Shear wave focusing for three-dimensional sonoelastography.

A new vibration scheme is shown to provide localized vibration fields for three-dimensional sonoelastography imaging. The theoretical vibration distributions of double strip loads vibrating normally to the surface of a semi-infinite elastic space are calculated. A localization or focusing of shear waves inbetween the double-strip loads is predicted. Experimentally, two parallel rigid rectangular cross-section bars are mounted on an electromagnetic shaker. Driven by the signal source, the bars vibrate against the surface of a tissue-mimicking phantom. The double-bar source is also used to propagate shear wave into an ex vivo prostate phantom with a 6 mm "tumor" in it. A combination of high frequencies (400-600 Hz) is used to drive the double-bar applicator. In the phantom experiments, a shear wave focal zone with higher vibration amplitude and uniformity predicted by the theory was confirmed. The position of the focal zone is controllable when adjusting the separation of the bars as the theory shows. When this vibration scheme was used in a prostate phantom experiment, high-resolution tumor images with clear boundaries are obtained. The parallel bar is an ideal applicator to create more uniform vibration within a controllable localized volume. The field has uniformity especially in the direction along the bars.