Establishing Surface Correspondence for Post-surgical Cortical Thickness Changes in Temporal Lobe Epilepsy

Abstract

In pre- and post-surgical surface shape analysis, establishing shape correspondence is necessary to investigate the postoperative surface changes. However, structural absence after the operation accompanies focal non-rigid changes, which leads to challenges in existing surface registration methods. In this paper, we present a fully automatic particle-based method to establish surface correspondence that can handle partial structural abnormality in the temporal lobe resection. Our method optimizes the coordinates of points which are modeled as particles on surfaces in a hierarchical way to reduce a chance of being trapped in a local minimum during the optimization. In the experiments, we evaluate the effectiveness of our method in comparison with conventional spherical registration (FreeSurfer) on two scenarios: cortical thickness changes in healthy controls within a short scan-rescan time window and patients with temporal lobe resection. The post-surgical scan is acquired at least 1 year after the presurgical scan. In region of interest-wise (ROI-wise) analysis, no changes on cortical thickness are found in both methods on the healthy control group. In patients, since there is no ground truth available, we instead investigated the disagreement between our method and FreeSurfer. We see poorly matched ROIs and large cortical thickness changes using FreeSurfer. On the contrary, our method shows well-matched ROIs and subtle cortical thickness changes. This suggests that the proposed method can establish a stable shape correspondence, which is not fully captured in a conventional spherical registration.

Keywords: cortical thickness; shape correspondence; surface; temporal lobe epilepsy.

Figure 1.

An example of failed shape correspondence using FreeSurfer. (a) parcellation map on the pre-surgical surface. (b) transferred parcellation map on the post-surgical surface. The parcellation map of pre-surgical surface is transferred to post-surgical surface using the correspondence established by FreeSurfer. The circle region shows that FreeSurfer failed to find the precise shape correspondence on post-surgical surface.

Figure 2.

A schematic overview of the proposed method. (a) the fixed particles on pre-surgical surface. (b) the initial particles on post-surgical surfaces. (c) the updated particles on post-surgical surfaces after optimization. (d) the established correspondence between two particles. The particles with the same index have same locations on pre- and post-surgical surface. (e) the icosahedral subdivision to get a better initial guess of higher levels of optimization

Figure 3.

Parcellation map on a randomly chosen subject. (a) is pre-surgical parcellation. (b) is the propagated parcellation on post-surgical surface using correspondence established by FreeSurfer. (c) shows the propagated parcellation on post-surgical surface using our established correspondence. The circled areas are mismatched, which means FreeSurfer could not establish precise correspondence between pre- and post-surgical surfaces. By showing parcellation map, our method can establish more precise correspondence.

Figure 4.

ROI-wise average cortical thickness changes on control group. (a) shows 39 ROIs from normal region. (b) shows 10 ROIs belonging to temporal lobe. The ROI-wise cortical thickness changes between first scan and second scan are small. There are no significant changes in cortical thickness for all ROIs.

Figure 5.

ROI-wise cortical thickness changes on patient group. There are 19 out of 39 ROIs that have significant thickness changes (red asterisk) using FreeSurfer. Lingual gyrus has significant cortical thickness changes (blue asterisk) using our method.

Figure 6.

Four ROIs that have weak correspondence using FreeSurfer. (a) is the pre-surgical ROIs. (b) is the transferred ROIs using correspondence established by FreeSurfer. (c) is the transferred ROIs using correspondence established by our method. Pre-surgical ROIs are not able to be transferred to correct post-surgical locations using weak correspondence as shown in (b). Since we do not observe obvious changes between pre- and post-surgical surface, the significant ROI-wise cortical thickness changes in some regions are likely caused by mismatched ROIs.