OCT

• OCT is a non contact, non invasive, micro resolution

cross-sectional study of retina which correlates very well

with the retinal histology.

• Histopathology without biopsy (Light is used instead of

knife)

• Developed by Huang and associates in 1991.

• Principle: Interferometry

- measures echo delay(time) and intensity of reflected

light from target structure.

• Wavelength used :

840 nm Posterior Segment

1310 nm Anterior segment

• Interference…?addition of two waves mathematically.

Subtract from each other to

create a wave of lesser

amplitude than either one.

Destructive Interference

Add together to create a

wave of greater amplitude

than either one.

Constructive interference

• Coherence..?– Correlation b/w physical quantities of a single wave, or

b/w several waves.– High energy electrons return to low energy level and

emit radiation (in a waveform).

– The peaks and troughs of the waves correspond (no interference) – constant phase (Coherence)

– Types : Spatial & Temporal

• Low Coherence infra red light coupled to a fibre optic travels

through a beam splitter directed through the ocular media to

the retina & a reference mirror

• Interference is measured by photo detector and processed into a signal.

• Light source moves resembles a histology section of retina.

• Forms 2 D image– Axial resolution =<10 microns– Transverse resolution is 20 microns

• Integrates data points over 2 mm depth

• Colour coding– White & Red : highly reflective structures– Black & blue : low reflective structures– Green : intermediate reflective structures.

UBM

B scan

Abdo.USG

Two types

Time Domain Fourier Domain

Swept source OCT Spectral OCT

Types of OCT..

• The X, Y and Z planes…

• X plane is the horizontal B-scan as it is acquired. The anatomical features as shown in the X plane are real since the eye movement is Negligible.

• Y plane is the vertical reconstructed B-scan. The eye movement in the reconstructed B-scan is quite noticeable.

• Z plane is a reconstructed en face image. It is also called the coronal scan or C-scan.

• Many A-scans arranged in a line or circle and form high definition B-scan (0.02 – 0.04 sec)

• Many B-scans compose 3D scan patterns.

Retinal scan patterns• Line scan• Cross hair scan• High density scan (line/cross hair)

– 4096 A-scans(4 times) and no averaging• Macular map scan• Enhanced macular map scan and 3d reference scan• Grid scan• Raster scan• Radial slicer scan & macular mapping 6 mm• 3D macular scan

• Line scan

Length :6mm

Default fixation

target : fovea

High definition

Bscan – 1024 Ascans

Frame averaging (16)

• Cross hair scanL: 6mm0.63 secDefault fixation :

foveal centreFrame averaging (8 Horizontal Bscan8 Vertical Bscan)

• Macular map(MM5)

– 5*5 sq with 0.5mm

spacing

– Central 3mm sq with

0.25mm spacing

– 19,500 Ascans

– Takes 0.78 sec

• Enhanced macular map scan(EMM5) and 3d reference scan– MM5 scan + extra raster scan in inferior macula (for

registration with 3D reference scan)

– 3D reference scan registered with EMM5 and create 7

mm sq of OCT SLO image

– To be done once for each patient

• Grid scan

– For cross hair scan, it is difficult to compare 2 visits

when the scan center has small shift due to movement

or poor vision.

– 5*5 lines

– Length of line is 8 mm

– Width of 5 lines is 2 mm

• Raster scan– Used to give 3D view in

short time (0.34sec)

– 17 parallel lines

– Length of line is 6 mm

– Width of Scan : 4 mm

• Radial slicer scan– 12 radial scans

• Macular Mapping scan– Special radial slicer

scan– Length is 6mm– Provides analysis of

retinal thickness map.

Normal AnatomyILM, RNFL, OPL,IPL Hyper Reflective

GCL, INL,ONL Hypo Reflective

Retinal Vessels Circular hyper reflective

Pre retinal

• Pre retinal membrane• Epi retinal membrane• Vitreo retinal strands or traction• NVE/NVD

Intra retinal

• Choroidal neovascularization• Diffuse intra retinal edema• CME• Hard Exudates• Scar

Sub retinal

• Choroidal neovascularization• RPE detachment• Drusens• Sub retinal fibrosis• Scar• RPE atrophy

Systemic approach

Identify RPE

Examine anterior to

RPE

Examine RPE

Examine posterior to

RPE

Systemic approach

Identify RPE

Examine anterior to

RPE

Examine RPE

Examine posterior to

RPE

• Irregularity• Fragmentation• Rupture• Interruption• Depression• Elevation• Thinning• Thickening

Systemic approach

Identify RPE

Examine anterior to

RPE

Examine RPE

Examine posterior to

RPE

• PED• Bruch’s membrane• Hyper reflectivity (fibrosis vs. RPE

atrophy)

Systemic approach

Identify RPE

Examine anterior to

RPE

Examine RPE

Examine posterior to

RPE

• Vitreous• Retinal thickness• Foveal contour• Sub retinal fluid• ELM• Intra retinal cysts• Inner retinal layers

AMD• Hard drusen

– discrete nodules of high reflectivity in RPE level.– RPE showing discontinuity

• Soft drusen– Elevations of RPE level– Hyperreflective than Photorecepter layer and

hyporeflective than RPE layer.– Confluent soft drusen have scattering deposits forming

‘dome like’ elevations.– IS/OS and ELM can be absent above the large RPE

elevations-photoreceptor impairment

• Geographic atrophy– Absence or thinning of overlying photoreceptors

– So bright areas of increase transmission (bright shadowing) seen.

WET AMD• CNVM

– Fusiform enlargement of RPE/Bruch’s membrane/ choriocapillaries reflective band.

Duplication with high scattering material b/w 2 bands

Sub-retinal fluid seen as optically empty space.

• Fibrovascular PED– Elevation of pigment epithelial layer with irregular surface– Space below RPE is filled with solid layers of medium reflectivity

separated by hypo-reflective clefts.

Sub retinal fluid

• Serous PED– Elevation of RPE with flat top– Homogenous hypo-reflective space under RPE.

• Haemorrhagic PED– Blood under the RPE appears hyper-reflective,

attenuating the signal from deeper structures.– Subretinal fluid can be observed as hyporeflective

spaces above the RPE (arrows).

SRF

RPE tear with rolled edge

• Application– Retinal thickness– Sub-retinal fluid– Response to therapy

Central Serous Chorioretinopathy• Elevated neurosensory retina + PED• Posterior surface of detached retina is smooth• Line corresponding IS/OS junction is not visible.

• After one month– Decrease sub retinal fluid– Small granules seen at posterior surface of detached retina– Inferior shift of fluid must be visible in vertical scan

• After 3 month– Thickness of photorecepter outer segment increase

• After the reattachment of retina– The IS/OS line become visible

• OCT at leakage point– PED (within or at margin of CSCR)– Size of PED is independent to the size of CSCR– Some eyes show residual PED even after the resolution

of CSCR

Epiretinal Membrane

Proliferation of glial cell or RPE cells over ILM and surrounded by a newly-formed extra cellular matrix with PVD (partial/complete)1. Macular cellophane

– Mild expression of disease– Usually no symptoms– Ophthalmoscope : nonhomogeneous enhanced reflex on the posterior pole

2. Macular pucker– More severe form– Decrease visual acuity, metamorphopsia– Wrinkles on the retinal surface, distortion of retinal vessel coarse and sometimes

associated with CME– ERM is thicker and adherent and contracted

• On OCT– Hyper-reflective line upon the retinal surface

• Thickened the ERM = more the reflectivity = more traction over retinal layers

• In long standing puckers, ERM can completely subvert the retinal anatomy

• Eventually intraretinal small cystic spaces develops

• With progression, they grow in dimension and extend through the retinal layers in the foveal region and form macular hole

(in the advance stages, ERM traction can rarely cause a detachment of neuroretina)

Cystoid Macular Edema• The shape, dimensions and extension of the cystic cavities depend on

the framework of retinal structures and layers in the macula that influence the pattern.

• The Henle fibers are arranged radially in star shaped and centered on the foveola.

• At the center of macula, muller cells binds photoreceptor cells together without which, photoreceptor cell layer with its horizontally radiating nerve fibers would be highly susceptible to disruption at the umbo.

• Muller cells are arranged radially and not horizontally. That is why on OCT imaging, cystic spaces are not confined to a single layer

• The retina is divided in vertical and horizontal structures that limit expansion of exudates, haemorrhages and edema.– Vertical structures: muller cells unite the inner and outer

limiting membrane, chains formed by photoreceptor cell to bipolar cells to ganglion cell

– Horizontal structures: inner limiting membrane, outer limiting membrane, IPL,OPL

• Two mechanisms

– Breakdown of blood retinal barrier

– Vitreomacular traction

• Early cystoid edema– One or more small rounded edema cells in

INL,ONL– Later numerous, small cavities are formed

which are seperated by thick septa, mainly seen in the ONL,INL and IPL

• Advanced cystoid edema– Small cavities coalesce, cystoid cells merge, their dimensions

increase, the wall get thinner and form large, smooth, regular, globular cysts.

• Largest cystic spaces are found centrally .• Smaller cystic spaces are seen parafoveally and extra foveally.• These aspects gives the major thickness to macular center.• So, measurement of retinal thickness in the center reflects , the size of

a few or a single central cyst rather than that of the entire retina.

• Regressed cystoid edema– Cavities decrease in dimension, triangular in shape with

sharp angles.– Walls are less regular, thinner, makes bifurcations and

granulated.

• ERM : tractional CME

– ERM + increase retinal thickness+ irregular cystic spaces

• Vitreomacular traction and CME– The tractional forces from the vitreous lead to edema formation.

• Macular hole with CME

– Full thickness macular hole with cysts of variable size.

Diabetic Retinopathy

• Why OCT?– For subtle macular edema

– To classify macular edema

– To predict outcome of treatment in DME

• Irreversible damage is seen on oct

Classification

• OCT sensitivity to detect DME is very high (98.6 %) Evaluation of the

OCT scans demonstrated five distinct patterns of DME.

• Type 1: focal macular thickening.

• Type 2: Diffuse thickening without cysts(sponge like retinal thickening)

• Type 3: diffuse cystoid macular edema.

• Type 4: Tractional macular edema.

• Type 5: DME from one of the previous types associated to a macular

serous retinal detachment.

• Diffuse thickening without cysts (sponge like retinal thickening)

– Increase retinal thickening and reduce intra retinal reflectivity

– May be seen as an isolated or along with CME/ serous retinal

detachment.

• Cystoid macular edema

– May be isolated or along with other variety

– 2A: 1 / 2 cysts in IPL with central foveal thickness is > 325 micron

– 2B: patelloid pattern of intraretinal cysts; foveal thickness >485 micron

– 2C: chronic edema, major intraretinal damage with large cavities

resembling retinoschisis; foveal thickness >405 micron

• Tractional retinal edema

– Focal or diffuse traction by thickened and taut posterior hyaloid,

thickened ILM, or by epiretinal membrane

– Tangential or antero-posterior forces can be demonstrated by OCT

4A: posterior hyaloid traction

4B: epiretinal membrane

4C: both posterior hyaloid and epiretinal membrane

• Serous retinal detachment

– Empty space b/w RPE and outer layer of photoreceptor cell

– Difficult to diagnose clinically

– Presence of serous RD was initially thought to signal a severe stage of

DME. But recent OCT based studies suggest that prognosis is depend

on integrity of photoreceptor cell layer and thickness of retina above

serous RD.

Vitreomacular traction syndrome

• As a person ages, vitreous liquifies and detaches from the retina.

• Posteriorly, vitreous is bound with a thin membrane of condensed collagen,

termed the posterior hyaloid.

• Vitreous is most adherent to the vit base > optic disc > macula> blood vessels.

• In some cases, adhesions b /w vit and macula are abnormally firm and as the

vit begins to separate from retina, traction forces are generated in areas of

vitreomacular adhesions.

– Macular hole

– Tractional macular edema

– Subretinal fluid and/or macular edema

This entity is commonly referred to as vitreomacular traction syndrome.

• On OCT, partially detached hyaloid appears as thin, moderately

reflective line anterior to neural retina.

• In some cases epiretinal membrane may be visualize as highly

reflective line just anterior to nerve fiber layer.

Macular Hole

• Why OCT is advisable in Macular hole?– For clinically useful classification– To determine if there is a concurrent ERM– To examine IS/OS– To evaluate post operative morphology of fovea– To differentiate it from other condition like

pseudohole, inner lamellar holes, cystoid macular edema.

ClassificationStage 1: Loss of the foveal depression.

1A: Foveolar detachment characterized by a loss of the foveal contour and a lipofuscin-colored spot.1B: Foveal detachment characterized by a lipofuscin-colored ring.

Stage 2: A full thickness break < 400µm in size. It might be eccentric with an inner layer “roof.” In most cases, the posterior hyaloid has been confirmed to be still attached to the fovea on OCT analysis.

Stage 3: A hole ≥400 µm in size. The posterior hyaloid is noted to be detached over the macula with or without an overlying operculum.

Stage 4: MH is characterized by a stage 3 MH with a complete posterior vitreous detachment and Weiss ring.

Lamellar holeA partial thickness. macular hole, where the inner layers of the macula are involved with traction and detached from the underlying layers.

• Pre operative evaluation1. Pre-op size of hole is inversely correlated to success rate2. It is good to know extent of ERM on OCT before OT.

• Post operative evaluation1. IS/OS junction indicates normal alignment which is necessary

for good visual outcome2. To classify healing pattern

Other classification

3. CME and ERM in post op OCT are useful findings to predict the risk of reopening

Type 1: closed without foveal neurosensory retinal defectType 2: closed with foveal neurosensory retinal defect

U – Normal foveal contourV- Steep foveal countour

W – foveal defect of neurosensory retina

Type I

Type II

V type

U type

Hole form factor

Macular hole index

MHI > 0.475 :

good visual

prognosis after

surgery

Tractional Hole Index

• THI = HM

• THI > 0.973 – good visual prognosis after suregery.

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