  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.
