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Published on: May 19, 2023
Micro-compartment specific T2* relaxation in the brain
Pascal Sati1, Peter van Gelderen, Afonso C Silva
1Translational Neuroradiology Unit, Neuroimmunology Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
Abstract:
MRI at high field can be sensitized to the magnetic properties of tissues, which introduces a signal dependence on the orientation of white matter (WM) fiber bundles relative to the magnetic field. In addition, study of the NMR relaxation properties of this signal has indicated contributions from compartmentalized water environments inside and outside the myelin sheath that may be separable. Here we further investigated the effects of water compartmentalization on the MRI signal with the goal of extracting compartment-specific information. By comparing MRI measurements of human and marmoset brain at 7T with magnetic field modeling, we show that: (1) water between the myelin lipid bilayers, in the axonal, and in the interstitial space each experience characteristic magnetic field effects that depend on fiber orientation (2) these field effects result in characteristic relaxation properties and frequency shifts for these compartments; and (3) compartmental contributions may be separated by multi-component fitting of the MRI signal relaxation (i.e. decay) curve. We further show the potential application of these findings to the direct mapping of myelin content and assessment of WM fiber integrity with high field MRI.
Insights
High-field MRI reveals distinct magnetic properties of water in different brain compartments. This allows for separating signals from myelin and axons to map myelin content and assess white matter integrity.
Area of Science:
- Neuroimaging
- Magnetic Resonance Imaging (MRI)
- Biophysics
Background:
- High-field MRI is sensitive to tissue magnetic properties, influenced by white matter (WM) fiber orientation.
- NMR relaxation studies suggest water compartmentalization within and around myelin contributes to the MRI signal.
Purpose of the Study:
- To investigate water compartmentalization effects on MRI signals.
- To extract compartment-specific information for enhanced brain imaging.
- To explore applications in myelin mapping and WM integrity assessment.
Main Methods:
- Comparing 7 Tesla MRI measurements in human and marmoset brains.
- Utilizing magnetic field modeling to analyze signal dependence on fiber orientation.
- Applying multi-component fitting to MRI signal relaxation (decay) curves.
Main Results:
- Water in different compartments (inter-bilayer, axonal, interstitial) exhibits orientation-dependent magnetic field effects.
- These effects lead to distinct relaxation properties and frequency shifts for each compartment.
- Compartmental contributions can be separated using multi-component signal fitting.
Conclusions:
- Water compartmentalization significantly impacts high-field MRI signals.
- Separating these compartmental contributions enables direct myelin mapping.
- This approach offers potential for assessing white matter fiber integrity with advanced MRI techniques.
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