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 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 including a subject-dependent phase influence and an MR system-dependent phase influence; supplying said MR phase image data set to a computerized processor and, in said processor, automatically determining said MR system-dependent phase influence by selecting a region to be examined that is represented within the MR phase image data set by selecting a contour around said region, and calculating the system-dependent phase influence 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 system-dependent phase influence from the acquired MR phase image data set to determine the subject-dependent phase influence, and making the subject-dependent phase influence available in electronic form at an output of said processor.