Shear wave propagation modulates quantitative ultrasound K-distribution echo envelope model statistics in homogeneous viscoelastic phantoms

In the context of tissue characterization, one may wonder what does the consideration of a quantitative ultrasound (QUS) feature of a medium under the propagation of a shear wave (SW) add to its discriminant power. This study presents the time-varying behavior of the K-distribution beta parameter - the reciprocal of the effective density of scatterers - under SW propagation and its relation with the viscoelasticity of the medium. Transient plane SW at 300 Hz central frequency was transmitted to three agar-gelatin phantoms at different concentrations. The amplitude of the B-mode backscatter echoes acquired with an 8 MHz probe was modeled with the K-distribution. The normalized range of beta (i.e., its range normalized by its mean value as the SW propagates) was determined by considering the B-mode images during SW propagation. Also, the storage (G´) and loss (G´´) moduli of each phantom were measured on samples with the RheoSpectris hyper-frequency instrument (Rheolution, Montreal, Canada). The time-evolution of the beta parameter and displacements (using cross-correlation) within tissue-mimicking phantoms under SW vibration suggest that the beta parameter can be used to track SW propagation. In-vitro results showed that the normalized range of beta is related to the viscoelasticity of phantoms. By increasing G´ and G´´, the normalized range of beta decreased. Thus, the consideration of the behavior of beta under SW propagation modifies the effective density of scatterers with respect to static conditions (i.e., without SW). This is new observation and a new step towards understanding statistical QUS behavior.