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Quantitative measurements are increasingly used in cancer staging and therapy monitoring. This work investigates the impact of resolution modeling and time-of-flight reconstruction on the accuracy and variability of two types of activity concentration (AC) measurements, mean and maximum, in spheres of different sizes and contrast. Results are compared with conventional OSEM reconstruction that does not include these additions. Positive bias was seen in nearly all maximum AC measurements, particularly with OSEM in larger, low contrast spheres with no or small amounts of post-filtering. Maximum AC was far less dependent on sphere size in high contrast with combined resolution modeling and time-of-flight. Resolution modeling and time-of-flight were shown to reduce the variability of maximum AC measurements for all sphere sizes and post-filters. Negative bias was seen in all mean AC measurements, with mean AC recovery being greatest in smaller spheres with resolution modeling and time-of-flight. Mean AC recovery was comparable in the larger spheres for all reconstruction algorithms. Either no or very small reductions in the variability of mean AC measurements were seen with resolution modeling and time-of-flight.