US Patent 7193415 Magnetic lens, method and focus volume imaging MRI

A ferromagnetic lens in the presence of a DC magnetic field and a smaller AC magnetic field creates a localized minimum of the magnitude of the magnetic field vector of the combined magnetic field in a volume defined in free space away from the lens referred to as the focus volume. The localized minimum can be nonzero, and can create conditions for spin resonance in the focus volume. The focus volume defined in free space away from the lens can be very small, providing excellent resolution for magnetic resonance imaging, for example.