Test-retest reliability of primary motor cortex mapping: Neuronavigated transcranial magnetic brain stimulation versus functional MRI

Introduction: In the past, presurgical functional brain mapping was mainly restricted to functional magnetic resonance imaging (fMRI). FMRI motor mappings previously showed rather low test-retest reliability,¹ especially for the face and tongue area.² Recently, neuronavigated Transcranial Magnetic Stimulation (nTMS) has been established as an alternative in clinical routine.^3,4 We here compared the reliability of both motor mapping approaches.

Fig. 1: ‘Heat map’ of the left cortical primary motor area (negative: gray; positive: red [MEP amplitude >50uV] / yellow [MEP amplitude >200uV] after mapping by navigated transcranial magnetic stimulation (nTMS). MEPs were recorded from the dominant abductor pollicis brevis muscle (APB).

Fig. 2: The cortical representation of the abductor pollicis brevis muscle (APB) appeared better reliable (interclass correlation rates of sessions 1 – 3 ranging from 0 [dark blue] to 1 [dark red] and more focused when investigated by navigated transcranial magnetic stimulation (nTMS), as compared to functional magnetic resonance imaging (fMRI).

Methods: We examined 10 healthy, right-handed subjects (5 male; age 24–34) on three days (d0, d3–5, week 3–5) by nTMS and fMRI. NTMS was applied at the dominant primary motor cortex at 110% of the resting motor threshold using eXimia 3.2.2. MEPs were recorded from the abductor pollicis brevis muscle (Figure 1), plantaris muscle, perioral muscles and the tongue. The fMRI (Siemens 3T Scanner) motor paradigms consisted of bilateral thumb abduction, unilateral toe flexion, pursing lips and tongue abduction. Euclidean distances (ED) between hotspots and centers of gravity (CoG) of the three sessions were calculated. Spatial reliability was tested by intersession overlaps and voxel-wise interclass correlation (ICC).⁵

Results: Pooled over all body parts, the mean ED of the hotspots was higher for nTMS (10.77±1.88mm) as compared to fMRI (6.2±1.1mm), whereas there was no difference in ED of the CoGs (mean 6.7mm). Regarding the spatial reliability (i.e., overlap volumes and ICC), nTMS was superior for hand (figure 2) and foot mappings but not for perioral and tongue mappings. The cortical representation of the perioral showed a broad overlap with the tongue area in both assessments.

Conclusions: CoGs were similarly reproducible by fMRI and nTMS. Both methods seem sufficiently reliable for clinical application. In terms of spatial reliability, nTMS seems to be superior to map the primary motor areas of hand and foot but less accurate for lips and tongue. Thus, both methods may well complement each other in the clinical routine.

Literatur: 1) Bennett CM, Miller MB. How reliable are the results from functional magnetic resonance imaging? Ann. N.Y. Acad. Sci. 2010 Mar; 1991: 133-155. 2) Havel P, Braun B, Rau S, Tonn JC, Fesl G, Bruckmann H, et al. Reproducibility of activation in four motor paradigms. An fMRI study. J. Neurol. 2006 Apr; 253(4): 471-6. 3) Picht T, Mularski S, Kuehn B, Vajkoczy P, Kombos T, Suess O. Navigated transcranial magnetic stimulation for preoperative functional diagnostics in brain tumor surgery. Neurosurgery. 2009 Dec; 65 (6 Suppl): 93-8; discussion 8-9. 4) Picht T, Schmidt S, Brandt S, Frey D, Hannula H, Neuvonen T, et al. Preoperative Functional Mapping for Rolandic Brain Tumor Surgery: Comparison of Navigated Transcranial Magnetic Stimulation to Direct Cortical Stimulation. Neurosurgery. 2011 Mar 23. 5) Shrout PE, Fleiss JL. Interclass correlations: Uses in assessing rater reliability. Psychol. Bull. 1979; 86:420-28.