1. A method of calibrating an emission tomography subsystem in a combined MR (magnetic resonance) and emission tomography imaging system, the method comprising: providing a phantom that is configured such that the phantom is visible on a MR image; providing an attenuation map of the phantom, wherein the attenuation map includes an attenuation of the phantom; obtaining the MR image of the phantom; obtaining a position of the phantom from the MR image; mapping the attenuation map with the position of the phantom; and calibrating the emission tomography subsystem using the attenuation map mapped with the position of the phantom.