Quantifying and reporting the precision of transcranial magnetic stimulation targeting

The precise positioning of the transcranial magnetic stimulation (TMS) coil on a person's head is crucial for the efficacy and reliability of the delivered stimulation protocol. Sophisticated techniques have been developed to define subject-specific stimulation targets, and advancements in the use of MRI-guided neuronavigation allows for real-time monitoring of the coil location during the TMS session. However, there is a need for TMS users to objectively quantify and report the accuracy of their targeting. Here, we share our technique (open-source scripts) that extracts the location of each TMS pulse delivered in a session from an MRI-guided neuronavigation system and outputs measures of targeting precision. Such measures include the variance in coil location over the duration of a session, detection of 'off-target' pulses, and the distance error relative to the intended cortical target. Reporting these metrics in publications may aid in the replicability of methodology and reproducibility of results of TMS research and clinical treatments. Furthermore, these measures can be used in training TMS operators. We encourage others to adapt our technique to their system(s) and specific needs and to report their targeting precision.