1. A computer-implemented method for penalized-likelihood reconstruction of a Positron Emission Tomography (PET) image, the computer including a processor, the method comprising: generating, by the processor, a regularization function in which a smoothing parameter is modulated by one or more data-independent spatially variable modulation factors to compensate for sensitivity variations in a PET voxel dataset; and reconstructing, by the processor, the PET image from the PET emission dataset using the regularization function.