Transcranial electrical stimulation motor threshold can estimate individualized tDCS dosage from reverse-calculation electric-field modeling

• Published in: Brain stimulation 2020-07, Vol.13 (4), p.961-969
• Main Authors: Caulfield, Kevin A., Badran, Bashar W., DeVries, William H., Summers, Philipp M., Kofmehl, Emma, Li, Xingbao, Borckardt, Jeffrey J., Bikson, Marom, George, Mark S.
• Format: Article
• Language: English
• Subjects: Adult; Cerebral Cortex - physiology; Electric field modeling; Female; Humans; Individualized dosing; Magnetic Resonance Imaging - methods; Male; Models, Neurological; Patient-Specific Modeling; tDCS; tDCS dosing; Transcranial direct current stimulation; Transcranial Direct Current Stimulation - methods; Transcranial electrical stimulation; Transcranial magnetic stimulation; Transcranial Magnetic Stimulation - methods
• ISSN: 1935-861X; 1876-4754
• DOI: 10.1016/j.brs.2020.04.007

Unique amongst brain stimulation tools, transcranial direct current stimulation (tDCS) currently lacks an easy or widely implemented method for individualizing dosage. We developed a method of reverse-calculating electric-field (E-field) models based on Magnetic Resonance Imaging (MRI) scans that can estimate individualized tDCS dose. We also evaluated an MRI-free method of individualizing tDCS dose by measuring transcranial magnetic stimulation (TMS) motor threshold (MT) and single pulse, suprathreshold transcranial electrical stimulation (TES) MT and regressing it against E-field modeling. Key assumptions of reverse-calculation E-field modeling, including the size of region of interest (ROI) analysis and the linearity of multiple E-field models were also tested. In 29 healthy adults, we acquired TMS MT, TES MT, and anatomical T1-weighted MPRAGE MRI scans with a fiducial marking the motor hotspot. We then computed a “reverse-calculated tDCS dose” of tDCS applied at the scalp needed to cause a 1.00 V/m E-field at the cortex. Finally, we examined whether the predicted E-field values correlated with each participant’s measured TMS MT or TES MT. We were able to determine a reverse-calculated tDCS dose for each participant using a 5 × 5 x 5 voxel grid region of interest (ROI) approach (average = 6.03 mA, SD = 1.44 mA, range = 3.75–9.74 mA). The Transcranial Electrical Stimulation MT, but not the Transcranial Magnetic Stimulation MT, significantly correlated with the ROI-based reverse-calculated tDCS dose determined by E-field modeling (R2 = 0.45, p