OPTICAL COHERENCE TOMOGRAPHYTYPES, INTERPRETATION AND USES

Manoj Aryal

B . Optometry

Institute Of Medicine,

Maharajgunj Medical Campus

PRESENTATION LAYOUT

IntroductionHistory Theories & PrinciplesTypes InterpretationClinical ApplicationsLimitations & AdvantagesLatest 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

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

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

PRINCIPLE

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

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.

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

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

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

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

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

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

OCT – POSTERIOR SEGMENT MODULE

Glaucoma

ONH analysis

Retina

Choroid

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

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 eyeOCT 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 withCompressed Color Scale in 8x8 Analysis Grid

Mean Thickness

Hemisphere Analysis withAsymmetry 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

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

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

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

GANGLION CELL COMPLEX

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

DISCIFORM SCAR

HYPO REFLECTIVE SCANS

Retinal atrophyIntraretinal/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

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

Anomalous structures in Pre-retinal area:

Pre-retinal membrane

Epi-retinal membrane

Vitreo-macular traction

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)

LAMELLAR MACULAR HOLE

FULL THICKNESS MACULAR HOLE WITHOUT PVD

DEFORMATIONS IN THE MACULAR PROFILE

Serous retinal detachment

DEFORMATIONS IN THE MACULAR PROFILE

Serous retinal pigment epithelial detachment

DEFORMATIONS IN THE MACULAR PROFILE

Hemorrhagic pigment epithelial detachment

INTRA-RETINAL ANOMALIES IN THE MACULAR PROFILE

Choroidal neovascular membraneDrusensHard exudatesScar tissueRPE 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

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.

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.

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 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

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

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

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

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

REFERENCES

INTERNET