Development of a Computational Method for Assessing Static Field Induced Torque on Medical Implants

The objective of this thesis is the development of a computational method for finding the torque induced on an object when placed in the static magnetic field of an MR scanner. As a preliminary step, the classic EM problems of a sphere and infinitely long cylinder of linear material was modeled in commercially available simulation software. Upon verification of the parameters implemented, the second step is the simulation of simple objects with realistic material properties, stainless-steel cylinders. Physical cylinders were machined to match those in the simulations and underwent the ASTM standard method for measuring induced torque. An adjacent study that was also performed was finding the measurement uncertainty in a prototype ASTM abiding apparatus, separate from the one used for experimental verification. It was found that the sphere and infinitely long cylinder models differed less than 5% from the analytical solutions. Implementing the correct material properties, magnetic susceptibility in particular, to the grades of stainless-steel used in this study was particularly challenging. However, when the experimentally measured results were used to find the necessary susceptibility values for the computational methods, it was found to be in agreement with literature values. The following computationally-found torque values agreed within 10% difference from the experimentally measured values. The induced torque increased linearly with the length of the cylinders and the square of magnetic susceptibility. In the uncertainty analysis of the torque measurement apparatus described in ASTM F2213-17, it was found that the apparatus described in the ‘Pulley Method’ offered a lower instrument uncertainty than the apparatus described in the ‘Torsional Spring Method’. This study emphasized on the contribution of static friction and is important to consider should the apparatus be used in the future to verify computational results

The effect of ELF electric fields on implantable cardioverter defibrillator

Through research and technological advances, modern society has developed a variety of implantable devices that helps to optimize human’s life quality. One of these devices is the implantable cardioverter defibrillator (ICD). Our unit, ERP Environmental Health, in previous investigations has found that, in some cases, the ICD receives certain disturbances under the power lines. Therefore it is important to study the degree of immunity that implants have against electromagnetic fields, because even small interference that result from EMF exposure could deliver a major public health impact. The aim of this master thesis is to study the effect of extremely low frequency (ELF) non-uniform electric fields on implantable cardioverter defibrillators (ICD). This test was conducted in the laboratory of High Voltage Technology, University of Tampere, in a controlled climate room. The experiment consisted in introducing the ICD in the interior of a Human-Shaped phantom which was filled with a saline solution to emulate the conductivity within the human body; the phantom was subsequently exposed to an electric field between two parallel plates. Following the experiment, we created a three-dimensional model of the electromagnetic fields generated by a phantom. This allowed us to corroborate the experiment and calculate quantities that it was impossible to obtain physically, such equipotential lines and the electric field lines between the parallel plates. We also calculated the total induced current and compared with experimental results, obtaining in a worse case a mean relative error of 10.53%, which suggests that our calculations are in agreement with the experimental result. However, an additional comprehensive study is needed to obtain more conclusive results

Feasibility of improving risk stratification in the inherited cardiac conditions

Fatal ventricular arrhythmias can occur in patients with Hypertrophic Cardiomyopathy, Brugada Syndrome and rarely in patients with normal cardiac investigations. Despite very different pathogeneses, we hypothesised that a common electrophysiological substrate precipitates these arrhythmias and could be used as a marker for risk stratification. In Chapter 3 of this thesis, we found that fewer than half the cardiac arrest survivors with Brugada Syndrome would have been offered prophylactic defibrillators based on current risk scoring, highlighting the need for better risk stratification. Our group previously used a commercially available 252-electrode vest which constructs ventricular electrograms onto a CT image of the heart to show exercise related differences in high-risk patients. In Chapter 4, we applied this method to Brugada patients, but could not reproduce prior results. Further investigation revealed periodic changes in activation patterns after exercise that could explain this discrepancy. An alternative matrix approach was developed to overcome this problem. Exercise induced conduction heterogeneity differentiated Brugada patients from unaffected controls, but not those surviving cardiac arrest. However, if considered alongside spontaneous type 1 ECG and syncope, inducible conduction heterogeneity markedly improved identification of Brugada cardiac arrest survivors. In Chapter 5 the method was shown to differentiate idiopathic ventricular fibrillation patients from those fully recovered from acute ischaemic cardiac arrest, implying a permanent electrophysiological abnormality. In Chapter 8, we showed prolonged mean local activation times and activation-recovery intervals in hypertrophic cardiomyopathy cardiac arrest survivors compared to those without previous ventricular arrhythmia. These metrics were combined into both logistic regression and support vector machine models to strongly differentiate the groups. We concluded that electrophysiological changes could identify cardiac arrest survivors in various cardiac conditions, but a single factor common pathway was not established. Prospective studies are required to determine if using these parameters could enhance current risk stratification for sudden death.Open Acces

Design, characterization and testing of a thin-film microelectrode array and signal conditioning microchip for high spatial resolution surface laplacian measurement.

Cardiac mapping has become an important area of research for understanding the mechanisms responsible for cardiac arrhythmias and the associated diseases. Current technologies for measuring electrical potentials on the surface of the heart are limited due to poor spatial resolution, localization issues, signal distortion due to noise, tissue damage, etc. Therefore, the purpose of this study is to design, develop, characterize and investigate a custom-made microfabricated, polyimide-based, flexible Thin-Film MicroElectrode Array (TFMEA) that is directly interfaced to an integrated Signal Conditioning Microchip (SCM) to record cardiac surface potentials on the cellular level to obtain high spatial resolution Surface Laplacian (SL) measurement. TFMEAs consisting of five fingers (Cover area = 4 mm2 and 16 mm2), which contained five individual microelectrodes placed in orthogonal directions (25-µm in diameter, 75-µm interelectrode spacing) to one another, were fabricated within a flexible polyimide substrate and capable of recording electrical activities of the heart on the order of individual cardiomyocytes. A custom designed SCM consisting of 25 channels of preamplification stages and second order band-pass filters was interfaced directly with the TFMEA in order to improve the signal-to-noise ratio (SNR) characteristics of the high spatial resolution recording data. Metrology characterization using surface profilometry and high resolution Scanning Electron Microscope (SEM) indicated the geometry of fabricated TFMEAs closely matched the design parameters \u3c 0.4%). The DC resistances of the 25 individual micro electrodes were consistent (1.050 ± 0.026 kO). The simulation and testing results of the SCM verified the pre-amplification and filter stages met the designed gain and frequency parameters within 2.96%. The functionality of the TFMEA-SCM system was further characterized on a TX 151 conductive gel. The characterization results revealed that the system functionality was sufficient for high spatial cardiac mapping. In vivo testing results clearly demonstrated feasibility of using the TFMEA-SCM system to obtain cellular level SL measurements with significantly improved the SNRs during normal sinus rhythm and Ventricular Fibrillation (VF). Local activation times were detected via evaluating the zero crossing of the SL electro grams, which coincided with the gold standard (dV/dt)min of unipolar electro grams within ± 1%. The in vivo transmembrane current densities calculated from the high spatial resolution SLs were found to be significantly higher than the transmembrane current densities computed using electrodes with higher interelectrode spacings. In conclusion, the custom-made TFMEASCM systems demonstrated feasibility as a tool for measuring cardiac potentials and to perform high resolution cardiac mapping experiments

Development of a high spatial selectivity tri-polar concentric ring electrode for Laplacian electroencephalography (LEEG) system

Brain activity generates electrical potentials that are spatio-temporal in nature. Electroencephalography (EEG) is the least costly and most widely used non-invasive technique for diagnosing many brain problems. It has high temporal resolution but lacks high spatial resolution. The surface Laplacian will enhance the spatial resolution of EEG as it performs the second spatial derivative of the surface potentials. In an attempt to increase the spatial selectivity, researchers introduced a bipolar electrode configuration using a five point finite difference method (FPM) and others applied a quasi-bipolar (tri-polar with two elements shorted) concentric electrode configuration. To further increase the spatial resolution, the nine-point finite difference method (NPM) was generalized to tri-polar concentric ring electrodes. A computer model was developed to evaluate and compare the properties of concentric bipolar, quasi-bipolar, and tri-polar electrode configurations, and the results were verified with tank experiments. The tri-polar configuration was found to have significantly improved spatial localization. Movement-related potential (MRP) signals were recorded from the left pre-frontal lobes on the scalp of human subjects while they performed fast repetitive movements. Disc, bipolar, quasi-bipolar, and tri-polar electrodes were used. MRP signals were plotted for all four electrode configurations. The SNR of four electrode configurations were studied and statistically analyzed using Bonferroni statistical tests. MRP signals were recorded from an array of 5X7 on the left hemisphere of the head. The SNR, spatial selectivity, and mutual information (MI) were compared among conventional disc electrodes, bipolar and tri-polar concentric ring electrodes. The tri-polar concentric electrodes showed more significant improvement in SNR than the all other electrode systems tested. Tri-polar concentric electrodes also had significantly higher spatial selectivity and spatial attenuation for global signals. The increased spatial selectivity significantly decreased the MI in between different channels which will be useful in different BCI system. The tri-polar and bipolar concentric ring electrode configuration was also shown to be appropriate for recording seizure electrographic activity. This higher spatial selectivity of tri-polar concentric electrodes may be useful for seizure foci detection and seizure stage determination

Computer assisted optimization of cardiac resynchronization therapy

The efficacy of cardiac resynchronization therapy (CRT) through biventricular pacing (BVP) has been demonstrated by numerous studies in patients suffering from congestive heart failure. In order to achieve a guideline for optimal treatment with BVP devices, an automated non-invasive strategy based on an electrophysiological computer model of the heart is presented. The presented research investigates an off-line optimization algorithm based on different electrode positioning and timing delays

Critical appraisal of technologies to assess electrical activity during atrial fibrillation: a position paper from the European Heart Rhythm Association and European Society of Cardiology Working Group on eCardiology in collaboration with the Heart Rhythm Society, Asia Pacific Heart Rhythm Society, Latin American Heart Rhythm Society and Computing in Cardiology

We aim to provide a critical appraisal of basic concepts underlying signal recording and processing technologies applied for (i) atrial fibrillation (AF) mapping to unravel AF mechanisms and/or identifying target sites for AF therapy and (ii) AF detection, to optimize usage of technologies, stimulate research aimed at closing knowledge gaps, and developing ideal AF recording and processing technologies. Recording and processing techniques for assessment of electrical activity during AF essential for diagnosis and guiding ablative therapy including body surface electrocardiograms (ECG) and endo- or epicardial electrograms (EGM) are evaluated. Discussion of (i) differences in uni-, bi-, and multi-polar (omnipolar/Laplacian) recording modes, (ii) impact of recording technologies on EGM morphology, (iii) global or local mapping using various types of EGM involving signal processing techniques including isochronal-, voltage- fractionation-, dipole density-, and rotor mapping, enabling derivation of parameters like atrial rate, entropy, conduction velocity/direction, (iv) value of epicardial and optical mapping, (v) AF detection by cardiac implantable electronic devices containing various detection algorithms applicable to stored EGMs, (vi) contribution of machine learning (ML) to further improvement of signals processing technologies. Recording and processing of EGM (or ECG) are the cornerstones of (body surface) mapping of AF. Currently available AF recording and processing technologies are mainly restricted to specific applications or have technological limitations. Improvements in AF mapping by obtaining highest fidelity source signals (e.g. catheter–electrode combinations) for signal processing (e.g. filtering, digitization, and noise elimination) is of utmost importance. Novel acquisition instruments (multi-polar catheters combined with improved physical modelling and ML techniques) will enable enhanced and automated interpretation of EGM recordings in the near future

Aerospace medicine and biology: A continuing bibliography with indexes (supplement 373)

This bibliography lists 206 reports, articles and other documents introduced into the NASA Scientific and Technical Information System during Feb. 1993. Subject coverage includes: aerospace medicine and physiology, pharmacology, toxicology, environmental effect, life support systems and man/system technology, protective clothing, exobiology and extraterrestrial life, planetary biology, and flight crew behavior and performance