1. A method of reconstructing an emission tomography image in a combined MR (magnetic resonance) and emission tomography imaging system, comprising: 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, using the raw emission tomography scan data, 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; reconstructing the emission tomography image using the raw emission tomography scan data and the final attenuation model; and wherein the using of the raw emission tomography scan data to obtain the missing part of the subject includes, using the raw emission tomography scan data to reconstruct a first emission tomography image with a first attenuation correction model, and obtaining the missing part from the first emission tomography image.