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OPTICAL COHERENCE
TOMOGRAPHY
TYPES, INTERPRETATION AND
USES
Manoj Aryal
B . Optometry
Institute Of Medicine,
Maharajgunj Medical Campus
PRESENTATION
LAYOUT
Introduction
History
Theories & Principles
Types
Interpretation
Clinical Applications
Limitations & Advantages
Latest Developments
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.
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
OCT images obtained by measuring
 echo time
 intensity of reflected light
Effectively ‘optical ultrasound’
Optical properties of ocular tissues, not a true
histological section
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
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
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
 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.
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
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
 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
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
Time domain-OCT
Types of OCT
Spectral Domain OCT
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
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
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
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
Glaucoma
ONH analysis
Retina
Choroid
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
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
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
 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
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
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
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
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
POSTERIOR POLE ASYMMETRY ANALYSIS
 Combines mapping of the posterior pole retinal
thickness with asymmetry analysis
Both eyes
Hemispheres of each eye
 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
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
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.
Case 2:
A 55-year-old female diagnosed with primary open
angle glaucoma OD
NEURO-OPHTHALMIC
In the evaluation of ONH
Optic disc edema
Optic neuritis
Optic atrophy
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
Collective term
 RNFL
 Ganglion cell layer and
 Inner plexiform layer
GCC thought to be affected in early glaucoma
HYPER REFLECTIVE SCANS
RNFL
ILM, RPE
RPE-
choriocapillaries
complex
PED
Drusen , ARMD
CNVM lesions
Anterior face of
hemorrhage
Disciform scars
Hard Exudates
Epiretinal
membrane
PED
Drusen
of the
Retina
HYPO REFLECTIVE SCANS
Retinal atrophy
Intraretinal/subretinal fluid
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
Regions:
 The Pre-retina
 The Epi-retina
 The Intra-retina
 The Sub-retina
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
Anomalous structures in Pre-retinal area:
Pre-retinal membrane
Epi-retinal membrane
Vitreo-macular traction
 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)
Serous retinal detachment
Serous retinal pigment epithelial detachment
Hemorrhagic pigment epithelial detachment
Choroidal neovascular membrane
Drusens
Hard exudates
Scar tissue
RPE tear
OCT deformations:
Concavity
 myopia
Convexity
 PED
 Subretinal cysts
 Subretinal tumors
Disappearance of foveal
depression
CSR
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
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.
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.
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.
 Misidentification of outer retinal layers: Commonly occurs in
outer retinal diseases such as central serous retinopathy ,AMD, CME and
geographic atrophy.
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
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
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
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.
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
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
 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
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
Some major limitations in the normative databases
Long term data on monitoring disease progression
with SD OCT unknown
Depends on operator skill
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
INTERNET

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Optical Coherence Tomography

  • 1. OPTICAL COHERENCE TOMOGRAPHY TYPES, INTERPRETATION AND USES Manoj Aryal B . Optometry Institute Of Medicine, Maharajgunj Medical Campus
  • 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
  • 7.
  • 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
  • 9.
  • 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
  • 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
  • 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
  • 34.
  • 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.
  • 36.
  • 37.
  • 38. Case 2: A 55-year-old female diagnosed with primary open angle glaucoma OD
  • 39.
  • 40.
  • 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
  • 43.
  • 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
  • 48.
  • 49.
  • 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
  • 55.
  • 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)
  • 57.
  • 58.
  • 59.
  • 60.
  • 61.
  • 63. Serous retinal 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

Editor's Notes

  1. 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
  2. Signal-to-Noise Ratio (SNR) : a quality measure of desired signal level divided by undesired noise Acquition: Something acquired or gained.
  3. In TD-OCT a mirror in the reference arm of the interferometer is moved to match the delay in various layers of sample The resulting interference signal 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 movement the detection system only collects signal from a narrow range of depth in the sample. This serial axial scanning is inefficient
  4. In FD-OCT, the reference mirror is kept stationary. The spectral pattern of the interference between the sample and reference reflections is measured The spectral interferogram is fourior transformed to provide an axial scan. The absence of moving parts allow 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.
  5. Vitreous anterior to retina is non reflective and is seen as a dark space. Posterior boundary of retina is also seen as a red layer representing highly refractive retinal pigment epithelium and choriocapillaries. Outer segment of retinal photoreceptors, being minimally reflective are represented by a dark layer just anterior to RPE- choriocapillaries complex. Vitreo-retinal interface is well defined due to contrast between the non reflective vitreous and the backscattering retina. Anterior boundary of retina formed by highly refractive RNFL is seen as a red layer due to bright backscattering.
  6. Alterations in the thickness of the retinal nerve fiber layer may be a powerful indicator of the onset of neurodegenerative diseases such as glaucoma. The NFL appears in the OCT images as a highly backscattering layer in the superficial retina and exhibits increased reflectivity compared to the deeper retinal layers. The observation of depressions from both the anterior and posterior margins of the NFL is a helpful indicator of actual thinning.
  7. Neurosensory detachments appear as a shallow elevation of the retina, with an optically clear space between the retina and RPE The backscattering from the normally minimally reflective photoreceptors is increased, resulting in a well-defined fluid-retina boundary. Serous detachments of the pigment epithelium have a distinctly different appearance . The reflective band corresponding to the RPE is focally elevated over an optically clear space. the detached RPE is more highly reflective than normal, perhaps due to a refractive index difference between serous fluid and the choriocapillaris, or due to decompensation and morphological changes in the RPE cells themselves
  8. Age related macular degeneration :OCT because of its high resolution capacity is able to image: Morphological changes in the non exudative ARMD Sub-retinal fluid, intraretinal thickening and sometimes, choroidal neovascularization in exudative ARMD This is especially helpful when vascularization of choroidal neovascularization is obscured on fluorescein angiography by a thin layer of fluid or hemorrhage
  9. Epi-retinal membrane:Thin, transparent membrane that are seen on the inner retinal surface in the macular area Epi-retinal membrane :classification 1. clearly separable where a clear space is visible between the epiretinal membrane and inner retinal surface 2. globally adherent where no area of separation can be seen easily between the epi-retinal membrane and inner retinal surface Vitreomacular traction may result in flattening or protrusion of the fovea Epi-retinal membrane: Highly reflective diaphanous membrane over the surface of retina
  10. Full thickness macular hole show a breach in all the layer of retinal while lamellar macular hole shows only partial loss of tissue with steep foveal contour
  11. OCT allows confirmation of diagnosis pf macular hole and differentiates it from the clinically simulating conditions such as lamellar hole, foveal pseudo cyst. Traction on the result in the formation of stage 1 macular hole where there is no visible hole only foveal detachment Continuous traction result in the formation of stage 2 hole with dehiscence of neural retina in a perifoveal location