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Published on: May 23, 2017
T2*-based fiber orientation mapping
Jongho Lee1, Peter van Gelderen2, Li-Wei Kuo2
1Advanced MRI Section, Laboratory of Functional and Molecular Imaging, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA; Department of Radiology, University of Pennsylvania, Philadelphia, Pennsylvania, USA.
Abstract:
Recent MRI studies at high field have observed that, in certain white matter fiber bundles, the signal in T(2)*-weighted MRI (i.e. MRI sensitized to apparent transverse relaxivity) is dependent on fiber orientation θ relative to B(0). In this study, the characteristics of this dependency are quantitatively investigated at 7 T using ex-vivo brain specimens, which allowed a large range of rotation angles to be measured. The data confirm the previously suggested variation of R(2)* (=1/T(2)*) with θ and also indicate that this dependency takes the shape of a combination of sin2θ and sin4θ functions, with modulation amplitudes (=ΔR(2)*) reaching 6.44±0.15 Hz (or ΔT(2)*=2.91±0.33 ms) in the major fiber bundles of the corpus callosum. This particular dependency can be explained by a model of local, sub-voxel scale magnetic field changes resulting from magnetic susceptibility sources that are anisotropic. As an illustration of a potential use of the orientation dependence of R(2)*, the feasibility of generating fiber orientation maps from R(2)* data is investigated.
Insights
High-field MRI reveals white matter T(2)*-weighted signal changes related to fiber orientation. This orientation dependency, explained by anisotropic magnetic susceptibility, could be used for generating brain fiber maps.
Area of Science:
- Neuroimaging
- Magnetic Resonance Imaging (MRI)
- Biophysics
Background:
- High-field MRI (7 Tesla) studies have observed orientation-dependent T(2)*-weighted MRI signals in white matter.
- This signal variation is linked to the angle (θ) of white matter fiber bundles relative to the main magnetic field (B(0)).
Purpose of the Study:
- To quantitatively investigate the characteristics of the orientation dependency of T(2)*-weighted MRI signals in white matter.
- To explore the potential of using this orientation dependency for generating fiber orientation maps.
Main Methods:
- Utilized ex-vivo human brain specimens for high-field (7 T) MRI.
- Measured T(2)*-weighted MRI signal across a wide range of fiber orientation angles (θ).
- Analyzed the relationship between R(2)* (1/T(2)*) and θ.
Main Results:
- Confirmed the previously suggested variation of R(2)* with fiber orientation θ.
- The dependency was characterized by a combination of sin2θ and sin4θ functions.
- Observed significant modulation amplitudes (ΔR(2)* up to 6.44±0.15 Hz) in major white matter tracts like the corpus callosum.
- This dependency is explained by anisotropic, sub-voxel scale magnetic field variations due to magnetic susceptibility.
Conclusions:
- The orientation dependence of R(2)* in white matter is quantitatively characterized at 7 T.
- This phenomenon can be attributed to anisotropic magnetic susceptibility at the sub-voxel level.
- The study demonstrates the feasibility of generating fiber orientation maps using R(2)* data, highlighting a potential new application in neuroimaging.
