Optical Coherence Tomography
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OPTICAL COHERENCE TOMOGRAPHYTYPES, INTERPRETATION AND USES
Manoj Aryal
B . Optometry
Institute Of Medicine,
Maharajgunj Medical Campus
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PRESENTATION LAYOUT
IntroductionHistory Theories & PrinciplesTypes InterpretationClinical ApplicationsLimitations & AdvantagesLatest Developments
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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.
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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
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THEORIES AND PRINCIPLE
OCT images obtained by measuring echo time intensity of reflected light
Effectively ‘optical ultrasound’
Optical properties of ocular tissues, not a true histological section
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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 laser1310 nm : AS-OCT
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PRINCIPLE
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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
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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
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COLOR CODING IN OCT 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.
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OCT VS USG
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
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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
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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
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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
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Time domain-OCT
Types of OCT
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Spectral Domain OCT
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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
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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
INTERPRETATION &CLINICAL APPLICATIONS
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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
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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
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OCT – POSTERIOR SEGMENT MODULE
Glaucoma
ONH analysis
Retina
Choroid
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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
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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
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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
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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
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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
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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
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Circle Scan
Differences betweeen average thickness in sectors(along the calculation circle) in each eyeOCT Scan with automatic
segmentation of RNFL
TSNIT RNFL thickness compared to normative database
RNFL Thickness in quadrants & sectors compared to normative database
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Posterior Pole Retinal Thickness Map withCompressed Color Scale in 8x8 Analysis Grid
Mean Thickness
Hemisphere Analysis withAsymmetry Gray Scale
OCT scan of macular region
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POSTERIOR POLE ASYMMETRY ANALYSIS
Combines mapping of the posterior pole retinal thickness with asymmetry analysis
Both eyes
Hemispheres of each eye
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INTERPRETATION OF ASYMMETRY ANALYSIS
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
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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
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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.
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Case 2:
A 55-year-old female diagnosed with primary open angle glaucoma OD
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NEURO-OPHTHALMIC
In the evaluation of ONH
Optic disc edema
Optic neuritis
Optic atrophy
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RETINAOCT 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
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GANGLION CELL COMPLEX
Collective term RNFL Ganglion cell layer and Inner plexiform layer
GCC thought to be affected in early glaucoma
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HYPER REFLECTIVE SCANS
RNFL ILM, RPE RPE-choriocapillaries complex
PED Drusen , ARMD
CNVM lesions Anterior face of hemorrhage
Disciform scars Hard Exudates Epiretinal membrane
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PED
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Drusen of the Retina
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DISCIFORM SCAR
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HYPO REFLECTIVE SCANS
Retinal atrophyIntraretinal/subretinal fluid
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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
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Regions:
The Pre-retina
The Epi-retina
The Intra-retina
The Sub-retina
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THE PRE-RETINAL PROFILEA 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
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Anomalous structures in Pre-retinal area:
Pre-retinal membrane
Epi-retinal membrane
Vitreo-macular traction
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DEFORMATIONS IN THE FOVEAL PROFILE
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)
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LAMELLAR MACULAR HOLE
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FULL THICKNESS MACULAR HOLE WITHOUT PVD
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DEFORMATIONS IN THE MACULAR PROFILE
Serous retinal detachment
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DEFORMATIONS IN THE MACULAR PROFILE
Serous retinal pigment epithelial detachment
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DEFORMATIONS IN THE MACULAR PROFILE
Hemorrhagic pigment epithelial detachment
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INTRA-RETINAL ANOMALIES IN THE MACULAR PROFILE
Choroidal neovascular membraneDrusensHard exudatesScar tissueRPE tear
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OCT deformations:
Concavity myopia
Convexity PED Subretinal cysts Subretinal tumors
Disappearance of foveal depression
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CSR
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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
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FOVEA
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.
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OCT: ARTIFACTS
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.
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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.
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Misidentification of outer retinal layers: Commonly occurs in outer retinal diseases such as central serous retinopathy ,AMD, CME and geographic atrophy.
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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
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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
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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
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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.
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OCT ARTIFACT AND WHAT TO DO?
OCT artifact Remedial measureInner 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
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NEW SPECTRALIS OCT FEATURES
Imaging of deeper tissue structuresDifficult 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
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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
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CONTD.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
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Some major limitations in the normative databases
Long term data on monitoring disease progression with SD OCT unknown
Depends on operator skill
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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
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REFERENCES
INTERNET