Abstract
Diffusion MRI has become an invaluable tool for studying white matter microstructure and brain connectivity. The emergence of quantitative susceptibility mapping and susceptibility tensor imaging (STI) has provided another unique tool for assessing the structure of white matter. In the highly ordered white matter structure, diffusion MRI measures hindered water mobility induced by various tissue and cell membranes, while susceptibility sensitizes to the molecular composition and axonal arrangement. Integrating these two methods may produce new insights into the complex physiology of white matter. In this study, we investigated the relationship between diffusion and magnetic susceptibility in the white matter. Experiments were conducted on phantoms and human brains in vivo. Diffusion properties were quantified with the diffusion tensor model and also with the higher order tensor model based on the cumulant expansion. Frequency shift and susceptibility tensor were measured with quantitative susceptibility mapping and susceptibility tensor imaging. These diffusion and susceptibility quantities were compared and correlated in regions of single fiber bundles and regions of multiple fiber orientations. Relationships were established with similarities and differences identified. It is believed that diffusion MRI and susceptibility MRI provide complementary information of the microstructure of white matter. Together, they allow a more complete assessment of healthy and diseased brains.
Highlights
Probing the microstructure of white matter has major applications in a number of neurological diseases and disorders
The abilities to map brain connectivity in vivo were initially made possible because of the discovery of diffusion anisotropy in the white matter (Moseley et al, 1990) and the development of a range of techniques based on this phenomenon, most notably the development of diffusion tensor imaging (DTI) and tractography (Basser et al, 1994, 2000; Conturo et al, 1999; Mori et al, 1999)
It is often reported that DTI, or more generally diffusion MRI, is the only method available for imaging white matter fiber tracts in vivo and non-invasively, which is a clear testament to the importance of DTI for studying brain connectivity
Summary
Probing the microstructure of white matter has major applications in a number of neurological diseases and disorders. By rotating intact mouse brains in small-bore animal 7T scanner, Liu showed that the magnetic susceptibility of white matter demonstrated strong orientation dependence (Liu, 2010). This orientation dependence has been observed by Lee et al on segments of corpus callosums from a postmortem human brain (Lee et al, 2010). Variations among tissue magnetic susceptibility cause a tissue dependent frequency shift that manifests as a phase shift in gradient echo images By measuring this frequency shift, a susceptibility tensor may be determined for each voxel of the brain. A recent study has utilized this relationship to perform fiber tracking in the mouse brain which has demonstrated similar tracks with DTI tractography in large fiber bundles (Liu et al, 2012)
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