Cross-correlation of ultrasound A-lines to obtain dynamic displacement profiles within poroelastic materials undergoing stress relaxation

One technique of elasticity imaging, elastography, uses cross- correlation between two ultrasound A-lines to obtain an axial strain image of a sample. Usually, great care is taken with respect to the assumption that the response of the sample is elastic (lossless). In this paper, we relax this assumption and extend elastography to estimate the time-varying displacement and strain status of small samples (of the order of 1 mm). Results are presented for gel phantoms and articular cartilage samples, and they are consistent with the current theories of poroelastic materials. For example, an effective Poisson's ratio of approximately 0.5 obtained at ramp completion indicates volume conservation since the ramp time was much shorter than the characteristic relaxation time of the material. Subsequent reduction in effective Poisson's ratio coincident with stress-relaxation confirms poroelastic mechanisms whereby fluid exudation dissipates internal fluid pressurization. Observed slower relaxation of strain at the center of the sample is also compatible with these types of models. Preliminary data obtained with articular cartilage also shows valuable potential of this technique to investigate tissue biomechanics.