Grey/white matter contrast in phase images: is it susceptibility or is it not?

Introduction Recently great interest has been devoted to the tissue contrast observed in phase images, not only in veins and iron rich regions (1) but also contrast between WM and cortical GM (2). The tissue phase variations are thought to originate from either tissue susceptibility variations (3) or variations of macromolecules content affecting the water chemical shift (4). The two mechanisms are expected to display different spatial patterns: the susceptibility contribution should be a convolution of the tissue distribution with the dipole field kernel (5) while the macromolecular contribution is expected to be characterized simply by the tissue distribution. In this study we aimed at estimating the susceptibility and/or macromolecular contribution to the gray matter (GM) / white matter (WM) phase contrast by taking advantage of this spatial information.