P 97 Individualized MRI are a valid alternative to individual MRI in navigated TMS studies

Introduction Navigated transcranial magnetic stimulation (nTMS) provides significant benefits over non-navigated TMS such as control for non-physiological TMS parameters and intracerebral electric field distribution (Ruohonen and Karhu, 2010; Schmidt et al., 2015). These benefits come at the cost of an often time consuming, expensive and not in all subjects feasible acquisition of individual structural magnetic resonance images (MRI). Spatial transformation is a well-established method that is used in functional neuroimaging to align individual MRI with template MRI (Friston, 1999). We hypothesize that the inverse procedure can be used to adjust template MRI to individual head shapes, and that TMS parameters do not differ between individual and individualized template MRI. Methods Seven healthy subjects were enrolled in the study (age 26.3 ± 4.5 years, 1 female). Individual anatomical MRI were acquired for all subjects. Individualized MRI were obtained through spatial transformation of a validated template MRI following a 5 point, 9 point and 36 point head shape registration (Lalys et al., 2009). Functional TMS parameters (motor thresholds, mean absolute MEP difference (MAD)) were compared between nTMS sessions with individual MRI (gold standard) and nTMS performed with each type of individualized MRI and non-navigated TMS. Bland-Altman analyses were used to compare functional results. Spatial consistency of hotspots was assessed by transformation to a common head coordinate system. Results Resting motor thresholds (RMT) obtained by non-navigated TMS or individualized nTMS were comparable to the gold standard (probability of fixed or proportional bias >.05 for all methods). There was no significant fixed or proportional bias for either of the 500  μ V threshold estimation methods except for that using 5 point registered individualized MRI (fixed bias of 1.29 %MSO, p  = .02). There was neither a fixed nor proportional MAD bias between methods. Yet, one MAD estimation in the 5 point registration condition lay beyond 95% confidence boundaries rendering this method by definition invalid. Only hotspots obtained using 36 point individualized MRI were spatially consistent with hotspots obtained using individual MRI (Euclidean distance Discussion Spatial transformation enables constructing individualized MRI for nTMS studies. Individualized nTMS yields functional results similar to individual nTMS when a sufficient number of registration points is used. Confirmatory data from larger data sets is required.