Functional MRI vs. navigated TMS to optimize M1 seed volume delineation for DTI tractography. A prospective study in patients with brain tumours adjacent to the corticospinal tract

Abstract

BackgroundDTI-based tractography is an increasingly important tool for planning brain surgery in patients suffering from brain tumours. However, there is an ongoing debate which tracking approaches yield the most valid results. Especially the use of functional localizer data such as navigated transcranial magnetic stimulation (nTMS) or functional magnetic resonance imaging (fMRI) seem to improve fibre tracking data in conditions where anatomical landmarks are less informative due to tumour-induced distortions of the gyral anatomy. We here compared which of the two localizer techniques yields more plausible results with respect to mapping different functional portions of the corticospinal tract (CST) in brain tumour patients.MethodsThe CSTs of 18 patients with intracranial tumours in the vicinity of the primary motor area (M1) were investigated by means of deterministic DTI. The core zone of the tumour-adjacent hand, foot and/or tongue M1 representation served as cortical regions of interest (ROIs). M1 core zones were defined by both the nTMS hot-spots and the fMRI local activation maxima. In addition, for all patients, a subcortical ROI at the level of the inferior anterior pons was implemented into the tracking algorithm in order to improve the anatomical specificity of CST reconstructions. As intra-individual control, we additionally tracked the CST of the hand motor region of the unaffected, i.e., non-lesional hemisphere, again comparing fMRI and nTMS M1 seeds. The plausibility of the fMRI-ROI- vs. nTMS-ROI-based fibre trajectories was assessed by a-priori defined anatomical criteria. Moreover, the anatomical relationship of different fibre courses was compared regarding their distribution in the anterior-posterior direction as well as their location within the posterior limb of the internal capsule (PLIC).ResultsOverall, higher plausibility rates were observed for the use of nTMS- as compared to fMRI-defined cortical ROIs (p < 0.05) in tumour vicinity. On the non-lesional hemisphere, however, equally good plausibility rates (100%) were observed for both localizer techniques. fMRI-originated fibres generally followed a more posterior course relative to the nTMS-based tracts (p < 0.01) in both the lesional and non-lesional hemisphere.ConclusionNTMS achieved better tracking results than fMRI in conditions when the cortical tract origin (M1) was located in close vicinity to a brain tumour, probably influencing neurovascular coupling. Hence, especially in situations with altered BOLD signal physiology, nTMS seems to be the method of choice in order to identify seed regions for CST mapping in patients.

Neurophysiology-Driven Parameter Selection in nTMS-Based DTI Tractography: A Multidimensional Mathematical Model

“…In the past, several studies have shown that ROI seeding based on nTMS data resulted not only in more specific tracts with less aberrant fibers, but reduced the variability between examiners compared to the seeding approach using anatomical landmarks (Frey et al, 2012;Krieg et al, 2012;Weiss Lucas et al, 2017). In order to improve the threshold selection, previous standardization attempts suggested a FL threshold of 110 mm and the implementation of a 100%-FAT criterium.…”

“…In order to improve the threshold selection, previous standardization attempts suggested a FL threshold of 110 mm and the implementation of a 100%-FAT criterium. The latter described the FA threshold at which only a thin fiber tract was visualized and all other FA values were described in relation to this 100%-FAT value as e.g., 75%-, 50%-or 25%-FAT (Frey et al, 2012;Weiss et al, 2015;Rosenstock et al, 2017;Weiss Lucas et al, 2017). The concept of performing fiber tracking with a FL threshold of 110 mm and a 75%-FAT resulted in a more objective tractography than the previous methods (Frey et al, 2012).…”

“…Nevertheless, this remains an arbitrary value selection which neither takes the interindividual patient characteristics into account nor the different types of fiber tracts (e.g., CST or language pathways). Furthermore, there currently exists contention concerning the ambiguity of the standardized values (i.e., 75%-, 50%-or 25%-FAT) to be used (Sollmann et al, 2016;Weiss Lucas et al, 2017). The proposed neurophysiology-driven approach for FA and FL threshold selection not only chooses the threshold parameters in a completely user-independent manner, but also considers the differences between patients.…”

“…threshold selection in deterministic DTI. The latest development impacting standard clinic care was the introduction of the nTMS in presurgical DTI-FT, introduced with the intention to standardize ROI seeding (Frey et al, 2012;Krieg et al, 2012;Raffa et al, 2017;Weiss Lucas et al, 2017). In nTMS, a neuronavigation system is used to define functional cortical spots that elicit electromyographic responses (i.e., MEP) in the contralateral limb following magnetic stimulation.…”

“…However, this approach leaves the thresholding problem unresolved. While some studies postulated using standardized FA and FL values (e.g., 75% of the individual FA and a FL threshold of 110 mm) (Frey et al, 2012), other studies recommend individual thresholds (Weiss Lucas et al, 2017). However, this is of particular importance as the threshold selection has a tremendous impact on the number of detected fibers.…”