A factor requiring careful consideration is the initial point of tract propagation, as this choice influences the relative effect of noise on the propagation itself. A frequently adopted solution is to use a number of equidistant seed points arranged on a grid space. This reduces the variance connected with the arbitrary choice of the seed point. However, a seed point acceptance criterion maybe applied to avoid selecting voxels not containing fibres. One useful criterion is a minimum FA value ensuring the presence of a distinct fibre at the seed point. It is worth stressing that, since the directions identified in each voxel by the diffusion tensor do not have an orientation (see Fig. 8.2), a forward and a backward pathway, lying on the same straight line but running in op posite directions, are consistently generated at seed points.
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