Scalp Measurements in TMS: The EEG Coordinate System
Scalp measurements in Transcranial Magnetic Stimulation (TMS) are based on the standardized EEG (electroencephalography) coordinate system, which uses anatomical landmarks including the nasion, inion, tragus points, and head circumference to localize stimulation targets. 1
Primary Measurement System
The EEG-based scalp measurement approach provides:
- A fast and inexpensive method for localizing the stimulation target 1
- Partial accommodation for individual head size through proportional measurements 1
- Standardized anatomical landmarks including nasion-inion distance, tragus-tragus distance, and head circumference 2
Specific Measurement Formulas
For targeting the dorsolateral prefrontal cortex (DLPFC), scalp measurements use the following calculations 2:
- Arc length measurements are derived from: Y = ((NI + TrTr)/2) × coefficient and X = HC × coefficient, where NI = nasion-inion distance, TrTr = tragus-tragus distance, and HC = head circumference 2
- These measurements are taken along the scalp contour to project the target location from underlying brain anatomy 3
Critical Limitations of Scalp-Based Measurements
The scalp-based approach has significant accuracy limitations that directly impact treatment outcomes:
- Does not account for inter-individual differences in brain anatomy, which can vary substantially between patients 1
- Has been shown not to be accurate for fine-grained targeting of specific cortical regions 1
- Produces the smallest behavioral effects compared to more sophisticated targeting methods 1
Alternative Targeting Methods
When scalp measurements are compared to other approaches 1:
- Functional hotspot method: Subject-specific but limited to motor cortex (muscle movement) or visual cortex (phosphenes) 1
- Frameless stereotactic neuronavigation with MRI: Uses individual anatomical MRI scans for precise coil positioning 1
- Individual functional MRI guidance: Associated with the largest treatment effects, though based on limited evidence 1
Common Pitfalls
- Assuming scalp measurements provide adequate precision for all cortical targets, when they are best suited for gross localization only 1
- Not recognizing that the 10-20 EEG system was designed for electrode placement, not for precise brain stimulation targeting 1
- Failing to consider that skull thickness and brain anatomy variations can cause the actual cortical target to be significantly displaced from the scalp measurement point 1
Clinical Recommendation
While scalp measurements based on the EEG coordinate system remain widely used due to accessibility and cost, neuronavigation with individual MRI should be strongly considered when available, particularly for non-motor cortical targets where precision directly impacts therapeutic efficacy. 1 The standardized EEG approach remains acceptable for initial motor threshold determination and basic motor cortex localization. 1