1. A method for producing an image of a subject with a magnetic resonance imaging (MRI) system, the steps of the method comprising: a) administering to the subject, an agent that includes magnetic particles; b) applying an electromagnetic drive field to the subject at a drive frequency so that the magnetic particles produce magnetic fields that oscillate at the drive frequency and harmonics thereof; c) applying a spin-lock pulse to the subject that includes a spin-locking radio frequency (RF) pulse having a spin-lock frequency that is a harmonic of the drive frequency, the spin-lock pulse sequence producing rotary saturation of nuclear spins affected by the magnetic fields produced by the magnetic particles; d) acquiring image data from the subject using the MRI system following the application of the spin-lock pulse sequence; and e) reconstructing an image of the subject from the acquired image data, the reconstructed image depicting an image contrast indicative of the rotary saturation produced in step c).