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Analysis of MR diffusion weighted images

Imaging Science and Biomedical Engineering, University of Manchester, Oxford Road, Manchester M13 9PT, UK

View larger version (87K): [in a new window]   Figure 1. The effect of diffusion sensitization of b=1800 s mm–2 on an axial echo planar image acquisition at the level of the internal capsule and optic radiations. Sensitization direction varies from frame to frame. Note signal attenuation is dependent on applied gradient direction and on the orientation of white matter fibre bundles.

View larger version (93K): [in a new window]   Figure 2. Axial diffusion tensor fractional anisotropy (FA) map.

View larger version (46K): [in a new window]   Figure 3. Diffusion tensor ellipsoids (above) and apparent diffusion coefficient (ADC) profiles (below). Eigenvalues ( x 10–6 mm2 s–1): (a) class I: 781, 670, 670; (b) class II: 1042, 529, 529; (c) class II: 1773, 164, 164; (d) class III: 931, 931, 283. Trace=2100 x 10–6 mm2 s–1. All ellipsoid axes in arbitrary spatial units. All ADC profile axes in 10–6 mm2 s–1.

View larger version (75K): [in a new window]   Figure 4. Principal eigenvector of the diffusion tensor (yellow needles) overlaid on orthogonal maps of fractional anisotropy. (a) Axial view through corpus callosum; (b) coronal view showing corticospinal tracts and corpus callosum; (c) sagittal view through corpus callosum and fornix.

View larger version (47K): [in a new window]   Figure 5. Synthetic apparent diffusion coefficient (ADC) profile for two crossing fibres, generated with a two tensor model of diffusion. Fractional anisotropy=0.9 for both fibres, which are present in equal proportions and crossing at 90°. Eigenvalues and trace as in Figure 3.

View larger version (124K): [in a new window]   Figure 6. Axial, coronal, and sagittal views through a region of the brain centred on the superior longitudinal fasciculus. Top: best estimates of fibre orientations obtained from the diffusion tensor, overlaid on a fractional anisotropy map. Bottom: regions of crossing fibres highlighted in white, as defined using a mixture model of two tensors, with two resulting estimates of fibre orientation. Light grey regions indicate the presence of anisotropic single tensor tissue (single fibre bundle); dark grey indicates isotropic diffusion (mostly grey matter). Green ovals indicate regions where crossing fibres may be wrongly interpreted as representing coherent single fibre structure in single tensor analysis.

View larger version (88K): [in a new window]   Figure 7. Medial sagittal view of a streamline reconstruction of the superior longitudinal fasciculus (left hemisphere), a long associative bundle composed of long and short fibres that connect the frontal lobe with the parietal, occipital, and temporal lobes (image courtesy Derek Jones).

View larger version (93K): [in a new window]   Figure 8. Connectivity (hotwire colour scale) to a voxel in the splenium of the corpus callosum, as defined using the fast marching tractography method. Overlay onto fractional anisotropy map. Note branching pattern, indicating a distributed pattern of connectivity to start voxel.

View larger version (66K): [in a new window]   Figure 9. Probability of connection to a region of interest in the left lateral geniculate nucleus (LGN), as defined using the multifibre probabilistic index of connectivity (PICo) method. Overlay onto axial T2 weighted images. Note multiple routes of connection identified from the LGN. OR, optic radiation.