A method and an system are disclosed for reconstructing an emission tomography image in a combined MR (magnetic resonance) and emission tomography imaging system. In at least one embodiment, the method includes obtaining an MR image of a subject, the subject being clipped in the MR image; obtaining raw emission tomography scan data of the subject; determining a missing part of the subject clipped in the MR image; using information of the MR image and the determined missing part to obtain a final attenuation model of the subject; and reconstructing the emission tomography image using the raw data and the final attenuation model.