Shear wave anisotropy imaging
Shear Wave Anisotropy Imaging 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 shear waves. The pulses 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 of shear wave propagation, the velocity changes and images are formed from the ratio and directions of the maximum and minimum shear wave speeds. The beam sequence and data acquisition are real time processes, however, data analysis and anisotropy imaging are performed off-line. 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 homogeneous phantom was imaged and isotropy was observed. Simulation results of an anisotropic 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 the homogeneous 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.
