1. A method for producing an image of a subject with a magnetic resonance imaging (MRI) system, the steps comprising: a) acquiring, using the MRI system, a first set of undersampled k-space data from the subject; b) acquiring, using the MRI system, a second set of undersampled k-space data from the subject; c) subtracting the first set of undersampled k-space data from the second set of undersampled k-space data in order to produce a differential k-space data set that is substantially more sparse than the first and second sets of undersampled k-space data; and d) reconstructing, from the differential k-space data and using a parallel image reconstruction method, an image of the subject in which background signal is substantially suppressed.