On-Axis Acoustic Radiation Force-based Stiffness Estimation in Phantoms

This research quantitatively estimates stiffness by measuring the time-to-peak displacement directly along the acoustic radiation force (ARF) axis in contrast to shear wave elasticity imaging (SWEI) that measures shear wave velocity at lateral locations. We have shown previously in simulation results that an advanced displacement estimator reduces variability in the final stiffness estimate. Here, we tested the on-axis approach in 10 simulations and 5 phantoms. We created a stiffness look-up table of the time-to-peak displacement as a function of depth. We generated the look-up table using a 3D FEM model coupled to Field II simulations. We evaluated both normalized cross-correlation (NCC) and the Bayesian displacement estimator. To evaluate the error of the on-axis method as compared to traditional shear wave methods, we computed a robust lateral time-of-flight-based shear wave speed and converted to a shear modulus for each phantom. The 5 phantoms had a mean shear modulus of 1.90 kPa and standard deviation of 0.18 kPa. The root mean square error between the lateral method and the on-axis method was 0.98 kPa for the Bayesian displacement estimator and 1.14 kPa for NCC on average in the depth of field. These phantom results show that on-axis methods coupled with a Bayesian displacement estimator produce stiffness estimates comparable to laterally offset shear wave methods.