Improving the accuracy of shear wave speed reconstructions using 4D directional filters in the presence of reflection artifacts

Reflected waves from stiffness boundaries can lead to artifacts in shear wave speed (SWS) reconstructions. 2D directional filters are commonly used with planar imaging systems to reduce in-plane reflected waves; however SWS artifacts arise from both in and out-of imaging plane reflected waves. Herein, we quantify the reduction in image artifacts afforded by the use of volumetric SWS monitoring and 4D directional filters. A Gaussian acoustic radiation force impulse was simulated in a phantom with a Young's modulus (E) of 3 kPa with a 5 mm spherical lesion with E = 6, 12 or 18.75 kPa. 2D, 3D, and 4D directional filters were applied to the displacement profiles to reduce in and out-of-plane reflected wave artifacts. SWS images were reconstructed and RMS error and CNR were calculated for each image to evaluate the image accuracy and quality. Applying 3D directional filters as compared to 2D led to larger improvements in image accuracy and quality than the improvements seen using 4D directional filters over 3D. This improvement in image accuracy is significant because the processing of these data could be performed on displacement data from a traditional 1D linear array with reasonable computational time and resources. Although 4D directional filters can further reduce the impact of large magnitude out-of-plane reflection artifacts in SWS images, computational overhead and transducer costs may outweigh the modest improvements in image quality.