Error Correction Methods

Tractography may yield anatomical reconstruction errors with any technique. These errors can be minimized using one of two methods of result analysis, one based on functional brain anatomy and one, a probabilistic method, using a standard space of brain coordinates. The first consists of using the anatomical data a priori by requiring a fibre tract to pass through at least two manually selected regions of interest (ROIs) [34, 42]. Using a single ROI, the reconstructed tract is more likely to contain different fibres, some representing trajectories belonging to the tract being studied and others generated by partial volume effects or noise. The latter can be eliminated by selecting multiple ROIs along the fibre tract being reconstructed so as to avoid an erroneous deviation of the reconstruction algorithm from the actual trajectory (Fig. 8.7). This method makes it possible to track simply and non-invasively the position of several tracts with a high level of confidence [34]. Its main drawback is that it cannot reconstruct bundles that are not well documented anatomically [42], and may also exhibit limitations in the presence of fibre deviations induced by brain disease.

Fig. 8.7 a, b. Use of the multi-ROI approach improves trac-tographic reconstruction of white matter tracts. Above, the addition of a second ROI (b) specifically selects the fibre tract of interest

The second, probabilistic method is based on the assumption that errors induced by partial volume effects or low SNR have a random distribution and are not reproduced consistently if multiple studies of the same object are performed and their results are superimposed. The same principle underpins a method that uses data from a large sample of subjects in a standard space of brain coordinates (e.g. the Talairach atlas). The first studies applying this method of normalization have yielded a high level of reproducibility for the large fibre bundles, but greater intersubject variability for the smaller bundles [24, 44-46].

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