1Department of Electrical Engineering, Faculty of Science and Technology, Keio University, 2Department of Electrical Engineering, Faculty of Science and Technology, Sagami Institute of Technology, 3Aloka Co., Ltd., 4Institute of Information Science and Electronics, University of Tsukuba
Ultrasound, Speckle, Velocity measurement
The Pulsed Doppler method which is able to measure the blood flow velocity noninvasively in real time is indispensable in the recent clinical field. But, the Pulsed Doppler is poor at measuring the extremely slow blood flow such as ones inside the small vessel and the heart. Therefore, the purpose of our study is to develop a new epochal technical method for measuring the slow blood flow in a human body. This paper proposes a new simple method based on the statistics of "SPECKLE fluctuation" appeared in the echo signal. In case of the blood flow, the speckle is caused by the random interference of the wavelets scattered by a large number of point scatterers (erythrocyte) inside the blood vessel. And, there is a phenomenon that the speckle fluctuates at a frequency corresponding to the blood flow velocity, and being independent of scatterers' density. Accordingly, we try to presume the blood flow velocity by counting the zerocrossing numbers which cope with the fluctuating rate of the speckle in the echo signal. In this paper, we describe the principle of the speckle velocimetry and demonstrate theoretically the relationship between the zerocrossing numbers and the blood flow velocity. To verify the availability of this method, the fundamental experiments using the pulse-echo system with single beam were performed in the slow flow region, and we insert the dextran micro-particles as the scatterers into the water flow. As a result, the good linear relationship between the flow velocity and the zerocrossing numbers is obtained. Consequently, this result suggests the applicability to the extremely slow blood flow measurement.