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Figure 1: An illustration of the links between high value diffusion-weighted data, Funk-Radon transform, diffusion ODF, and fiber ODF: the slice cuts the corpus callosum, whose fibers approximately follow the direction. The figure proposes the aligned T1- and T2-weighted slices and a number of diffusion-weighted slices. When the diffusion gradient is not orthogonal to the fibers (unit vector under corresponding slice), most of the MR signal is destroyed by water diffusion: the corpus callosum is black (row of slices at the bottom). When the diffusion gradient is orthogonal to the fibers, water diffusion is restricted by the axonal membranes: some noisy signal survives in the corpus callosum. Hence, the sum of the raw signal along the equator around the axis leads to a peak of the -ball indicating the fiber direction. In this figure, the -balls. are scaled according to their anisotropy.