1. A method for determining an attenuation map for use in positron emission tomography (PET) and for determining homogeneity information relating to the magnetic resonance (MR) magnetic field within the framework of a single magnetic resonance image recording, the method comprising: executing, by a MR-PET device, a single three-dimensional MR gradient echo sequence having a first echo time and a second echo time during a single excitation; recording, by the MR-PET device, a first and a second image data record of the single three-dimensional MR gradient echo sequence during the first and the second echo time, respectively, a phase difference between a water and a fat signal amounting to zero during the first echo time and amounting to 180 degrees during the second echo time; and determining, by the MR-PET device, the attenuation map for use in correcting PET data from fat/water ratios obtained from the first and second image data records by way of Dixon technology; excluding all voxels with a signal intensity below a first threshold value from at least the second image data record by using a mask; and determining, by the MR-PET device, homogeneity information from phase differences of adjacent voxels, taking into account only non-excluded voxels of the first and second image data records whereby both the homogeneity information and the attenuation map are determined from a single three-dimensional gradient echo sequence.