A method of image reconstruction corrected for attenuation is provided for use with radioemission-based imaging, such as SPECT and PET. This method includes collecting measured emission projection data. The emission projection data, a reconstruction of the emission projection data, and a priori organ information are collectively analyzed to generate a body region estimate of the imaged subject. Each voxel of the body region estimate is then homogenously assigned an attenuation coefficient to generate an initial attenuation map estimate. An initial emission assumption is also generated based on a reconstruction of the emission projection data. The initial emission assumption and initial attenuation map estimate are then processed and refined to produce an image reconstruction.