1. A computer-implemented method for partial volume correction in Positron Emission Tomography (PET) image reconstruction, the computer including a processor, the method comprising: receiving, by the processor, emission data related to an activity distribution; reconstructing, by the processor, the activity distribution from the emission data by maximizing a penalized-likelihood objective function to produce a reconstructed PET image; quantifying, by the processor, an activity concentration in a region of interest of the reconstructed PET image to produce an uncorrected quantitation; and correcting, by the processor, the uncorrected quantitation based on a pre-calculated contrast recovery coefficient value to account for a partial volume error in the uncorrected quantitation.