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.