Magnetic Susceptibility Imaging: White Matter Fiber Model
Daeun Kim1
1University of Southern California, United States

Synopsis

In MRI, magnetic susceptibility-induced contrast has been widely used due to its sensitivity and accessibility to magnetic properties of biological tissues. For example, in white matter of the brain, magnetic susceptibility variations are affected by both microstructural and molecular arrangements, which provides unique insights into understanding biologically important tissue features such as myelin. The main goal of this lecture is to 1) understand the magnetic susceptibility effects of white matter microstructure and 2) review a hollow cylinder fiber model for white matter that characterizes the magnetic susceptibility effects to explain susceptibility-induced contrast in white matter.

Magnetic Susceptibility Imaging

Magnetic susceptibility refers to the degree of magnetization in a material due to interaction with an external magnetic field1. When materials are placed in an external magnetic field, magnetic susceptibility variations due to different magnetic properties induce the perturbation of local magnetic fields, and this perturbation generates a shift of the resonance frequency. The resonance frequency shifts can provide useful information about the magnetic properties of the materials (e.g., biological tissue properties in MRI) 2-4 or lead to undesired artifacts5 in the NMR signal.

In MRI, magnetic susceptibility imaging aims to exploit such useful information about biological tissue magnetic properties, and studying susceptibility-induced contrasts have been of great interest leading to a variety of applications6-13. For example, in the brain, magnetic susceptibility imaging has provided unique insights into studying neuronal activities (measured by the blood-oxygen-level-dependent (BOLD) signal)7, vasculature8, concentrations of iron9 and myelin4,10-11, white matter microstructure and fiber orientation12-13, etc. Among these applications, this lecture will focus on the susceptibility effects of white matter and will discuss modeling approaches that properly account for the complexity of white matter microstructure.

Magnetic Susceptibility in White Matter

Susceptibility-induced local field variations are highly reflected in gradient echo (GRE) or free induction decay (FID) signal, resulting in signal losses in the magnitude signal and resonance frequency shifts in the phase signal14. In the magnitude signal, the signal loss is characterized by the decaying constant 1/T2* = 1/T2 + 1/T2' (or R2* = R2 + R2’) in which the susceptibility effect is represented by T2'. In the phase signal, the susceptibility effect is represented by the mean local frequency after removing undesired global phase effects. In white matter, it has been well-recognized that both T2* and the mean local frequency are sensitive to not only microstructure and fiber orientations but also molecular arrangements15-17. At the microscopic level (i.e., the cellular level), three water compartments (in the myelin sheath, the intra-axonal space, and the extra-axonal space) are taken into account, and the susceptibility-induced contrast can be affected by the distribution of those compartments. It has been observed that the dominant source of the susceptibility-induced contrast is myelin that exists in myelin sheath as a hollow cylinder. The anisotropic distribution of the myelin sheath introduces orientation-dependence of the T2* and frequency relative to the external magnetic field18-22. In addition to the microstructural anisotropic effect, the T2* and frequency are affected by an anisotropic susceptibility at the molecular level12-13. In white matter, the anisotropic susceptibility arises from lipid bilayers in the myelin sheath.

Hollow Cylinder Fiber Model

Susceptibility-induced contrast in white matter can be better understood by accurately modeling white matter accounting for its anisotropic microstructure and anisotropic susceptibility. For the modeling, a hollow cylinder model has been developed23-25. In this lecture, the details of the hollow cylinder modeling will be reviewed. The hollow cylinder model adopts the water compartmentalization in which the myelin sheath is modeled as an infinite hollow cylinder, with the intra-axonal water in an inner space and the extra-axonal water in an outer space. This model further assumes that the magnetic susceptibility within the myelin sheathe is anisotropic. Under this assumption, fiber bundles are simulated at a certain orientation relative to the B0-field. It has been shown that this type of modeling can accurately predict the magnitude and phase behaviors of the susceptibility-induced MRI signal, potentially revealing important microstructural information related to neuronal pathology.

Acknowledgements

No acknowledgement found.

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Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)