Blood flow stream line imaging by direct visualization of echo trajectories

Ultrasonic blood flow imaging is useful for diagnosis of the cardiovascularsystem. Doppler-based blood flow measurements are widely used in clinicalsituations to estimate blood flow velocity. However, Doppler-based method doesnot show the direction of blood flow. Recently, a new method called B-Flow,which shows echoes from blood particles, was introduced to directly observe thedirection of blood flow. In this study, we further investigated ultrasonic bloodflow imaging to visualize stream lines of blood flow. In conventional bloodflow imaging, ultrasonic pulses emitted at each beam position for several times,typically 8 times, to increase signal-to-noise ratios (SNRs) of weak echoesfrom blood particles. In this case, 0.8 ms is required to create one scan linewhen the pulse repetition frequency is 10 kHz, and the frame rate would be 17 Hzwhen the number of scan lines is 75. Under such condition, blood particles moveby 60 mm during one frame interval at a typical blood flow velocity in thecarotid artery of 1 m/s. Therefore, blood particles observed in a certain framemove out from the imaging region in the next frame, and echoes from the sameblood particles cannot be observed continuously between frames. In this study,parallel receive beamforming with plane wave transmission was used to increasethe frame rate over 3 kHz. Blood particles moves by only 0.3 mm at a frame rateof 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. Eachexcitation was coded with the 5-bit Barker code to increase SNRs of received RFsignals. In the measurement of a carotid artery of a 33-year-old male, envelopesof RF echoes were averaged for 20 ms in the direction of frame. This averagingprocedure corresponded to lithographic exposure in photography, and trajectoriesof echoes during 20 ms were visualized. In this study, we developed a novelmethod to visualize blood flow stream lines by imaging trajectories of echoesfrom blood particles based on very high frame rate imaging.