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.