Shear wave anisotropy imaging

Shear wave anisotropy imaging (SWAI) is a novel method that images local variations in tissue shear wave velocity. A commercial ultrasound scanner is used to generate and track propagating shear waves. Radiation force from a brief high-energy pulse generates the shear waves. The pulse and resultant shear waves are steered in order to launch the waves at oblique angles. The Helmholtz equation is used to extract the shear propagation velocity. Depending on the direction shear wave propagation, the propagation speed changes and images are formed from the ratio of shear wave speeds. We describe simulation and experimental studies of this method in phantoms. Finite element methods were employed to test the feasibility and calibration of the method. A homogenous phantom was imaged and isotropy was observed. Simulation results of an anistropic medium provided an estimate of anisotropy consistent with expectations. Shear wave speed images were made for each sample in all three planes and inspected for shear speed variations between propagation angles. Shear wave speeds in homogenous phantom were quite uniform with an average level of anisotropy of 1.15, indicating little organization within the sample. Upon a ninety degree rotation of the transducer, the average level of anisotropy was 1.14.