Oct(optical cohorence tomography)

1. Principles
of
Optical coherence tomography (OCT)
DR MD AFZAL MAHFUZULLAH
MCPS,FCPS,FELOW VITREO-RETINA
ASSISTANT PROFESSOR(VITREO-RETINA)
BANGABANDHU SHEIKH MUJIB MEDICAL UNIVERSITY

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.

3. Basic Principles of Optical Coherence Tomography.
It accomplishes this by directing –
A beam of near-infrared light from a broadband coherent light source
at target tissue.
Capturing light that is back-scattered from that tissue.

4. Scattering is a fundamental property of a
heterogeneous medium, and occurs because of
variations in the refractive index within tissue.
4

7. 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.

8.  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.

9.  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.

10. Formation of OCT image

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

13. 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.

14. 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

15. 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.

16.  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.

17.  Time domain Optical Coherence
tomography:
 Fourier domain Optical Coherence
tomography:

18. 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

19. Spectral OCT
 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.

20.  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

22.  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.
Applications: