Effect of interstitial fluid pressure on shear wave elastography: an experimental and computational study.

An elevated Interstitial Fluid Pressure (IFP) can lead to strain-induced stiffening of poroelastic biological tissues. As Shear Wave Elastography (SWE) measures functional tissue stiffness based on the propagation speed of acoustically induced shear waves, the Shear Wave Velocity (SWV) can be used as an indirect measurement of the IFP. The underlying biomechanical principle for this stiffening behavior with pressurization is however not well understood, and we therefore studied how IFP affects SWV through SWE experiments and numerical modeling.
Approach. For model set-up and verification, SWE experiments were performed while dynamically modulating IFP in a chicken breast. To identify the confounding factors of the SWV-IFP relationship, we manipulated the material model (linear poroelastic vs. porohyperelastic), deformation assumptions (geometric linearity vs. nonlinearity), and boundary conditions (constrained vs. unconstrained) in a finite element model mimicking the SWE experiments.
Main results. The experiments demonstrated a statistically significant positive correlation between the SWV and IFP. The model was able to reproduce a similar SWV-IFP relationship by considering an unconstrained porohyperelastic tissue. Material nonlinearity was identified as the primary factor contributing to this relationship, whereas geometric nonlinearity played a smaller role. The experiments also highlighted the importance of the dynamic nature of the pressurization procedure, as indicated by a different observed SWV-IFP for pressure buildup and relaxation, but its clinical relevance needs to be further investigated.
Significance. The developed model provides an adaptable framework for SWE of poroelastic tissues and paves the way towards non-invasive measurements of IFP.