FDT

Overview

Research at FMRIB has concentrated on developing a statistical framework in which to address the tractography problem. By considering the problem of inferring on voxel-wise models of diffusion in the presence of image noise and incomplete signal modelling we are able to construct probability density functions pdfs on the voxelwise PDD and therefore on the inferred local mean fibre orientation. Comparison with empirical measurements of these distributions constructed by repeated acquisition of the diffusion-weighted data reveals a strong correspondance between the distributions recovered from the two different techniques. In the case when there is uncertainty in the local fibre orientation, it is no longer possible to trace determistic pathways through the data field. Instead, we have developed a generalisation of diffusion tractography to this case. We estimate a pdf on the location of the fibre trajectory (a connectivity distribution). We are able to quantify our belief in the location of the pathway and hence quantify our belief in the existence of axonal connections between brain regions. By removing the need to make a deterministic decision at every step in the tractography process, we are able to trace beyond regions of low diffusion anisotropy deep into grey matter structures (see figure (a)).

The availability of such a rich source of connectivity information has allowed us to address new questions with diffusion tractography. For example, the anatomical connectivity pattern of a brain region constrains its function. It should therefore be possible, by examining connectivity patterns derived from diffusion tractography, to identify functionally distinct subunits in the brain. Taking the thalamus as a specific example, by generating connectivity distributions from every voxel in thalamus we have been able to compute the probability of connection from every thalamic voxel to each of seven predefined cortical zones. We have used this information to segment thalamus into putative thalamic nuclei on the basis of connectivity information alone These connectivity defined regions form the basis of an atlas of thalamo-cortical connectivity which we have used to provide the first functional/anatomical validation of diffusion tractography. For example, in figure (d), the thalamic region defined by a high probability of connection with prefrontal cortex corresponds well with the location of FMRI/PET activation centres in executive memory tasks.

Papers

A paper on probabilistic tractography has been published in MRM; also see a related technical report (PS/PDF/HTML).
A paper on the application of probabilistic tractography to the segmentation of the thalamus has been published in Nature Neuroscience.
A paper on the extension of probabilistic tractography to modelling intra-voxel crossing fibres has been published in NeuroImage.

Software

FDT is available as part of the FMRIB Software Library.

Also available is the Thalamo-cortical Connectivity Atlas.