High-frame-rate velocity vector imaging echocardiography: an in vitro evaluation

High-frame-rate ultrasound imaging using diverging waves has demonstrated its potential as a useful cardiac imaging method. It has been shown that the compounding of steered beams improves static images quality. In the presence of high-velocity tissue motion, however, the combination of successive steered diverging waves is incoherent and thus deteriorates the image contrast. Motion compensation (MoCo) integrated in the coherent compounding process has recently shown to be a very promising technique to provide high-contrast B-mode images of the cardiac muscle. Ultrafast cardiac motion estimation based on speckle tracking could greatly benefit from this original method. In this study, Velocity Vector Imaging (VVI) was applied on high-frame-rate envelope images performed with MoCo to estimate the myocardium 2-D motion. The method was investigated in vitro, using a rotating disk. With sequences of steered diverging waves (pulse repetition frequency = 4500 Hz) generated by the full aperture of a 2.5 MHz phased array, high-contrast high-resolution images were constructed at 500 FPS using MoCo. Standard cross-correlation and phase correlation in the Fourier domain were applied to generate VVI on the pre-scanned envelope images at 100 images/s. The estimated in vitro velocity vectors were consistent with the expected values, with an average normalized error of 6.0% +/-0.4% in the radial direction, and 13.1% +/-1.2% in the cross-range direction. These results make us confident to pursue the study with in vivo investigations.