Using bioimpedance spectroscopy parameters as real-time feedback during tDCS.

An exploratory analysis is carried out to investigate the feasibility of using BioImpedance Spectroscopy (BIS) parameters, measured on scalp, as real-time feedback during Transcranial Direct Current Stimulation (tDCS). TDCS is shown to be a potential treatment for neurological disorders. However, this technique is not considered as a reliable clinical treatment, due to the lack of a measurable indicator of treatment efficacy. Although the voltage that is applied on the head is very simple to measure during a tDCS session, changes of voltage are difficult to interpret in terms of variables that affect clinical outcome. BIS parameters are considered as potential feedback parameters, because: 1) they are shown to be associated with the DC voltage applied on the head, 2) they are interpretable in terms of conductive and capacitive properties of head tissues, 3) physical interpretation of BIS measurements makes them prone to be adjusted by clinically controllable variables, 4) BIS parameters are measurable in a cost-effective and safe way and do not interfere with DC stimulation. This research indicates that a quadratic regression model can predict the DC voltage between anode and cathode based on parameters extracted from BIS measurements. These parameters are extracted by fitting the measured BIS spectra to an equivalent electrical circuit model. The effect of clinical tDCS variables on BIS parameters needs to be investigated in future works. This work suggests that BIS is a potential method to be used for monitoring a tDCS session in order to adjust, tailor, or personalize tDCS treatment protocols.

Preprocessing and parameterizing bioimpedance spectroscopy measurements by singular value decomposition.

In several applications of bioimpedance spectroscopy, the measured spectrum is parameterized by being fitted into the Cole equation. However, the extracted Cole parameters seem to be inconsistent from one measurement session to another, which leads to a high standard deviation of extracted parameters. This inconsistency is modeled with a source of random variations added to the voltage measurement carried out in the time domain. These random variations may originate from biological variations that are irrelevant to the evidence that we are investigating. Yet, they affect the voltage measured by using a bioimpedance device based on which magnitude and phase of impedance are calculated.By means of simulated data, we showed that Cole parameters are highly affected by this type of variation. We further showed that singular value decomposition (SVD) is an effective tool for parameterizing bioimpedance measurements, which results in more consistent parameters than Cole parameters. We propose to apply SVD as a preprocessing method to reconstruct denoised bioimpedance measurements. In order to evaluate the method, we calculated the relative difference between parameters extracted from noisy and clean simulated bioimpedance spectra. Both mean and standard deviation of this relative difference are shown to effectively decrease when Cole parameters are extracted from preprocessed data in comparison to being extracted from raw measurements.We evaluated the performance of the proposed method in distinguishing three arm positions, for a set of experiments including eight subjects. It is shown that Cole parameters of different positions are not distinguishable when extracted from raw measurements. However, one arm position can be distinguished based on SVD scores. Moreover, all three positions are shown to be distinguished by two parameters, R0/R∞ and Fc, when Cole parameters are extracted from preprocessed measurements. These results suggest that SVD could be considered as an effective technique for overcoming the variability of bio-impedance spectroscopy measurements.

Outside access: advances in tDCS could provide a mainstream clinical tool for noninvasive neuromodulation.

A treatment for depression, chronic pain, cravings, and more--this is the emerging field of transcranial direct current stimulation (tDCS). In tDCS, transcranial means through the skull, and direct current is a weak stimulation current delivered to the brain using noninvasive electrodes placed on the scalp [Figure 1(a)].

Bioimpedance spectroscopy method for investigating changes to intracranial dose during transcranial direct current stimulation.

Tissue resistance changes upon application of DC current. We posit that in a similar fashion, that scalp and skull resistances during trancranial direct current stimulation (tDCS) are variable, resulting in changes to intracranial dose. Transcranial magnetic stimulation (TMS), electoencephelogram (EEG), functional magnetic resonance imaging (fMRI), proton magnetic resonance spectroscopy ((1)H MRS) and functional near infrared spectroscopy (fNIRS) are technologies used to measure individual neural response to tDCS. These technologies are complex and may not be directly correlated to intracranial dose. We therefore present a bioimpedance spectroscopy method of measuring changes to the intracranial dose in vivo. Scalp resistance changes are measured during tDCS. Current flow through the scalp is calculated as the ratio of voltage measured on the scalp and scalp resistance. Variation of intracranial current is indirectly calculated from changes in the current shunted through the scalp. We thus demonstrate a novel methodology of on-line monitoring of scalp resistance and current as an objective feedback of estimated individual tDCS dose.