Shear wave arrival time estimates correlate with local speckle pattern

The displacements involved in Acoustic Radiation Force Impulse (ARFI) Shear Wave Elastography (SWE) are small, with associated high echo correlation. Quantitative SWE techniques estimate the velocity of shear waves by tracking tissue motion at one or more locations with ultrasound beams of width comparable to the dimensions of the shear wave. These shear wave arrival time estimates are known to be noisy even at high echo SNR. We hypothesize that the observed arrival time variations are largely due to the underlying speckle pattern, and call the effect speckle bias. Field II was used to simulate imaging of both diffuse and point targets subject to shear wave motion. A 5.3 MHz linear array with transmit apertures of f/2-8 and receive apertures of f/2-4 was modeled, with a range of shear wave amplitudes (5-20μm) and profiles simulated. The local speckle pattern was determined through swept-receive-aperture imaging of the transmit beam. Arrival time bias found to be strongly (r=0.7) correlated with the location of the lateral peak of the swept-receive speckle pattern. The results suggest that high RF echo correlation does not equate to an accurate shear wave arrival time estimate, and that RF echo correlation is weakly related to arrival time accuracy. The variation in shear wave arrival time bias at a given location over the range of shear wave amplitudes considered is an order of magnitude smaller than the variation with different speckle realizations obtained along a given tracking vector.