Abstract
Diffusion MRI has the potential to provide important information about the connectivity and microstructure of the human brain during normal and abnormal development, noninvasively and in vivo. Recent developments in MRI hardware and reconstruction methods now permit the acquisition of large amounts of data within relatively short scan times. This makes it possible to acquire more informative multi-shell data, with diffusion sensitisation applied along many directions over multiple b-value shells. Such schemes are characterised by the number of shells acquired, and the specific b-value and number of directions sampled for each shell. However, there is currently no clear consensus as to how to optimise these parameters. In this work, we propose a means of optimising multi-shell acquisition schemes by estimating the information content of the diffusion MRI signal, and optimising the acquisition parameters for sensitivity to the observed effects, in a manner agnostic to any particular diffusion analysis method that might subsequently be applied to the data. This method was used to design the acquisition scheme for the neonatal diffusion MRI sequence used in the developing Human Connectome Project (dHCP), which aims to acquire high quality data and make it freely available to the research community. The final protocol selected by the algorithm, and currently in use within the dHCP, consists of 20 b=0 images and diffusion-weighted images at b = 400, 1000 and 2600 s/mm2 with 64, 88 and 128 directions per shell, respectively.
Highlights
Diffusion MRI has been the focus of intense research over the last 20 years, holding great promise for investigation of tissue microstructure due to the technique's unique sensitivity to the micron-scale diffusion of water
We propose a means of optimising multi-shell acquisition schemes by estimating the information content of the diffusion MRI signal, and optimising the acquisition parameters for sensitivity to the observed effects, in a manner agnostic to any particular diffusion analysis method that might subsequently be applied to the data
Clear structure can be observed in the corresponding weights maps for at least the first four components
Summary
Diffusion MRI (dMRI) has been the focus of intense research over the last 20 years, holding great promise for investigation of tissue microstructure due to the technique's unique sensitivity to the micron-scale diffusion of water. More recent work has investigated the use of additional contrast mechanisms, such as spherical and planar tensor encoding,[17,18,19,20] more complex Q-space trajectories,[21] variable echo times[22] and variable inversion times,[23,24] among others. Many of these entail a considerable increase in scan time, and the use of custom sequences, limiting their widespread use in the immediate future. We decided to focus on optimisation of the multi-shell pulsed gradient spin echo (PGSE) sequence, as it is likely to remain the most widely used acquisition strategy for the foreseeable future, especially for large longitudinal and cross-sectional studies
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