Electromagnetic coupling simulagions for a magnetic induction sensor for sleep monitoring

Magnetic induction (MI) method has been extensively used in non-destructive testing of materials. In biomedical applications, it attracted lots of attention for the contact-less advantages it provides. Sleep monitoring through detecting conductivity changes in lungs and heart during breathing and cardiac activity is the purpose of our studies. The low conductivity of biological tissues increase the complexity of the design of such systems. One challenge is to separate the effects of magnetic field from the electric field; achieving a pure magnetic contribution is difficult since the received signal is contaminated by the unwanted capacitive coupling. Our hypothesis is that for a periodic vital sign monitoring like breathing and heart activity, part of this secondary coupling could be considered as a desired effect to take the advantage of both contributions. In this paper, the coupling mechanisms existed in our system have been simulated and studied using finite element and Orcad simulations to estimate different contributions we would have in the developed MI system.Peer ReviewedPostprint (author’s final draft

Simulation of a magnetic induction method for determining passive electrical property changes of human trunk due to vital activities

Postprint (published version

Simulation of a magnetic induction method for determining passive electrical property changes of human trunk due to vital activities

Determining the changes in passive electrical properties of human tissues and the geometry changes of the body parts due to breathing and cardiac activity could be a method for monitoring these vital signs. We have designed a model with COMSOL Multiphysics to simulate a magnetic induction method for determining changes of these characteristics. Given the large number of parameters and variables, difficulties of anatomical modeling and various tissue properties, we are facing some numerical challenges. In addition, using magnetic induction method, we have to consider safety issues, standards and limitations. We used COMSOL to simulate the induced currents in the human body and the resulting perturbation in the magnetic field for different tissue properties and geometries. As a result, we obtained the expected changes during breathing and we know the minimum distances and maximum excitation currents that could be used to meet the international safety standards.Postprint (published version

Signal-to-motion artifact ratio versus frequency for impedance pneumography

We measured transthoracic impedance between 12.5 and 185 kHz in nine adults. We used a system with two impedance channels, both simultaneously detecting the real part of impedance at two different frequencies. We used only two electrodes in the midaxillary line, connecting both channels in parallel. The amplitude relation between the two channels was measured for different maneuvers and frequencies. Results show for normal breathing an increase of the signal of 20% and a decrease in motion artifacts from 12.5 to 185 kHz. We conclude that, for the maneuvers studied, it is better to work at higher frequencies than the ones commonly used. Also, we suggest a method to further increase the signal-to-motion artifact ratio based on measurement at two frequencies.Peer Reviewe

Biological tissue characterization by magnetic induction spectroscopy (MIS): requirements and limitations

Magnetic induction spectroscopy (MIS) aims at the contactless measurement of the passive electrical properties (PEP) σ, ε, and μ of biological tissues via magnetic fields at multiple frequencies. Whereas previous publications focus on either the conductive or the magnetic aspect of inductive measurements, this article provides a synthesis of both concepts by discussing two different applications with the same measurement system: 1) monitoring of brain edema and 2) the estimation of hepatic iron stores in certain pathologies. We derived the equations to estimate the sensitivity of MIS as a function of the PEP of biological objects. The system requirements and possible systematic errors are analyzed for a MIS-channel using a planar gradiometer (PGRAD) as detector.We studied 4 important error sources: 1) moving conductors near the PGRAD; 2) thermal drifts of the PGRAD-parameters; 3) lateral displacements of the PGRAD; and 4) phase drifts in the receiver. All errors were compared with the desirable resolution. All errors affect the detected imaginary part (mainly related to σ ) of the measured complex field much less than the real part (mainly related to ε and μ). Hence, the presented technique renders possible the resolution of (patho-) physiological changes of the electrical conductivity when applying highly resolving hardware and elaborate signal processing. Changes of the magnetic permeability and permittivity in biological tissues are more complicated to deal with and may require chopping techniques, e.g., periodic movement of the object.Peer Reviewe

In-bed vital signs monitoring system based on unobtrusive magnetic induction method with a concentric planar gradiometer

Significance. Unobtrusive vital signs monitoring is of major importance for various medical areas such as detection and treatment of sleep disorders, monitoring neonates and burned victims, home health care and smart home applications and wearables among others. Such applications call for monitoring methods in which the patient's natural state is less interfered with. An ideal device would be non-invasive, minimally restrictive, robust enough to compensate movements of the patients, and would operate without relying on the patient's full cooperation. Objective. This paper focuses on the design and development of an unobtrusive vital signs monitoring system particularly suited for long-term monitoring placed under the mattresses. Approach. The system is based on the magnetic induction sensing method, designed to infer presence on the bed, breathing and cardiac activity, and consists of two coils for excitation and detection. The new detection coil is based on a concentric planar gradiometer for canceling the primary field. The signal acquisition system has been designed using simple electronics to avoid ending up with a complex and expensive system. The experimental results were compared with reference signals coming from other known sensors with different technical bases for benchmarking and identifying the advantages and/or drawbacks of the new system regarding other techniques. The designed system was also studied in regards to safety standards and limitations for the exposure to the magnetic fields. Main results. Experimental results confirm the suitability and safety of the sensor for long-term cardiac and respiratory monitoring. The system is able to detect respiration and cardiac activity as well as the presence on the bed and changes in position.Peer ReviewedPreprin

Transmural versus non-transmural in situ electrical impedance spectrum for healthy, ischemic, and healed myocardium

Electrical properties of myocardial tissue are anisotropic due to the complex structure of the myocardial fiber orientation and the distribution of gap junctions. For this reason, measured myocardial impedance may differ depending on the current distribution and direction with respect to myocardial fiber orientation and, consequently, according to the measurement method. The objective of this study is to compare the specific impedance spectra of the myocardium measured using two different methods. One method consisted of transmural measurements using an intracavitary catheter and the other method consisted of nontransmural measurements using a four-needle probe inserted into the epicardium. Using both methods, we provide the in situ specific impedance spectrum (magnitude and phase angle) of normal, ischemic, and infarcted pig myocardium tissue from 1 kHz to 1 MHz. Magnitude spectra showed no significant differences between the measurement techniques. However, the phase angle spectra showed significant differences for normal and ischemic tissues according to the measurement technique. The main difference is encountered after 60 min of acute ischemia in the phase angle spectrum. Healed myocardial tissue showed a small and flat phase angle spectrum in both methods due to the low content of cells in the transmural infarct scar. In conclusion, both transmural and nontransmural measurements of phase angle spectrum allow the differentiation among normal, ischemic, and infarcted tissue.Peer Reviewe

Transoesophageal detection of heart graft rejection by electrical impedance: using Finite Element Method simulations

Previous studies have shown that it is possible to evaluate heart graft rejection level using a bioimpedance technique by means of an intracavitary catheter. However, this technique does not present relevant advantages compared to the gold standard for the detection of a heart rejection, which is the biopsy of the endomyocardial tissue. We propose to use a less invasive technique that consists in the use of a transoesophageal catheter and two standard ECG electrodes on the thorax. The aim of this work is to evaluate different parameters affecting the impedance measurement, including: sensitivity to electrical conductivity and permittivity of different organs in the thorax, lung edema and pleural water. From these results, we deduce the best estimator for cardiac rejection detection, and we obtain the tools to identify possible cases of false positive of heart rejection due to other factors. To achieve these objectives we have created a thoracic model and we have simulated, with a FEM program, different situations at the frequencies of 13, 30, 100, 300 and 1000 kHz. Our simulation demonstrates that the phase, at 100 and 300 kHz, has the higher sensitivity to changes in the electrical parameters of the heart muscle.Peer ReviewedPostprint (author’s final draft

Total body water changes using segmental bioimpedance in healthy population with similar anthropometry

Electrical bioimpedance was measured in 7 body segments, and also with the standard right-side method, using 11 electrodes at 6 different frequencies in 8 healthy male subjects with similar anthropometry. Our objective was to determine the capability of segmental bioimpedance measurements to estimate small changes of water on each segment (TWsegi)and total body water (TBW) in comparison with the standard right-side method. Water was also estimated with 40K and DXA. Volunteers were measured before and after a 3.5% water load of their individual TBW. The expected TBW mean increment after water load was 1.45 l. The estimator with lower Standard Error SE was the weight of the subject (0.15 l). For impedance methods, the SE of the segmental method was 0.94 l vs. 1.41 l for right-side. Segmental volume changes obtained by DXA and Impedance compared with expected values showed maximum differences of almost 2 l for DXA and 0.5 l for Z in the abdomen. In conclusion, in a healthy sample with similar anthropometry, such as astronauts and athletes, the use of a segmental impedance method improves the accuracy of the right-side method to estimate TBW. Changes in water segments estimated by impedance where more close to expected values than using DXA.Postprint (author’s final draft

Multisite recording of extracellular potentials produced by microchannel-confined neurons in-vitro

Towards establishing electrical interfaces with patterned in vitro neurons, we have previously described the fabrication of hybrid elastomer-glass devices polymer-on-multielectrode array technology and obtained single-electrode recordings of extracellular potentials from confined neurons (Claverol-Tinturé et al., 2005). Here, we demonstrate the feasibility of spatially localized multisite recordings from individual microchannel-guided neurites extending from microwell-confined somas with good signal-to-noise ratios (20 dB) and spike magnitudes of up to 300 μV. Single-cell current source density (scCSD) analysis of the spatio-temporal patterns of membrane currents along individual processes is illustrated.Peer Reviewe