OPTICAL COHERENCE TOMOGRAPHY

Group:48Solanki Ujjval

PRESENTATION LAYOUT

IntroductionPrinciplesTypes InterpretationClinical ApplicationsLimitations & AdvantagesLatest Developments

INTRODUCTION

Optical coherence tomography, or OCT is a non-contact, noninvasive imaging technique used to obtain high resolution 10 micro 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.

OPTICAL COHERENCE TOMOGRAPHY

Optical Coherence Tomography, or OCT, is a noncontact, noninvasive imaging technique used to obtain high resolution cross-sectional images of the retina and anterior segment.

Three-dimensional imaging technique with ultrahigh spatial resolution

Measures reflected light from tissue discontinuities

Based on interferometry.

OPTICAL COHERENCE TOMOGRAPHY-THE PROCESS IS SIMILAR TO THAT OF ULTRASONOGRAPHY, EXCEPT THAT LIGHT IS USED INSTEAD OF SOUND WAVES. 

Analog to ultrasound

PRINCIPLEOCT 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

Light from Reference arm & Sample arm combinedDivision of the signal by wavelengthAnalysis 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.

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

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

1. Time domain-OCT

Types of OCT

2. 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

SCANNING TIPS1. Communicate with the doctor regarding the size

and location of the pathology of interest.2. Refer to other images of the pathology, e.g. color

photos and FA.3. Review past OCT exams and repeat scan types

used before.4. Dilate the eye well.5. The patient must keep the forehead against the

bar and the chin in the chinrest, with teeth together.  Use the marker on the headrest to align the patient vertically.  The outer canthus should be even with the line.

6.Use the two buttons near the joystick for freezing and saving scans.  This saves you from having to juggle the joystick and the mouse.

7.Minimize patient fatigue by keeping scan time to a minimum.  Never scan an eye for more than 10 minutes (FDA regulation).

8.Keep the cornea lubricated.  Use artificial tears and have the patient blink when you are not saving a scan pass.

9.Move the instrument on the x and y axis (using the joystick) to work around opacities.

INTERPRETATION &CLINICAL

APPLICATIONS

1. SPECTRALIS-Anterior Segment Module

2. OCT – Posterior Segment Module

SPECTRALIS-ANTERIOR SEGMENT MODULE

New dimension to anterior segment imagingCorneaAngle structure Iris details

Consists of Add-on lens and dedicated software

Compatible with all SPECTRALIS SD-OCT models

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

ANTERIOR SEGMENT OPTICAL COHERENCE TOMOGRAPHY (OCT)

•High-speed anterior segment optical coherence tomography (OCT) offers a non-contact method for high resolution cross-sectional and three-dimensional imaging of the cornea and the anterior segment of the eye.

•Anterior Segment Optical Coherence Tomography enhances surgical planning and postoperative care for a variety of anterior segment applications by expertly explaining how abnormalities in the anterior chamber angle, cornea, iris, and lens can be identified and evaluated

ON THE LEADING EDGE OF ANTERIOR SEGMENT

IMAGING: Mapping of corneal thickness and keratoconus evaluation

Measurement of LASIK flap and stromal bed thickness

Visualization and measurement of anterior chamber angle and diagnosis of narrow angle glaucoma

Measuring the dimensions of the anterior chamber and assessing the fit of intraocular lens implants

Visualizing and measuring the results of corneal implants and lamellar procedures

Imaging through corneal opacity to see internal eye structures

IMAGE SHOWS AN ANTERIOR-CHAMBER ANGLE AS VIEWED WITH GONIOSCOPY AND THE OCT

The latter replaces subjective evaluation with objective measurement.

A NARROW ANGLE IS APPARENT WITH OCT IMAGING, IN THIS CASE 9.5°.

With the increase in popularity of anterior chamber imaging, and anterior segment OCT proving to be the best tool for high resolution biometry, Anterior Segment Optical Coherence Tomography is a must-have for anterior segment, refractive, cornea, and glaucoma surgeons.

OCT – POSTERIOR SEGMENT MODULE

Glaucoma

ONH analysis

Retina

Choroid

GLAUCOMADiagnosis 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

POSTERIOR POLE ASYMMETRY ANALYSIS

Combines mapping of the posterior pole retinal thickness with asymmetry analysis

Both eyes

Hemispheres of each eye

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

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

MACULAR CYST

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 septaeInitially 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.

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 scanOff center artifact Retake the scan/manually plot

the foveaMotion artifact Retake the scanBlink 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 OCTBest axial resolution available so far

Scans various ocular structures

Tissue sections comparable to histopathology sections

Easy to operate

Short scanning time

REF. Internet books >optical coherence tomography- Carmen puliafito

and michael Hee >optical coherence tomography- Gangjun liu Important links: http://www.intechopen.com/books/optical-

coherence-tomography