Oct

1. OPTICAL COHERENCE
TOMOGRAPHY
Arjun Sapkota
Optometry
Maharajgunj
Medical Campus

2. Introduction
History
Theories & Principles
Types
Interpretation
Clinical Applications
Limitations & Advantages
Latest Developments
Outline

3. INTRODCUTION
OPTICAL

LIGHT
COHERENCE 
SPECEFIC
PROPERTY OF
LIGHT
(MONOCHROMAT
C LIGHT )
TOMOGRAPHY 
LOOKING THE
TISSUE IN SLICES
(CROSS SECTIONS )

4. Optical Coherence Tomography (OCT) is an optical
imaging modality that uses to
create high-resolution images of tissue microstructure
• non-contact, noninvasive imaging technique
• Reflected light is used instead of sound waves
• Infrared ray of 830 nm with 78D internal lens
•  Safe, non-ionizing radiation
• Image resolutions of 1–15 μm
Key
Features:
OCT

5. Analog to
ultrasound
OCT- the process is similar to that of USG,
except that light is used instead of sound

6. Qualitative
description by location,
a description of form & structure,
identification of anomalous structures,&
observation of the reflective qualities of the retina
Allows both qualitative & quantitative
analysis

7. Specifically retinal thickness & volume
Nerve fiber layer thickness
Possible due to the OCT software is able to
identify & trace two key layers of the
retina,the RNFL& RPE
Quantitative

8. RESOLUTION
When all of the A-scans are combined into one image,
the 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
OCT USG

9. HISTORY- OCT TIMELINE
 1991–Concept of OCT
in ophthalmology
• 1993 - First in vivo
retinal OCT images
• 1994-OCT prototype
• 1994-Anterior
segment/Cornea OCT
• 1995-The First
Clinical Retinal OCT
• 1995-The First
Glaucoma OCT
• 2002 – Time domain OCT (e.g. Stratus)
• 10 µm axial resolution
• scan velocity of 400 A-scans/sec
• 2004 – Concept of spectral domain OCT
introduced
• 2007 – Spectral domain OCT
• 1-15 µm axial resolution
• up to 52,000 A-scans/sec

10. 1998 20082002
A-Scans/sec 100 400 27,000
Axial resolution 15 microns 10 microns 5
microns
Contrast &
Image quality + +++ +++++
10 years of progress in OCT Imaging

11. OCT images obtained by measuring
 echo time
 intensity of reflected light
Effectively ‘optical ultrasound’
Optical properties of ocular tissues, not a true
histological section
Principle

12. Laser output from OCT is low, using a near-infra-
red broadband light source
Measures backscattered or back-reflected light
Source of light: 830nm diode laser
1310 nm : AS-OCT

13. • 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

14. Light from Reference arm &
Sample arm combined
Division of the signal by
wavelength
Analysis of signal
Interference pattern
A-scan created for
each point
B-Scan created
by combining A-scans

15. RESOLUTION OF AN OCT
Axial resolution
-Wavelength and
-Bandwidth of the light source
Long wavelength - visualisation of
choroid, laminar pores, etc
 Transverse resolution
•Based on spacing of A-scans
•Limited by optics of eye and
media opacity

16. 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
High definition & High resolution

17. Speed of acquisition
 Faster acquisition speed in the newer generation
OCT
 Increased signal-noise ratio
 Reduced motion artifacts
Spectral domain OCT :1-15 µm axial resolution &
Up to 52,000 A-scans/sec

18. Displacement of mirror
placed on the path of the
reference light beam
Analysis of structures
situated at various depths
during each light echo
acquisition
Time-Domain OCT

19.  Interferences obtained
in the entire spectrum
 Interference signal -
function of wavelength
 All the echoes of light from
the various layers of the
retina measured
simultaneously
Spectral-Domain
OCT

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

21. Optovue and Cirrus : Anterior eye imaging capabilities in
addition to posterior eye
Spectralis : Require special lens and anterior segment
module for anterior eye imaging
Spectral-domain OCTs: –
Spectralis (Heidelberg)
Cirrus (Zeiss)
RTVue (Optovue)

22. SPECTRALIS

23. SPECTRALIS OCT : FEATURES
TruTrackTM technology
Fovea-to-disc (FoDi) alignment

24. TruTrackTM technology
 Two separate laser beams to capture two images
simultaneously
 Reference laser constantly image and track the retinal
vessel location (TruTrack)
 Eliminates motion and blink artifacts

25. Fovea-to-disc (FoDi) alignment

26. SPECTRALIS-ANTERIOR SEGMENT
MODULE
 New dimension to anterior segment imaging
 Cornea
 Angle structure
 Iris details
 Consists of Add-on lens and dedicated software
 Compatible with all SPECTRALIS SD-OCT models

27. 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 of narrow angle glaucoma
 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 procedures
 Imaging through corneal opacity to see internal eye structures

28. Anterior segment imaging OCT was first
demonstrated in 1994 using light of wavelength 830
nm

29. AS-OCT using light of wavelength 1310 nm
Better detail of non-transparent tissues
 increased penetration & illumination power
High-speed Fourier domain optical depth scanning
 Scan speed of 2000 A scans/second
Axial resolution – 18 micron
Transverse resolution – 60 micron
Use of real-time anterior segment optical coherence
tomography

30. 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).
Conical cornea with central stromal
thinning
Tumor of the iris
Obscuring the angleTumor of ciliary body

31. A study comparing AS-OCT with Goniscopy
 AS-OCT detected more closed angles than gonioscopy
Disparity to attributed
 Possible distortion of the anterior segment by contact
gonioscopy
 Differences in illumination
Image shows an AC angle as viewed with gonioscopy & the
OCT
Scleral spur is more reflective
Ciliary body is less reflective

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

33. OCT – POSTERIOR
SEGMENT MODULE
Glaucoma
ONH analysis
Retina
Choroid

34. Retinal nerve fiber layer thickness (RNFLT), optic disc
parameters
Posterior segment lesions like detection of fluid within
the retinal layers or under the retina which may not be
visible clinically
Anomolous structures seen in pre retinal area
Deformations in foveal profile
Intraretinal and subretinal anomalies
Unexplained visual loss
Indiaction For Posterior Segment Scan

35. GLAUCOMA
Diagnosis of glaucoma difficult in early stage
 Infrequency of episodes of rise in the IOP
 Visual field tests not being sensitive enough
Glaucoma diagnosis traditionally performed by
examining
 optic nerve cupping
 width of the neuroretinal rim

36. Visual field loss late clinical findings
Detected only after significant loss of retinal
nerve fibers
Difficult to differentiate early glaucoma from
normal
Limitations of Visual Field Tests:

37. Ganglion cells outside the paramacular region
 Not multilayered
 Early losses more readily detected by VF testing
Not central visual field defects
However, losses of ganglion cells possibly
occur in
 Paramacular region
 Outside the paramacular region simultaneously

38.  Multiple layers of
ganglion cells in the
paramacular &
macular region
 Loss 5 layers of these cells
before the visual fields show
abnormality in central area

39.  Any decrease in the overall retinal thickness
 an indicator of a loss of the ganglion cell layer and RNFL
 OCT detect nerve fiber layer thinning before the onset
of visual changes
 Potential of diagnosing glaucoma early
 examining the retinal thickness in the macular area
 Nerve fiber layer thickness, as measured by OCT, has
been shown to correspond to visual function
ROLE OF OCT IN GLAUCOMA-
RECENT ADVANCES

40. 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

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

43. Circle Scan
Differences betweeen average
thickness in sectors
(along the calculation circle) in
each eye
OCT Scan with automatic
segmentation of RNFL
TSNIT RNFL thickness
compared to narmative database
RNFL Thickness in quadrants
& sectors compared to
normative database

44. Posterior Pole Retinal Thickness
Map with
Compressed Color Scale in
8x8 Analysis Grid
Mean Thickness
Hemisphere Analysis with
Asymmetry Gray Scale
OCT scan of macular region

45.  Combines mapping of the posterior pole retinal
thickness with asymmetry analysis
 Both eyes
 Hemispheres of each eye
Posterior pole asymmetry analysis

46.  Posterior Pole Retinal Thickness
Map- Retinal thickness over the entire
posterior pole for each eye
 Compressed Color Scale
Highlight early retinal loss too small to be
detected with standard color scales
 8x8 Analysis Grid- Positioned along the
fovea to disc axis Mean retinal thickness is
given for each cell
Interpretation of asymmetry
analysis

47. Asymmetry Maps
Compare relative macular thickness between
corresponding grid
Gray Scale
 Gray: thickness less than the corresponding
cell
 White :thickness the same or greater than the
corresponding cell
Hemisphere (S-I and I-S) Asymmetry
Compares thickness of cells between
hemispheres of the same eye
Mean Thickness – Mean retinal thickness for
the entire grid area and for each hemisphere

49. Case 1
 A 53 year old female patient :
glaucoma suspect due to
borderline IOP of 23 mm Hg
 Right optic nerve: 0.5 cup with
an infero-temporal RNFL loss
(arrows)
 The visual fields normal in both
eyes along with the rest of the
eye examination.
Novel Strategies in Glaucoma Diagnosis and Management
Sanjay Asrani, MD,Associate Professor of Ophthalmology
Head, Glaucoma OCT Reading Center, Duke University Eye Center ,
2010

50. Case 2
A 55-year-old female diagnosed with
primary open angle glaucoma OD

51. NEURO-OPHTHALMIC
In the evaluation of ONH
Optic disc edema
Optic neuritis
Optic atrophy

52. RETINA

53. • The vitreous - black space on the top of the image
• fovea - normal depression
• Umbo- central hyper reflective dot within foveola
• The nerve fiber layer (NFL) and the retinal pigment
epithelium (RPE) - highly reflective than the other
layers of the retina ( red – yellow)
• RNFL – thicker on nasal side of macula
• Areas of minimal signals ( blue – black)
• ONL – thickest portion
Retinal anatomy compared to OCT

54. Collective term
 RNFL
 Ganglion cell layer and
 Inner plexiform layer
GCC thought to be affected in early glaucoma
Ganglion cell complex

55. Red & White - High reflectivity
Black & Blue – low reflectivity
Green - Intermediate reflectivity
Pseudo color in OCT

56. Retinal nerve fiber layer
Internal limiting membrane
Junction between inner and outer segments of PRs
Retinal pigment epithelium-Bruch’s membrane –
choriocapillaries complex
Normal ocular tissues which show high
reflectivity (represented by red color
on OCT scans and printouts) are:

57. Inner nuclear layer
Outer nuclear layer
Ganglion cell layers
Photoreceptors
Normal ocular tissues which show low
reflectivity (represented by black
color on OCT scans and printouts)
are:

58. Inner plexiform layers
Outer plexiform layers
External limiting membrane
Normal ocular tissues which show
intermediate reflectivity
(represented by Green color on OCT
scans and printouts) are:

59. Epiretinal and vitreal membranes
Exudates and hemorrhages which produce underlying
shadow effect
Cotton wool spots
High reflectivity
Superficial lesions
Intraretinal lesions
Hemorrhages
Hard exudates
Retinal fibrosis and disciform degenerative scars

60. Drusen
Retinal pigment epithelial hyperplasia
Intraretinal and subretinal neovascular
membranes
Scarring following choroiditis, trauma or laser
treatment
Hyperpigmented choroidal nevi
Deep lesions

61. Atrophic RPE (loss of pigment)
Cystic or pseudocystic areas containing
serous fluid
Cystoid edema, serous neural retinal detachment
and RPE detachment Such lesions appear as
black,optically empty spaces
Hypo or low reflectivity

62. OCT retinal morphology (form and structure)
can be subdivided into four "profiles": Each
profile has it's own set of deformations and
anomalous structures
 1. pre-retinal profile
 2. overall retinal profile
 3. foveal profile
 4. macular profile
Profiles

63. A normal pre-retinal profile is black space
Because the normal vitreous space is translucent,
meaning it has minimal reflective properties
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 visualize
low reflective structures
The pre-retinal profile

64.  Pre-retinal membrane
A pre-retinal membrane with traction on the fovea
2. epi-retinal membrane
3. vitreo-retinal traction
4. vitreo-retinal strands
5. pre-retinal neovascular
membrane
6. pre-papillary neovascular
membrane
Anomalous structures observed in the pre-
retinal profile include

65. The normal over-all retinal profile has a slightly concave curvature
that you would expect from observing the surface of a globe
Abnormal profiles would include exaggerated concavity and
convexity
Retinal folds would also result in an abnormal over-all profile
The over-all retinal profile

66. Demonstrates an abnormal convexity in the over-all
retinal profile.
In this case, a pigment epithelial detachment is causing
the convexity.
Demonstrates an abnormal concavity to the over-all
retinal profile
Aside from the retinal detachment, notice the underlying
concave curvature of the retina, suggesting the long eye
of a significant myope

67. The normal foveal profile is a slight depression in the surface
of the retina
The foveal profile

68.  Macular pucker
Macular lamellar hole
Deformations in the foveal profile

69. Macular hole, stage 4
(complete hole, edema at
margins, complete PVD)
Macular hole, stage 1( no
depression, cyst present)
Macular hole, stage 2 (partial
rupture of retina, incraesed
thickness)
Macular hole stage 3 (hole
extends to RPE, increased
thickness, some fluid)

70. The macular profile can, and often does, includethefovea as it's
center
Therefore, a common OCT scan length of 6 mm would include
3 mm of the macula on each side of the fovea
The macular profile

71. it is a PED because the fluid (black space around the arrow) is pushing up
underneath the retinal pigment epithelium, identified by the relatively highly
reflective (red and orange) line (arrow)
1. serous retinal pigment epithelial detachment
(PED)

72. 2. hemorrhagic pigment
epithelial detachment
3. serous retinal
detachment (RD)

73. Intra-retinal anomalies that have been
identified in the macular profile include:
1. choroidal neovascular membrane
2. diffuse intra-retinal edema
3. cystoid macular edema
4. drusen
5. hard exudates
6. scar tissue
7. atrophic degeneration
8. sub-retinal fibrosis
9. RPE tear

74. Cystoid Macular Edema
OCT is capable of detecting small, fluid-filled,
cystic spaces within the macula

75. Central Serous Chorioretinopathy
Central serous chorioretinopathy is characterized by
the presence of fluid between the RPE and
neurosensory retina

76. Diabetic Retinopathy
Exudates appear as accumulation of dense
material within the neurosensory retina.

77. Patterns of Diabetic macular edema in
OCT:
 Sponge like thickening of retinal layers:
Mostly confined to the outer retinal layers due to backscattering from
intraretinal fluid accumulation
 Large cystoid spaces involving variable depth of the
retna with intervening septae
Initially confined to outer retina mostly
 Serous detachment under fovea
 Tractiional detachment of fovea
 Taut posterior hyaloid membrane

78. is characterized by the presence of fluid between
the RPE and neurosensory retina

79. CSR may be
distinguished from a
PED on OCT by
observation of the
reflective layer
corresponding to the
RPE and choriocapillaris.
 Elevation of this
reflection above an
optically clear space
occurs when the
pigment epithelium is
detached.

80. Drusen
of the
Retina

83. ARTIFACTS
 Notice the large gap in the middle of the scan above
 This is an artifact caused by a blink during scan acquisition
 The was a high resolution scan, which takes about a second for
the scan pass, which is plenty of time to record a blink
 Artifacts in the OCT scan are anomalies in the scan that are
not accurate images of actual physical structures, but are
rather the result of an external agent or action

84. The scan below has waves in the retinal contour
These are not retinal folds, but rather movement of
the eye during the scan pass

85. OCT artifact Remedial measure
Inner layer misidentification Manual correction
Outer layer misidentification Manual correction
Mirror artifact Retake the scan in the area of interest
Degraded image Repeat scan after proper positioning
Out of register scan Repeat the scan after realigning the
area of interest
Cut edge artifact Ignore the first scan
Off center artifact Retake the scan/manually plot the
fovea
Motion artifact Retake the scan
Blink artifact Retake the scan
How to removed

86. NEW SPECTRALIS OCT FEATURES
Imaging of deeper tissue structures
Difficult due to :
 Pigment from the Retinal Pigment Epithelium (RPE)
 Light scattering from the dense vascular structure of the choroid
 Enhanced Depth Imaging (EDI) :
 New imaging modality on the Spectralis OCT
 Provides an enhanced visualization of the deeper structures,
like choroid
 Particularly useful for imaging pigmented lesions in the choroid
such as naevi and melanomas

87. LIMITATIONS OF OCT
 Penetration depth of OCT is limited
 Limited by media opacities
 Dense cataracts
 Vitreous hemorrhage
 Lead to errors in RNFL and retinal layer
segmentation
 Each scan much be taken in range and in
focus
 must be examined for blinks and motion artifacts
 Axial motion is corrected with computer
correlation software
 transverse motion cannot be corrected

88. CONTD.
Unable to visualize
 neovascular network or analyze if a CNV is active
 fluorescein angiography still has a significant role
OCT images cannot be interpreted in isolation
 must be correlated with red-free OCT fundus image and
photography/ophthalmoscopy
Aligning the scanning circle around the optic disc
may be difficult in patients with abnormal disc
contours

89. Some major limitations in the normative databases
Long term data on monitoring disease progression
with SD OCT unknown
Depends on operator skill

90. ADVANTAGES OF OCT
Best axial resolution available so far
Scans various ocular structures
Tissue sections comparable to histopathology
sections
Easy to operate
Short scanning time

91. References
Guide To Interpreting Spectral Domain Optical Coherence
Tomography BRUNO LUMBROSO MARCO RISPOLI
INTERNET
Novel Strategies in Glaucoma Diagnosis and Management
Sanjay Asrani, MD,Associate Professor of Ophthalmology
Head, Glaucoma OCT Reading Center, Duke University Eye Center ,
2010