Adaptive beamforming applied to transverse oscillation

In ultrasonic tissue motion estimation, the accuracy in the lateral motion estimate is worse than the axial estimate. For improvement of the accuracy in the lateral motion estimate, the transverse oscillation (TO) was introduced to increase the lateral spatial frequency of the ultrasonic field. In the present study, adaptive beamforming was introduced to the TO method to further improve the accuracy in motion estimation. TO is realized by the apodization scheme in delay-and-sum (DAS) beamforming. However, the apodization scheme has not yet been introduced in adaptive beamforming, i.e., rectangular apodization is used. In the present study, the apodization scheme was introduced to an adaptive beamformer proposed by our group. In the present study, TO was done by DAS and adaptive beamforming both with apodization composed of two Hanning functions. A phase-sensitive 2D motion estimator was used for investigation of the effect of adaptive TO on the accuracy in motion estimation. The proposed apodized adaptive beamformer was applied to the ultrasound data obtained by ultrafast plane wave imaging (1302 fps) without compounding. Compared with the B-mode image obtained by DAS beamforming with apodization composed of a single Hanning function, the lateral spatial frequency was increased by TO with DAS beamforming. Furthermore, a sharper oscillation in the lateral direction is realized by adaptive beamforming than TO with DAS beamforming. In validation experiments, relative motion velocities of 2 mm/s (lateral) and 1 mm/s (axial) between the ultrasonic probe and phantom were produced by moving the probe by an automatic stage. The bias errors and standard deviations in the estimated lateral velocities were decreased by the proposed adaptive transverse oscillation. In addition, the proposed adaptive beamformer required computation time of only 3.5 times of the conventional DAS beamformer.