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Tumor avidity for Ga-67 is known to be correlated with the histopathologic stage of disease, yet quantitative imaging has remained elusive because Ga-67 images are seriously degraded by several factors. We have previously optimized and evaluated on projection images a general spectral (GS) approach to compensate for patient scatter and Pb x-rays from the collimator. We also recently developed a joint, multienergy iterative algorithm (OSEM-J) to compensate for nonuniform attenuation and for distance- and energy-dependent collimator resolution. In the work reported here, we evaluated the overall activity quantitation performance achieved by combining GS with OSEM-J, and compared this to the (almost ideal) performance obtained when reconstructing primary unscattered photons, as well as to that obtained with another proposed iterative approach (OSEM-S) in which the sum of the two lowest photopeak windows was reconstructed using a single, energy-weighted attenuation map. We simulated 96 projections of an anthropomorphic phantom containing seven 2-cm-diameter tumors with Ga-67 concentration values 4 to 8 times that of the local background. Sixteen noise realizations containing lesions and 16 without lesions were generated, and least-squares estimates of tumor activity were calculated for each reconstruction method. The mean bias of tumor estimates for primary+OSEM-J, GS+OSEM-J, and GS+OSEM-S was, respectively, 1.2%, 1.6%, and 8.0% The average precision of the corresponding estimates was 15.0%, 19.1%, and 24.8% These results suggest that use of a spectral-based estimate of contaminant photons within a joint multiple-energy iterative reconstruction leads to nearly optimal performance in quantitation of Ga-67 tumor activity.

We determined theoretical bounds on performance in estimating /sup 99m/Tc and /sup 123/I 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 /sup 99m/Tc-Trodat and /sup 123/I-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 /spl radic/2 at high count levels. From physical considerations alone, simultaneous /sup 99m/Tc//sup 123/I 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.