1. For a subject respirating in successive respiratory cycles, each comprising inspiration comprising mechanical intake of fresh gas and expiration comprising mechanical expulsion of breathed gas, a method for generating a quantitative indicator of ventilation that indicates a relation between said intake and said expulsion, said method comprising: operating a magnetic resonance (MR) data acquisition unit to acquire at least two lung-representing first MR image data sets from the subject, with one of said first MR data sets being acquired in an inspiratory phase of said respiratory cycles and another of said first MR data being acquired in expiratory phase of said respiratory cycles, each of said first MR image data sets comprising pixels having respective intensity values produced by MR signals originating from lung tissue in a lung of the subject; in a processor, automatically determining a difference in respective intensities between pixels in a region in said one of said first MR image data sets and pixels in a same region in said another of said first MR image data sets, said difference representing a density change of said lung tissue in said region due to said respirating; in said processor, also automatically determining a volume, selected from the group consisting of a volume of a lung of the subject and a volume of the thorax of the subject, from two of said first MR image data sets or from at least one second image data set acquired from the subject; and in said processor, automatically calculating a quantitative indicator of ventilation for said region, that is spatially-resolved by the pixels in said region, as a predetermined mathematical relationship of said density change of said lung tissue in said region and said volume of a lung, and making said spatially-resolved quantitative indicator of ventilation available as an output from said processor in a humanly-perceptible form.