Estimation of individually induced e-field strength during transcranial electric stimulation using the head circumference.

Background: Head and brain anatomy have been related to e-field strength induced by transcranial electrical stimulation (tES). Individualization based on anatomic factors require high-quality structural magnetic resonance images, which are not always available. Head circumference (HC) can serve as an alternative means, but its linkage to electric field strength has not yet been established.

Methods: We simulated electric fields induced by tES based on individual T1w- and T2w-images of 47 healthy adults, for four conventional ("standard") and four corresponding focal ("4x1") electrode montages. Associations of electric field strength with individual HC were calculated using linear mixed models.

Results: Larger HC was associated with lower electric field strength across montages. We provide mathematical equations to estimate individual electric field strength based on the HC.

Conclusion: HC can be used as an alternative to estimate interindividual differences of the tES-induced electric field strength and to prospectively individualize stimulation dose, e.g., in the clinical context.