Susceptibility Effects and Phase Contrast

Introduction Magnetic susceptibility effects resulting from the heterogeneous magnetic properties of living systems are of obvious importance for high field magnetic resonance imaging and spectroscopy. The relatively large differences in the magnetic susceptibility of water, bone and air, gives rise to the large length-scale magnetic field inhomogeneity, which causes distortion and signal attenuation in MRI [1], as well as a variety of other off-resonance effects. Smaller differences in the susceptibility of other MR-signal generating tissues (e.g. grey matter, white matter and cerebrospinal fluid) give rise to local variation in the phase of gradient echo images, which forms the basis of susceptibility weighted imaging (SWI) and phase contrast [2-12] both of which offer great promise for high field MRI. This presentation will: (i) describe the electromagnetic theory that links susceptibility and magnetic field; (ii) show how the spatial variation of magnetic susceptibility translates into a spatially varying NMR frequency; (iii) briefly discuss the artefacts caused by such variation before focussing on the exploitation of the small, local frequency offsets in phase contrast and SWI.