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.