Page 386 - Handbook of Properties of Textile and Technical Fibres

Structure and behavior of collagen fibers 359

ocular response analyzer (ORA), optical coherence elastography (OCE), and OCT
with vibrational analysis that are quite promising. Classical methods such as constant
rate-of-strain deformation as well as incremental stressestrain analysis are useful but
prove to be too destructive to tissue and therefore have limited value for measuring
tissue properties in vivo.

11.9.1 Magnetic resonance elastography

MRE has been used to calculate values of the modulus of tissues. In this method,
mechanical excitation is produced by pneumatic, electromechanical, or piezoelectric
stimulators positioned next to the body (Low et al., 2016). The tissue is loaded by
one of these means and then the magnetic resonance imaging (MRI) signal is collected.
The phase shift in the MRI signal is used to calculate a value of the modulus; however,
the workers assume that Poisson’s ratio is 0.5 and that the tissue density is 1.0 g/cc.
These assumptions create calculation errors since Poisson’s ratio has been shown to
vary from 0.5 for tissues (Shah et al., 2016). The value of this technique is that it
can be used noninvasively in real time; however, use of this technique requires correc-
tion for Poisson’s ratio and strain-rate effects to be entirely accurate.

11.9.2 Ocular response analyzer

The ORA is a clinical device that uses a high-speed air puff to deform the cornea.
Changes in shape of the anterior surface are tracked using an infrared beam reflected
from the surface and aligned with the geometry of a detector (see Ruberti et al., 2014
for a review). In this technique, corneal deformation is tracked after the air puff is
applied to the corneal surface. Differences in the pressures between the inward and
outward flattening of the cornea are reported as the corneal hysteresis. Changes in
the corneal hysteresis are correlated with disease states anecdotally. The noninvasive-
ness of this technique is a positive attribute of this method. However, the inability to
relate the results to standard mechanical testing parameters limits the utility of this
method.

11.9.3 Optical coherence elastography

OCE has been used for the analysis of tissue mechanical properties (Kennedy et al.,
2014a,b; Wang and Larin, 2015). This technique uses light that is reflected off a sur-
face andcomparedtothe nonreflected light to create an image; the image is used to
measure displacement after the tissue undergoes a small deformation. Mathematical
modeling is used to calculate the tissue modulus assuming the tissue is a linear elastic
solid and that Poisson’s ratio is 0.5. This technique is noninvasive and can be used to
evaluate tissue in situ. However, the values of moduli obtained from the models used
appear lower than those calculated from destructive testing, suggesting that the
strains introduced are not large enough to deform the collagenous components of
the tissue.