Individualized Template MRI Is a Valid and Reliable Alternative to Individual MRI for Spatial Tracking in Navigated TMS Studies in Healthy Subjects

Objectives: Navigated transcranial magnetic stimulation (nTMS) provides significant benefits over classic TMS. Yet, the acquisition of individual structural magnetic resonance images (MRI_individual) is a time-consuming, expensive, and not feasible prerequisite in all subjects for spatial tracking and anatomical guidance in nTMS studies. We hypothesize that spatial transformation can be used to adjust MRI templates to individual head shapes (MRI_warped) and that TMS parameters do not differ between nTMS using MRI_individual or MRI_warped. Materials and Methods: Twenty identical TMS sessions, each including four different navigation conditions, were conducted in 10 healthy subjects (one female, 27.4 ± 3.8 years), i.e., twice per subject by two researchers to additionally assess interrater reliabilities. MRI_individual were acquired for all subjects. MRI_warped were obtained through the spatial transformation of a template MRI following a 5-, 9-and 36-point head surface registration (MRI_{warped_5}, MRI_{warped_9}, MRI_{warped_36}). Stimulation hotspot locations, resting motor threshold (RMT), 500 μV motor threshold (500 μV-MT), and mean absolute motor evoked potential difference (MAD) of primary motor cortex (M1) examinations were compared between nTMS using either MRI_warped variants or MRI_individual and non-navigated TMS. Results: M1 hotspots were spatially consistent between MRI_individual and MRI_{warped_36} (insignificant deviation by 4.79 ± 2.62 mm). MEP thresholds and variance were also equivalent between MRI_individual and MRI_{warped_36} with mean differences of RMT by -0.05 ± 2.28% maximum stimulator output (%MSO; t ₍₁₉₎ = -0.09, p = 0.923), 500 μV-MT by -0.15 ± 1.63%MSO (t ₍₁₉₎ = -0.41, p = 0.686) and MAD by 70.5 ± 214.38 μV (t ₍₁₉₎ = 1.47, p = 0.158). Intraclass correlations (ICC) of motor thresholds were between 0.88 and 0.97. Conclusions: NTMS examinations of M1 yield equivalent topographical and functional results using MRI_individual and MRI_warped if a sufficient number of registration points are used.