Non-Linear Phase Evolution: The Dominant Source of the Average Frequency Difference between Grey Matter & White Matter in Gradient Echo MRI?

Samuel James Wharton¹, Richard Bowtell¹

There is an ongoing debate about the source of the small difference in the frequency of signals from cortical grey matter (GM) and adjacent white matter (WM) that is manifested in phase images of the human brain. Recently, it was shown that the evolution of the phase with TE in WM is non-linear. Here, we assess the contribution of this non-linear phase evolution to the average GM/WM frequency difference at 7 T. The results show that non-linear phase effects may be the dominant source of the GM/WM frequency difference seen in phase images acquired using long echo times (> 17ms).