Layer-specific strain and strain rate : Estimation using miniature transducers attached to the epicardium
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Reliable methods to asses ventricular function during and after cardiac surgery are essential tools to evaluate patient prognosis. We presented an ultrasound system to monitor cardiac motion using miniature transducers attached directly to the epicardial surface. The aim is both as a research tool for detailed studies of cardiac mechanics, and to develop a continuous, real time system for perioperative evaluation of heart function. Two 3 mm diameter, 10 MHz ultrasound transducers were sutured to the epicardial surface. As the epicardial surface is the reference for the velocity and strain estimations, this procedure compensates for the motion of the heart. The short distance allows use of high frequencies and pulse repetition rates. The system was driven in pulse-echo mode, using electronics developed for the application, and RF-lines were recorded at pulse repetition rate 2500 s-1 and sampling rate 40 MS/s. This thesis presents the data processing methods used for heart disease diagnosis and in clinical research for the clinicians. We have categorized the work in this thesis into four main topics: Prove the feasibility of the measurement system in monitoring regional myocardial deformation. Look for suitable signal processing methods, especially velocity estimationmethods with appropriate parameters for the specific measurement system. Build a simple simulation model in order to validate the data processing methods. The simulation model imitates the motion of the heart including contraction/expansion and rotationmotions. Make the analysis tools available for clinicians and other researchers, to use in heart disease diagnosis. The main contributions are: Feasibility of themeasurement system in getting information about regional motion of myocardium was proved. This goal was achieved by employing the time delay estimation method to estimate tissue velocity and regional radial strain from experimental data recorded on animal models. The simulation model of insonified myocardiumwas built up and implemented it in Matlab (TheMathWorks Inc., Natick,MA, USA). The axial deformation and rotation motion of the myocardium were considered in the simulation model. The graphic user interface was made to easily manage the input for the simulation, making the tool available for other researchers. The modifications of two standard velocity estimation methods (Time Delay Estimation (TDE) and phase shift Dopplermethods)were presented. Themethodswere first tested in the simulation model and then on data from animal experiments. The evaluation of these methods was performed using Bland-Altman test between the results from the methods and the ground truth from the simulation model. The data processing method was implemented inMatlab (TheMathWorks Inc., Natick, MA, USA). The Input and Output interfaces were supported to aid clinicians in entering the input parameters for data processing and analyzing and interpreting the output data, such as tissue velocity, radial strain, andmyocardial layer tracking. The signal processing methods were applied ultrasound RF recordings on animal models, to test the feasibility of obtaining new clinical information, such as measurement of the transmural strain profiles at different sites inside myocardium of the left ventricle, and effect of myocardial perfusion on the end-systolic radial strain at the apex.
Består avPaper A: Nguyen, T. T., Espinoza, A. W., Hyler, S.,Remme, E. W., D’hooge, J., & Hoffa, L. (Accepted). Estimating Regional Myocardial Contraction Using Miniature Transducers on the Epicardium. (Aaccepted for publishing in Ultrasound in Medicine and Biology)
Paper B: Nguyen, T. T., Espinoza, A. W., Hyler, S., Remme, E. W., D’hooge, J., & Hoffa, L. (Submitted). Myocardial strain measured by epicardial transducers : Comparison between velocity estimators. (Submitted to Ultrasound in Medicine and Biology)
Paper C: Nguyen, T. T., Espinoza, A. W., Hyler, S., Remme, E. W., D’hooge, J., & Hoffa, L. (2011). Transmural Myocardial Strain Distribution Measured at High Spatial and Temporal Resolution (pp. 696-699). In IEEE International Ultrasonics Syposium Proceedings.
Paper D: Nguyen, T. T., Espinoza, A. W., Hyler, S., Remme, E. W., D’hooge, J., & Hoffa, L. (2012). Transmural Strain Distribution Across the Cardiac Wall and Its Dependency on Measurement Site (pp. 1-4). In IEEE International Ultrasonics Syposium Proceedings.
Paper E: Thuy Thu Nguyen, A., Espinoza, Hyler, Remme, Dhooge, & Hoff. (2012). Effect of myocardial perfusion on end-systolic radial strain at the apex. 2012 IEEE International Ultrasonics Symposium, 1-4.
Nguyen, T., & Hoff, L. (Unpublished). Implementation and Use of the Software : Data Processing Algorithms and Simulation of Myocardial Motion