Quantitative Susceptibility Mapping: Contrast Mechanisms and Clinical Applications

Abstract

Quantitative susceptibility mapping (QSM) is a recently developed MRI technique for quantifying the spatial distribution of magnetic susceptibility within biological tissues. It first uses the frequency shift in the MRI signal to map the magnetic field profile within the tissue. The resulting field map is then used to determine the spatial distribution of the underlying magnetic susceptibility by solving an inverse problem. The solution is achieved by deconvolving the field map with a dipole field, under the assumption that the magnetic field is a result of the superposition of the dipole fields generated by all voxels and that each voxel has its unique magnetic susceptibility. QSM provides improved contrast to noise ratio for certain tissues and structures compared to its magnitude counterpart. More importantly, magnetic susceptibility is a direct reflection of the molecular composition and cellular architecture of the tissue. Consequently, by quantifying magnetic susceptibility, QSM is becoming a quantitative imaging approach for characterizing normal and pathological tissue properties. This article reviews the mechanism generating susceptibility contrast within tissues and some associated applications.

Imaging whole-brain cytoarchitecture of mouse with MRI-based quantitative susceptibility mapping

“…For example, QSM at the resolution achieved here allows delineation of the olfactory bulb layers, retina layers, hippocampus, and cerebellum cell layers with whole brain coverage. The major contributors to magnetic susceptibility contrast in these tissues are related to the high iron concentration in the cell bodies, storage proteins and myelin (Duyn, 2013; Liu et al, 2015;Wang and Liu, 2015), as illustrated by the histological studies in Figs. 5 and 6.…”

“…Axon fiber bundles that are oriented perpendicular to the applied magnetic field appear more diamagnetic than when the bundles are oriented parallel to the field (Liu, 2010; Liu et al, 2015). In this study, the specimen was scanned with the long axis (rostral–caudal axis) of the mouse brain oriented perpendicular to the main magnetic field direction.…”

“…Quantitative susceptibility mapping (QSM) addresses this limitation by computing the spatial distribution of the underlying source of the phase, i.e. magnetic susceptibility (Bilgic et al, 2012; Carpenter et al, 2016; Li and van Zijl, 2014; Liu, 2010; Liu et al, 2014, 2015; Schweser et al, 2010; Wei et al, 2016; Xie et al, 2015). QSM reveals excellent image contrast and quantifies the magnetic properties of brain tissue, affected by e.g., iron in the cell body and myelin in the axons (Argyridis et al, 2014; Benner et al, 2013; Bilgic et al, 2012; Lee et al, 2012; Liu et al, 2012;Wang and Liu, 2015), which indicates that magnetic susceptibility may be used to improve the spatial resolution and contrast for cytoarchitecture of the whole brain.…”

An illustrated comparison of processing methods for phase MRI and QSM: removal of background field contributions from sources outside the region of interest

“…the brain) depends on its biochemical composition but usually varies only in the sub-ppm range. Hence, background fields are often one to two orders of magnitude stronger than tissue-related field variations from within the ROI and typically dominate the measured field [7, 8, 9]. …”

Characterizing Brain Iron Deposition in Patients with Subcortical Vascular Mild Cognitive Impairment Using Quantitative Susceptibility Mapping: A Potential Biomarker

“…Consequently, QSM is becoming a quantitative imaging approach for characterizing normal and pathological tissue properties by quantifying magnetic susceptibility (Liu et al, 2015c). Therefore, QSM is being evaluated in a growing number of clinical applications, including: (1) the separation of diamagnetic calcium from paramagnetic iron; (2) the quantification of myelination in the white matter; and (3) the quantification of iron deposition and blood by-products (Liu et al, 2015c). In recent studies, QSM has become increasingly prominent in the search for a quantitative biomarker for assessing the iron deposition.…”