Syllabus overview

Biological structure on the cellular scale affects the thermal motion of water molecules. This provides an opportunity for diffusion MRI combined with biophysical modeling to map specific microstructural information with direct biological relevance. Work during the last 2 decades or so has converged towards a broadly accepted understanding of diffusion in brain white matter as described by the “Standard Model” (SM), which has been extensively validated with various means such as histology, computer simulations, and functional behavior/predictive power. Its parameters have been demonstrated to capture a variety of pathological changes. However, estimating them from a standard “clinical diffusion data” set is prone to fitting instabilities, resulting in high sensitivity to noise and starting values for the optimization. Fortunately, combination with advanced pulse sequences and echo time variation adds the missing information to constrain parameter estimation, making it possible to estimate them robustly and reproducibly on clinical scanners. Additional information available beyond SM is an active research frontier, as is developing the understanding of diffusion in brain gray matter.

Acknowledgements

I would like to thank Dmitry Novikov, Santiago Coelho, and Noam Shemesh for discussions, and Leif Østergaard for support. I am grateful to CFIN/MINDLab, Aarhus University, Lundbeck Foundation, and the Independent Research Fund Denmark for financial support.