US Patent 7480525 Positive contrast MRI using positive and negative field inhomogeneity

The acquisition of multiple images at slightly different time shifts from the spin echo (or from TE=0 for gradient echo sequences), allows the separation of on-resonance spins from off-resonance spins by encoding this information in the received signal. The excitation pulse can be a standard broadband excitation that will excite all spins. The separation of on- and off-resonance spins is then performed on the received signal. The polar and equatorial lobes of a magnetic particle such as SPIO produce signals from excited water molecules near the particle which are frequency offset above and below the frequency of signals from water molecules unaffected by the particle.