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 based on a smoothing parameter 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; calculating, by the processor, an aggregate certainty value associated with the region of interest based on the emission data; calculating or receiving, by the processor, an estimated background activity concentration in the region of interest; calculating, by the processor, a normalized smoothing parameter by dividing the smoothing parameter associated with the region of interest by the calculated aggregate certainty value or dividing the smoothing parameter associated with the region of interest by the calculated aggregate certainty value and the estimated background activity concentration; calculating, by the processor, a size of a reconstructed lesion associated with the region of interest based on the reconstructed PET image; receiving or calculating, by the processor, at least one of an estimated size of a lesion associated with the region of interest and a local perturbation response in the region of interest of the reconstructed PET image; 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, wherein correcting the uncorrected quantitation comprises: selecting, by the processor, the pre-calculated contrast recovery coefficient based on at least one of the estimated size of the lesion, the uncorrected quantitation, the size of the reconstructed lesion, the local perturbation response, the normalized smoothing parameter and the estimated background activity concentration; and calculating, by the processor, a corrected quantitation as a function of: 
 ((the uncorrected quantitation−the estimated background activity concentration)/the pre-calculated contrast recovery coefficient value)+the estimated background activity concentration.