1. interference pattern, but only if light from both

1.     Introduction
to Optical Coherence Tomography

Optical
coherence tomography (OCT)
is an imaging technique that uses coherent light to capture cross-sectional image of the
optical scattering media (biological tissues) with micrometer-resolution.
It is used for medical imaging and
industrial nondestructive testing (NDT). Long wave length
light is used to penetrate through scattering medium. Optical Coherent
Tomography is a type of Optical Tomographic Techniques. OCT is similar to
ultrasound imaging, but the different is here we use light waves instead of
sound waves.

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OCT performs high-resolution,
cross-sectional tomographic imaging using backscattered or backreflected light
in internal microstructure in material and biological systems. Imaging can be
performed in situ and in real time.
OCT imaging is widely used in biomedical and clinical applications. OCT was
initially used in the imaging in the eye. OCT enables
the noncontact, non-invasive imaging of the anterior eye as well as imaging of
morphologic features of the human retina including the fovea and optic disc.
Nowadays, advanced in technology enables OCT to perform imaging in
non-transparent tissues and leads to wide range of applications. Imaging depth
is limited by the optical attenuation from scattering and absorption in
tissues. When compared to ultra sound imaging, imaging depth is limited to
ultrasound. But resolution of OCT is few times greater than ultrasound. Eventhough
there is a limit to depth of penetration in OCT, it can provide images of
tissue pathology in vivo with spatial
resolution of few micrometers. The dominant application of OCT is currently
ophthalmology.

The basis of OCT is
low-coherence interferometry. In conventional interferometry with long coherence length, for
example laser interferometry, interference of light occurs over a distance of
meters. In OCT, this interference is limits within the range of micrometers,
owing to the use of broad-bandwidth light sources.

Light in an OCT
system is broken into two arms—a sample arm (containing the item of interest)
and a reference arm (usually a mirror). The combination of reflected light from
the sample arm and reference light from the reference arm gives rise to an
interference pattern, but only if light from both arms have traveled the
“same” optical distance (“same” meaning a difference of
less than a coherence length). By scanning the mirror in the reference arm, a
reflectivity profile of the sample can be obtained (this is time domain OCT).
Areas of the sample that reflect back a lot of light will create greater
interference than areas that don’t. Any light that is outside the short
coherence length will not interfere.16 This reflectivity profile,
called an A-scan, contains information about the
spatial dimensions and location of structures within the item of interest. A
cross-sectional tomograph (B-scan) may be achieved by laterally
combining a series of these axial depth scans (A-scan). A face imaging at an
acquired depth is possible depending on the imaging engine used.

2.   Principle of Operations

The function
principle of OCT imaging is light interference. Hence, a light interference
setup is a core requirement of any OCT system. There are many types of
interference configurations. In OCT system, the light from a low coherence
source is split into two paths by a coupler directing it along two different
arms of an interferometer. One arm is a reference arm and other is a sample
arm. When the light exits the fiber end of either arm, it is shaped by
various optical components such as mirror and lens to control specific beam
parameters such as shape, depth of focus and the intensity distribution of the
light. In the reference arm, the light is back-reflected by a reference mirror
and it returns into the interference system. Likewise, the same process happens
in the sample arm. The returning light from both arms recombine at the coupler
and generate an interference pattern and it is recorded by the detector.

For a particular position of the
reference mirror, the light propagating in the reference arm travels a certain
optical distance and forms the corresponding interference pattern only with
light that travelled the same optical distance along the sample arm, including
the portion of the distance travelled inside the sample. Therefore, when the
reference mirror is translated along the propagation direction of light, for
different positions of the mirror, the returning reference generates
interference patterns with light backscattered from corresponding depths within
the sample. From this, the dependence on depth of the intensity of light
backscattered from below the sample surface can be measured.

depth scan or an A-scan is known
as the OCT signal recorded by the detector during a complete travel of the
reference mirror. To form an OCT image, the sample beam has to be translated
across the sample surface and also A-scan should be recorded at each position
of the beam. A set of consecutive A-scans that are obtained forming an OCT
image is called the B-Scan. Generally, the sample itself affects the maximum
possible depth that can be probed by an OCT imaging system.

Low-coherence semiconductor Super – Luminescent
Diode (SLD) is the light source in an OCT system. The characteristics of the
SLD are an important design parameter since the axial resolution, also known as
the depth resolution or the coherence gate, is the coherence length of the
source, an intrinsic parameter of the source which is inversely proportional to
its spectral bandwidth. When the spectral distribution of the SLD is Gaussian,
the axial resolution ?z is given as:

Where:

? – Central wavelength of SLD

?? – Bandwidth of SLD

Therefore, broadband optical
sources are required in order to achieve high axial resolution. Axial
resolutions in the micron and sub-micron range can be achieved by using sources
with very large spectral band such as femto-second pulsed lasers or white-light
sources such as halogen lamps.

The axial and transverse
resolutions are independent in OCT imaging. The transverse resolution is
determined by the minimum spot size of the focused probing beam, a parameter
which is inversely proportional to the numerical aperture (NA) of the focusing
lens:

In relation (2), d is the spot size of the probing beam as it is
projected on the objective lens and f is the focal length of
the objective lens, i.e. the lens or lens system located at the end of the
sample arm. An important detail that needs mentioning is that there is a
trade-off between transverse resolution and depth of field. A high numerical
aperture features a great focusing power translating into excellent transverse
resolution, i.e. small diameter of the focused beam, with a corresponding short
depth of field. Meanwhile, a low NA offers a greater diameter
of the beam at the focal point but a large depth of field. Most OCT imaging is
performed with a low NA lens in order to ensure a depth of
field of the order of millimetres, much longer than the coherence length of the
source. Commercial OCT systems typically use transverse resolutions of
20–25 ?m.

3.   Quality Assurance

Quality Index(QI) is an OCT image quality parameter
that is used to assess the quality of the image. QI can be calculated based on
image histogram information using a software program. As a first step to find
the Quality Index, four new parameters were identified based on reflectivity
values of the pixels that comprise each image.

·      Low: the first (lowest) percentile
of all recorded reflectivity values in a given image.

·      Noise: the reflectivity value that
corresponds to the 75th percentile of all recorded reflectivity values in a
given image. (Signal values up to this point in the scan were considered
extraneous signal from sources other than the retina).

·      Saturation: the reflectivity value
representing the 99th percentile of all recorded reflectivity values in a given
image.

·      Middle: the mean value of noise and
saturation.

Quality Index is comprising of two
components. They are Intensity Ratio(IR) and Tissue Signal Ratio (TSR). These
two components are calculated by the above four parameters.

Intensity Ratio (IR)

 

IR is analogous to signal to
noise ratio (SNR) provided by the manufacturer. The manufacturer’s SNR is the
maximum SNR value among all a?scans. IR is calculated by information on histogram
that takes entire image into account. SNR calculation requires preprocessed
signal data that are proprietary and not available to OCT users. We therefore
calculated an SNR equivalent, IR, using OCT raw data that any user can export.

(2) Tissue signal
ratio (TSR):

This calculates the ratio of the number of highly
reflective pixels versus those with lower reflectivity. The numerator is the
number of pixels between the “Middle” and “Saturation” intensity values. The
denominator is the number of pixels between the “Noise” and “Middle” intensity
values.

Our new parameter, QI, was then constructed as the product
of the IR and the TSR described above:

Ultrahigh resolution
is obtained in the ophthalmic imaging using OCT. The axial resolution is
approximately 3m, which is determined by measuring the full-width half maximum
of isolated reflections from structures in the retina. Index of refraction
normalizes the axial dimension of the OCT
image to convert optical delay into geometrical distances. The transverse
resolution is determined by aperture of the pupil, which is approximately 15 ?m
in all of the retinal OCT images. Ultrahigh-resolution OCT enables visualization of the foveal and
optic disc contour, as well as internal architectural morphology of the retina
and choroid that is not resolvable with conventional resolution OCT. Ultrahigh-resolution
OCT images can be processed using segmentation algorithms to identify and
quantitatively measure intraretinal structure relevant for the early diagnosis
and monitoring of ophthalmologic diseases. In addition to the retina,
ultrahigh-resolution OCT can also image the anterior segment with unprecedented
resolution. In this case, the transverse resolution can be improved
significantly because the pupil does not limit the numerical aperture for beam
focusing. An in vivo ultrahigh-resolution OCT
image of the cornea of a normal human subject with approximately 2 ?m axial and
6 ?m transverse resolutions is shown in Fig. 4. The corneal
epithelium, Bowman’s layer, intrastromal morphology (for example, corneal
lamellae) can clearly be differentiated.

 

 

Retinal nerve
fiber layer (RNFL) thickness, a measure of glaucoma progression, can be
measured in images acquired by spectral domain optical coherence tomography
(OCT). The accuracy of RNFL thickness estimation, however, is affected by the
quality of the OCT images. In this paper, a new parameter, signal deviation
(SD), which is based on the standard deviation of the intensities in OCT
images, is introduced for objective assessment of OCT image quality. Two other
objective assessment parameters, signal to noise ratio (SNR) and signal
strength (SS), are also calculated for each OCT image. The results of the
objective assessment are compared with subjective assessment. In the subjective
assessment, one OCT expert graded the image quality according to a three-level scale
(good, fair, and poor). The OCT B-scan images of the retina from six subjects
are evaluated by both objective and subjective assessment. From the comparison,
we demonstrate that the objective assessment successfully differentiates
between the acceptable quality images (good and fair images) and poor quality
OCT images as graded by OCT experts. We evaluate the performance of the
objective assessment under different quality assessment parameters and
demonstrate that SD is the best at distinguishing between fair and good quality
images. The accuracy of RNFL thickness estimation is improved significantly
after poor quality OCT images are rejected by automated objective assessment
using the SD, SNR, and SS.