Measurement of Secondary Waves Generated by Nonlinear Vibration of Microbubbles in The Crossed Beams of Two Ultrasonic Frequencies

Abstract

The contrast echo imaging is based on detecting second order harmonic components in echo signals, because nonlinear vibration of microbubbles in the blood vessels gives second order harmonic components in scattered waves. However, the separation of the echoes from the microbubbles in the blood flow to the echoes from biological soft tissues is an important issue, because the harmonic components are also generated during the propagation through biological soft tissues as well as the nonlinear oscillation of the microbubbles. In order to solve this problem we propose a novel contrast echo imaging method using the sum or difference frequency component in the echoes reflected from the contrast agents in the crossing area of the two ultrasonic beams with different frequencies, which we call Crossed Beams Contrast Echo (CBCE) method. In CBCE method, the sum or difference frequency components are generated only in the crossing area of beams. Therefore it is capable of separating the microbubble echoes from the soft tissue echoes, so that the microvascular blood flow information can be obtained in detail by using these specific frequency components. In this study, the sum frequency components which were generated by the commercially available contrast agents “Sonazoid” in the flow tube were measured by transmitting two ultrasonic waves of 2.8 MHz and 2.0 MHz. As a result, it is indicated that CBCE method is applicable for the blood flow measurement, especially for the separation of the microbubble echoes from the tissue echoes.