We have developed a system capable of tracking acoustic radiation force impulse (ARFI) induced shear wave propagation in 3D using ultrasound. In contrast to existing systems, which use 1D array transducers, a 2D matrix array is used for tracking shear wave displacements. A HIFU piston is used for ARFI excitation. This system allows shear wave propagation in all directions away from the push to be observed, which enables:
- characterization of anisotropic mechanical properties
- acquisition of greater amounts of data for stiffness estimation
- additional flexibility in tracking beam locations
An annular focused HIFU piston (H-101, Sonic Concepts, Bsystem_diagram_0.pngothell, WA) is used for ARFI excitation (1.1 MHz, F/1, 63 mm focus). A 2D matrix array transducer (4Z1C on a SC2000 scanner, Siemens Healthcare, Ultrasound Business Unit, Mountain View, CA) is used for monitoring shear wave propagation.
Shear wave propagation in homogeneous isotropic phantom. The 4Z1C/HIFU transducers are located at the top of the volume. The push is focused at 60 mm in the axial direction and at the center of the volume. The shear wave displacement amplitude and the corresponding B-mode image in 3 cross-sectional planes are shown. The coronal plane (shown in red) is orthogonal to the push axis and parallel to the direction of shear wave propagation. The shear wave travels with the same speed in all directions.
Shear wave propagation in excised canine muscle (semitendinosus). The shear wave speed is faster along the muscle fibers (green plane) than across the fibers (yellow plane).
Top: Shear wave speed (SWS) as a function of direction in the coronal plane for the homogenous isotropic phantom. The SWS is the same in all directions. Bottom: SWS in the coronal plane for the excised muscle. The SWS has the shape of an ellipse, consistent with a transverse isotropic model of elasticity. The muscle fiber orientation measured independently from B-mode ultrasound is shown by the red arrow. The fastest SWS (the ellipse major axis) occurs parallel to the fibers, and the slowest SWS (the ellipse minor axis) occurs perpendicular to the fibers.
We have worked on implementing 3D ARFI/SWEI with a single transducer, the Volumetric transducer. It is a 3D phased array with 440 elements.
We built custom connectior board to couple the transducer with two Verasonics Vantage System. The two verasonics systems were synchronized by a single clock cycle, and get triggered to transmit and receive by a single trigger signal. In this way, we are able to control up to 512 elements at once.
Below shows two images of the same phantom. The left image was taken by doing tx and rx on only 184 elements on the volumetric probe with one verasonics system. The right image was taken by doing tx on 440 elements (2 verasonics systems) and rx on the same 184 elements as the left image (1 verasonics system). The image on the right is brighter, without destructive interference. This indicates that the 2 verasonics systems are well synchronized.