1. A method of image reconstruction for positron emission tomography (PET), the method comprising: obtaining raw PET data from a PET scanner; analyzing the raw PET data to provide estimates of both emission activity and photon attenuation; wherein the analyzing the raw PET data comprises maximum likelihood expectation maximization (MLEM); wherein a first set of MLEM coordinates relates to voxels in a PET image; wherein a second set of MLEM coordinates is defined by a partition of each line of response (LOR) into LOR cells having equal length l; wherein MLEM computations assume all intersections between LOR cells and voxels have an intersection length of l.