Page 387 - Handbook of Properties of Textile and Technical Fibres

360 Handbook of Properties of Textile and Technical Fibres

11.9.4 Ultrasound elastography

Ultrasound elastography is referred to by a number of terms including strain elastog-
raphy, compression elastography, sonoelastography, and real-time elastography
(Drakonakietal.,2012). Using these techniques a low-frequency compression is
applied to the tissue, frequently via the hand held transducer. The applied compres-
sion induces a strain and the modulus is estimated from the change in the echo before
and after the force is applied. Zaleska-Dorobisz et al. (2014) review the use of ultra-
sound to calculate the modulus values of tissues for different clinical applications.
This technique assumes that the tissue is a linearly elastic solid that has a Poisson’s
ratio of 0.5; the technique does not measure the modulus directly. Clinically, this
technique has been used to identify pathologic changes in a number of diseases.
However, the data obtained from UE will depend on the frequency of sound used
in the measurements and the assumptions made in converting the displacement to
elastic modulus.

11.9.5 Vibrational analysis and optical coherence tomography
Pulsed laser excitation has been used to create a surface wave and estimate the
modulus; this is accomplished using an equation that relates the surface wave velocity
to the modulus. Song et al. (2015) used ultrasound to create a shear wave and used
OCE to measure the properties of tissue. The above studies assumed a value for Pois-
son’s ratio and a density to calculate the mechanical properties. The assumption of a
value of 0.49 for Poisson’s ratio leads to calculation errors as discussed above. These
methods are noninvasive and if modified to correct for viscoelasticity and incompres-
sibility would give improved results.
Shah et al. (2016) used vibrational analysis in concert with OCT to measure the
resonant frequency of decellularized human dermis (Fig. 11.8). They applied an acous-
tic vibration to the samples under tension and showed that the resonant frequency
squared obtained from the change in frequency of the reflected light was directly
related with the tensile modulus obtained in an incremental stressestrain experiment
(Fig. 11.9). Their method did not rely on the assumption of a value of Poisson’s ratio;
this technique would be of value clinically if the measurements could be made nonin-
vasively in situ.
These new methods are quite promising for measuring the mechanical properties
of tissues in vivo and advancing the materials science of collagenous tissues. Howev-
er, for these techniques to provide accurate information about tissues they must
consider nonlinear behavior, strain-rate dependence, and volumetric effects that occur
during mechanical loading. It is well known that fluid flow during cartilage and bone
deformation is an important mechanism for energy dissipation as well as a stimulator
of tissue mechanotransduction (Kim et al., 1995; Fritton and Weinbaum, 2009). Fluid
flow from tissues under load is an important contributor to nonlinear viscoelastic
behavior. To ignore these effects limits the relevance of any technique used to deter-
mine the mechanical properties and limits the accuracy of the results obtained using
these methods.