Towards a molecular level understanding of elasticity in tissues and hydrogels

In elasticity imaging, tissues are stimulated with mechanical forces while spatiotemporal strain responses are observed. The basis for diagnostic imaging is that disease processes characteristically alter the structure of connective tissues that determine viscoelastic properties. Time-varying strains for step-like stress stimuli were measured in gelatin hydrogels and normal breast tissue. The medium's mechanical response function - the retardance-time spectrum - was computed. This spectrum is the continuous distribution of time constants that characterizes viscoelastic behavior. Spectra were parameterized using low order discrete rheological models from linear viscoelastic theory to reduce data dimensionality yielding parameters related to stiffness and viscosity: elastic strain and two retardation time constants. Broadband, continuous, bi-modal spectra was obtained for gelatin samples. Similar spectra with narrow bandwidth were found for breast tissue. Both characteristic of lightly crosslinked amorphous polymers. Measured time constants in gelatin indicated fast (1-10 s) fluidic behavior and a slower (50-400 s) matrix restructuring. Corresponding parameters in breast were 3.2 +/- 0.8 s and 42.0 +/- 28 s. Phantom imaging studies showed that these parameters provided consistently high target contrast. Although the ultra-structure of collagen within gelatin and breast stroma is different, their mechanical behavior is quite similar. Creep in both media are consistent with the molecular theory of entanglement coupling proposed to explain amorphous polymer behavior.