Samuel James Wharton1, Richard Bowtell1
1Sir Peter Mansfield Magnetic Resonance Centre, University of Nottingham, Nottingham, United Kingdom
Phase images generated at high field show exquisite anatomical contrast resulting from small changes of the NMR frequency linked to variation of the local magnetic susceptibility across tissues. When a significant contribution to the average susceptibility comes from NMR-invisible inclusions, the average NMR frequency offset is not however simply proportional to the average susceptibility. Here, we derive a simple expression based on the use of the conventional sphere of Lorentz, which allows the average NMR frequency offset to be calculated for compartments containing inclusions of varying shape and concentration. The expression is tested by comparison with the results of simulations.