Automatic Valve-Rejection Algorithm for Cardiac Doppler Ultrasound Systems

In recent years, blood flow diagnosis using Doppler ultrasound systems has become popular. Using these systems, the peak velocity of blood flow is automatically traced. However, because valve signals are mixed with the blood flow signals in a heart chamber, automatic measurements of blood flow are not correctly recorded. To solve this problem, we developed a novel method that adopted system identification. We applied a mathematical model with an electrocardiographic waveform as the input and a trace waveform of the peak velocity as the output. Several mathematical models with different structures and orders were compared to select the optimal model. Using this model, we developed a system that could automatically eliminate the valve signal. We also evaluated our valve-rejection algorithm using simulations based on actual clinical data. 1. Introduction Doppler ultrasound systems are widely used for the diagnoses of the heart, abdomen, and other organs and systems. Because blood flow measurement is very complicated, methods for automatic measurements have been developed [1–4]. For example, in the carotid artery, blood flow volume, peak velocity, pulsatility, and so forth. are measured automatically. However, in the heart, because valve motions overlap blood flow, automatic measurements become difficult. Blood flow measurements, such as left ventricular outflow, have been made by manual procedures due to the influence of the aortic valve [5, 6]. We developed a method for automatic left ventricular outflow measurements and verified its performance using simulations. 2. Trends and Problems with Blood Flow Measurements A Doppler ultrasound diagnostic image for left ventricular outflow is shown in Figure 1. The upper part of the figure shows the ultrasound echo image (B-mode image). The Doppler range gate was set at the tip of the aortic valve, and the blood flow information at this point is shown in the lower part of this figure as a Doppler spectrum image. In this image, the horizontal axis represents time, whereas the vertical axis represents blood flow velocity that corresponds to the Doppler shift frequency. The Doppler spectrum image expresses the time-velocity distribution of blood flow. In addition, the Doppler ultrasound system automatically traces the peak velocity waveform , which is superimposed as a bright yellow line. Figure 1: Doppler ultrasound image for left ventricular outflow. There are two noise sources that affect cardiac blood flow measurements: cardiac wall with a strong, low velocity signal and a valve with a strong, high velocity