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Susceptibility-weighted imaging (SWI) is a magnetic resonance imaging (MRI) technique that enhances image contrast by using the susceptibility differences between tissues. It is created by combining both magnitude and phase in the gradient echo data. SWI is sensitive to both paramagnetic and diamagnetic substances which generate different phase shift in MRI data. SWI images can be displayed as a minimum intensity projection that provides high resolution delineation of the cerebral venous architecture, a feature that is not available in other MRI techniques. As such, SWI has been widely applied to diagnose various venous abnormalities. SWI is especially sensitive to deoxygenated blood and intracranial mineral deposition and, for that reason, has been applied to image various pathologies including intracranial hemorrhage, traumatic brain injury, stroke, neoplasm, and multiple sclerosis. SWI, however, does not provide quantitative measures of magnetic susceptibility. This limitation is currently being addressed with the development of quantitative susceptibility mapping (QSM) and susceptibility tensor imaging (STI). While QSM treats susceptibility as isotropic, STI treats susceptibility as generally anisotropic characterized by a tensor quantity. This article reviews the basic principles of SWI, its clinical and research applications, the mechanisms governing brain susceptibility properties, and its practical implementation, with a focus on brain imaging.

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Frequency shift of gradient-echo MRI provides valuable information for assessing brain tissues. Recent studies suggest that the frequency and susceptibility contrast depend on white matter fiber orientation. However, the molecular underpinning of the orientation dependence is unclear. In this study, we investigated the orientation dependence of susceptibility of human brain in vivo and mouse brains ex vivo. The source of susceptibility anisotropy in white matter is likely to be myelin as evidenced by the loss of anisotropy in the dysmyelinating shiverer mouse brain. A biophysical model is developed to investigate the effect of the molecular susceptibility anisotropy of myelin components, especially myelin lipids, on the bulk anisotropy observed by MRI. This model provides a consistent interpretation of the orientation dependence of macroscopic magnetic susceptibility in normal mouse brain ex vivo and human brain in vivo and the microscopic origin of anisotropic susceptibility. It is predicted by the theoretical model and illustrated by the experimental data that the magnetic susceptibility of the white matter is least diamagnetic along the fiber direction. This relationship allows an efficient extraction of fiber orientation using susceptibility tensor imaging. These results suggest that anisotropy on the molecular level can be observed on the macroscopic level when the molecules are aligned in a highly ordered manner. Similar to the utilization of magnetic susceptibility anisotropy in elucidating molecular structures, imaging magnetic susceptibility anisotropy may also provide a useful tool for elucidating the microstructure of ordered biological tissues.

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