Multi-Dimensional Ultrasonic Shear Wave Reconstructions: Improving the Accuracy of Viscoelastic Parameter Estimation

Hepatocellular carcinoma (HCC) lesions are often preceded by liver cirrhosis or Hepatitis C, and patients with these diagnoses are monitored every six months with an ultrasound screening. B-mode ultrasound is an ideal imaging modality for regular screening; however, ultrasound has demonstrated a low sensitivity for detecting small, early stage HCCs. Studies using ultrasonic elasticity methods have shown increased HCC lesion contrast compared to B-mode ultrasound. This thesis presents the preliminary work of shear wave elasticity imaging (SWEI) methods to improve estimates of viscoelastic parameters in the context of liver screening for tumors, with the goals of (1) using multi-dimensional directional filtering and shear wave reconstruction to reduce reflection artifacts, (2) evaluating bias introduced from small depth-of-field (DOF) excitations into frequency dependent shear wave speed (SWS) and attenuation estimates and (3) evaluating the feasibility of combining on-axis and off-axis elasticity methods to screen the entire liver.

Interfaces of different shear stiffness causes propagating shear waves to be reflected, which can lead to artifacts in SWS reconstructions due to the reflections both in and out of the imaging plane. Two-dimensional (2-D), three-dimensional (3-D), and four- dimensional (4-D) directional filters were applied to shear wave data, and SWS images were reconstructed with 2-D and 3-D shear wave reconstruction methods to quantify the reduction in image artifacts. For 2-D SWS image reconstructions, 3-D directional filters showed greater improvements in image quality than 2-D filters, and 4-D directional filters showed marginal improvement over 3-D filters. The 4-D directional filters have the largest impact in reducing reflection artifacts in 3-D SWS volumes.

Commercial scanners reconstruct shear wave speeds for a region of interest using time-of-flight (TOF) methods reporting a single SWS (or elastic modulus) to the end user under the assumptions that tissue is elastic and independent of frequency. Human tissues are known to be viscoelastic (VE), resulting in dispersion and attenuation. Existing methods to quantify shear wave dispersion and attenuation commonly make an assumption that the acoustic radiation force excitation acts as a cylindrical source with a known geometric shear wave amplitude decay. The bias in shear dispersion and attenuation estimates associated with making this cylindrical wave assumption (up to 15% for dispersion and 41% for attenuation) when applied to shear wave sources with finite depth extents in realistic focal geometries is greater for more tightly-focused acoustic radiation force sources with smaller DOF.

Curvilinear transducers are the standard probe used in ultrasound HCC screenings; however, previous studies using curvilinear arrays performing liver SWEI have been limited by penetration depth of the acoustic radiation force excitation. In order for SWEI to be feasible as an imaging method to screen for and detect HCC lesions, large, low frequency arrays designed for deep abdominal imaging must be used. A prototype low frequency deep abdominal curvilinear array and a proposed low frequency matrix array were simulated, and the combined dynamic on-axis response in the region of excitation (ROE) and the propagating shear wave (off-axis) response were used to create quantitative shear wave images of a spherical lesion using sparse acoustic radiation force excitations. The on-axis behavior in the ROE is related to the underlying stiffness of the material and a lookup table (LUT) approach can be used to determine a SWS in this region. Combined with traditional off-axis TOF methods, the combined methods can reduce the sparsity that would otherwise occur inside the ROE, which allows a larger field-of-view (FOV) to be interrogated with the same number of excitations. The on-axis and off-axis methods can be combined in either 2-D or 3-D reconstructions. The performance of the deep abdominal curvilinear array was comparable to the matrix array for 2-D SWEI imaging. A benefit of the curvilinear array over a large matrix array is its ability to image through intercostal acoustic windows. If there is not a sufficient subcostal acoustic window to use the matrix array to image the entire liver, a combination of both the matrix array and the curvilinear array can be used to scan the entire organ.