All fibre tracking algorithms that use a seed point require a stopping criterion to terminate the propagation process. The most intuitive criterion is the FA value itself; in grey matter FA is low (0.1-0.2 on a 0-1 scale), so the orientation of the principal eigenvector of the diffusion tensor is random and unrelated to that of the fibre tract. A useful stopping criterion may thus be an FA threshold (usually 0.2) below which the propagation is halted, preventing reconstruction of fibres that are not organized into bundles, like grey matter fibres. However, this criterion may also halt the elongation of a line in those white matter voxels which, albeit containing fibre tracts, have a low FA because of the lack of a main direction (see below).
Another possible stopping criterion is the curvature of the reconstructed fibre tract. The method used to calculate the diffusion tensor assumes the absence in the voxels of sharp curves, in line with the fundamental hypothesis of the Gaussian nature of the diffusion process in all directions. A criterion halting tract propagation in the presence of sharp angles is thus useful, but is difficult to apply to the shorter and more tortuous tracts, where the low spatial resolution of the image does not enable reconstruction of the real course of the fibres.
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