1. A magnetic resonance imaging (MRI) method comprising: performing imaging of a subject using an MRI machine, the subject having been administered with a contrast agent demonstrating a magnetic-field dependent variation in one or more MRI properties, wherein a polarizing magnetic field strength of said MRI machine is shifted during an MRI pulse sequence prior to image acquisition; and processing acquired images to yield an enhanced contrast, difference image, wherein said performing comprises: shifting the strength of the polarizing magnetic field of the MRI machine in one direction during a relaxation portion of the MRI pulse sequence and thereafter acquiring an image; and then shifting the strength of the polarizing magnetic field of the MRI machine in an opposite direction during another relaxation portion of the MRI pulse sequence and thereafter acquiring an image, wherein said shifting comprises conditioning an electromagnet of said MRI machine to generate magnetic field pulses of opposite polarities to shift the polarizing magnetic field in the one direction and in the opposite direction.