Optical Coherence Tomography By DR Tushya Om Parkash, Vydehi institute of medical sciences,Bangalore

1. OPTICAL COHERENCE TOMOGRAPHY
Dr Tushya Om Parkash
Dr Om Parkash Eye Institute

2. INTRODUCTION
• OCT is a non contact , non invasive , micron resolution cross sectional
study of retina which correlates very well with the retinal histology
• It was unbelievable that histopathology without biopsy of a structure
which was literally untouchable.(Retina)

3. It goes back in 1991 when first OCT paper was published by Huang et al
First in-vivo studies of human retina started in 1993
Evolution
HISTORY OF OCT

4. 10 years of progress in OCT Imaging
1998 2002 2008
A-Scans/sec 100 400 27,000
Axial resolution 15 microns 10 microns 5 microns
Contrast &
Image quality + +++ +++++

5. OCT
1995
OCT2 2000
OCT3
Stratus OCT
2002
Cirrus HD-OCT
2007
100 A-scans x
500 points
100 A-scans x
500 points
512 A-scans
x1024 points
4096 A-scans x
1024 points
100
100
500
27,000
20
20
10
5
Single line scan
Scans/
second
Resolution
(microns)

6. OCT VS USG
• OCT image has a resolving power of about 10 microns vertically
and 20 microns horizontally
• Compare that to the resolution of a good ophthalmic ultrasound
at 100 microns
• USG needs contact with the tissue under study whereas OCT does
not require any contact

7. Optical Coherence Tomography
THE PRINCIPLE
• 2 or 3 dimentional cross sectional imaging of retina by
measuring echo delay and intensity of back reflected infra red
light from internal tissue structures
• Combination of low coherence interferometry with a special broadband

8. Based on Principle of Michelson Interferometry
• Low coherence infra red light
coupled to a fibre optic travels
through beam splitter and is
directed through the ocular media
to the retina
and a reference mirror
• The distance between the beam
splitter and the reference mirror is
continuosly varied
• When the distance between light
source and retinal tissue =
distance
between light source and
reference mirror , the reflected light
and the
refrence mirror interacts to
produce an interference pattern

9. TYPES OF OCT
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
• Further more, at each moment the detection system only collects signal from

10. 2. FOURIER/SPECTRAL DOMAIN 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
• Furthermore,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

11. Difference between Time and spectral domain
• Spectral domain mesures retinal thickness from RPE to ILM
• Time domain meares retinal thickness from IS/OS to ILM

12. THE OCT MACHINE
THE OCT SYSTEM comprises
• Fundus viewing unit
• Interferometric unit
• Computer display
• Control Panel
• Color inkjet printer

13. PROCEDURE
• Patient is asked to look inside the ocular lens - internal fixation-onto
the green target light inside the red rectangular field or external
fixation- onto the external target by the other eye in patients with
poor vision. The patient is encouraged to blink in between scan
acquisition.
• There is no discomfort to the patient and an experienced operator
can acquire the required scans within 1-3 mins in each eye.
The actual time taken by the machine is 1 sec - the additional time
is for patient positioning and optimising scan quality.

14. PROCEDURE
• Switch on the system: This activates all the components and takes
45 secs to start window
• The menu and toolbar in the main window has several options inclu
-ding select patient,acquisition protocol,analysis protocol
• Appropriate category can be selected. Data entry made for a new
patient. Apprpriate protocol is selected

15. PROCEDURE
• A 3mm pupil is necessary for adequate visualisation
• Patient is seated with his chin on the chin rest and eye at the level
of the mark on the side of the frame
• Once the patient is seated comfortably, the OCT machine is moved slowly • Then the z offset of the image is optimised to bring the image to the
centre. The polarisation is optimised next to create a clear image
• The signal strength of 5 and above gives a clear image

16. PROCEDURE
• Normally the patient can look at this field for several minutes at a
time without discomfort
• During scan alignment , the patient sees the scan pattern in motion
on the red field.
• During scan acquisition , the patient sees a bright greenish-white
flash light, when the scan image is stored into the camera
• It is possible to acquire the scans without the flash, which is more comfortable

18. PRODUCTION AND DISPLAY OF IMAGE
• On Z axis, 1024 points are captured over a 2mm depth to create a
tissue density profile, with resolution of 10microns
• On X-Y axis, the tissue density profile is repeated unto 512 times
every 5-60 microns to generate cross sectional image. Several data
points over 2mm of depth are integrated by the interferometer to
construct a tomogram of retinal structures.
• Image thus produced has an axial resolution of 10 microns and a
transverse resolution of 20 microns
• The tomogram is displayed in either grey scale or false scale on
a high resolution computer screen.
• X and Y(north-south and east-west) and Z axis(depth)

19. OCT
• The Interference is measured by a photodetector and processed
into a signal. A 2D image is built as the light source moves along
the retina , which resembles a histology section
• Digital processing aligns the A scan to correct for eye motion.
Digital smoothing techniques further improve the signal to noise
ratio
• The small faint bluish dots in the pre-retinal space is noise
• This is an electronic aberration created by increasing the sensitivity
of the instrument to better visualise low reflective structures
• Intraretinal cross sectional anatomy is displayed with an axial
resolution <10 microns and transverse resolution of 20 microns

20. • The Interferometer integrates several data points over 2mm depth
to construct a tomogram of retinal structures
• It is a real time tomogram with false colours
• Different colours represent degree of light scattering from different depths • Highly Reflective structures are shown in bright colours (white and
red) and those with low reflectivity are represented by dark colours
(black and blue). Intermediate reflectivity by green colour

21. High definition and High resolution
Axial resolution,or
definition,
determines which
retinal layers can be
distinguished. Axial
resolution is
determined by the
light source.
Transverse resolution determines accuracy with which size and separation of
features (such as drusen) can be identified. Transverse resolution is determined
by optics of the eye, as limited by pupil size, and as corrected by the scanner.

22. INTERPRETATION OF OCT IN CLINICAL CONDITIONS

23. TYPES OF MACHINE SCANS
• Posterior segment scan
1.Macular cube scan
2.Glaucoma RNFL thickness analysis scan
• Anterior segment scan

24. 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

25. OCT INTERPRETATION
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

26. 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.

27. • Retinal layers are represented as below
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 retinal pigment epithelium(RPE) and chorio capillaries
3.Outer segment of retinal photoreceptors, being minimally reflective are
represented by dark layer just anterior to RPE-Choriocapillaries complex
• Different intermediate layers of neurosensory retina between the dark
layer of photoreceptors and red layer of RNFL are seen as an alternating
layer of moderate and low reflectivity

28. Cirrus HD-OCT Healthy Macula
NFL ILM GCL
IPL INL OPL ONL
ELM IS IS/OS OS RPE
Choroid
• NFL and plexiform layers are highly reflective due to horizontal oriented axonal structure
• RPE and Choriocapillaries due to high melanin and vascular content respectively
• Retinal thickness is directly proportional to reflectivity
• IS/OS junction is also hyper reflective and plexiform layers to some extent
• Reflectivity Red-green-yellow-blue-black
• 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.

29. The Foveal Profile
The normal foveal profile is a slight depression in the surface
of the retina

30. • Hyperreflective areas(white and red)
1.Superficial- ERM, Haemorrhage,cotton-wool spots
2.Intraretinal- hard exudates,haemorrhage
3.Deep- Drusen,SRNV,nevi,RPE Hyperplasia
• Hyporeflective areas(black and blue)
1.Intraretinal- Fluid,Cysts
2.Deep- RPE Detachments
3.Shadow areas- screened
4.Anterior-Asteroid bodies, Vitreous Haemorrhage

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

32. Macular Change Analysis
ETDRS grid with thickness values is
overlaid on retinal thickness maps.
Change analysis map shows variance
from baseline, in micrometers, and
represented in color

33. Advanced Visualization
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

34. Automatic fovea finder

35. Automatic fovea finder
Fovea center = 255, 71 Scan center = 255, 64
Macula Thickness Analysis is
aligned with fovea location
(left)
Resulting analysis may differ
from analysis aligned on
scan center (right)

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

38. REGIONS
For purpose of analysis , the OCT image of the retina can be
subdivided vertically into four regions
• The Pre-retina
• The Epi-retina
• The Intra-retina
• The Sub-retina

39. INDICATIONS FOR POSTERIOR SEGMENT SCAN
1. Anomolous structures seen in pre retinal area
• Epi retinal membrane
• Vitreo-retinal traction
• Posterior vitreous detachment

40. 2. Deformations in foveal profile
• Macular pucker
• Macular pseudo-hole
• Macular lamellar hole
• Macular cyst
• Macular hole, stage 1(no depression,cyst present)
• Macular hole stage 2(partial rupture of retina,increased thickness)
• Macular hole stage 3(extends to RPE,increased thickness,some fluid)
• Macular hole,stage 4(complete hole,edema at margins, complete PVD)

41. 3. Intraretinal and subretinal anomalies
• Choroidal neovascular membrane
• Diffuse intraretinal oedema
• Cystoid macular edema
• Drusen
• Hard exudates
• Scar tissue
• Atrophic degeneration’
• Sub-retinal fibrosis
• RPE tear

42. Posterior Vitreous Detachment
• Syneresis of vitreous gel
• liquified vitreous gains entry to the retro hyaloid space through a defect in the
posterior hyaloid face
• Seen as thin faint hyper reflective line above the surface of the retina

43. Macular Hole stage 1
• Hyperreflective Vitreo macular traction band
• With fovea and foveolar detachment causing a schisis cavity

44. Macular stage 2
• Dehiscence of the wall of schisis cavity associated with vitreomacular traction

45. Macular stage 3
• Formation of operculum

46. Macular stage hole 4
• Full thickness macular hole with complete posterior vitreous detachment

47. Epi-Retinal Membrane
• Normal foveal contour is lost
• HYper reflective band seen over the ILM suggestive of epi retinal membrane

48. Epiretinal Membrane HD Images
Thickness
Map Overlay
Fundus Image
OCT Image
Thickness
Map
ILM Layer
Fly-through movie

49. Retinal pigment epithelial detachment
• Loss of normal foveal contour
• Non reflective area suggesting of serous fluid which is causing the PED

50. 3D Volume Rendering
Cube can be
manipulated for
visualization of
various aspects
Advanced Visualisation

51. Advanced visualisation
3D Volume Rendering
with RPE layer exposed

52. Cystoid macular edema
• Loss of foveal contour
• Increased thickness in neurosensory retina
• Cystoid spaces- honey comb like

53. Central sereous chorioretinopathy
• Loss of normal foveal contour
• Non reflective area which is separating the neurosensory retina from RPE

54. Choroidal neovascular membrane
• Hyperreflective band beneath the RPE causing its detachment
• Posterior shadowing towards the choroid suggestive of cnvm

55. Sub retinal fibrosis
• Hyperreflective band seen beneath the RPE with irregularity of RPE

56. Drusens- seen between bruch’s membrane and RPE
• hyperreflective bumpy RPE with localised PED

57. NORMAL OCT OF OPTIC DISC
• TSNIT graph
• Double Hump PAttern
• OCT helps in detecting RNFL loss even with no VF defects in disc suspects and
ocular hypertensives(in stages of undetectable and asymptomatic) before
progressing to stage of functional impairment
• Clinically inferior and average RNFL thickness are most commonly used as
baseline measurement and follow up of glaucoma suspects
• In manifest glaucoma patient, RNFL region with least measurements is followed
up

58. NORMAL OCT OF OPTIC DISC
• The ONH analysis depends upon automated detection of the ends of RPE by
software and the distance between these ends is taken as the optic disc
diameter

59. Glaucoma - RNFL thickness analysis
Identifying and Monitoring RNFL Loss
OPTIC DISC CUBE SCAN
The 6mm x 6mm cube is captured with
200 A-scans per B-scan, 200 B-scans.
CALCULATION CIRCLE
AutoCenter™ function automatically centers the
1.73mm radius peripapillary calculation circle
around the disc for precise placement and
repeatable registration. The placement of the
circle is not operator dependent. Accuracy,
registration and reproducibility are assured.

61. DIFFICULTIES AND LIMITATIONS
• Limited by intraocular media opacities , which attenuate • Non cooperative patient
• Expensive

62. ARTIFACTS
• Artifacts in the OCT scan are anomalies in the scan that are not accurate • Notice the large gap in the middle of the scan below. This is an artifact caused

63. The scan below has waves in the retinal contour. These are not retinal

64. Anterior Segment OCT

65. Cirrus HD-OCT Anterior Segment Imaging, a new
indication for use, received FDA clearance in May,
2009.
“…It is indicated for in-vivo viewing, axial cross-sectional,
and three-dimensional imaging and measurement of anterior
and posterior ocular structures, including cornea, retina,
retinal nerve fiber layer, macula, and optic disc. . .”
Two new scan patterns
Anterior Segment 5-line raster 3
mm length, adjustable rotation and
spacing
Anterior Segment 512x128 cube
scan. 4mmx 4mm

66. INDICATIONS FOR ANTERIOR SEGMENT SCAN
•Mapping of corneal thickness and keratoconus evaluation
•Measurement of LASIK flap and stromal bed thickness
•Visualization and measurement of anterior chamber angle and diagnosis •Measuring the dimensions of the anterior chamber and assessing
the fit of intraocular lens implants
•Visualizing and measuring the results of corneal implants and lamellar •Imaging through corneal opacity to see internal eye structures

68. Cirrus HD-OCT scan of normal cornea. Layers identified with colored arrows as follows: tear film (blue),
epithelium (white), Bowman’s layer (red), Descemet’s/endothelium (green).

69. an anterior-chamber angle as viewed with gonioscopy Scleral spur is more reflective
Ciliary body is less reflective

71. Corneal ectasia
• Diffuse corneal thinnig probabaly suggestive of Post lasik ectasia

72. Keratoconus
• Conical cornea with central stromal thinning

73. Tumor of the iris
• Obscuring the angle

74. Tumor of ciliary body

75. Narrowing of angle of anterior chamber

76. •Scleral spur (red arrow)more
reflective
•Schlemm’s canal (blue arrow)
•Schwalbe’s line (green arrow)

77. RECENT ADVANCES
• OPMI LUMERA 700 and RESCAN 700 from ZEISS
Now with integrated intraoperative OCT(Inbuilt OCT in microscope)
A new dimension in visualization
Innovation in eye care starts with the desire to see more. With the first surgical microscope See more:
• during surgery
• with real-time HD-OCT
• for better decision making

78. THANK YOU