Biophotonics_Concepts_to_Applications

Example 10.10What is the measurement procedure needed for the case of

phase-sensitive OCE?

Solution: Only two OCT scans need to be made in this setup [ 40 ]. One scan

is made when there is no mechanical load on the tissue sample, whereas the

other scan is performed when there is a compressive load resulting from a

known applied axial stressσ. The local axial strainεis then determined from

the relationshipε=Δd/Δz, where

Dd¼DUk 0 =ð 4 pnÞ

is the change in axial displacement over the depth rangeΔz. HereΔΦis the

change in the measured OCT phase between the loaded and unloaded scans,

λ 0 is the mean free-space wavelength of the OCT system, and n is the

refractive index of the tissue.

A wide variety of mechanical loading methods have been used in various OCE
techniques. These methods include static, quasi-static, continuous wave dynamic,
or pulsed dynamic loadings. The loadings can be applied to the tissue either
internally or externally. In each case, the displacement resulting from the loading is
measured and a mathematical model of mechanical deformation is used to estimate
a mechanical property of the tissue.

10.5 Photoacoustic Tomography

Photoacoustic tomography(PAT) is a hybrid biophotonics imaging modality that
combines the processes of optical absorption in tissue with a resulting sound wave
generation [ 42 – 46 ]. The procedure in PAT is to irradiate a localized tissue volume
of interest with a short-pulsed laser beam. Some of the light is partially converted
into heat in regions where enhanced optical absorption takes place. For example,
some forms of diseased tissues and certain endogenous or exogenous chromophores
exhibit strong light absorption properties. Strongly absorbing endogenous chro-
mophores are molecules such as melanin, water, oxygenated hemoglobin, and
deoxygenated hemoglobin that originate or are produced within an organism, tissue,
or cell. As a result of their absorption properties, such endogenous chromophores
can be used for examining features such as vascular structures, hemoglobin oxygen
saturation, and bloodflow. Exogenous chromophores are externally introduced
substances such as dyes, nanoparticles, and reporter genes.
The strong light absorption by molecules from the short laser pulse results in a
transient localized pressure rise due to a thermoelastic expansion effect. The sudden
pressure rise creates an ultrasonic wave, which is known as aphotoacoustic wave.
An ultrasonic probe then measures this resultant ultrasonic wave and a computer is
used to analyze the data and form an image of the targeted region. The amount of

312 10 Optical Imaging Procedures