Poroelastography: Imaging the poroelastic properties of tissues

In the field of elastography, biological tissues are conveniently assumed t o be purely elastic solids. However, several tissues, including brain, cart ilage and edematous soft tissues, have long been known to be poroelastic. T he objective of this study is to show the feasibility of imaging the poroel astic properties of tissue-like materials. A poroelastic material is a mate rial saturated with fluid that flows relative to a deforming solid matrix. In this paper, we describe a method for estimating the poroelastic attribut es of tissues. It has been analytically shown that during stress relaxation of a poroelastic material (i.e., sustained application of a constant appli ed strain over time), the lateral-to-axial strain ratio decreases exponenti ally with time toward the Poisson's ratio of the solid matrix. The time con stant of this variation depends on the elastic modulus of the solid matrix, its permeability and its dimension along the direction of fluid flow. Rece ntly, we described an elastographic method that can be used to map axial an d lateral tissue strains. In this study, we use the same method in a stress relaxation case to measure the time-dependent lateral-to-axial strain rati o in poroelastic materials. The resulting time-sequenced Images (poroelasto grams) depict the spatial distribution of the fluid within the solid at eac h time instant, and help to differentiate poroelastic materials of distinct Poisson's ratios and permeabilities of the solid matrix. Results are shown from finite-element simulations (C) 2001 World Federation for Ultrasound i n Medicine & Biology.