Biophotonics_Concepts_to_Applications

two-dimensional B-scan. A series of cross-sectional B-scans at different points
along the x-direction is referred to as aC-line scan. That is, multiple B-scans along
the x-axis form a three-dimensional C-scan image.

Example 10.1Consider a scanning system that uses a rotating mirror that

can scan at an 8 kHz rate over several mm. How much time is needed to

acquire 500 A-scans (columns of data)?

Solution: The time needed to acquire 500 A-scans is

tscan¼ðnumber of scansÞ=ðscan rateÞ¼ð500 scansÞ=ð8000 scans=sÞ

¼ 62 :5ms

As noted in Chap. 1 (Fig.1.6), a spectral range with low optical absorption loss
exists between about 600 and 1600 nm. In this range the imaging depth with light is
several millimeters into nontransparent tissue. Thus OCT is used in this operating
region because here light attenuation is more dependent on scattering processes
than on absorption processes. The key operational characteristic of OCT is that
ranging-based optical imaging can be carried out in biological tissue because a
portion of the incident light is reflected by each different material interface when-
ever there is a change in refractive index in the biological tissue. Low-coherence
light from a super-luminescent diode or other broadband optical source is needed to
measure the time delay of the reflected light.
Several variations of the OCT process have been devised. The original setup is
known astime domain OCT(TD-OCT). The TD-OCT method uses a low coher-
ence interferometer with a scanning reference arm, which detects echo delay signals
from tissue in the time domain. This method is described in Sect.10.1.1. The
second major OCT category performs detection in the Fourier domain by measuring
the interferometric spectrum. The advantages ofFourier domain OCT(FD-OCT)
are an increased sensitivity and a higher signal-to-noise ratio. This results because
the Fourier domain detection principle measures all the echoes of light from dif-
ferent cross-sectional planes in the tissue sample simultaneously, thereby yielding

A-scan

B-scan

Illumination to

various depths

Reflectance from

various depths

Tissue sample

x

C-scan

y

z

Fig. 10.2 Concept of
A-scans (1D), B-scans (2D),
and C-scans (3D)

10.1 Optical Coherence Tomography 293