Improved diffusion MRI with an ultra-strong gradient head insert
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Demyelination and dysmyelination diseases cause damage to the white matter in the central nervous system by disrupting the myelin water sheaths on axons. Diffusion MRI has been suggested to be crucial in uncovering microstructural alterations in white matter. However, conventional diffusion MRI methods are unable to detect myelin water due to its short transverse relaxation time (T2). The use of ultra-strong gradients can significantly shorten the echo time thereby regaining sensitivity to myelin water, but presents technical difficulties including expensive setup and risk of peripheral nerve stimulation. In this work, the potential of using a gradient insert that can be interfaced with existing scanners is assessed to boost diffusion MRI experiments. First, the magnetic force experienced by the cables that supply power to the coil had to be reduced to enable strong diffusion weightings. An optimal cable configuration for the gradient insert coil is presented along with a proposed mechanical design to reduce the magnetic force on cables. This enabled the acquisition of healthy human brain images with strong diffusion weightings. Next, a preprocessing pipeline was developed to address deviations from the imposed diffusion weighting resulting from nonlinearities in the ultra-strong coil's magnetic field, and to correct image distortions resulting from linear eddy currents and susceptibility fields. Finally, the feasibility of quantifying myelin water diffusion with the gradient insert was investigated in simulations.