Elasticity Imaging of Polymeric Media

Abstract

Viscoelastic properties of soft tissues and hydropolymers depend on the strength of molecular bonding forces connecting the polymer matrix and surrounding fluids. The basis for diagnostic imaging is that disease processes alter molecular-scale bonding in ways that vary the measurable stiffness and viscosity of the tissues. This paper reviews linear viscoelastic theory as applied to gelatin hydrogels for the purpose of formulating approaches to molecular-scale interpretation of elasticity imaging in soft biological tissues. Comparing measurements acquired under different geometries, we investigate the limitations of viscoelastic parameters acquired under various imaging conditions. Quasistatic (step-and-hold and low-frequency harmonic) stimuli applied to gels during creep and stress relaxation experiments in confined and unconfined geometries reveal continuous, bimodal distributions of respondance times. Within the linear range of responses, gelatin will behave more like a solid or fluid depending on the stimulus magnitude. Gelatin can be described statistically from a few parameters of low-order rheological models that form the basis of viscoelastic imaging. Unbiased estimates of imaging parameters are obtained only if creep data are acquired for greater than twice the highest retardance time constant and any steady-state viscous response has been eliminated. Elastic strain and retardance time images are found to provide the best combination of contrast and signal strength in gelatin. Retardance times indicate average behavior of fast (1-10 s) fluid flows and slow (50-400 s) matrix restructuring in response to the mechanical stimulus. Insofar as gelatin mimics other polymers, such as soft biological tissues, elasticity imaging can provide unique insights into complex structural and biochemical features of connectives tissues affected by disease.

Magnetomotive nanoparticle transducers for optical rheology of viscoelastic materials

“…Viscoelastic materials, as their name suggests, have both elastic and viscous properties. Namely, they store a portion of the energy they receive from external sources as potential energy, while the other portion is damped [1–5]. Their mechanical properties are of crucial importance for their function and reliability, a fact well known in the materials science field where high-performance materials are widely sought.…”

“…Current methods for the measurement of mechanical properties of viscoelastic materials include ultrasound elastography [5–8], magnetic resonance elastography [9–12], vibro-acoustography [13,14], atomic force microscopy (AFM) [15], and optical coherence elastography [16–22]. These methods are either static or dynamic, and aim to detect displacements in samples when an external or internal stress is applied.…”

Fractional derivative models for ultrasonic characterization of polymer and breast tissue viscoelasticity

“…Both algorithms were designed to minimize strain noise for relatively large (>1%) deformations. Later generation algorithms [12], [13] provide superior strain estimates for the very small displacements (<0.1%) associated with radiation force stimuli [14] and viscoelastic creep imaging techniques [15]–[17]. For each volume element of tissue imaged, a time sequence of strain estimates is measured from which VE imaging parameters are extracted.…”

Assessing composition and structure of soft biphasic media from Kelvin–Voigt fractional derivative model parameters

“…This is expected from a viscoelastic matrix embedded in some fluid. Relaxation of a viscoelastic solid depends on the degree and type of molecular bonds in the matrix connections [19, 28]. The extracellular matrix in tissues and the matrix in these emulsion samples have very different collagen-component structures, and yet both naturally self-assemble in fluids to respond viscoelastically.…”

“…In such media, we generally find that α > 0.2 [19]. In contrast, pure gelatin has tightly bound water that strongly adheres to exposed charged sites in the denatured collagen molecules, so it responds as an elastic solid [28, 29]. For pure gelatin we find α ≈0.…”