Reliability for non-invasive somatosensory cortex localization: Implications for pre-surgical mapping.

OBJECTIVES: In patients with epilepsy or space occupying tumors in cortical regions, surgical resection is often considered as the primary treatment. Pre-surgical neuroimaging can provide a detailed map of pathological and functional cortex, leading to safer surgery. Mapping can be achieved non-invasively using magnetoencephalography (MEG), and is concordant with invasive findings. However, the reliability of MEG mapping between sessions is not well established. The inter-session reliability is an important property in pre-surgical mapping to establish resection margins, but repeated scans are impracticable. The present study sought to quantify the intersession reliability of MEG localization of somatosensory cortex (S1). PATIENTS AND METHODS: Eighteen healthy individuals underwent MEG sessions on 3 consecutive days. Five participants were excluded due to technical issues during one of the three days. Each session included clinical-style S1 localization using electrical stimuli to each median nerve at sub-motor thresholds. The 35 ms peak of the somatosensory evoked field was used for localizing S1 in each session using a single equivalent current dipole model. Intersession reliability was quantified using two methods. Average Euclidean Distance (AED) quantified the difference in localization between each session and the inter-session mean localization. Session Euclidean Distance (SED) quantified the difference in localization between each pair of sessions. RESULTS AND DISCUSSION: Results showed the AED was 4.8 ± 1.9 mm, whereas the SED was 8.3 ± 3.4mm. While the AED values obtained parallel those reported previously in smaller samples, the SED values were substantially larger.
CONCLUSION: Clinicians should consider up to an 8mm confidence interval around the estimated location of S1 based on MEG pre-surgical mapping.