Ophthalmoogy

1. An Introduction to Optical Coherence
Tomography
Eranda Wannigama

2. Overview
• History and evolution.
• What is OCT ?
• OCT Physics
• What is OCT Angiography
• Interpretation of OCT.
• Limitations.

3. History
• OCT was first introduced in 1991 and has found many uses outside of
ophthalmology, where it has been used to image certain non-transparent
tissues.
• Due to the transparency of the eye (i.e. the retina can be viewed through the
pupil), OCT has gained wide popularity as an ophthalmic diagnostic tool.
Optical coherence tomogram of a
fingertip. It is possible to observe
the sweat glands, having
"corkscrew appearance"

4. James G.
Fujimoto, 1991

5. What is OCT ?
• Optical Coherence Tomography (OCT) is a non-invasive,
noncontact, technique for obtaining sub-surface images of
translucent or opaque materials at a resolution equivalent to a low-
power microscope.
• Reflected light is used instead of sound waves(It is an ‘optical
ultrasound’, imaging reflections from within tissue to provide
cross-sectional images)
• it provides tissue morphology imagery at much higher resolution
(better than 10 µm) than other imaging modalities such as MRI or
ultrasound.

6. Comparison of various imaging methods according to their resolutions and penetration depths.
The length of the “pendulum” represents the penetration depth while the diameter of the circle
represents the spatial resolution of the imaging

7. • Piezoelectric effect( curie brothers 1880)
• Came in to ophthalmology in 1956
• Resolution 50 micro m
• High frequency 35-50 MHz
• Can combine with Doppler / movement and flow within a tissue / colour
coded for high and low flow
• Penetrates hazy media
B -SCAN

8. • Contact
• Poor resolution
• Can image when media hazy
• Less costly
B-SCAN VS OCT

9. The key benefits of OCT over B scan:
Advantages
• Non-contact
• Minimal cooperation needed
• Resolution ~ 10 μm
• Pick up earliest signs
of disease
• Quantitatively monitor
disease/staging
Disadvantages
• Best for optically transparent tissues
• Diminished penetration through
Retinal/subretinal hemorrhage
• Requires pupil diameter > 4 mm

10. • Fiber-based OCT system in a Michelson configuration. The light (Infrared light
840nm)from a low-coherence source is split in two by the coupler with each part
traveling along a separate arm of the interferometer,
• the reference and the sample arm. The light backscattered from the reference
mirror and from the sample recombine at the coupler and generate an interference
pattern, which is recorded by a single point detector
OCT Basic principle

11. • Interferometry is used to record the optical path length of received
photons allowing rejection of most photons that scatter multiple times
before detection.
• Thus OCT can build up clear 3D images of thick samples by rejecting
background signal while collecting light directly reflected from
surfaces of interest.

12. Axial resolution, or
definition,
determines which
retinal layers can be
distinguished. Axial
resolution is determined
by the
light source.
Transverse resolution is determined by optics of the eye, as limited by
pupil size, and as corrected by the scanner

13. OCT Platforms
• Time domain (TD-OCT)
• Fourier-domain OCT (FD-OCT).
Fourier-domain OCT imaging can also be done in two ways:
spectral-domain OCT (SD-OCT)
swept-source OCT (SS-OCT).
• High definition- HD
• Array of detectors/ 100000 scans per second ( 200 times faster than
time domain)

14. 1. Time Domain OCT
• In TD-OCT a mirror in the reference
arm of the inter-ferometer is moved
to match the delay in various layers of
the sample
• The resulting interference is
processed to produce the axial scan
waveform.
• The reference mirror must move one
cycle for each axial scan. The need
for mechanical movement limits the
speed of image acquisition.

15. Fourier-domain OCT (FD-OCT)
• In FD-OCT the reference mirror is kept
stationary. The spectral pattern of the
interference between the sample and the
reference reflections is measured • The spectral
interferogram is fourier transformed to provide
an axial scan.
• The absence of moving part allows the image to be
acquired very rapidly .
• the full spectral bandwidth sets the axial
resolution.
• Reflections from all layers in the sample are
detected simultaneously.
• This parallel axial scan is much more efficient,
resulting in both greater speed and higher signal-
to-noise ratio.

16. TD-OCT vs FD-OCT
• Spectral domain measures retinal thickness from RPE to ILM
• Time domain measures retinal thickness from IS/OS to ILM

17. Swept-source OCT (SS-OCT)
• SS-OCT has improved acquisition
speed, volume and depth of ocular
tissue measurements compared with
SD-OCT technology.
• has a scan speed of 100,000 A-scans
per second and uses a 1050nm
wavelength to pass through
cataracts and retinal hemorrhages.
• It is the first ophthalmic diagnostic
technology to demonstrate the entire
structure of the posterior
precortical vitreous pocket
(PPVP) in vivo.
.

18. Swept-source OCT (SS-OCT)
• The roles of the PPVP in
physiological posterior vitreous
detachment and Vitreoretinal
interface disorders have now been
elucidated
• Deeper penetration of SS-OCT has
made it possible to view the choroid.
• It also has an important role in
central serous chorioretinopathy
and uveitis.
• Treat Harada disease by monitoring
the choroidal thickness.

19. OCT Angiography
• Optical coherence tomography
angiography (OCT-A) emerged as a
non-invasive technique for imaging
the microvasculature of the retina
and choroid, and the first clinical
studies using this innovative
technology were published in 2014.
OCT-A technology uses laser light reflectance of the
surface of moving red blood cells to accurately
depict vessels through different segmented areas of
the eye, thus eliminating the need for intravascular
dyes

20. OCT interpretation
2 MODES OF INTERPRETATION - Objective & Subjective
For accurate interpretation both have to be combined
OCT reading must be done in 2 stages :
1.Qualitative and quantitative analysis
2.Deduction and synthesis

21. Qualitative Analysis
• Morphological studies -
- Overall retinal structural changes, changes in retinal outline , retinal
structural changes and morphological changes in the post layers
-Anomalous structures- pre/epi/intra/sub retinal
• Reflectivity study - hyper/hypo/ shadow areas
Quantitative Analysis
• Thickness, Volumetery and shadow areas

22. Color coding in OCT
Those with low reflectivity are represented by dark colors (black and blue).
photoreceptor layer,choroid,vitreous fluid or blood
Intermediate reflectivity is shown Green.
Highly reflective structures are shown in bright colures (white and red)
Nerve Fiber Layer, RPE,choriocapillaris

23. Interpretation of retinal scan
• Vitreous anterior to retina is non reflective and is seen as a dark space.
• Vitreo retinal interface is well defined due to contrast between the non
reflective vitreous and backscattering retina

24. 1. Anterior boundary of retina formed by highly reflective RNFL is seen as a red layer due to
bright back scattering.
2. Posterior boundary of retina is also seen as a red layer representing highly reflective (RPE) and
chorio capillaries
3.Outer segment of retinal photoreceptors, being minimally reflective are represented by dark layer
just anterior to RPE
4.Different intermediate layers of neurosensory retina seen as an alternating layer of moderate and
low reflectivity.
Retinal layers are represented as below

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Macular Change Analysis
ETDRS grid with thickness values is
overlaid on retinal thickness maps.

28. Macular Thickness Normative data
Macular thickness is compared to an
age-matched normative database as
indicated by a stop-light color code

29. Automatic fovea finder
Macula Thickness Analysis is
aligned with fovea location
(left)

30. xxx

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32. Advanced Vaisualization
The Tissue Layer image allows you to isolate
and visualize a layer of the retina. The thickness
and placement of the layer are adjustable. This
provides an optical biopsy of the retina by
extracting the layer of interest

33. Each high definition line is comprised of 4096 A-scans
Rotation, length of lines and height of scan area can be adjusted
Custom 5-Line Raster Scan

34. Limitations
• OCT utilizes light waves (unlike ultrasound which uses
sound waves) media opacities can interfere with optimal
imaging.
eg; vitreous hemorrhage, dense cataract or corneal
opacities.
• Patient movement can diminish the quality of the image.
• The quality of the image is also dependent on the
operator.