Exploiting GPUs to investigate an inversion method that retrieves cardiac conductivities from potential measurements

Accurate cardiac bidomain conductivity values are essential for realistic simulation of various cardiac electrophysiological phenomena. A method was previously developed that can determine the conductivities from measurements of potential on a multi-electrode array placed on the surface of the heart. These conductivities, as well as a value for fibre rotation, are determined using a mathematical model and a two-pass process that is based on Tikhonov regularisation. Using simulated potentials, to which noise is added, the inversion method was recently shown to retrieve the intracellular conductivities accurately with up to 15% noise and the extracellular conductivities extremely accurately even with 20% noise. Recent work investigated the sensitivity of the method to the choice of the regularisation parameters. Such a study only became possible due to modifications that were made to the C++ code so that it could run on graphical processing units (GPUs) on the CUDA platform. As the method required the solution of a large number of matrix equations, the highly parallel nature of GPUs was exploited to accelerate execution of the code. Reorganisation of the code and more efficient memory management techniques allowed the data to completely fit in the GPU memory. Comparison between the execution time on the GPU versus the original CPU code shows a speedup of up to 60 times. In the future, the speedup could be further increased with greater use of shared memory, which has a much lower latency (access time) than global memory. References Clayton, R. H., Bernus, O., Cherry, E. M., Dierckx, H., Fenton, F. H., Mirabella, L., Panfilov, A. V., Sachse, F. B., Seemann, G., and Zhang, H. Models of cardiac tissue electrophysiology: Progress, challenges and open questions. Progress in Biophysics and Molecular Biology, 104(1–3):22–48, 2011. doi:10.1016/j.pbiomolbio.2010.05.008 Arthur, R. M. and Geselowitz, D. B. Effect of inhomogeneities on the apparent location and magnitude of a cardiac current dipole source. IEEE Transactions on Biomedical Engineering, 17:141–146, 1970. doi:10.1109/TBME.1970.4502713 Clerc, L. Directional differences of impulse spread in trabecular muscle from mammalian heart. Journal of Physiology, 255:335–346, 1976. http://jp.physoc.org/content/255/2/335 Roberts, D. E., Hersh, L. T., and Scher, A. M. Influence of cardiac fiber orientation on wavefront voltage, conduction velocity and tissue resistivity in the dog. Circ. Res., 44:701–712, 1979. doi:10.1161/01.RES.44.5.701 Roberts, D. E. and Scher, A. M. Effects of tissue anisotropy on extracellular potential fields in canine myocardium in situ. Circ. Res., 50:342–351, 1982. doi:10.1161/01.RES.50.3.342 Hooks, D. A. Myocardial segment-specific model generation for simulating the electrical action of the heart. BioMedical Engineering OnLine, 6(1):21–21, 2007. doi:10.1186/1475-925X-6-21 MacLachlan, M. C., Sundnes, J., and Lines, G. T. Simulation of ST segment changes during subendocardial ischemia using a realistic 3-D cardiac geometry. IEEE Transactions on Biomedical Engineering, 52(5):799–807, 2005. doi:10.1109/TBME.2005.844270 Roth, B. J. Electrical conductivity values used with the bidomain model of cardiac tissue. IEEE Transactions on Biomedical Engineering, 44(4):326–328, April 1997. doi:10.1109/10.563303 Johnston, P. R. and Kilpatrick, D. The effect of conductivity values on ST segment shift in subendocardial ischaemia. IEEE Transactions on Biomedical Engineering, 50(2):150–158, February 2003. doi:10.1109/TBME.2002.807660 Johnston, P. R. Cardiac conductivity values–-a challenge for experimentalists? Noninvasive Functional Source Imaging of the Brain and Heart and 2011 8th International Conference on Bioelectromagnetism (NFSI and ICBEM), pages 39–43, 13-16 May 2011. doi:10.1109/NFSI.2011.5936816 Hooks, D. A. and Trew, M. L. Construction and validation of a plunge electrode array for three-dimensional determination of conductivity in the heart. IEEE Transactions on Biomedical Engineering, 55(2):626–635, 2008. doi:10.1109/TBME.2007.903705 Trew, M. L., Caldwell, B. J., Gamage, T. P. B., Sands, G. B., and Smaill, B. H. Experiment-specific models of ventricular electrical activation: Construction and application. In 30th Annual International IEEE EMBS Conference, pages 137–140, 2008. doi:10.1109/IEMBS.2008.4649109 Caldwell, B. J., Trew, M. L., Sands, G. B., Hooks, D. A., LeGrice, I. J., and Smaill, B. H. Three distinct directions of intramural activation reveal nonuniform side–to–side electrical coupling of ventricular myocytes. Circulation: Arrhythmia and Electropysiology, 2:433–440, 2009. doi:10.1161/CIRCEP.108.830133 Pollard, A. E., Ellis, C. D., and Smith, W. M. Linear electrode arrays for stimulation and recording within cardiac tissue space constants. IEEE Transactions on Biomedical Engineering, 55(4):1408–1414, 2008. doi:10.1109/TBME.2007.912401 Pollard, A. E. and Barr, R. C. A biophysical model for cardiac microimpedance measurements. American Journal of Physiology-Heart and Circulatory Physiology, 298:H1699–H1709, 2010. doi:10.1152/ajpheart.01131.2009 Johnston, B. M. Design of a multi–electrode array to measure cardiac conductivities. ANZIAM Journal, 54:C271–C290, 2013. http://journal.austms.org.au/ojs/index.php/ANZIAMJ/article/viewFile/6278/1694 Johnston, B. M. and Johnston, P. R. A multi-electrode array and inversion technique for retrieving six conductivities from heart potential measurements. Medical and Biological Engineering and Computing, 51(12):1295–1303, 2013. doi:10.1007/s11517-013-1101-2 Johnston, B. M. Using a sensitivity study to facilitate the design of a multi–electrode array to measure six cardiac conductivity values Mathematical Biosciences, 244:40–46, 2013. doi:10.1016/j.mbs.2013.04.003 Plonsey, R. and Barr, R. The four-electrode resistivity technique as applied to cardiac muscle. IEEE Transactions on Biomedical Engineering, 29(7):541–546, 1982. doi:10.1109/TBME.1982.324927 Johnston, B. M., Johnston, P. R., and Kilpatrick, D. A new approach to the determinination of cardiac potential distributions: Application to the analysis of electrode configurations. Mathematical Biosciences, 202(2):288–309, 2006. doi:10.1016/j.mbs.2006.04.004 Johnston, B. M., Johnston, P. R., and Kilpatrick, D. Analysis of electrode configurations for measuring cardiac tissue conductivities and fibre rotation. Annals of Biomedical Engineering, 34(6):986–996, June 2006. doi:10.1007/s10439-006-9098-4 Kuntsevich, A. and Kappel, F. SolvOpt: The solver for Local Nonlinear Optimisation Problems, version 1.1 in C. Technical Report, Institute for Mathematics: Karl–Franzens University of Graz, 1997. http://uni-graz.at/imawww/kuntsevich/solvopt/ps/manual.pd

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

High Fidelity Bioelectric Modelling of the Implanted Cochlea

Cochlear implants are medical devices that can restore sound perception in individuals with sensorineural hearing loss (SHL). Since their inception, improvements in performance have largely been driven by advances in signal processing, but progress has plateaued for almost a decade. This suggests that there is a bottleneck at the electrode-tissue interface, which is responsible for enacting the biophysical changes that govern neuronal recruitment. Understanding this interface is difficult because the cochlea is small, intricate, and difficult to access. As such, researchers have turned to modelling techniques to provide new insights. The state-of-the-art involves calculating the electric field using a volume conduction model of the implanted cochlea and coupling it with a neural excitation model to predict the response. However, many models are unable to predict patient outcomes consistently. This thesis aims to improve the reliability of these models by creating high fidelity reconstructions of the inner ear and critically assessing the validity of the underlying and hitherto untested assumptions. Regarding boundary conditions, the evidence suggests that the unmodelled monopolar return path should be accounted for, perhaps by applying a voltage offset at a boundary surface. Regarding vasculature, the models show that large modiolar vessels like the vein of the scala tympani have a strong local effect near the stimulating electrode. Finally, it appears that the oft-cited quasi-static assumption is not valid due to the high permittivity of neural tissue. It is hoped that the study improves the trustworthiness of all bioelectric models of the cochlea, either by validating the claims of existing models, or by prompting improvements in future work. Developing our understanding of the underlying physics will pave the way for advancing future electrode array designs as well as patient-specific simulations, ultimately improving the quality of life for those with SHL

Engineering Principles

Over the last decade, there has been substantial development of welding technologies for joining advanced alloys and composites demanded by the evolving global manufacturing sector. The evolution of these welding technologies has been substantial and finds numerous applications in engineering industries. It is driven by our desire to reverse the impact of climate change and fuel consumption in several vital sectors. This book reviews the most recent developments in welding. It is organized into three sections: “Principles of Welding and Joining Technology,” “Microstructural Evolution and Residual Stress,” and “Applications of Welding and Joining.” Chapters address such topics as stresses in welding, tribology, thin-film metallurgical manufacturing processes, and mechanical manufacturing processes, as well as recent advances in welding and novel applications of these technologies for joining different materials such as titanium, aluminum, and magnesium alloys, ceramics, and plastics

Future Trends in Advanced Materials and Processes

The Special Issue “Future Trends in Advanced Materials and Processes” contains original high-quality research papers and comprehensive reviews addressing the relevant state-of-the-art topics in the area of materials focusing on relevant or innovative applications such as radiological hazard evaluations of non-metallic materials, composite materials' characterization, geopolymers, metallic biomaterials, etc

Proceedings of the 2018 Canadian Society for Mechanical Engineering (CSME) International Congress

Published proceedings of the 2018 Canadian Society for Mechanical Engineering (CSME) International Congress, hosted by York University, 27-30 May 2018

NASA SBIR abstracts of 1992, phase 1 projects

The objectives of 346 projects placed under contract by the Small Business Innovation Research (SBIR) program of the National Aeronautics and Space Administration (NASA) are described. These projects were selected competitively from among proposals submitted to NASA in response to the 1992 SBIR Program Solicitation. The basic document consists of edited, non-proprietary abstracts of the winning proposals submitted by small businesses. The abstracts are presented under the 15 technical topics within which Phase 1 proposals were solicited. Each project was assigned a sequential identifying number from 001 to 346, in order of its appearance in the body of the report. Appendixes to provide additional information about the SBIR program and permit cross-reference of the 1992 Phase 1 projects by company name, location by state, principal investigator, NASA Field Center responsible for management of each project, and NASA contract number are included