Blood flow stream line imaging by direct visualization of echo trajectories

Abstract

Ultrasonic blood flow imaging is useful for diagnosis of the cardiovascular system. Doppler-based blood flow measurements are widely used in clinical situations to estimate blood flow velocity. However, Doppler-based method does not show the direction of blood flow. Recently, a new method called "B-Flow", which shows echoes from blood particles, was introduced to directly observe the direction of blood flow. In this study, we further investigated ultrasonic blood flow imaging to visualize stream lines of blood flow. In conventional blood flow imaging, ultrasonic pulses emitted at each beam position for several times, typically 8 times, to increase signal-to-noise ratios (SNRs) of weak echoes from blood particles. In this case, 0.8 ms is required to create one scan line when the pulse repetition frequency is 10 kHz, and the frame rate would be 17 Hz when the number of scan lines is 75. Under such condition, blood particles move by 60 mm during one frame interval at a typical blood flow velocity in the carotid artery of 1 m/s. Therefore, blood particles observed in a certain frame move out from the imaging region in the next frame, and echoes from the same blood particles cannot be observed continuously between frames. In this study, parallel receive beamforming with plane wave transmission was used to increase the frame rate over 3 kHz. Blood particles moves by only 0.3 mm at a frame rate of 3 kHz, and echoes from the same blood particles can be observed continuously. In this study, receiving beams were created at three different steering angle (-5, 0, 5 degrees) to avoid the beam-to-flow angle being 90 degrees. Each excitation was coded with the 5-bit Barker code to increase SNRs of received RF signals. In the measurement of a carotid artery of a 33-year-old male, envelopes of RF echoes were averaged for 20 ms in the direction of frame. This averaging procedure corresponded to lithographic exposure in photography, and trajectories of echoes during 20 ms were visualized. In this study, we developed a novel method to visualize blood flow stream lines by imaging trajectories of echoes from blood particles based on very high frame rate imaging.