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) applying with the MRI system, a spin-lock radiofrequency (RF) field that establishes a spin-lock condition in transverse magnetization such that the transverse magnetization in the spin-lock condition is saturated by magnetic fields generated by oscillatory motion of the contrast agent, wherein the magnetic fields generated by the oscillatory motion of the contrast agent oscillate at a frequency of the oscillatory motion; c) acquiring with the MRI system, image data from magnetic resonance signals that are indicative of the transverse magnetization in the spin-lock condition as it is saturated by the magnetic fields generated by the oscillatory motion of the contrast agent; and d) reconstructing, from the acquired image data, an image indicative of the contrast agent.