Tie at center

With the default processing, each streamline is resampled so that it has the same number of points, spread evenly along its length. This prescribes 2 points of correspondence – one at either end. However, since the ends of the streamlines typically have higher variability (in FA estimates, geometric position, etc.), it can be desirable to prescribe an extra tie-down point somewhere in the middle of the tract. To do this, simply flag the tie_at_center option to trk_interp like this:

tracks_interp = trk_interp(tracks, nPts, [], 1);

Algorithm

The way this method prescribes the central correspondence point is as follows:

1. The number of desired interpolation points is modified to be an odd number, if needed.
2. The mean tract geometry is calculated using the standard interpolation method.
3. The midpoint vertex is identified on the mean tract geometry.
4. For each original streamline, the vertex lying closest to this tract midpoint is identified.
5. The original streamlines are then resampled in a modified way, such that (nPts-1)/2 vertices fall on one side of the midpoint vertex, and (nPts-1)/2 vertices fall on the other.

Comparison

Using the example data, we can compare the default processing (left) with the modified version using the tie_at_center flag (right). The top panels show correspondence plots, where like colors indicate vertices that will be grouped together in the analysis. The bottom panels show the along-tract variation in FA for individual streamlines as well as the mean ± SD. Note the tighter coherence in the central portion of the tract when using the tie_at_center option.

Original processing	Tie-at-center processing