Direct Saturation MRI: Theory and Application to Imaging Brain Iron

摘要

When applying RF saturation to tissue, MRI signal reductions occur due to magnetization transfer (MT) and direct saturation (DS) effects on water protons. It is shown that the direct effects, often considered a nuisance, can be used to distinguish gray matter regions with different iron content. Direct saturation effects were selected by reducing the magnitude and duration of RF irradiation to minimize confounding MT effects. Contrary to MT saturation spectra, direct water saturation spectra are characterized by a symmetric Lorentzian-shaped frequency dependence that can be described by an exact analytical solution of the Bloch equations. The effect of increased transverse relaxation, e.g. due to the presence of iron, will broaden this saturation spectrum. As a first application, direct saturation ratio (DSR) images were acquired to visualize gray matter structures in the human brain. Similar to T2*w images, the quality of DSR images was affected by local field inhomogeneity, but this could be easily corrected for by centering the saturation spectrum on a voxel-by-voxel basis. The results show that, contrary to commonly used T2*-weighted and absolute R2 images, the DSR images visualize all gray matter structures, including cortex. A direct correlation between DSR and iron content was confirmed for these structures.