1. A method for correcting attenuation of positron emission tomography (PET) image data using magnetic resonance (MR) imaging data, the method comprising the steps of: a) acquiring PET sinogram data of a subject; b) acquiring MR image data of the subject that is registered with respect to the PET sinogram data; c) producing, using the PET sinogram data and MR image data, an estimate attenuation sinogram using a data consistency condition and joint entropy based prior; and d) reconstructing a PET image using the estimated attenuation sinogram.