal size. In all cases, simultaneous imaging led to better performance by a factor which approached the theoretical limit of √2 at high count levels. From physical considerations alone, simultaneous 99mTc/123I brain imaging is expected to lead to improved performance in striatal activity estimation compared to sequential imaging, for the same total imaging time. Moreover, simultaneous imaging offers physiological advantages over sequential imaging.