Speckle-Enhanced Cardiac Blood Flow Imaging with High Frame Rate Ultrasound

Blood flow imaging in echocardiography helps users to evaluate the pumping function of the human heart. As a typical method, color Doppler imaging has been widely used to obtain the information of intracardiac blood flow. Color Doppler imaging, however, does not show the direction of blood flow stream because only flow velocity along a scan line is estimated. Echocardiographic particle image velocimetry (E-PIV) technique gives flow characteristic information, such as velocity vector fields and stream lines of blood flow, which are calculated based on motions of ultrasonic echoes from contrast agent. However, E-PIV is an invasive approach forcing physical and mental burden on patients because of intravenous injection of a contrast agent. To overcome this problem, we propose a non-invasive method for cardiac blood flow imaging by visualization of motions of echoes from blood particles. High frame rate measurement by parallel receive beamforming with spherically diverging wave was used for continuous observation of echoes from the same blood particles between frames. In addition, a coded excitation with 5-bit Barker code and clutter filtering were also used to enhance weak echoes from blood particles. Moreover, the power of echo signal was weighted by coherence calculated from demodulated signals to suppress the random noise which remained after clutter filtering. Through in vivo measurement of a 26-year-old healthy male, B mode image overlaid with enhanced echo speckle of blood particle was obtained in the three chamber view. Moreover, velocity vectors of blood flow could be estimated by applying enhanced echoes of blood particles with speckle tracking technique. The estimated velocity vector showed that blood is flowed out to the aorta in systole and is flowed into the left ventricular cavity in diastole. The in vivo result shows a potential of non-invasive imaging of blood flow pattern at high temporal and spatial resolution using our proposed method.