Biophotonics_Concepts_to_Applications

combined with optical coherence tomography, which is known asoptical coherence
elastography(OCE), were investigated to improve the image resolution [ 36 – 41 ].
The goal of OCE is tofind the Young’s modulus of a tissue, which will indicate
the tissue elasticity (or stiffness). Deviations in the Young’s modulus from that of
healthy tissue can help identify diseased cells. A challenge in applying OCE is that
biological tissues are anisotropic and their response to an external mechanical
loading has both viscous and elastic components. Thus elaborate mathematical
techniques are needed for elastic modulus imaging.
However, a basic idea of the phenomenological process can be obtained by
assuming that a tissue is a uniform linear elastic solid with isotropic mechanical
properties. The linearity assumption is usually valid because the strain level applied
in elastography is typically less than 10 % of the elastic limit. Thus when a uniform
stressσis applied in the axial direction, a local stress results that is constant
throughout the sample tissue and is equal to the applied stress. For a uniaxial load
(compression along one axis), the local stress can be used tofind the Young’s
modulus through the simple relationship

E¼

r

e

ð 10 : 20 Þ

where E is the Young’s modulus,σis the axial stress, andεis the axial strain. This
equation shows that stiffer materials, for example, tissues of higher density such as
bones, require a higher stress to induce identical deformations and thus have a
larger Young’s modulus than less dense materials such as skin, arterial walls, or red
blood cells. In addition, diseases can modify both the constituent elements and the
structure of tissue, which results in variations in mechanical properties between
healthy and diseased tissue. The value of the Young’s modulus varies over more
than eight orders of magnitude ranging from tens to billions of Pascals. This
illustrates that a high contrast between different tissue constituents, structures, and
whole organs can be obtained from elastograms.

Piezoelectric

transducer

Tissue sample

Probing

light beam Mechanical loading

on the tissue sample

Fig. 10.14Example of an OCE setup using a circular piezoelectric transducer and a central light
beam probe

10.4 Optical Coherence Elastography 311