OCT uses interferometry to perform non-invasive imaging of biological tissues. The first OCT images of the retina were obtained in 1990. Time domain OCT works by scanning a reference mirror to measure echo time delays, while Fourier domain OCT measures spectral interference patterns without scanning. Fourier domain OCT allows for much faster acquisition speeds compared to time domain OCT. Integrating OCT with scanning laser ophthalmoscopy enables localization of OCT scans on fundus images.

2. Introduction
 OCT is noncontact noninvasive technique for imaging
biological tissues.
 In 1990, Fercher presented cross-sectional
topographic image of the retinal pigment epithelium
(RPE) of a human eye.
 The first commercial instrument, OCT 1,was launched
in 1996.

5. Scattering is a fundamental property of a
heterogeneous medium, and occurs because of
variations in the refractive index within tissue.
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6. Principle of OCT
 Interferometry is the
technique of superimposing
(interfering ) two or more
waves, to detect differences
between them.
 Interferometry works
because two waves with the
same frequency that have
the same phase will add
each other while two waves
that have opposite phase
will subtract.

7.  Light from a source is
directed onto a partially
reflecting mirror and is
split into a reference
and a measurement
beam.
 The measurement beam
reflected from the
specimen with different
time delays according to
its internal
microstructure.

8.  The light in the reference
beam is reflected from a
reference mirror at a
variable distance which
produces a variable time
delay.
 The light from the
specimen, consisting of
multiple echoes, and the
light from the reference
mirror, consisting of a
single echo at a known
delay are combined and
detected.

9. Formation of OCT image

10. A-SCAN, B-SCAN, 3D-SCAN
B-scan
A-scan

12. Time Domain OCT
 The Michelson interferometer splits the light from the
broadband source into two paths, the reference and
sample arms.
 The interference signal between the reflected reference
wave and the backscattered sample wave is then recorded.
 The axial optical sectioning ability of the technique is
inversely proportional to its optical bandwidth.

13. Time Domain OCT
 Transverse scanning of the sample is achieved via
rotation of a sample arm galvonometer mirror.
 In order to measure the time delays of light echoes
coming from different structures within the eye, the
position of the reference mirror is changed so that the
time delay of the reference light pulse is adjusted
accordingly

14. Fourier domain OCT
 In FD-OCT ,the detector arm of the Michelson
interferometer uses a spectrometer instead a single
detector.
 The spectrometer measures spectral modulations
produced by interference between the sample and
reference reflections.

15.  No physical scanning of the reference mirror is
required; thus, FD-OCT can be much faster than
TDOCT.
 The simultaneous detection of reflections from a
broad range of depths is much more efficient than
TD-OCT, in which signals from various depths are
scanned sequentially.
 FD-OCT is also fast enough for sequential image
frames to track the pulsation of blood vessels during
the cardiac cycle.

16.  Time domain Optical
Coherence tomography:
 Fourier domain Optical
Coherence tomography:

17. Time vs Fourier domain OCT
Time domain OCT
 A scan generated
sequentially, one pixel at a
time of 1.6 seconds
 Moving reference mirror
 400 scans/sec
 Resolution – 10 micron
 Slower than eye movement
Fourier domain OCT
 Entire A scan is generated at
once based on Fourier
transformation of
spectrometer analysis
 Stationary reference mirror
 26,000 scans/sec
 Resolution – 5 micron
 Faster than eye movement
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18. Spectral OCT/SLO
 Limitation of OCT technology was difficulty in
accurately localizing the cross-sectional images
and correlating them with a conventional en face
view of the fundus.
 To localize and visually interpret the images,
integrating a scanning laser ophthalmoscopy
(SLO) into the OCT was needed.
 This rationale was used by OTI technologies
(Toronto, Canada) to develop the Spectral
OCT/SLO.

19.  The Spectral OCT/SLO is a computerized optical
scanner device providing high-resolution, high-definition
images of the fundus anatomy.
 It integrats SLO’s confocal imaging principles with
OCT’s high resolution tomographic images.
 The system simultaneously produces SLO and OCT
images that are created through the same optical path,
and therefore correspond pixel to pixel.
 It produces a new image format called as C scan

21.  The technique has already become
established as a standard imaging modality
for imaging of the eye.
 The application of OCT imaging to other
biomedical areas such as endoscopic imaging
of gastro-intestinal and cardiovascular
systems is currently an active field of
research.