Diffusion MRI at Ultra High Field

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Diffusion MRI at Ultra-High Field******Diffusion magnetic resonance imaging (MRI) measures parameters describing water diffusion (i.e. random molecular motion) in tissue. Diffusion is strongly affected by cell membranes, which allows diffusion MRI to non-invasively probe microscopic tissue characteristics. Accordingly, diffusion MRI has provided a unique ability to study changes in the brain that occur during development and in disease. ******The diffusion of water in tissue has a complex dependence on the cellular properties of the tissue. However, conventional diffusion MRI uses a simplified signal model that does not have a direct physiological interpretation. Oscillating gradient spin-echo (OGSE) diffusion MRI extends the experiment to also measure the frequency dependence of diffusion. This can allow the estimation of specific cell properties like size or density, but this method is time consuming and requires high signal strength. ******Diffusion MRI at ultra high fields increases the signal strength; however, diffusion MRI suffers from severe distortions as magnetic field strength is increased. Some progress towards solving this distortion issue has been made, but these methods drastically increase scan time. Accordingly, diffusion MRI at ultra high field strengths is not commonly performed. The objectives of this proposal are to develop methods for distortion-free diffusion MRI at ultra high field (7 Tesla) and time-efficient OGSE diffusion MRI. This work will involve the design of new data sampling schemes and exploitation of data redundancies to reduce distortions and decrease scan times (i.e., "compressed sensing"), and the development of “model-based reconstructions” that model the physics of the MRI acquisition to remove any remaining distortions.******While these methods will be developed at ultra high field, they will be translatable to typical field strengths. Even at these lower field strengths, distortions are problematic in some regions of the brain. Accordingly, this work will open the door to a whole new set of neurological investigations in these regions, both at ultra high field and typical field strengths. In addition, changes in basic diffusion MRI parameters (e.g., “fractional anisotropy”) have been observed in aging, training, and many neurological conditions. While this tells us something has changed in the brain and potentially when it occurred, the answer to what has changed is usually not clear. Quantifiable tissue parameters, like cell size or density, enabled by OGSE can likely improve our basic physiological understanding of these changes, which would lead to improved understanding of the healthy brain and help to inform on possible interventions for neurological conditions.