Fig. 1.

Focal comparisons between fibre density and fractional anisotropy in lesions and healthy white matter. (A) Lesions of differing severity from a single MS patient and the corresponding region in a healthy control illustrates the pathological effects on fibre orientation distributions (FOD) and corresponding fixel fibre density (FD). In lesion 1 with severe axonal loss there is a complete loss of fibre density (no white matter fibre orientation distributions detected). There is also some evidence of disrupted fibre directions in the lateral perilesional white matter compared to the control subject. In lesion 2 there is a more moderate reduction of FOD amplitude and preservation of fibre directions compared to the healthy control. (B) Shows voxelwise comparisons between total fibre density and fractional anisotropy within the same two lesions and the surrounding white matter. The spatial maps showing the difference between normalised total fibre density and normalised fractional anisotropy show regions of large differences between fibre density and fractional anisotropy, particularly in regions of crossing fibres in the healthy white matter. The corresponding scatter plots (line through unity) further illustrate the differences between the measures.