Clinical utilization of shear wave dispersion imaging in diffuse liver disease

Abstract

Shear wave (SW) dispersion imaging is a newly developed imaging technology for assessing the dispersion slope of SWs, which is related to tissue viscosity in diffuse liver disease. Our preclinical and preliminary clinical studies have shown that SW speed is more useful than dispersion slope for predicting the degree of fibrosis and that dispersion slope is more useful than SW speed for predicting the degree of necroinflammation. Thus, dispersion slope, which reflects viscosity, may provide additional pathophysiological insight into diffuse liver disease.

Keywords: Dispersion; Elasticity; Liver; Shear wave elastography; Ultrasonography; Viscosity.

Fig. 1.. Relationship between shear wave speed and frequency in viscoelastic tissue.

In perfectly elastic tissue, shear wave speed is constant regardless of shear wave frequency. However, in viscoelastic tissue such as that found in the human body, shear wave speed does vary depending on shear wave frequency. The graph shows the relationship between shear wave speed and shear wave frequency in viscoelastic tissue. The charts are formulated based on the Voight model. If shear elasticity is fixed at 2.0 kPa and shear viscosity varies from 0.1 to 0.5 Pa·sec, the slope becomes higher according to the shear viscosity level. The slope itself is not the viscosity coefficient, but they correlate with the viscosity coefficient.

Fig. 2.. Schematic diagram of shear wave dispersion (SWD) map processing.

SWD processing involves four steps. First, the displacement induced by the shear waves (SWs) is obtained using a technique based on color Doppler scanning. Second, the displacement at each location is transformed from the time domain to the frequency domain by a Fourier transform in order to estimate the phase changes in the SWs at several frequencies. Third, the SW speed is calculated using the phase-difference method. Fourth, the gradient of the SW speed is calculated based on the distribution of SW speed versus frequency. The calculated gradient values are then superimposed on the measurement locations to create a dispersion map. FFT, fast Fourier transform.

Fig. 3.. Quad-view for shear wave elastography/shear wave dispersion quantification.

Upper left, elasticity map; upper right, dispersion map; lower left, grayscale; lower right, propagation map.

Fig. 4.. Shear wave speed and dispersion slope in inflammation and fibrosis model rats.

Boxplots show shear wave (SW) speed (A) and dispersion slope (B) (median and interquartile range) for each group (G0-G4). Kruskal-Wallis test: P<0.01; Jonckheere-Terpstra test: P<0.01. ^a)Wilcoxon signed-rank test: P<0.05. G0, rats that were not treated and served as control; G1, rats that received an intraperitoneal injection of 50% carbon tetrachloride (CCl₄) twice a week for 1 week; G2, rats that received an intraperitoneal injection of 50% CCl₄ 4 times a week for 1 week; G3, rats that received an intraperitoneal injection of 50% CCl₄ twice a week for 6 weeks; G4, rats that received an intraperitoneal injection of 50% CCl₄ twice a week for 10 weeks.

Fig. 5.. Histopathologic and ultrasonography parameter changes in patient with nonalcoholic steatohepatitis.

Histopathologic changes in a patient with nonalcoholic steatohepatitis and type 2 diabetes mellitus, comparing findings from before glucagon-like peptide-1 (GLP-1) therapy and 1 year after therapy. As the lobular inflammation grade improved from grade 3 to 2, the dispersion slope also improved from 15.5 to 12.5 m/sec/kHz. As the fibrosis stage improved from stage 4 to 3, shear wave speed also improved from 1.89 to 1.40 m/sec.

Fig. 6.. Pitfall on dispersion slope measurements.

Dispersion slope values vary according to the location of the region of interest. Color hotspots should be avoided to ensure accurate dispersion measurements.