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OCT – Introduction & Macula
Dr. Sajjan SangaiDept. Of OphthalmologyMGM Medical College and Hospital, Aurangabad
Introduction
Retina is a multi layered tissue with each layer having a different reflectance pattern thus by this principle OCT permits recognition of multiple retinal layers in vivo.
Optical Coherence Tomography
Optic
al Related
to light and optics
Cohe
renc
e Two light beams of the same wavelength in phase To
mog
raph
ySectioning /Cutting
Non-Excisional Optical Biopsy
Normal Retinal Anatomy
Macular Anatomy
Retina Framework
Horizontal Structures • Inner and Outer
Plexiform layer • Inner and Outer
Limiting Membrane• Pigment epithelium
Vertical Structures• Müllers fibres • Cell chains
containing of photoreceptor to bipolar and ganglion cells
Hyper reflective Hypo reflective
Retinal Anatomy & OCT
Optical Principles of OCT
Imaging with OCT is based on Michelson's Interferometer and includes complex analysis of reflections of LOW COHERENCE LIGHT from the ocular tissue ( low coherence interferometry)
Optical Principles of OCT
Low coherence infrared light 840 nm
Reflected by different retinal tissue layers
840 nm
Optical Principles of OCT
The reflected light from the retinal
tissue and reference mirror interacts to form
an
Step 3
Distance between the light source
and reference mirror
Step 2
Distance between the light source
and retinal tissue
Step 1
INTERFERENCE PATTERNThis interference pattern is processed into a signal
Image formation
The signal is analogous to that obtained with A Scan Ultrasonography.
A 2D image is built as the light source is moved across the retina and then the series of stacked and aligned A scans produce a 2dimensional cross sectional image resembling a HISTOLOGIC SECTION
Image formationTransverse scanning at multiple sequential points (A-Scans)
2 Dimensional data collection
Generation of Cross sectional map
Display in pseudo colours
Basic Concepts
The infrared image has a field of 30˚ .
OCT operates like a fundus camera but resolves like a USG machine .USG OCT
Source Sound waves Infrared light waves
Resolution 150µ 10µPatient Contact Needed Non- Invasive
Indications of OCT
Macular hole
Macular edema
Central serous retinopathy
Epiretinal membranes
Diabetic retinopathy
Age-related macular degeneration
Scars, Foveal burns
http://www.aao.org/eye-health/treatments/what-does-optical-coherence-tomography-diagnose
Advantages of OCT
Non contact Non invasive
High Resolution
Real time Imaging
Limitations of OCT
Corneal Opacities, Cataract
Poor patient cooperation
Asteroid hyalosis, Vitreous
Haemorrhage
Intraocular Gas
(SF6 ,C2F6,C3F8 )
Types of OCT
Time domain OCT Spectral domain OCT
Optical Principle – TD vs. SD OCT
Time Domain OCT - Principle Time-domain devices can provide
400 A scans per second with a maximal axial resolution of 8-10µm
High quality images require longer time to create. Therefore time is the major limitation of this technique.
Spectral Domain OCT- Principle Absence of movable mirror speeds up
the image acquisition up to 50 times. This technique enables to obtain large numbers of A-scans that allows creating high resolution images.
SD devices can provide 20.000-52.000 A-scan per second with a 5-7µm resolution
Such a speed reduces the eye movement artefacts.
TD OCT vs. SD OCT
TD OCT vs. SD OCTTIME DOMAIN OCT SPECTRAL DOMAIN OCT
RESOLUTION : 10µ 5-7µm2 Dimensional images 2 D and 3 D
Low coherence Interferometry Low coherence interferometryUses fixed wavelength Broad wavelength spectrum
Lower speed 400-512 A scan/sec High speed : 52000 A scans/ sec
Scanning protocolsTOPCON
Scanning protocols
Zeiss
Retinal scan protocolScan Utility
Line Multiple line scans can be obtained without returning to main window
Radial Lines Determines entire macular thickness/ volume
Raster lines Entire area of pathology can be scanned
Repeat scan Monitoring change during follow-up
Macular thickness Determines entire macular thickness/ volume
Fast macular thickness Allows comparative thickness/ volume analysis
Analysis
Qualitative •Morphological changes•Reflectivity•Shadowing•Structure microstructure segmentation- alteration/ deformation/ loss of structure.
Quantitative
•Retinal Map•Retinal Thickness•Retinal Volume
Patterns of Abnormalities
Retinal edema-1.CME2. Vitreoretinal traction leading to edema
Retinal atrophy secondary to 1.Laser2.Trauma3.Inflammations
Increased
Thickness
Decreased
Thickness
Patterns of AbnormalitiesHigh Reflectivity:(Red and Yellow): Superficial: 1. Epiretinal/ vitreal membranes2. Sub hyaloid / Sub ILM haemorrhage3. Cotton Wool Spot’s 4. Myelinated nerve fibres.
Epiretinal MembraneSub Hyaloid hemorrhageCotton Wool spots
Patterns of Abnormalities Intraretinal: 1. Hard exudates, 2. Intraretinal haemorrhages 3. fibrosis and scarring
Intraretinal Hemorrhage
Patterns of Abnormalities Deep:1. Drusen 2. RPE Hyperplasia3. Scarring , atrophy4. Sub retinal neovascular membrane 5. Deep pigmented lesion e.g. Nevus
Drusen
Patterns of AbnormalitiesLow reflectivity: ( Black and
Blue)1. Gross separation of cellular
elements and fluid present either in form of cystoid spaces
2. Neurosensory detachment3. RPE detachment
results in decreased reflectivity
Patterns of AbnormalitiesShadowing: Dense highly reflective elements
produce a kind of blockage of light waves by attenuation
This appears a shadow that conceals the element lying behind it
E.g. Haemorrhage's, Hard exudates, cotton wool spots, dense pigmented lesion or scar, retained foreign body.
Diseases of Macula Age related Macular Degenerationa) Dry / non-neovascular AMD: Drusenb) Wet / neovascular AMD: CNV and PED
Epiretinal Membrane
Full Thickness Macular Hole
Central Serous Chorioretinopathy
Cystoid Macular Edema
Age related Macular Degeneration Degenerative disorder affecting
macula Clinical classification of AMD:Category Definition, based on lesion ( within 2DD of
fovea)No apparent ageing change
No drusenNo AMD pigmentary abnormalities
Normal ageing changes
Only drupletsNo AMD pigmentary abnormalities
Early AMD Medium drusen (>63 m - < 125 m)No AMD pigmentary abnormalities
Intermediate AMD
Large drusen (>125m)Any AMD pigmentary abnormalities
Late AMD Neovascular AMD and/or any geographic atrophy
Drusen Extracellular deposits located at the
interface between the RPE and Brusch membrane.
Derived from immune mediated and metabolic processes in RPE.
On OCT:Medium sized and large drusen are seen
as hyper reflective irregular nodules beneath the RPE
Drusen• Small drusen/ Hard drusen• Well defined , whitish yellow lesion• < 63 m
• Fairly well defined yellow white focal deposits
• Measuring between 63 m - 125 mIntermediate Drusen
• Large drusen/ Soft drusen• Less well delineated yellow white
deep retinal lesion • >125m
Choroidal Neovascularisation Causes:
Degenerative
Inflammatory
Traumatic
Neoplastic
Brusch memb./ RPE
Compromised by
Choroidal Neovascularisation ( CNV) Types: Classified according to Macular
Photocoagulation Study (MPS): Based on FA
Classic CNV (20%): well defined lacy pattern during early transit of dye subsequently leaking to sub retinal space.
Occult CNV (80%): Limits cannot be fully defined on FA
Predominantly / Minimally classic CNV: Classic element is grater or less than 50 % of total lesion
Choroidal Neovascularisation ( CNV)On OCT: CNV is shown as 1. thickening and fragmentation
of RPE and Choriocapillaries2. Sub retinal, Sub-RPE fluid, 3. Blood and scarring are demonstrated.
Choroidal Neovascularisation ( CNV)
Retinal Pigment Epithelial Detachment (PED) Thickened and dysfunctional Brusch
membrane impending movement of fluid from RPE towards the choroid PED.
On OCT : PED shows Separation of RPE from the Brusch membrane by an optically empty area
Clinically: Orange dome shaped lesion
FA: Well demarcated Hyperfluroscent pooling
ICGA: HypofluroscenceOCT: Optically empty area below RPE
Epiretinal Membrane
Sheet like fibro cellular structure that develops over surface of retina.
Proliferation of cellular component and contraction of membrane leads to visual symptoms.
On OCT-o Highly reflective surface layer
associated with retinal thickening.o Useful to exclude significant VMT .
Epiretinal Membrane
Full Thickness Macular Hole Pathogenesis:The vitreofoveal traction is central to development of a full thickness macular hole. Gass: Proposed that contraction of
prefoveolar cortical vitreous results in tangential traction
Full Thickness Macular Hole Classification:1. Gass classification scheme on
Biomicroscopy, 2. New OCT based classification- IVTS-
(International Vitreomacular Traction Study)
Idiopathic Macular Hole
Gass Classification
Gass Classification
Full Thickness Macular Hole
Full Thickness Macular Hole
Differential Diagnosis1. Lamellar hole2. Pseudo hole3. ERM4. CME5. CSR with central yellow spot
Central Serous Chorioretinopathy Idiopathic Characterised by local serous
detachment of the sensory retina at the macula secondary to leakage from choriocapillaries through one or more hyper permeable RPE sites
Affects young , middle aged men Risk factors: Steroid administration,
Cushing syndrome, H. Pylori infection, pregnancy, psychological stress, sleep apnoea.
Central Serous ChorioretinopathyOn OCT:Optically empty neurosensory elevation,Other findings – one or more smaller RPE detachments , precipitates on posterior surface of detached retina , thickened choroid.On FA:Hyperfluroscent spot that enlarges- Ink BlotVertical column- Smoke stack
Central Serous Chorioretinopathy
Cystoid Macular Edema
Accumulation of fluid in outer plexiform layer and inner layers of retina with formation of tiny cyst like cavities.
Fluids may initially accumulate intracellularly in Müller cells with subsequent rupture.
Cystoid Macular Edema-Causes:1. Ocular Surgery and Laser.2. Retinal Vascular Disease: Diabetic
retinopathy, Retinal Vein occlusions.3. Inflammation: Intermediate Uveitis4. Drug induced: topical prostaglandin
derivatives.5. Retinal dystrophies: Retinitis pigmentosa6. Conditions having VMT: ERM7. CNV8. Tumours: Retinal capillary haemangioma9. Systemic diseases : CRF
Cystoid Macular Edema
On OCT:Retinal thickening with cystic hypo reflective spaces, and loss of foveal depression.Lamellar holes may be demonstrated in advanced cases.On FA:A petalloid pattern is seen due to dye accumulation in microcytic spaces in outer plexiform layer
Recent Advances
Various newer OCT systems are:1. Ultra high resolution OCT2. Doppler OCT3. CAS OCT- Visante OCT4. Combined FFA and en-face OCT5. Intraoperative OCT
Ultra high resolution OCT (UHR OCT) Axial resolution of 3 µm Transverse resolution of 15-20 µm Useful for visualization ofa) External limiting membraneb) Ganglion cell layerc) Photoreceptor layer
Colour Doppler OCT
Technique that combines laser doppler velocimetry and OCT for imaging the depth , diameter flow rate, retinal haemodynamic characteristic
Only possible in larger vessels Not well suited for angiography of
retinal and choroidal microvasculature, where vessels are nearly perpendicular to the OCT beam.
Intraoperative OCT
Surgical microscope integrated with OCT to perform simultaneous imaging and en face visualization
Uses are: Macular hole surgery,ERM peeling,Sub retinal surgery
Zeiss OPMI LUMERA® 700 and RESCAN™ 700
OCT Angiography
Doppler OCT uses the Doppler phase shift to quantify blood flow in larger vessels and measure total retinal blood flow
OCTA is more concerned about separating moving scatters from static background tissue to create angiograms.
OCT Angiography
Bibliography Principles and Practices of Ophthalmology, Vol. 2 ,
Third edition, Albert and Jacobiec’s. RETINA, Vol.1 , Fifth edition, Ryan.S.J. Kanski’s Clinical Ophthalmology, 8th edition,
Bowling B. Practical Handbook of OCT , Lumbroso.B, Rispoli M. Step by Step Optical Coherence Tomography.
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