Abstract: Radionuclide emission imaging is improved by correcting emission-transmission data for attenuation along calculated path lengths and through calculated basis material. X-ray transmission data are used to develop an attenuation map through an object which is then used in reconstructing an image based on emission data, Radiation detection circuitry is provided which has different operating modes in detecting the x-ray and emission photons passing through the object. An iterative process is used to reconstruct the radionuclide distribution using the radionuclide projection data and the attenuation map based on physical characteristics of the object being imaged. Subsets of the complete radionuclide projection data are used to reconstruct image subsets of the radionuclide distribution. The image subsets can be generated concurrently with the acquisition of the radionuclide projection data or following acquisition of all data.