1. A method to separate a magnetic resonance (MR) system-dependent phase influence from a subject-dependent phase influence in phase values of an MR phase image data set of an examination subject, comprising the steps of: operating an MR data acquisition unit to acquire MR phase image data, comprising phase values, from an examination subject in the MR data acquisition unit, wherein two different tissue types with respectively different resonance frequencies supply a signal contribution to at least some phase values in said MR phase image data set, said phase values each comprising a subject-dependent phase value contribution to the respective phase value that is produced by the examination subject and an MR system-dependent phase value contribution to said respective phase value that is produced by the physical attributes of said MR data acquisition unit; supplying said MR phase image data set to a computerized processor and, in said processor, automatically determining said MR system-dependent phase value contribution by selecting a region to be examined that is represented within the MR phase image data set by designating a contour around said region, and calculating the MR system-dependent phase value contribution in said region using the selected contour and assuming that a spatial curve of background phase corresponds to a harmonic or quasi-harmonic function; and in said processor, subtracting the determined MR system-dependent phase value contribution from the acquired MR phase image data set to determine the subject-dependent phase value contribution, and making the subject-dependent phase value contribution available in electronic form at an output of said processor.