While diffusion MRI (dMRI) promises an insight into white matter integrity in vivo, the axonal pathways that connect different brain regions together can only partially be segmented using current methods. Here we present a novel method for estimating the tissue composition of each voxel in the brain from dMRI data, thereby providing a foundation for computing the volume of different pathways in both health and disease. With our method, white matter (WM) is segmented by removing the ambiguity caused by the isotropic partial volumes (PV): both grey matter (GM) and cerebrospinal fluid (CSF). Apart from the volume fractions of all three tissue types we also obtain estimates of fibre orientations for tractography as well as diffusivity and anisotropy parameters which serve as proxy indices of pathway coherence. We assume Gaussian diffusion of water molecules for each tissue type. The resulting three-tensor model comprises one anisotropic (WM) compartment modelled by a cylindrical tensor and two isotropic compartments (CSF and GM).We model the measurement noise as Rician distributed. Markov chain Monte Carlo sampling techniques are used to estimate posterior distributions over the model's parameters. In particular, we employ Metropolis Hastings sampler with a custom burn-in and proposal adaptation to ensure good mixing and efficient exploration of the high-probability region. This way we obtain not only point estimates of quantities of interest, but also a measure of their uncertainty (posterior variance). We observe that the volume maps produced with our method show plausible and well delineated structures for all three tissue types. Estimated white matter fibre orientations also agree with known anatomy and align well with those obtained using the current methods. Importantly, we are able to disambiguate the volume and anisotropy information thus alleviating patrial volume effects and providing measures superior to the currently ubiquitous fractional anisotropy.
|Publication status||Published - 2012|
|Event||SINAPSE Annual Scientific Meeting - Royal College of Physicians and Surgeons of Glasgow, Glasgow, United Kingdom|
Duration: 30 May 2012 → …
|Conference||SINAPSE Annual Scientific Meeting|
|Period||30/05/12 → …|