OCT

1. OPTICAL COHERENCE
TOMOGRAPHY
TYPES, INTERPRETATION AND
USES
Manoj Aryal
B . Optometry
Institute Of Medicine,
Maharajgunj Medical Campus

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

3. INTRODUCTION
Optical coherence tomography, or OCT is a non-
contact, noninvasive imaging technique used to
obtain high resolution 10 cross sectional images of
the retina and anterior segment.
Reflected light is used instead of sound waves.
Infrared ray of 830 nm with 78D internal lens.

4. 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

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

6. 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

8. 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

10. Digital processing aligns the A-scan to correct for eye
motion.
 Digital smoothing techniques further improves 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 visualize low reflective structures

11.  Highly reflective structures are shown in bright colures (white and red) .
 Those with low reflectivity are represented by dark colours (black and blue).
 Intermediate reflectivity is shown Green.

12. Advantages
 Non-invasive
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
OCT

13. Advantages
 Resolution of ~ 50 μm
 Anterior segment of the
eye
 Not limited to optically
transparent tissues
i.e. opaque corneas
Disadvantages
Direct contact
 Penetration of only
4-5 mm
 Image influenced by
 Plane of section
 Distance to anterior
chamber
 Orientation of the probe
 Room illumination
 Fixation
 Accommodative effort
USG

14.  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

15. 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

16. Time domain-OCT
Types of OCT

17. Spectral Domain OCT

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

19. 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
INTERPRETATION &
CLINICAL APPLICATIONS

20. 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
 Reduced motion artifact
SD-OCT using light of
wavelength 830nm
 Axial resolution of 5
micron
 Higher resolution allows
better visualization of
cornea and angle and it’s
structures
 Provided a scan depth
greater than 6.30nm-
allowing imaging of entire
AC depth
 Reduced overlap artifacts

21. 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

22. Glaucoma
ONH analysis
Retina
Choroid

23. 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

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

25. 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

26.  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

27. 3-dB sensitivity loss at a single location in the
perifoveal area on Humphrey visual field testing
 associated with loss of approximately 230 ganglion cells
compared with loss of 10 ganglion cells in the peripheral
posterior pole
 retinal thickness losses correlated more strongly with the severity
of optic nerve cupping than with visual field changes

28. ROLE OF OCT IN GLAUCOMA-RECENT
ADVANCES
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

29. 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 normative database
RNFL Thickness in quadrants
& sectors compared to
normative database

30. 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

31. POSTERIOR POLE ASYMMETRY ANALYSIS
 Combines mapping of the posterior pole retinal
thickness with asymmetry analysis
Both eyes
Hemispheres of each eye

32.  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

33. 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

35. 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.

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

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

42. OCT image display,
 Highest reflectivity - red
 nerve fiber layer
 retinal pigment epithelium
and
 choriocapillaris
 Minimal reflectivity appear
blue or black
 photoreceptor layer
 choroid
 vitreous fluid or blood

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

45. HYPER REFLECTIVE SCANS
RNFL
ILM, RPE
RPE-
choriocapillaries
complex
PED
Drusen , ARMD
CNVM lesions
Anterior face of
hemorrhage
Disciform scars
Hard Exudates
Epiretinal
membrane

46. PED

47. Drusen
of the
Retina

50. HYPO REFLECTIVE SCANS
Retinal atrophy
Intraretinal/subretinal fluid

51. Yes shadows (cone effect): No shadows.
Superficial layers
Normal retinal blood vessels Serous collections
Dense collection of blood Scanty hemorrhage
Cotton wool exudates
Deep layers
Hard exudates (lipoproteins)
RPE hyperplasia
Intraocular foreign body
Dense pigmented scars
Choroidal nevi
Thick SRNVM

52. Regions:
 The Pre-retina
 The Epi-retina
 The Intra-retina
 The Sub-retina

53. A normal pre-retinal profile is black space
Normal vitreous space is translucent
The small, faint bluish dots in the pre retinal space is
noise
This is an electronic alteration created by increasing the
sensitivity of the instrument to better visualize low
reflection structures

54. Anomalous structures in Pre-retinal area:
Pre-retinal membrane
Epi-retinal membrane
Vitreo-macular traction

56.  Macular pucker
 Macular lamellar hole
 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)
 Macular hole, stage 4 (complete hole, edema at margins,
complete PVD)

62. Serous retinal detachment

63. Serous retinal pigment epithelial detachment

64. Hemorrhagic pigment epithelial detachment

65. Choroidal neovascular membrane
Drusens
Hard exudates
Scar tissue
RPE tear

66. OCT deformations:
Concavity
 myopia
Convexity
 PED
 Subretinal cysts
 Subretinal tumors
Disappearance of foveal
depression

67. CSR

68. 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

69. Loss of foveal photoreceptors can be assessed with
OCT, as occurs with
full-thickness macular holes
 central scarring or fibrosis
Steepening of the foveal contour
 epiretinal membranes and
 macular pseudoholes or lamellar holes .
 Loss or flattening of the foveal contour
impending macular holes
 foveal edema or foveal neurosensory detachments.

70. Artifacts in the OCT scan are anomalies in the scan
that are not accurate the image of actual physical
structures, but are rather the result of an external
agent or source
Misidentification of inner retinal layer:
Occurs due to software breakdown, mostly in
eyes with epiretinal membrane vitreomacular
traction or macular hole.

71. Mirror artifact/inverted artifact:
Noted only in spectral domain OCT machines.
Subjects with higher myopic spherical equivalent, less
visual acuity and a longer axial length had a greater
chance of mirror artifacts.

72.  Misidentification of outer retinal layers: Commonly occurs in
outer retinal diseases such as central serous retinopathy ,AMD, CME and
geographic atrophy.

73. Out of register artifact:
Out of register artifact is defined as a condition where
the scan is shifted superiorly or inferiorly such that some
of the retinal layers are not fully imaged.
 This is generally an artifact, which is operator
dependent and caused due to misalignment of the scan

74. Degraded image:
 Degraded images are due to poor image acquisition.
 These images were generally associated with non-retinal diagnosis.
Cut edge artifact:
 This is an artifact where the edge of the scan is truncated.
 Result in abnormality in peripheral part of the scan and do not affect
the central retinal thickness measurements

75. Off center artifact:
Happens due to a fixation error.
 Happens mostly with subjects with poor vision,
eccentric fixation or poor attention.
Motion artifact:
 Noted due to ocular saccades, change of head position
or due to respiratory movements

76. Blink artifacts:
These are noted when the patient blinks during the
process of scan which are noted as areas of blanks in the
rendered en-face image and macular thinning on
macular map.

77. OCT ARTIFACT AND WHAT TO DO?
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

78. 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 visualisation of the deeper structures, like choroid
 Particularly useful for imaging pigmented lesions in the choroid such as
naevi and melanomas

79.  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

80. Unable to visualise
 neovascular network or analyse 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

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

82. 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

83. INTERNET