Large Field-of-View Shear Wave Imaging for Hepatocellular Carcinoma Screening

Patients with liver cirrhosis or Hepatitis C have an increased likelihood of developing hepatocellular carcinoma (HCC) and are screened every six months with ultrasound; however, B-mode has demonstrated a low sensitivity for detecting small, early stage HCC. Studies using ultrasonic elasticity methods have shown increased HCC lesion contrast compared to B-mode, but current elasticity systems are limited to a depth penetration of 8 cm. A proposed, large-aperture, matrix array is presented for increasing depth penetration of acoustic radiation force excitations. Sparsity in the reconstructed shear wave elasticity imaging (SWEI) occurs in the region of excitation (ROE). Our previously proposed on-axis method uses a lookup table (LUT) and relates the ROE time-to-peak (TTP) displacement to underlying material stiffness. The ROE-TTP methods were extended to include the near-field region (3 mm) of the shear wave source and can be combined with traditional off-axis time of flight methods. The same field-of-view (FOV) using only SWEI would require 60% more excitations. Finite element simulations in homogeneous, elastic phantoms were used to create a LUT. Imaging of a 20 mm diameter spherical lesion in three liver disease states was simulated, including an HCC in a fibrotic liver. Reconstructions combining on-axis and offaxis methods in a simulated fibrotic liver with an HCC lesion had an average CNR of 4.49. The ROE-TTP methods have increased bias in high stiffness materials increasing from 4% bias in SWEI estimates up to 15% bias in ROE-TTP estimates in a 60 kPa phantom. Even with increased bias, on-axis methods can reduce the sparsity that would otherwise occur inside the ROE, which allows a larger FOV to be interrogated with a reduced number of excitations.