Ultrafast compound imaging for 2D displacement vector measurements: application to transient elastography and color flow mapping

This paper presents a new technique for 2D imaging of the two-dimensional motion vector at a very high frame rate with ultrasound. Its potential is experimentally demonstrated for transient elastography. To date, only axial displacements induced in human tissues by low frequency vibrators were measured. This new technique allows us to follow both axial and lateral displacements during the shear wave propagation and thus should improve the Young's modulus image reconstruction. The process is a combination of several ideas well known in ultrasonic imaging: ultra-fast imaging, multi-synthetic aperture beamforming, 1D speckle tracking and compound imaging. Classical beamforming in the transmit mode is here replaced by a single plane wave illumination. The beamforming process is achieved only in the receive mode on two independent subapertures. Comparison of successive frames by a 1D speckle tracking algorithm allows to estimate displacements along two different directions linked to the sub-apertures beams. The variance of the deduced estimates is finally improved by tilting the emitting plane wave at each illumination. Beyond transient elastography, this technique could be of great interest for complex flows imaging.