Susceptibility phase imaging with comparison to R2 mapping of iron-rich deep grey matter.

Magnetic resonance imaging with susceptibility phase is seeing increasing use, especially at high magnetic fields. Tissue susceptibility can produce unique phase contrast for qualitative or quantitative imaging of iron-rich deep grey matter. However, phase imaging has several established sources of error including inherent susceptibility field effects and artifacts from background phase removal. These artifacts have led to inconsistent findings in past works relating iron to phase in healthy deep grey matter. This study seeks to determine the relative artifactual contributions from inherent susceptibility fields and from high pass phase filtering, currently the most common and accessible background phase removal method. In simulation, phase is compared to a known susceptibility distribution, while R2 maps are used as the in vivo gold standard surrogate for iron in healthy volunteers. The results indicate phase imaging depends highly on filtering, structure size, shape and local environment. Using in vivo phase and R2 profiles, it is shown that different filtering values, commonly seen in the literature, can lead to substantially different phase measures. Correlations between phase and R2 mapping are shown to be highly variable between structures. For example, using a standard filter of 0.125 the slopes and correlation coefficients were 4.28×10(-4) ppm s and R=0.88 for the putamen, 0.81×10(-4) ppm s and R=0.08 for the globus pallidus, 5.48×10(-4) ppm s and R=0.72 for the red nucleus, and -14.64×10(-4) ppm s and R=0.54 for the substantia nigra. To achieve the most effective correlation to R2 we recommend using a filter width of 0.094 for the globus pallidus and putamen and 0.125 for the substantia nigra and red nucleus. The baseline phase measure should be obtained directly adjacent to the substantia nigra, and red nucleus to yield the most accurate phase values as demonstrated in simulation and in vivo. Different regression slopes are seen between subROIs within structures suggesting that regional iron accumulation within a structure is best studied with subROIs between different subject groups, not differences in phase values relative to the overall phase in one structure. Phase imaging with the standard high pass filter method has the potential to differentiate subtle iron changes in pathological processes compared to normal tissues with more reliability if specific filter strengths and measurement areas are appropriately applied on a structure dependent basis.