Automated assessment of tDCS electrode placement accuracy based on structural MRI

PurposeAccurate electrode placement is essential for studies using transcranial direct current stimulation (tDCS) since it determines whether - and to which extent - the intended brain region was modulated. Precise electrode placement also reduces between-subject variability. Using in-scanner tDCS, accuracy and precision of electrode placement can be determined. Previous studies manually segmented electrode positions from structural MRI, a process that is time-consuming and prone to error. We therefore developed an algorithm that automatedly extracts electrode coordinates from structural MRI. MethodsWe applied this algorithm to 49 datasets acquired at diferent field strengths (3 T and 7 T), with two image resolutions and electrode montages (bifrontal or ring-shaped) targeting the dorsolateral prefrontal cortex. We validated its accuracy by comparing the automatedly extracted coordinates with manual annotations from two independent readers. We then used the actual and intended electrode coordinates to simulate how electrode placement deviations influenced the resulting electric field. ResultsFor all setups and scanners, the median localization error of the algorithm was 2.3 mm. It was within 1 mm of the variability between human readers (2.7 mm), indicating human-level reliability. Large deviations (> 20 mm) from intended positions led to substantial changes in the electric field - up to 35% for the ring-shaped montage -highlighting the need for placement verification in tDCS studies. ConclusionThe proposed algorithm obviates the need for manual electrode segmentation and allows an objective evaluation of electrode placement accuracy and precision for in-scanner tDCS. It may be used to relate electrode position errors to stimulation efects, thereby supporting reproducible tDCS research.