Dynamic Simulation of Viscoelastic Soft Tissue in Acoustic Radiation Force Creep Imaging

Abstract

Acoustic radiation force (ARF) creep imaging applies step ARF excitation to induce creep displacement of soft tissue, and the corresponding time-dependent responses are used to estimate soft tissue viscoelasticity or its contrast. Single degree of freedom (SDF) and homogeneous analytical models have been used to characterize soft tissue viscoelasticity in ARF creep imaging. The purpose of this study is to investigate the fundamental limitations of the commonly used SDF and homogeneous assumptions in ARF creep imaging. In this paper, finite element (FE) models are developed to simulate the dynamic behavior of viscoelastic soft tissue subjected to step ARF. Both homogeneous and heterogeneous models are studied with different soft tissue viscoelasticity and ARF configurations. The results indicate that the SDF model can provide good estimations for homogeneous soft tissue with high viscosity, but exhibits poor performance for low viscosity soft tissue. In addition, a smaller focal region of the ARF is desirable to reduce the estimation error with the SDF models. For heterogeneous media, the responses of the focal region are highly affected by the local heterogeneity, which results in deterioration of the effectiveness of the SDF and homogeneous simplifications.

On the Quantitative Potential of Viscoelastic Response (VisR) Ultrasound Using the One-Dimensional Mass-Spring-Damper Model

“…It is important to note that these FEM models simulate each element in the mesh as a distributed network of masses, springs, and dampers in three dimensions [24]. The interconnected nature of the elements creates complex behavior and multiple degrees of freedom in the simulated environment that, in general, cannot be directly recapitulated using a SDF model, such as the Voigt or MSD model [15]. However, in the case of an idealized point force, as described in sections IV.A, the MDF FEM simulation approximately reduces to a SDF system at the focal point.…”

“…One approach to viscoelastic property assessment by ultrasound involves using acoustic radiation force (ARF) to induce compressive and shear waves at depth in tissue, and the associated dynamic tissue deformations are monitored and related to viscoelasticity. Shear wave dispersion (as in Shearwave Dispersion Ultrasound Vibrometry (SDUV) [10] and Shear Wave Spectroscopy[2]), shear wave attenuation [11], [12], and tissue creep behavior (as in Kinetic Acoustic Vitreoretial Examination (KAVE) [13], Monitored Steady-State Excitation Recovery (MSSER) [14], ARF Creep imaging [15], hybrid creep with SDUV [16] and Viscoelastic Response (VisR) imaging [9]) have been exploited to estimate the viscoelastic parameters of tissue using ARF.…”

“…1(a). The three-dimensional nature of the ARF excitation induces displacement in tissue extending beyond the tracking focal region, and the complex system inertia is not accurately accounted for by a single degree of freedom (SDF) model such as the MSD, as demonstrated by Zhao and Pelegri [15]. The unaccounted complex system inertia extends the duration of the force experienced at the tracking focus beyond that of the applied ARF excitation.…”

“…While FEM analysis models tissue as a MDF system, the MSD model describes only a SDF [15]. Thus, the fundamental assumptions associated with applying the MSD model to describing the viscoelastic properties of tissue are that: 1) the induced motion is highly localized, 2) the tissue response reflects only the local distribution of mechanical properties, and 3) the tissue is homogeneous.…”

“…Thus, the fundamental assumptions associated with applying the MSD model to describing the viscoelastic properties of tissue are that: 1) the induced motion is highly localized, 2) the tissue response reflects only the local distribution of mechanical properties, and 3) the tissue is homogeneous. Importantly, these assumptions break down in the context of the volumetric nature of ARF excitations due to the associated complex and interrelated three-dimensional system inertia they induce [15]. In tissue (and in FE simulation) the system inertia arises from complex interactions of both compressive and shear dynamics; however, the MSD model accounts for only uniaxial displacements from compressive stresses, neglecting shear phenomena.…”