Comparison of Personalised Dosage Between HD-tDCS and Conventional tDCS

105 

Abstract Submission 

Irtisha Chakraborty¹, Rajan Kashyap², S. Sreeraj³, Faheem Arshad⁴, B.K. Yamini⁵, Rose Bharath², P. T. Sivakumar³, Suvarna Alladi⁴, Ganesan Venkatsubramanian³, T N Sathyaprabha¹, Kaviraja Udupa¹, Sagarika Bhattacharjee¹

¹Department of Neurophysiology, NIMHANS, Bengaluru, Karnataka, ²Department of Neuroimaging and Interventional Radiology, NIMHANS, Bengaluru, Karnataka, ³Department of Psychiatry, NIMHANS, Bengaluru, Karnataka, ⁴Department of Neurology, NIMHANS, Bengaluru, Karnataka, ⁵Speech Pathology and Audiology Department, NIMHANS, Bengaluru, Karnataka

Irtisha Chakraborty  
Department of Neurophysiology, NIMHANS
Bengaluru, Karnataka

Rajan Kashyap, Phd  
Department of Neuroimaging and Interventional Radiology, NIMHANS
Bengaluru, Karnataka

S. Sreeraj  
Department of Psychiatry, NIMHANS
Bengaluru, Karnataka

Faheem Arshad, Assistant professor  
Department of Neurology, NIMHANS
Bengaluru, Karnataka

B.K. Yamini  
Speech Pathology and Audiology Department, NIMHANS
Bengaluru, Karnataka

Rose Bharath, Professor  
Department of Neuroimaging and Interventional Radiology, NIMHANS
Bengaluru, Karnataka

P. T. Sivakumar  
Department of Psychiatry, NIMHANS
Bengaluru, Karnataka

Suvarna Alladi, Professor  
Department of Neurology, NIMHANS
Bengaluru, Karnataka

Ganesan Venkatsubramanian, Professor  
Department of Psychiatry, NIMHANS
Bengaluru, Karnataka

T N Sathyaprabha, Professor  
Department of Neurophysiology, NIMHANS
Bengaluru, Karnataka

Kaviraja Udupa, Professor  
Department of Neurophysiology, NIMHANS
Bengaluru, Karnataka

Sagarika Bhattacharjee, Assistant professor  
Department of Neurophysiology, NIMHANS
Bengaluru, Karnataka

The variability in the effects of transcranial Direct Current Stimulation (tDCS) is seen due to the inter-individual differences in brain anatomy causing variability in the current intensity reaching the Region of Interest (ROI). Evans et al. showed that variability in current intensity within the ROI can be reduced by varying the dose (amount of current applied to the scalp) from outside the brain [3]. It is known that current intensity at target ROI also depends on montage configurations like conventional (1x1) and high-definition (HD) tDCS (4x1) as HD-tDCS is thought to be more focal [1]. The present study investigates the variability in the personalised dose needed to obtain a constant intensity at the target ROI across individuals for both HD-tDCS and Conventional tDCS. This will be investigated in dementia patients, known to have severely atrophied brains, compared against a control major depressive disorder group expected to have relatively lesser brain atrophy.

T1 weighted MRI images of 125 subjects (78 Dementia and 47 Depression), were used to simulate 2 language pathway montages (dorsal CP5_Cz and ventral TP7_Nape of the neck) [2] using 2 montage configurations (Conventional and HD electrode) and 2mA input current through ROAST software. ROAST final output gives electric field distribution for both montages [4]. ROAST output i-SATA (MNI) software [5] estimates Average Current Density (ACD) across individual cortical lobes and gyri in two ROIs for each dorsal [Inferior parietal lobule (IPL) and angular gyrus (AG)] and ventral [middle temporal gyrus (MTG) and inferior temporal gyrus (ITG)] pathways of language (Figure 1). Using ACD data, we calculate the required dose (RD) for conventional and HD-tDCS by following formulae.[3,5]

Required Dose= 2/(simulated current at ROI )× Desired intensity at ROI

The desired intensity at the target ROI was determined as the intensity obtained at the same ROI after simulating MNI brain with 2 mA as the current dose.

DTDI (Dose-target determination index) measured the probability of target ROI receiving the maximum current, was calculated by following formulae [5]

DTDI=(ACD at the target ROI)/(Maximum value of ACD formed at any ROI)

A DTDI value of 1 show that the maximum stimulation intensity (ACD) is generated at the target ROI. However, the peak intensity may be received at non-targeted ROI, resulting in a DTDI value <1. A DTDI value of 0 will indicate no stimulation of the target ROI.

Three separate Linear regression analysis was performed to understand the relationship between the dose, ACD at ROIs and DTDI index as dependent variable and montage configuration (conventional vs HD) as the independent variable while controlling for age and sex.

For all 4 target ROIs in both the groups (dementia and depression), the ACD at ROI was significantly higher for conventional tDCS than HD-tDCS. For the dementia group, the dorsal and ventral pathway ROIs needed significantly lesser doses for conventional tDCS than HD tDCS. However, such a difference in the dosage requirement was seen only for dorsal pathway ROIs in the control depression group. The DTDI values were almost similar between the two montage configurations in both the groups, except IPL(both dementia and depression) and AG (only dementia) ROIs showed a higher probability of reaching peak current for conventional montage (Figure 2).

We found that significantly higher ACD is received at the target ROI for conventional montage than HD-tDCS, consistent across different ROIs and disease groups. However, depending on the disease population (with varying degrees of atrophy), conventional tDCS requires a lower individual current dose (within the maximum tolerable dosage of 4mA) outside the scalp to generate a consistent current intensity at the target ROI across individuals. However, the probability of hitting the target region with peak current remains unchanged, irrespective of the montage configuration.

Non-invasive Magnetic/TMS

TDCS ¹

Neurodegenerative/ Late Life (eg. Parkinson’s, Alzheimer’s) ²

Psychiatric (eg. Depression, Anxiety, Schizophrenia)

Language Comprehension and Semantics

Aphasia

Degenerative Disease

Other - tDCS simulation