Stability metrics for optic radiation tractography: Towards damage prediction after resective surgery

- The alignment of streamlines is quantified by fiber-to-bundle coherence measures.
- Reliable ML-TP distance measurement by removal of spurious (deviating) streamlines.
- Parameter estimation to remove spurious streamlines and to retain the Meyer's loop.
- The validity of ML-TP distance is estimated by pre and postoperative OR comparisons.
- The stability metrics are promising to relate OR damage to a visual field deficit.

BackgroundAn accurate delineation of the optic radiation (OR) using diffusion MR tractography may reduce the risk of a visual field deficit after temporal lobe resection. However, tractography is prone to generate spurious streamlines, which deviate strongly from neighboring streamlines and hinder a reliable distance measurement between the temporal pole and the Meyer's loop (ML-TP distance).New methodStability metrics are introduced for the automated removal of spurious streamlines near the Meyer's loop. Firstly, fiber-to-bundle coherence (FBC) measures can identify spurious streamlines by estimating their alignment with the surrounding streamline bundle. Secondly, robust threshold selection removes spurious streamlines while preventing an underestimation of the extent of the Meyer's loop. Standardized parameter selection is realized through test-retest evaluation of the variability in ML-TP distance.ResultsThe variability in ML-TP distance after parameter selection was below 2 mm for each of the healthy volunteers studied (N = 8). The importance of the stability metrics is illustrated for epilepsy surgery candidates (N = 3) for whom the damage to the Meyer's loop was evaluated by comparing the pre- and post-operative OR reconstruction. The difference between predicted and observed damage is in the order of a few millimeters, which is the error in measured ML-TP distance.Comparison with existing method(s)The stability metrics are a novel method for the robust estimate of the ML-TP distance.ConclusionsThe stability metrics are a promising tool for clinical trial studies, in which the damage to the OR can be related to the visual field deficit that may occur after epilepsy surgery.