The Importance of Tissue Identity in Modeling Transcranial Direct Current Stimulation

Abstract

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique where

electrodes applied to the scalp deliver electrical currents and modify the neuronal cell environment. Research shows that tDCS can improve cognitive functions in older adults by modulating brain function via delivered increasing current in the brain. We aimed to investigate the impact of individual tissue identities on the delivered current density value in the brain. Six models (M1–M6) were created for analysis. M6 was assigned as the reference model, incorporating the six most used tissue types (i.e., white matter, grey matter, CSF, skin, and bone). The M2–M5 models were compared to the M6 reference model, with each model incorporating one additional tissue type to assess its specific contribution to current density (J) (i.e., blood, eye, fat, and separating bone into cancellous and cortical bone, respectively). M1 included all eleven tissues tested. Magnetic resonance images (MRIs) were acquired in four older adults (2M, 2F, M_age = 73 years) and were segmented into each of the six models using a combination of automatic and manual tissue segmentation. Electrodes were placed using a F3- F4 montage, with a 2mA input current. Each tissue was assigned unique conductivity values and electric fields were generated using the COMSOL AC/DC module. The white matter and grey matter regions were isolated and combined to compute J values in the brain (Jbrain). Jbrain in M1-M5 was compared to M6 using a Welch’s t-test in RStudio. We found that, among the six models, M1 (median Jbrain = 0.020381 Am-2), M4 (median Jbrain = 0.018377 Am-2), and M5 (median Jbrain = 0.015608 Am-2) were statistically significantly (p-value <0.001) different from the reference model M6. This research offers an understanding of how various head tissues contribute to the current density in the brain, aiding in the planning of future tDCS modeling using these techniques.

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