FIG. 1.

Cartoon illustrations of ambiguities in mapping diffusion to axon geometry, and their consequences for tractography. Top left: Different axon geometries can lead to a similarly oriented tensor. The Tensor's principal direction is the same for all cases, but modeling crossing fibers helps distinguish a few of the cases. Modeling fiber fanning separates the top two geometries. Further modeling the polarity of a fanning can help separate all cases. Top right: Illustration of the asymmetry in tracking when fanning polarity is modeled locally. Bottom left: Illustration of a case of kissing fibers, where the local model is one of crossing fibers. Tractography will lead to jumping between the tracts, causing false positives. Bottom right: case of ambiguities near the cortex. Both axon configurations lead to the same diffusion profile (and hence the same tracking results), but have very different implications in terms of the actual connectivity.