A method is disclosed for calculating a spatially resolved value of an absorption parameter for a positron emission tomography (PET) scan of an examination object via magnetic resonance tomography (MRT). Magnetic resonance data is acquired within a first region lying within a field of view of a magnetic resonance system and within a second region bordering on the first and lying at the edge of the field of view. The method includes the spatially resolved calculation of a first value of the absorption parameter from the first MR data within the first region and of a second value from the second MR data within the second region. A three-dimensional parameter map is obtained from the first value. This parameter map is extended by the second value such that within the first region and the second region the parameter map has the value of the absorption parameter in spatially resolved form.