Fast strain tensor imaging using beam steered plane wave ultrasound transmissions

Ultrasound strain imaging can be used to assess local mechanical properties of tissue. From conventional non-steered 2D ultrasound data, the axial (along the beam) displacements and strains can be estimated precisely, whereas lateral (perpendicular to the ultrasound beam) displacements and corresponding strains are more complicated to estimate. The lateral displacements/ strains can be estimated more precisely by adding data from acquisitions at various large beam steering angles, although frame rates are reduced. Plane wave ultrasound transmission enables ultrasound acquisition at high frame rates. This study investigates beam steered plane wave ultrasound transmission for full strain tensor estimation at high frame rates. Using finite element modeling (FEM) and Field II, ultrasound radio frequency data of a vessel with a vulnerable plaque were generated before and after the vessel underwent an intraluminal pressure increase of 4 mmHg. RF data were simulated for a linear array transducer (3-11 MHz, fs = 39 MHz, pitch = 135 μm) that either transmitted focused pulses or plane waves at beam steering angles of -30°, 0° and 30°. In receive dynamic focusing was applied. Band limited noise was added to obtain a signal-to noise ratio of 20 dB. Displacements were iteratively estimated using 2D cross-correlation. Next, principal strains were derived using ID least squares strain estimators. The absolute differences between the estimated principal strains and the FEM principal strains were determined to compare the transmission methods. It was found that plane wave beam steering enabled a fast and more precise estimation (Wilcoxon, P<;0.001) of the full strain tensor than conventional 0° strain imaging. Although focused beam steering provided slightly more precise estimates, the main advantage of the plane wave approach is that it suffers less from motion artifacts when imaging tissue in vivo, due to its at least 50 times higher frame rate.