System dependent sources of error in time-of-flight shear wave speed measurements
This work discusses eight sources of uncertainty and bias arising from system dependent parameters in ultrasound shear wave speed (SWS) measurement. Each of the eight error sources of error we have identified is discussed in the context of a linear, isotropic, elastic, homogeneous medium, combining previously reported analyses with Field II simulations, full-wave 2D acoustic propagation simulations and experimental studies. Errors arising from both spatial and temporal sources lead to errors in SWS measurements. Arrival time estimation noise, speckle bias, master clock jitter, and phase aberration cause uncertainties (variance) in SWS measurements, while pulse repetition frequency and beamforming errors, as well as coupling medium sound speed mismatch cause biases in SWS measurements (accuracy errors). Calibration of these sources of error is an important step in the development of shear wave imaging systems. In a well-calibrated system, where the sources of biases are minimized, and averaging over an ROI is employed to reduce the sources of uncertainty, a SWS error < 3% can be expected.
