Monte Carlo diffusion simulations disambiguate the biophysical mechanisms of diffusion hinderance along tracts
Kleinnijenhuis M., Mollink J., Kinchesh P., Lam WW., Galinsky VL., Frank LR., Smart SC., Jbabdi S., Miller KL.
PURPOSE – Diffusion imaging at long diffusion times can inform on microstructural features of tissue at scales spanning several hundreds of micrometers. At these scales the common approximation of axons as straight cylinders might not hold, even for tissues that are generally assumed to be coherently organized. The human corpus callosum is such a tract. It is often used as a reference structure for simple fibre configuration. Nevertheless, evidence from electron microscopy and histology suggests that corpus callosum fibres are far from coherent1,2. Fibres not only bend along the tract, but also twist and undulate, effects that could lead to specific signatures of hinderance “along” the tract. In this study, we investigated the diffusion time dependence of the apparent diffusion coefficient (ADC) along the direction of the fibres in the corpus callosum. Possible biophysical mechanisms of this dependence are explored by means of Monte Carlo simulations of various axon models.