No abstract

Two challenges to optimizing transcranial direct current stimulation (tDCS) are selecting between, often similar, electrode montages and accounting for inter-individual differences in response. These two factors are related by how tDCS montage determines current flow through the brain considered across or within individuals. MRI-based computational head models (CHMs) predict how brain anatomy determines electric field (EF) patterns for a given tDCS montage. Because conventional tDCS produces diffuse brain current flow, stimulation outcomes may be understood as modulation of global networks. Therefore, we developed a network-led, rather than region-led, approach. We specifically considered two common “frontal” tDCS montages that nominally target the dorsolateral prefrontal cortex; asymmetric “unilateral” (anode/cathode: F4/Fp1) and symmetric “bilateral” (F4/F3) electrode montages. CHMs of 66 participants were constructed. We showed that cathode location significantly affects EFs in the limbic network. Furthermore, using a finer parcellation of large-scale networks, we found significant differences in some of the main nodes within a network, even if there is no difference at the network level. This study generally demonstrates a methodology for considering the components of large-scale networks in CHMs instead of targeting a single region and specifically provides insight into how symmetric vs asymmetric frontal tDCS may differentially modulate networks across a population.

Two challenges to optimizing transcranial direct current stimulation (tDCS) are selecting between, often similar, electrode montages and accounting for inter-individual differences in response. These two factors are related by how tDCS montage determines current flow through the brain considered across or within individuals. MRI-based computational head models (CHMs) predict how brain anatomy determine electric field (EF) patterns for a given tDCS montage. Because conventional tDCS produces diffuse brain current flow, stimulation outcomes may be understood as modulation of global networks. Therefore, we developed network-led, rather than region-led, approach. We specifically considered two common frontal tDCS montages that nominally target the dorsolateral prefrontal cortex; asymmetric unilateral (anode/cathode: F4/Fp1) and symmetric bilateral (F4/F3) electrode montages. CHMs of 66 participants were constructed. We showed that cathode location significantly affects EFs in the limbic network. Furthermore, using a finer parcellation of large-scale networks, we found significant differences in some of main nodes within a network, even if there is no difference at the network level. This study generally demonstrates a methodology for considering the components of large-scale networks in CHMs instead of targeting a single region and specifically provides insight into how symmetric vs asymmetric frontal tDCS may differentially modulate networks across a population.

Introduction: Transcranial direct current stimulation (tDCS) has been studied as an adjunctive treatment option for substance use disorders (SUDs). Alterations in brain structure following SUD may change tDCS-induced electric field (EF) and subsequent responses. However, group-level differences between healthy controls (HC) and participants with SUDs in terms of EF and its association with cortical architecture have not yet been modeled quantitatively. Objective: We provided a methodology for group-level analysis of computational head models (CHMs) to investigate the influence of cortical morphology metrics on EFs. Method: Whole-brain surface-based morphology was conducted and cortical thickness, volume, and surface area were compared between participants with CUD (n=20) and age-matched HC (n=22). We also simulated EFs for bilateral tDCS over DLPFC. Effects of structural alterations on EF distribution were investigated based on individualized CHMs. Results: In terms of EF, no significant difference was found within the prefrontal cortex. However, EFs were significantly different in left-postcentral and right-superior temporal gyrus (P < 0.05) with higher level of variance in CUD compared to HC (F39,43=5.31,P<0.0001,C =0.95). We found significant differences in cortical area (caudal anterior cingulate and rostral middle frontal), thickness (lateral orbitofrontal), and volume (paracentral and fusiform) between two groups. Conclusion: Brain morphology and tDCS-induced EFs may be changed following CUD. However, differences between CUD and HCs in EFs do not always overlap with brain areas that showed structural alterations. To sufficiently modulate stimulation targets, it should be checked if individuals with CUD need to be given different stimulation dose based on tDCS target location.

No abstract

Several phantoms have been fabricated for reflection measurements from human tissues in the range of 26.5-40 GHz. For the first time, a novel composition is introduced for fabrication of low-cost simple tissue-equivalent phantoms. The effective medium theory is applied to estimate the dielectric properties of phantoms consisting of polyethylene (PE) powder and the results are verified by dielectric properties measurement on the proposed phantoms. It is shown that mixing theory can estimate the dielectric properties of the PE-based phantoms with minimum error. Therefore, the amount of the ingredients can be estimated theoretically.

We designed a rectangular waveguide exposure system to study the effects of mobile phone frequency (940 MHz) electromagnetic fields (EMF) on luciferase structure and activity. The luciferase activity of exposed samples was significantly higher than that of unexposed samples. Dynamic light scattering of the exposed samples showed smaller hydrodynamic radii compared to unexposed samples (20 nm vs. 47 nm ± 5%). The exposed samples also showed less tendency to form aggregates, monitored by turbidity measurements at l = 360 nm. A microwave dielectric measurement was performed to study the hydration properties of luciferase solutions with a precision network analyzer over frequency ranges from 0.2 to 20 GHz before and after exposure. The change in the dielectric properties of the exposed luciferase solution was related to the disaggregation potency of the applied field. Together, our results suggested that direct interactions with luciferase molecules and its dipole moment were responsible for the reduced aggregation and enhanced luciferase activity upon exposure to the EMF.

Fracture healing happens naturally in most bone break cases. Occasionally prolongation of restoration period or non‐union of the fracture may occur, where electrical stimulation has been shown to facilitate bone restoration by stimulating osteoblasts. Despite clinical use, a comprehensive computational model linking the applied currents to the stimulating field in the fracture has been missing. In this paper, we investigate the input current needed to stimulate osteoblasts in a fracture in the human forearm. Optimal current is computed for various fracture configurations, and sensitivity to frequency and inter/intrapersonal variance in dielectric properties are analyzed. Stimulation thresholds at the fracture site are based on detailed review of experimental studies. Our results show that for a 1 mm thick 30° fracture with a 15 Hz sinusoidal field, the input current amounts to a maximum of 3.77 µA. Minimum and maximum required current levels are plotted versus fracture parameters, all of which comply with the ICNIRP standard. Simulation results are supported by several experimental reports. Our model is useful for understanding the effects of various geometrical and electrical factors on clinical outcome, and serves as a theoretical aid in the design of more efficient systems. Bioelectromagnetics. 40:128–135, 2019. © 2019 Bioelectromagnetics Society