1. Method determine a background phase in phase values of a phase image data set that is acquired from an examination subject, wherein the background phase is determined in a partial region of the phase image data set, comprising the following steps of: providing magnetic resonance phase image data set acquired from the examination subject to a computerized processor; through an input unit in communication with the computerized processor, manually establishing a substantially closed, planar contour in the phase image data set around the partial region, the planar contour having a contour area with a width of at least one pixel of the phase image data set; in said computerized processor, automatically determining the phase values in the partial region from an assumption that the spatial curve of the background phase is a harmonic or quasi-harmonic function, by determining the phase values of the pixels in the partial region based on the phase values in the planar contour.