High frame rate ultrasonic imaging of the heart by placing virtual point sources in front of array

We previously developed a method for high frame rate echocardiography [frame rate (FR) > 300 Hz] using a transmit beam, which was spherically diverging from a virtual point source behind the array. Although such diverging beam illuminates a wider area compared with a focused beam or plane wave, sound pressure starts to decrease just after emission. Another type of a diverging beam, which is diverging from a focal point of a conventional focused beam, has already been proposed for synthetic aperture imaging. This type of beam can increase sound pressure around the focal point, and it would be useful for applications which require higher sound pressure, such as blood flow imaging. In the present study, the feasibility of high frame rate echocardiography using diverging beams from conventional focused beams was examined. In the present study, multiple focused receiving beams were created using diverging beams in front of focal points of conventional focused transmit beams. Smaller focal distance achieves a wider diverging angle and the number of transmissions can be reduced. In the proposed method, wave fronts were assumed to be spherical, and dynamic focusing was used in receive. Since multiple receiving beams were created for each transmission, overlapping receiving beams were averaged after beamforming. The proposed method was evaluated using an ultrasound imaging phantom. Spatial resolutions (width at half maximum) of parallel beamforming with plane wave and diverging beams from virtual point sources at distances of 100 mm behind array and 20 mm in front of array were 1.17, 1.04, and 1.08 mm (at 44 mm depth), respectively. B-mode images could be obtained with good spatial resolutions using transmit beams diverging in front of focal points.