1. A method of generating an emission image data attenuation correction map for an object, comprising: obtaining emission image data of said object from a nuclear medicine imaging modality; subjecting the obtained emission image data to a predetermined threshold to obtain thresholded emission image data; back-projecting the thresholded emission image data to obtain a 3D image; initializing the back-projected 3D image to an initial attenuation correction map obtained from a second imaging modality, with attenuation values outside a field of view of said second imaging modality being estimated based on said emission image data; forward-projecting the initial attenuation correction map; obtaining regularization parameters for the forward-projected initial attenuation correction map; applying the obtained regularization parameters to said initial attenuation correction map to obtain an optimized initial attenuation correction map; replacing said initial attenuation correction map with said optimized initial attenuation correction map; generating a 3D estimate of an emission image by applying attenuation correction factors generated from said optimized attenuation correction map; and generating a 3D estimate of an attenuation correction map by performing a maximum-likelihood calculation using said generated 3D emission image estimate.