Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 162

optical pulse remitted from scattering sites within the sample is localized by low-
coherence interferometry (LCI) [ 4 - 9 ]. This is typically achieved with a Michelson
interferometer. The sample rests in one arm of the interferometer, and a scanning
reference optical delay line is in the other arm. In LCI, light interferes at the detector
only when light reflected from the sample is matched in optical path length with that
reflected from the scanning reference mirror. A single scan of the reference mirror
thus provides a one-dimensional depth-reflectivity profile of the sample. Two-
dimensional cross-sectional images are formed by laterally scanning the incident
probe beam across the sample. The reconstructed OCT image is essentially a
map of the changes of reflectivity that occurs at internal interfaces, similar to the
discontinuities in acoustic impedance in ultrasound images.
The principal difference between ultrasound and optical imaging is that the
velocity of light is approximately a million times faster than that of sound. For
this reason, the distance within the materials or tissues with a resolution of 10 m
by measuring the echo time delay of backreflected or backscattered light wave
corresponds to a time resolution of 30 fs .10 15 /, which is well beyond the limits
of currently available electronic detection system. But for ultrasound, the echo time
delay is approximately 100 ns, which can be easily realized with modern electronic
detection systems. Thus, in OCT, each time delay has to be measured indirectly,
normally by correlation techniques, in which the backscattered light is compared
to the reference light with known optical path length. OCT uses the low-coherence
interferometric method to measure this echo time delay with high dynamic range
and sensitivity.

5.1.1 Advantages of OCT over Other Imaging Technologies

Recently, OCT has attracted much attention in many clinical and basic research
fields due to its high sensitivity for noninvasive high-resolution imaging at cellular
level. The diagnostic capability of OCT has revolutionized many fields such as
ophthalmology, dermatology, cardiology, etc. In terms of other medical imaging
devices, OCT is the one best offered currently. OCT is becoming rapidly an
important biomedical tool for imaging tissues and engineered tissues [ 10 ]. OCT
has critical advantages over other medical imaging systems. Microscopes work
well for examining small tissue samples and cells but not for examining biological
tissues inside the body [ 11 ]. The ultrasound, CT, and MRI can peer inside the
body; however, they do not have sufficient resolution to capture cellular detail
[ 11 ]. Electron microscopy can pick up extremely fine details; however, it is
not able to view living samples within the body [ 11 ]. Figure 5.1 shows the
comparison of OCT with presently existing clinical imaging modalities [ 12 ]. The
two major advancements which assist both scientists and medical physicians are
the improvement in the depth and clarity at which they can view tissues. Current
OCT systems have resolutions at 1-20 m compared to 110 m for high-frequency
ultrasound [ 13 ]. The advancement of medical imaging systems is illustrated by the
technology inherent in OCT, which allows images of living tissue to become clearer
and more defined in both structure and detail. Using information inherent to the