No abstract

No abstract

Non-invasive ultrafast imaging for human cardiac applications is a big challenge to image intrinsic waves such as electromechanical waves or remotely induced shear waves in elastography imaging techniques. In this paper we propose to perform ultrafast imaging of the heart with adapted sector size by using diverging waves emitted from a classical transthoracic cardiac phased array probe. As in ultrafast imaging with plane wave coherent compounding, diverging waves can be summed coherently to obtain high-quality images of the entire heart at high frame rate in a full field-ofview. To image shear waves propagation at high SNR, the field-of-view can be adapted by changing the angular aperture of the transmitted wave. Backscattered echoes from successive circular wave acquisitions are coherently summed at every location in the image to improve the image quality while maintaining very high frame rates. The transmitted diverging waves, angular apertures and subapertures size are tested in simulation and ultrafast coherent compounding is implemented on a commercial scanner. The improvement of the imaging quality is quantified in phantom and in vivo on human heart. Imaging shear wave propagation at 2500 frame/s using 5 diverging waves provides a strong increase of the Signal to noise ratio of the tissue velocity estimates while maintaining a high frame rate. Finally, ultrafast imaging with a 1 to 5 diverging waves is used to image the human heart at a frame rate of 900 frames/s over an entire cardiac cycle. Thanks to spatial coherent compounding, a strong improvement of imaging quality is obtained with a small number of transmitted diverging waves and a high frame rate, which allows imaging the propagation of electromechanical and shear waves with good image quality.

Shear wave imaging was evaluated for the in vivo assessment of myocardial biomechanical properties on ten open chest sheep. The use of dedicated ultrasonic sequences implemented on a very high frame rate ultrasonic scanner ( > 5000 frames per second) enables the estimation of the quantitative shear modulus of myocardium several times during one cardiac cycle. A 128 element probe remotely generates a shear wave thanks to the radiation force induced by a focused ultrasonic burst. The resulting shear wave propagation is tracked using the same probe by cross-correlating successive ultrasonic images acquired at a very high frame rate. The shear wave speed estimated at each location in the ultrasonic image gives access to the local myocardial stiffness (shear modulus μ). The technique was found to be reproducible (standard deviation ) and able to estimate both systolic and diastolic stiffness on each sheep (respectively μ(dias) ≈ 2 kPa and μ(syst) ≈ 30 kPa). Moreover, the ability of the proposed method to polarize the shear wave generation and propagation along a chosen axis permits the study the local elastic anisotropy of myocardial muscle. As expected, myocardial elastic anisotropy is found to vary with muscle depth. The real time capabilities and potential of Shear Wave Imaging using ultrafast scanners for cardiac applications is finally illustrated by studying the dynamics of this fractional anisotropy during the cardiac cycle.