Shearwave Dispersion Ultrasound Vibrometry (SDUV)

Shearwave Dispersion Ultrasound Vibrometry (SDUV) is a quantitative method to measure stiffness and viscosity of soft tissue non-invasively. SDUV uses a focused ultrasound beam within the FDA power limits to stimulate (within the studied tissue) cylindrical harmonic shear waves propagating outwards from the beam axis. Shear wave propagation is tracked by a separate ultrasound beam in pulse/echo mode. The phase of shear wave at two locations along its traveling path is used to calculate the shear wave propagation speed. The speed of shear waves at different frequencies is measured and fit with a theoretical Voigt dispersion model to inversely solve for tissue viscosity and elasticity.

Figure 1 shows an example of in vivo SDUV measurement in pig liver. Phases of shear waves of four different frequencies are measured at five locations (left panel). The circles in the right panel are calculated from the phase measurements at location 0 and 4 mm. The solid line is the LMS fit by the Voigt dispersion model to these circles, which gives a stiffness of 2.4 kPa and a viscosity of 2.1 Pa*s for the liver. Repeated measurements give 2.2±0.63 kPa for stiffness and 1.96±0.34 Pa*s for viscosity. These values are close to values reported by MRE measurements in human livers (2.1±0.26 kPa for stiffness and 1.7±0.15 Pa*s for viscosity) and rat livers (1.76±0.37 kPa for stiffness and 0.51±0.04 Pa*s for viscosity) [1,2]. Research is currently conducted to implement SDUV in commercial ultrasound scanners.

References

• Laurent Huwart et al., NMR in Biomedicine, 19(2): 173-179, 2006.
• Najat Salameh et al., Journal of Magnetic Resonance Imaging, 26:956-962, 2007.