Jose Gomez-Tames

A simulation study: Effect of the inter-electrode distance, electrode size and shape in Transcutaneous Electrical Stimulation

Transcutaneous Electrical Stimulation (TES) has been used widely to recover motor functions in neurologically impaired individuals by artificially activating skeletal muscles using superficial electrodes. Some simulation studies have investigated the percentage of fibers activated in denervated skeletal muscles, the comfort and selectivity, and the influence of fat thickness in the case of obese people, to optimize the inter-electrode distance and electrode size. However, the effect of the inter-electrode distance, electrode shape and electrode size might be further analyzed using the selectivity, activation depth and activation volume. In this regard, we developed a 3D multi-layer (skin, fat, muscle, and nerve) thigh model coupled with a mammalian nerve model using a finite element method for optimization of TES therapy. Different evaluation indices (motor threshold, activation depth, selectivity and activation volume) were inspected to compare different TES parameters in terms of nerve activation. The simulation results agreed with experimental data and new insights were obtained: selectivity is better in small electrodes; nevertheless, in high current stimulation, small electrodes and large electrodes have similar selectivity.

A human-phantom coupling experiment and a dispersive simulation model for investigating the variation of dielectric properties of biological tissues

Variation of the dielectric properties of tissues could happen due to aging, moisture of the skin, muscle denervation, and variation of blood flow by temperature. Several studies used burst-modulated alternating stimulation to improve activation and comfort by reducing tissue impedance as a possible mechanism to generate muscle activation with less energy. The study of the effect of dielectric properties of biological tissues in nerve activation presents a fundamental problem, which is the difficulty of systematically changing the morphological factors and dielectric properties of the subjects under study. We tackle this problem by using a simulation and an experimental study. The experimental study is a novel method that combines a fat tissue-equivalent phantom, with known and adjustable dielectric properties, with the human thigh. In this way, the dispersion of the tissue under study could be modified to observe its effects systematically in muscle activation. We observed that, to generate a given amount of muscle or nerve activation under conditions of decreased impedance, the magnitude of the current needs to be increased while the magnitude of the voltage needs to be decreased.

Influence of Different Geometric Representations of the Volume Conductor on Nerve Activation during Electrical Stimulation

Volume conductor models with different geometric representations, such as the parallel layer model (PM), the cylindrical layer model (CM), or the anatomically based model (AM), have been employed during the implementation of bioelectrical models for electrical stimulation (FES). Evaluating their strengths and limitations to predict nerve activation is fundamental to achieve a good trade-off between accuracy and computation time. However, there are no studies aimed at clarifying the following questions. (1) Does the nerve activation differ between CM and PM? (2) How well do CM and PM approximate an AM? (3) What is the effect of the presence of blood vessels and nerve trunk on nerve activation prediction? Therefore, in this study, we addressed these questions by comparing nerve activation between CM, PM, and AM models by FES. The activation threshold was used to evaluate the models under different configurations of superficial electrodes (size and distance), nerve depths, and stimulation sites. Additionally, the influences of the sciatic nerve, femoral artery, and femoral vein were inspected for a human thigh. The results showed that the CM and PM had a high error rate, but the variation of the activation threshold followed the same tendency for electrode size and interelectrode distance variation as AM.

An experimental study on the effect of fat conductivity on voltage distribution and muscle recruitment using tissue-equivalent phantoms

The current transfer through tissues have been investigated at different electrical stimulation parameters to improve Functional Electrical Stimulation (FES). However, there are no experimental studies that investigate the effect of tissue conductivity on the recruitment. This is due to the difficulty of isolating the effect of one tissue from the others, and comparing the same tissue but with different conductivity in vivo. This study focused on the tissues parameters. Our idea is to investigate the effect of conductivity of one single tissue using tissues-equivalent phantoms setting fixedly on the skins surface of subjects. Voltage distribution over tissues and muscle recruitment were measured in both constant current and constant voltage stimulation settings. As a result, we found that the electrical configuration containing the fat tissue with lower conductivity resulted in higher recruitment. This experimental finding agrees with the results of our simulation studies in the constant current stimulation setting.

Needle detection by electro-localization for a needle EMG exam robotic simulator

Needle EMG (Electromyogram) Exam (NEE) is an important neurological exam, and neurology interns and novice medical need repetitive training to gain the necessary skill to perform the exam. However, until now it has been difficult to reproduce multiple pathological conditions for their training, since in most cases, trainees serve as human subjects for each other. A robotic simulator that could reproduce behavior with various pathological disorders can be of great help for NEE skill training. Needle tip localization is a key component of the robotic simulator, since position-dependent-EMG is the signal source for skilled neurologists to determine the pathological situation. The needle tip localization has been investigated for many medical tests and applications, such as prostate brachytherapy, etc. However, only few studies have been reported on the process of needle operation in muscle based on EMG signals dependent on needle tip position. Our idea is to apply a tissue-like conductive phantom so as to realize both physical sense of insertion, and needle localization for the NEE robotic simulator. A pair of electrodes was designed to generate a near-linear voltage distribution along the depth direction of the tissue-like phantom, by which the inserted needle could be localized. The influence of the shape of phantom and electrodes on detection accuracy were investigated by a set of measurement experiment and a computer simulation. The results showed that, the estimated depth values agreed with that of the computer simulation model, and the curved phantom had a much steeper distribution in the deeper region (better accuracy for needle tip detection).

Ultrasound imaging and semi-automatic analysis of active muscle features in electrical stimulation by optical flow.

Ultrasound imaging is an effective way to measure the muscle activity in electrical stimulation studies. However, it is a time consuming task to manually measure pennation angle and muscle thickness, which are the benchmark features to analyze muscle activity from the ultrasound imaging. In previous studies, the muscle features were measured by calculating optical flow of the pennation angle by using only fibers of a muscle from the ultrasound, without carefully considering moving muscle edges during active and passive contraction. Therefore, this study aimed to measure the pennation angle and muscle thickness by using the edges and fibers of a muscle in a quantitative way in a semi-automatic optical flow based approach. The results of the semi-automatic analysis were compared to that of manual measurement. Through the comparison, it is clear that the proposed algorithm could achieve higher accuracy in tracking the thickness and pennation angle for a sequence of ultrasound images.

Influence of fat thickness and femur location on nerve activity computation during electrical stimulation

Cylindrically and anatomically based electrophysiological models have been used to study how different stimulation parameters influence electrical stimulation for muscle activation. The former are less computationally expensive because of their symmetrical properties and simpler geometry. However, the availability of employing simplified models instead of anatomically based models in the prediction of nerve activity has not been investigated. In this study, we compared an anatomically based model of the thigh with its corresponding cylindrically based model to study the difference in nerve activation with respect to stimulation site, the role of fat thickness, bone location, and the presence of the sciatic nerve and blood vessels. As a result, it is clear that the location of the femur and the non-uniformity of fat thickness through different stimulation sites make it difficult to use a cylindrically based model to predict nerve activation in the same way as with the anatomically based model. Nevertheless, the geometry of the cylindrically based model gives a general insight of the nerve activation and could be optimized to match the anatomically based model.

Ultrasound Imaging and Analysis of Muscle Activity in Lower Limb

This study aimed to investigate the possibility of using ultrasound images as a tool of quantitative analysis for muscle activity. One expected advantage of ultrasound imaging is that it could provide a non-invasive way to investigate the activities of both outer and inner muscles. In the literature, there have been fewer studies exploring the ultrasound image analysis for quantitative analysis of muscle activity, i.e., the estimation of spatiotemporal aspect of muscle contraction, such as, the power, the latency. This is why this study attempted to investigate different features extracted from the ultrasound images, to represent the muscle activity. For this purpose we measured the ultrasound images, as well as Electromyogram, and joint angle simultaneously, during the voluntary contraction, and muscle activity activated by the Achilles tendon reflex, in which temporal aspect of muscle activity should be precisely investigated. We were able to quantify the inner and outer muscle activity by measuring the muscle thickness, pennation angle and long-axis displacement.

Simulation of the Muscle Recruitment by Transcutaneous Electrical Stimulation in a Simplified Semitendinosus Muscle Model

A new simplified version of the Semitendinosus muscle was used to study muscle activation initiated at the nerve axons near the vicinity of the endplates due to transcutaneous electrical stimulation. Recruitment duration curves were obtained to evaluate the feasibility of the proposed model. As result, the recruitment duration curves agreed with available experimental data.

Three-dimensional needle-tip localization by electric field potential and camera hybridization for needle electromyography exam robotic simulator.

As one of neurological tests, needle electromygraphy exam (NEE) plays an important role to evaluate the conditions of nerves and muscles. Neurology interns and novice medical staff need repetitive training to improve their skills in performing the exam. However, no training systems are able to reproduce multiple pathological conditions to simulate real needle electromyogram exam. For the development of a robotic simulator, three components need to be realized: physical modeling of upper limb morphological features, position-dependent electromyogram generation, and needle localization; the latter is the focus of this study. Our idea is to couple two types of sensing mechanism in order to acquire the needle-tip position with high accuracy. One is to segment the needle from camera images and calculate its insertion point on the skin surface by a top-hat transform algorithm. The other is voltage-based depth measurement, in which a conductive tissue-like phantom was used to realize both needle-tip localization and physical sense of needle insertion. For that, a pair of electrodes was designed to generate a near-linear voltage distribution along the depth direction of the tissue-like phantom. The accuracy of the needle-tip position was investigated by the electric field potential and camera hybridization. The results showed that the needle tip could be detected with an accuracy of 1.05±0.57 mm.