Introduction imaging modality that uses coherent light

Introduction Optical Coherence Tomography (OCT)
is an imaging modality that uses coherent light to capture 2D and 3D images of micrometer
resolution from within optical scattering media such as biological tissue. It
is used for medical imaging and industrial non-destructive testing. OCT is
based on low-coherence interferometry, generally employing near-infrared light.
In laser interferometry which is the conventional interferometry with long
coherence length, interference of light occurs over a distance of meters. In
OCT, this interference is shortened to a distance of micrometers, due to the
use of broad bandwidth light sources. The use of relatively long wavelength
light allows it to penetrate into the scattering medium. The concept of OCT
imaging is based on the measurement of echo time delay and the magnitude of
backscattered light. OCT enables the real time visualization of the tissue which
makes it a powerful imaging tool for medical applications.  Basic
Tissue Optics The two basic optical properties of
living tissues are absorption and scattering. The absorption coefficient ?a
(m?1) and the the scattering coefficient ?s (m?1)
are defined as follows:Absorption
coefficient – the
probability of absorption of a photon at an infinitesimal distance ?d when the photon
propagates over the infinitesimal distance i.e. for an absorption event the
mean free path is 1/?a Scattering
coefficient – the
probability of scattering of a photon at an infinitesimal distance ?d when the photon
propagates over the infinitesimal distanceWhen
individual scattered photons are detected in OCT, the relevant light from the
source collected by the interferometer detector travels in the tissue a
distance of 2d which is the sum of two distances: the distance in the tissue
from the source to the point where the light is backscattered or reflected and the
distance from the point where the light is backscattered or reflected back to
the detector. These two distances are equal and the light is attenuated twice
over that distance. Lamber-Beer’s
law describes the light attenuation in a non-scattering media:

where   I(d)
– the intensity at a distance d I0 – the light intensity incident on
the tissueLambert–Beer’s law is used to calculate the
total attenuation using the total attenuation coefficient ?t.?t
= ?a + ?s        The
OCT System 

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OCT interferometer system for catheter/endoscope
imaging The objective of the OCT technique
is to measure the echo time delay of light using interferometric techniques. Through
the use of interferometric techniques, the tissue backscattered light signal is
correlated with the light that has travelled an already known reference path
length. Since the speed of light is faster than the speed of sound, the use of
interferometry techniques to determine the echo time delay of light is crucial.
Both the magnitude of backscattering light properties and the echo time delay
can be measured using interferometry. A frequently used detection method is the
Michelson interferometer which applies a scanning reference delay arm. OCT uses
interferometric techniques to perform high resolution performance of light
echoes. In an OCT system, the fiber-optic coupler of the interferometer is
analogous to an optical beam splitter and divides the input light into a
reference arm and a measurement arm. A catheter or other image device is
connected to the optical fiber in the measurement arm. The catheter scans the
transverse position of the measurement beam and focuses the beam onto the
tissue that is being imaged. The catheter collects the light echoes as
backscattered by the tissue. The collected light echoes are returned back to
the measurement arm. A retro-reflecting mirror at a calibrated distance is
attached in the reference arm. The echo light of the reference arm is returned
with a calibrated delay. At the fiber coupler the two echoes of light, one from
the tissue and one from the reference arm are combined. A high-speed
photo-detector detects the intensity of the interference and finally, the
electronic signal is processed in order to extract a measurement of echo time
delay. Several
detection methods are used to measure the echo time delay of light. There are two
interferometric techniques used in OCT systems to perform measurements:·        
Time
– Domain OCT (TD-OCT): applies an interferometer with a low-coherence light
source and scanning reference delay is used·        
Fourier
– Domain OCT (FD-OCT): applies a narrow bandwidth, frequency-swept light source
and a stationary reference delay interferometer is used      Time – Domain OCT (TD-OCT) 

Interferometer (low-coherence) by the Time – Domain OCT
technique  The Time – Domain OCT (TD-OCT)
systems are based on a low-coherence interferometer, which is actually a
Michelson-type interferometer. The back scattered light echoes from the tissue
are correlated with scattered light. The light travels a known reference path
delay. Interference can be observed when light from the reference arm arrives
at the same time as light from the tissue when using a low-coherence light
source. By detecting the envelope of the modulated interference signal axial
information can be obtained. As the reference path is scanned, the echoes are
measured sequentially at different depths. A beam splitter splits the beam
from a light source into two beams. The first light beam is directed to the
tissue and is backscattered from the structures of tissue at different depths.
From the backscattered light beam consisting of multiple echoes, relevant
information can be derived about the depth or range of the different tissue structures.
A reference mirror reflects the second light beam. The position of the
reference mirror in time, thus the reference beam has a variable time delay. At
the beam splitter the reference beam and the measurement beam are interfered
with. A photodetector detects the output. When the light beam is considered as
light pulse, if the tissue backscattered light pulse and the mirror reflected
light pulse arrive synchronously within the pulse duration then the two pulses
will coincide. This happens if the two distances, the distance that light
travels in the interferometer measurement arm when it is backscattered from the
tissue and the distance that light travels in the interferometer reference path
are similar. A modulation in intensity is produced when the two light pulses
interfere. This happens when the light pulses coincide. The modulation in
intensity is measured by a photodetector. The approach in order to measure the
time delays of light echoes coming from different structures of the tissue and
from different depths is to scan mechanically the position of the reference
mirror so that the time delay of the reference light pulse varies continuously.
In conclusion, the key idea of the Time – Domain interferometer is to measure
the time delays of optical echoes sequentially and it is done by scanning a
reference path, so that different echo delays are measured at different times.      Fourier – Domain OCT (FD-OCT) 

Interferometer by the Fourier – Domain OCT technique Fourier – Domain OCT (FD-OCT)
systems use a narrow bandwidth light source. This light source is frequency
swept in time and the reference arm in the interferometer that is used is
stable. The backscattered tissue light echoes are interfered with scattered
light. This light travels an already known reference path delay. The two light beams:
the one coming from the tissue and the other traveling the reference path have
different frequencies. This is because of the fact that the light coming from
the tissue is delayed in time when compared to the light traveling the
reference path. The interference varies according to the differences in
frequency. By applying the Fourier transform in the signal of the detector,
axial scan information is obtained. In the Fourier – Domain detection, all the
echoes of light are measured at once instead of sequentially as in Time –
Domain detection. Thus, the imaging speed of the Fourier – Domain detection is
higher than the imaging speed of the Time – Domain detection. This is the most
important difference between the Fourier – Domain detection and the Time –
Domain detection.   OCT Image
Resolution  The high resolutions of OCT images play
an important role in its clinical use. Different mechanisms determine the
resolution of an OCT image in the axial and in the traverse directions. The
focused spot size of the optical beam determines the traverse resolution. The
resolution of the measurement for the echo time delay determines the resolution
of the image in the axial direction. In fact, the light source used for the
measurement determines this axial resolution. If a low-coherence light source
is used to measure the echo time-delay, the coherence length of the source
determines the axial resolution. If a frequency light is used then the axial
resolution is determined by the tuning range of the light. The axial resolution
for standard OCT systems is 10-15 ?m.
An OCT image of the human retina with high resolution provides better differentiation
of intra-retinal layers. Thus, more detailed features of the photoreceptor
layer can be extracted which enables the diagnosis of an early disease. For an
intravascular OCT image, improvements to increase transverse pixel density and improvements
to increase the speed of imaging are very important.In an OCT image, the number of
axial scans determines the number of pixels in the transverse direction. The
image acquisition time increases proportionately to the number of transverse
pixels. By increasing the transverse pixel density, the image quality can be
improved. High-definition images are produced by high axial scan density. In these
images, the improvement of visualization is apparent. FD-OCT systems use
Fourier – Domain detection methods that enable higher axial scan densities
providing high quality OCT images by reducing the examination time.  Applications
of Optical Coherence Tomography OCT was initially applied for
imaging in ophthalmology. Advances in OCT technology have made it possible to
use OCT in a wide variety of applications with the medical applications still being
the dominating ones. Speci?c
advantages of OCT compared to alternative optical techniques are •
depth resolution is independent of the sample beam aperture•
the coherence gate can substantially improve the probing depth in scattering
mediaThe
advantages of OCT compared to non-optical imaging modalities are  •
high depth and transversal resolution•
contact-free and non-invasive operation and the possibility to create• function dependent image contrast
– Related contrasting techniques are based on Doppler frequency shift,
polarization        and
wavelength-dependent backscattering The
main disadvantage of OCT compared to alternative imaging modalities in medicine
is its limited penetration depth in scattering media.  

An OCT image from a coronary artery acquired using an
FD-OCT system Ophthalmology OCT is heavily used by
ophthalmologists to obtain high resolution images of the eye’s anterior
segment and retina. Because of its cross-sectional capabilities, OCT provides a
straightforward method of assessing axonal integrity in multiple sclerosis and glaucoma. OCT is also well suited to assess macular degeneration and is considered the new standard for the assessment
of diabetic macular oedema. More recently, ophthalmic OCT devices have been
engineered to perform angiography and have been used to assess retinal
microvasculature pathology in diseases such as glaucoma and diabetic
retinopathy. Cardiology In the context of cardiology, OCT
is used to image coronary arteries in order to visualize vessel wall
lumen morphology and microstructure at a resolution ten times higher than the other
existing modalities such as intravascular ultrasounds and X-ray angiography.
For this kind of application – Intracoronary Optical Coherence Tomography,
approximately 1 mm in diameter fiber-optics catheters are used to access
artery lumen through semi-invasive interventions, that is percutaneous
coronary intervention.The higher imaging speed
of FD-OCT enabled the widespread adoption of this imaging technology for
coronary artery imaging. Recent developments of intravascular OCT included the
combination with other optical imaging modalities. OCT has been combined
with fluorescence molecular imaging to enhance its capability to
detect molecular/functional and tissue morphological information at the same
time. Similarly,
combination with near-infrared spectroscopy has been also demonstrated. Oncology 

Endoscopic OCT has been applied to
the detection and diagnosis of cancer and precancerous
lesions such as Barrett’s esophagus and esophageal dysplasia.