Power law dispersion from shear wave images

Abstract

Many normal soft tissues exhibit dispersion of shear wave phase velocity that is consistent with power law behavior. This rheology is consistent with the fractional Zener model under assumptions that appear to be realistic for normal liver and other tissues. The phenomenon can be measured using multi-frequency reverberant shear wave elastography (R-SWE). A simultaneous multi-frequency R-SWE field can be accomplished by applying an array of external sources that operate at multiple frequencies, for example, 50, 100, 150…500 Hz, all contributing to the shear wave field produced in the target organ. With estimates of phase velocity across this frequency band, the R-SWE approach can obtain a 2-D power law coefficient (PLC) image. The clinical feasibility of this method has been analyzed by assessing the shear wave speed (SWS) and PLC in phantoms and in vivo human organs. For in vivo liver cases, mean SWS (1.99 m/s, 2.29 m/s, and 2.38 m/s) and mean PLC (0.23, 0.44, and 0.43) were estimated for a thin patient, and for two obese patients, respectively. For these cases, the PLC results may be an additional parameter that could help to differentiate the viscoelastic properties in liver tissue and enhanced image contrast in cases where lesions or pathologies show an altered viscoelastic response compared with normal tissue.