Local and global fiber tractography in patients with epilepsy

Abstract

Background and purpose: Fiber tractography is increasingly used in the preoperative evaluation of endangered fiber bundles. From a clinical point of view, an accurate and methodologically transparent procedure is desired. Our aim was to evaluate the recently described global tracking algorithm compared with other established methods, such as deterministic and probabilistic tractography.

Materials and methods: Twenty patients, candidates for excision of epileptogenic lesions, were subjected to higher-angular resolution diffusion imaging-based fiber tractography. Seed points were created without manual bias, predominantly by FreeSurfer and voxel-based atlases. We focused on 2 important fiber bundles, namely the descending motor pathways and the optic radiation. Postoperatively, the accuracy of the predicted fiber route was controlled by structural MR imaging and by inflicted functional deficits.

Results: Among the 3 evaluated methods, global tracking was the only method capable of reconstructing the full extent of the descending motor pathways, including corticobulbar fibers from the area of face representation. Still, probabilistic tractography depicted the optic radiation better, especially the Meyer loop. The deterministic algorithm performed less adequately.

Conclusions: The probabilistic method seems to be the best balance between computational time and effectiveness and seems to be the best choice in most cases, particularly for the optic radiation. If, however, a detailed depiction of the fiber anatomy is intended and tract crossings are implicated, then the computationally time-consuming global tracking should be preferred.

Fig 1.

Depiction of the left descending motor pathways on the healthy side in patient 3 and the fan angle measurement performed. FACT (A), probabilistic (B), and global (C and F) tractography show approximately the same route and extent along the internal capsule. Note differences in the reconstruction of fibers arising from the lateral area of face cortical representation. These corticobulbar fibers are best depicted by global tracking, as this method is least susceptible to fiber crossings in the semiovale center (D) (orientation distribution function of the diffusion signal) and (E) (principal direction of the diffusion tensor shown as color-coded map) between the corpus callosum (1), corona radiata (2), and SLF (3). FACT and probabilistic tractography cannot overcome the crossing with the SLF (3, green in F, oriented perpendicular to the plane of the figure), with the results shown in On-Line Fig 2 in mind.

Fig 2.

Assessment of the OR in patient 14 with a right hippocampal sclerosis. Results of probabilistic tractography (A) for the right Meyer loop (arrowhead) and (B) for the main part of the OR are shown. The preoperative OR reconstruction by the probabilistic algorithm is superimposed onto postoperative images (C and D, asterisk = resection area, arrow = anterior extent of Meyer loop). Postoperatively, the patient presented with a partial visual field defect in the left upper quadrant.

Fig 3.

Coronal sections through the OR at the level of the trigone of the lateral ventricle in patient 15 with a perinatal stroke. Scheme drawing (A) of the functional organization of the OR on the healthy side (redrawn), indicating the course of the posterior (p), anterior (a), and central (c) bundles, carrying fibers of the lower quadrant, upper quadrant, and center of the contralateral visual field, respectively. Preoperative T1WI of the pathologic side (right half, A). Postoperatively, the patient had a lower right quadrant anopia. Preoperative tractography superimposed on the postoperative coronal T1 (B–D). The arrowhead indicates that the posterior bundle of the OR was within the resection area. It is missed by FACT (B) but is depicted by probabilistic (C) and global (D) tractography.