Development of the Hand Knob Position in the 10-20 System for a More Accurate Identification of the M1-HAND

The international 10-20 system's C3/C4 positions are standard for locating the primary motor hand area (M1-HAND) in tDCS and as TMS motor mapping references. While cost-effective, their accuracy for M1-HAND localization is limited as compared to specialized methods. This study aimed to establish a novel position within the 10-20 system for accurately identifying the anatomical hand knob, thereby providing an accurate indirect reference for the M1-HAND. We analyzed 116 MRI scans to define standardized X and Y coordinates representing the scalp projection of the anatomical hand knob. These coordinates were converted into percentages (X% and Y%) based on the nasion-inion and tragus-to-tragus distances, respectively. Initial testing revealed X% didn't improve accuracy and thus was excluded. However, Y% (13% of the tragus-to-tragus distance) significantly enhanced targeting by shifting the optimal site medially relative to C3/C4 placements. We then evaluated the accuracy of this new position against C1/C2, C3h/C4h, and C3/C4. While X coordinates of the corresponding anatomical area of the hand knob on the scalp and C3/C4 positions were similar, their Y coordinates differed significantly. The new position (hand knob position) demonstrated the closest proximity to the anatomical hand knob area on the scalp among all evaluated positions. Our study establishes an accurate position within the 10-20 system for identifying the corresponding anatomical area on the scalp of the hand knob. The Y% value (13% of tragus-to-tragus distance) notably enhances the specificity of M1-HAND localization (via its anatomical correlate) as compared to traditional 10-20 system positions.