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3D shear wave imaging of anisotropic mechanical properties of muscle using a 2D matrix array transducer

A 2D matrix array is used to monitor acoustic radiation force impulse (ARFI) induced shear wave propagation in 3D in excised canine muscle. From a single acquisition, both the shear wave phase and group velocity can be calculated within a plane of symmetry to estimate the shear wave speed (SWS) along and across the fibers, as well as the fiber orientation in 3D. The true fiber orientation found using the 3D Radon Transform (3D RT) on B-mode volumes of the muscle was used to verify the fiber direction. For the simplified imaging case when the ARFI push can be oriented perpendicular to the fibers, the error in estimating the fiber orientation using phase and group velocity measurements was 3.5 ± 2.6° and 3.4 ± 1.4° (mean ± standard deviation), over six acquisitions in different muscle samples. For the more general case when the push is oblique to the fibers, the angle between the push and the fibers (or orientation of the plane of symmetry) is found using the dominant orientation of the shear wave displacement magnitude. In 30 acquisitions on six different muscle samples with oblique push angles up to 40°, the median error in the estimated fiber orientation using phase and group velocity measurements was 5.2° and 4.7°, respectively, after accounting for the additional unknown push angle. Either the phase or group velocity measurements can be used to estimate fiber orientation and SWS along and across the fibers in a plane of symmetry. Although it is possible to estimate the orientation of the plane of symmetry when the push is oblique to the fibers, inaccurate measurements of fiber orientation and SWS along and across the fibers occurred for highly oblique push angles (>25°).