Toward in vivo histology: A comparison of quantitative susceptibility mapping (QSM) with magnitude-, phase-, and R₂^*-imaging at ultra-high magnetic field strength

Quantitative magnetic susceptibility mapping (QSM) has recently been introduced to provide a novel quantitative and local MRI contrast. However, the anatomical contrast represented by in vivo susceptibility maps has not yet been compared systematically and comprehensively with gradient (recalled) echo (GRE) magnitude, frequency, and R₂^* images. Therefore, this study compares high-resolution quantitative susceptibility maps with conventional GRE imaging approaches (magnitude, frequency, R₂^*) in healthy individuals at 7T with respect to anatomic tissue contrast. Volumes-of-interest were analyzed in deep and cortical gray matter (GM) as well as in white matter (WM) on R₂^* and susceptibility maps. High-resolution magnetic susceptibility maps of the human brain exhibited superb contrast that allowed the identification of substructures of the thalamus, midbrain and basal ganglia, as well as of the cerebral cortex. These were consistent with histology but not generally visible on magnitude, frequency or R₂^*-maps. Common target structures for deep brain stimulation, including substantia nigra pars reticulata, ventral intermediate nucleus, subthalamic nucleus, and the substructure of the internal globus pallidus, were clearly distinguishable from surrounding tissue on magnetic susceptibility maps. The laminar substructure of the cortical GM differed depending on the anatomical region, i.e., a cortical layer with increased magnetic susceptibility, corresponding to the Stria of Gennari, was found in the GM of the primary visual cortex, V1, whereas a layer with reduced magnetic susceptibility was observed in the GM of the temporal cortex. Both magnetic susceptibility and R₂^* values differed substantially in cortical GM depending on the anatomic regions. Regression analysis between magnetic susceptibility and R₂^* values of WM and GM structures suggested that variations in myelin content cause the overall contrast between gray and white matter on susceptibility maps and that both R₂^* and susceptibility values provide linear measures for iron content in GM. In conclusion, quantitative magnetic susceptibility mapping provides a non-invasive and spatially specific contrast that opens the door to the assessment of diseases characterized by variation in iron and/or myelin concentrations. Its ability to reflect anatomy of deep GM structures with superb delineation may be useful for neurosurgical applications.