3D Elasticity Imaging With Acoustic Radiation Force

Acoustic radiation force impulse (ARFI) based elasticity imaging methods have been under development for the past 15 years, and both qualitative on-axis (ARFI imaging) and quantitative shear wave speed (SWEI) methods have been introduced into the commercial market. On-axis methods employ less processing and provide higher spatial resolution, whereas SWEI methods employ reconstruction algorithms that afford higher contrast and provide quantitative estimates of the underlying material stiffness. Each approach can be optimized through custom beam sequences and processing algorithms for specific clinical applications. We discuss three specific applications herein: 1) hepatic fibrosis and steatosis staging through SWEI, 2) 3D SWEI for characterizing material anisotropy, and 3) 3D prostatic SWEI and ARFI imaging. 1) SWEI in hepatic disease: The potential for employing SWEI methods to stage liver disease has been demonstrated clinically through the correlation of group shear wave velocity with hepatic fibrosis stage when applying elastic material assumptions. Through Fourier analysis of the broadband propagating shear wave generated by ARFI excitation, dispersion of wave propagation can be characterized, allowing the use of higher order, viscoelastic material models. In a cohort of 119 Non-Alcoholic Fatty Liver Disease (NAFLD) patients, a comparison of hepatic disease characterization using a one-parameter elastic material model to derive shear wave speed (SWS) with the two-parameter Voigt model to derive elasticity (1) and viscosity (2) is performed. AUROC analysis demonstrates that no improvement in fibrosis stage characterization is achieved through the use of the Voigt model. However, threshold SWS values for the differentiation of advanced liver disease (>= F3) from mild to moderate disease (<= F2) are shown to increase from 1.7 to 2.5 m/s with increasing frequency over the range from 0400 Hz. In addition, no correlation is observed between any of the shear wave parameters and steatosis. 2) 3D SWEI: We have developed an integrated volumetric shear wave imaging system using a HIFU piston for ARFI excitation, and a 2D matrix array (4Z1C) and a Siemens SC2000 scanner to perform volumetric imaging. In isotropic materials, this system allows improved shear wave estimate precision over that obtained with planar imaging. In anisotropic materials such as skeletal muscle, this system enables accurate determination of muscle fiber orientation and the principal axes of propagation. Through Fourier analysis of 3D propagation, we observe decreasing shear wave speed anisotropy with increasing shear wave frequency. 3) 3D prostate imaging: Using a linear, transrectal ultrasound array, an external automated rotation stage, and 16: 1 parallel processing on an SC2000 scanner, on-axis (ARFI) and off-axis (SWEI) data is obtained concurrently for each excitation throughout the entire prostate volume in vivo. Good agreement is observed between the ARFI and SWEI data, with the ARFI images providing better resolution. Prostate structures and some regions of pathology are well visualized in pre-operatively acquired ARFI images, as determined by correlation with pre-operative MR images and whole-mount histology data obtained post radical-prostatectomy.