1. A method for producing an image of a subject with a magnetic resonance imaging (MRI) system, the steps comprising: a) administering a contrast agent to the subject; b) inducing an oscillatory motion of the contrast agent by applying, with the MRI system, a spin-lock radiofrequency (RF) field; c) acquiring, with the MRI system, image data from magnetic resonance signals that are saturated responsive to the oscillatory motion of the contrast agent; and d) reconstructing, from the acquired image data, an image indicative of the contrast agent.