Noninvasive assessment of liver viscoelasticity by acoustic radiation force with a rat model

Most ultrasound elastographic methods utilize a purely elastic model to describe the liver mechanical properties. However, to describe tissue that is dispersive and to obtain an accurate measure of tissue elasticity, the tissue viscoelastic property should be examined. The objective of this study was to investigate the shear viscoelastic characteristics, as measured by ultrasound elastography, of liver fibrosis in a rat model and to evaluate the diagnostic accuracy of viscoelasticity for liver fibrosis at different stages. Liver fibrosis was induced in 37 rats using carbon tetrachloride (CCI4) with 6 rats as controls. Liver viscoelasticity was measured in vitro using shear waves induced by acoustic radiation force. The measured mean values of liver elasticity and viscosity ranged from 0.84 to 3.45 kPa and from 1.12 to 2.06 Pa·s for the F0-F4 fibrosis stages, respectively. The area under receiver operating characteristic (AUROC) values were 0.97 (≥F2), 0.91 (≥F3), and 1.00 (F4) for elasticity and 0.91 (≥F2), 0.79 (≥F3), and 0.74 (F4) for viscosity, respectively. The results confirmed that the shear wave velocity was dispersive in frequency, suggesting a viscoelastic model to describe liver fibrosis. The study suggests that although viscosity is not as good as elasticity for fibrosis staging, it is important to consider viscosity for the accurate estimation of elasticity and it may provide other mechanical insights into liver tissues.