Towards a consensus on rheological models for shear waves in soft tissues

TL;DR

This paper advocates for the adoption of fractional derivative models as the most effective and meaningful rheological models for describing shear wave behavior in soft tissues, aiming to unify diverse measurement approaches.

Contribution

It provides a comprehensive argument favoring fractional derivative models over traditional models for soft tissue rheology, promoting consensus across related scientific fields.

Findings

Fractional models capture multi-scale tissue behavior effectively.

Traditional models may lack the succinctness and physical relevance.

A consensus can improve comparability of tissue measurements.

Abstract

A rising wave of technologies and instruments are enabling more labs and clinics to make a variety of measurements related to tissue viscoelastic properties. These instruments include elastography imaging scanners, rheological shear viscometers, and a variety of calibrated stress-strain analyzers. From these many sources of disparate data, a common step in analyzing results is to fit the measurements of tissue response to some viscoelastic model. In the best scenario, this places the measurements within a theoretical framework and enables meaningful comparisons of the parameters against other types of tissues. However, there is a large set of established rheological models, even within the class of linear, causal, viscoelastic solid models, so which of these should be chosen? Is it simply a matter of best fit to a minimum mean squared error of the model to several data points? We argue that the long history of biomechanics, including the concept of the extended relaxation spectrum, along with data collected from viscoelastic soft tissues over an extended range of times and frequencies, and the theoretical framework of multiple relaxation models which model the multi-scale nature of physical tissues, all lead to the conclusion that fractional derivative models represent the most succinct and meaningful models of soft tissue viscoelastic behavior. These arguments are presented with the goal of clarifying some distinctions between, and consequences of, some of the most commonly used models, and with the longer term goal of reaching a consensus among different sub-fields in acoustics, biomechanics, and elastography that have common interests in comparing tissue measurements.