Optical Coherence Tomography

OPTICAL COHERENCE TOMOGRAPHY

Dr Tushya Om ParkashDr Om Parkash Eye Institute

INTRODUCTION

• OCT is a non contact , non invasive , micron resolution cross sectional study of retina which correlates very well with the retinal histology

• It was unbelievable that histopathology without biopsy of a structure which was literally untouchable.(Retina)

It goes back in 1991 when first OCT paper was published by Huang et alFirst in-vivo studies of human retina started in 1993

Evolution

HISTORY OF OCT

1998 20082002

A-Scans/sec 100 400 27,000

Axial resolution 15 microns 10 microns 5 microns

Contrast & Image quality + +++ +++++

10 years of progress in OCT Imaging

OCT 1995

OCT2 2000

OCT3 Stratus OCT 2002

Cirrus HD-OCT 2007

100 A-scans x 500 points

100 A-scans x 500 points

512 A-scans x1024 points

4096 A-scans x 1024 points

100

100

500

27,000

20

20

10

5

Single line scanScans/ second

Resolution (microns)

OCT VS USG• OCT image has a resolving power of about 10 microns vertically and 20 microns horizontally

• Compare that to the resolution of a good ophthalmic ultrasound at 100 microns

• USG needs contact with the tissue under study whereas OCT does not require any contact

Optical Coherence Tomography

THE PRINCIPLE

• 2 or 3 dimentional cross sectional imaging of retina by measuring echo delay and intensity of back reflected infra red light from internal tissue structures

• Combination of low coherence interferometry with a special broadband width light

Based on Principle of Michelson Interferometry• Low coherence infra red light

coupled to a fibre optic travels through beam splitter and is directed through the ocular media to the retina

and a reference mirror

• The distance between the beam splitter and the reference mirror is

continuosly varied

• When the distance between light source and retinal tissue = distance

between light source and reference mirror , the reflected light and the refrence mirror interacts to produce an interference pattern

TYPES OF OCT1. TIME DOMAIN OCT

• In TD-OCT a mirror in the reference arm of the inter -ferometer is moved to match the delay in various layers of the sample• The resulting interference is processed to produce the axial scan waveform• The reference mirror must move one cycle for each axial scan.The need for mechanical movement limits the speed of image acquisition• Further more, at each moment the detection system only collects signal from a narrow range of depth in the sample. This serial scanning is inefficient

2. FOURIER/SPECTRAL DOMAIN OCT

• In FD-OCT the reference mirror is kept stationary.The spectral pattern of the interference between the sample and the reference reflections is measured• The spectral interferogram is fourier transformed to provide an axial scan. The absence of moving part allows the image to be acquired very rapidly• Furthermore,reflections from all layers in the sample are detected simultaneously. This parallel axial scan is much more efficient, resulting in both greater speed and higher signal-to-noise ratio

Difference between Time and spectral domain

• Spectral domain mesures retinal thickness from RPE to ILM• Time domain meares retinal thickness from IS/OS to ILM

THE OCT MACHINE

THE OCT SYSTEM comprises

• Fundus viewing unit

• Interferometric unit

• Computer display

• Control Panel

• Color inkjet printer

PROCEDURE

• Patient is asked to look inside the ocular lens - internal fixation- onto the green target light inside the red rectangular field or external fixation- onto the external target by the other eye in patients with poor vision. The patient is encouraged to blink in between scan acquisition.

• There is no discomfort to the patient and an experienced operator can acquire the required scans within 1-3 mins in each eye. The actual time taken by the machine is 1 sec - the additional time is for patient positioning and optimising scan quality.

PROCEDURE

• Switch on the system: This activates all the components and takes 45 secs to start window

• The menu and toolbar in the main window has several options inclu -ding select patient,acquisition protocol,analysis protocol

• Appropriate category can be selected. Data entry made for a new patient. Apprpriate protocol is selected

PROCEDURE

• A 3mm pupil is necessary for adequate visualisation

• Patient is seated with his chin on the chin rest and eye at the level of the mark on the side of the frame

• Once the patient is seated comfortably, the OCT machine is moved slowly towards the eye to within 1cm, with the joystick till an image appears on the screen

• Then the z offset of the image is optimised to bring the image to the centre. The polarisation is optimised next to create a clear image

• The signal strength of 5 and above gives a clear image

PROCEDURE

• Normally the patient can look at this field for several minutes at a time without discomfort

• During scan alignment , the patient sees the scan pattern in motion on the red field.

• During scan acquisition , the patient sees a bright greenish-white flash light, when the scan image is stored into the camera

• It is possible to acquire the scans without the flash, which is more comfortable to the patient

PRODUCTION AND DISPLAY OF IMAGE

• On Z axis, 1024 points are captured over a 2mm depth to create a tissue density profile, with resolution of 10microns

• On X-Y axis, the tissue density profile is repeated unto 512 times every 5-60 microns to generate cross sectional image. Several data points over 2mm of depth are integrated by the interferometer to construct a tomogram of retinal structures.

• Image thus produced has an axial resolution of 10 microns and a transverse resolution of 20 microns

• The tomogram is displayed in either grey scale or false scale on a high resolution computer screen.

• X and Y(north-south and east-west) and Z axis(depth)

OCT

• The Interference is measured by a photodetector and processed into a signal. A 2D image is built as the light source moves along the retina , which resembles a histology section

• Digital processing aligns the A scan to correct for eye motion. Digital smoothing techniques further improve 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 visualise low reflective structures

• Intraretinal cross sectional anatomy is displayed with an axial resolution <10 microns and transverse resolution of 20 microns

• The Interferometer integrates several data points over 2mm depth to construct a tomogram of retinal structures

• It is a real time tomogram with false colours

• Different colours represent degree of light scattering from different depths of retina

• Highly Reflective structures are shown in bright colours (white and red) and those with low reflectivity are represented by dark colours (black and blue). Intermediate reflectivity by green colour

Axial resolution,or definition, determines which retinal layers can be distinguished. Axial resolution is determined by the light source.

Transverse resolution determines accuracy with which size and separation of features (such as drusen) can be identified. Transverse resolution is determined by optics of the eye, as limited by pupil size, and as corrected by the scanner.

High definition and High resolution

INTERPRETATION OF OCT IN CLINICAL CONDITIONS

TYPES OF MACHINE SCANS

• Posterior segment scan 1.Macular cube scan 2.Glaucoma RNFL thickness analysis scan

• Anterior segment scan

OCT INTERPRETATION

• 2 MODES OF INTERPRETATION - Objective & Subjective For accurate interpretation both have to be combined

• OCT reading must be done in 2 stages :1.Qualitative and quantitative analysis2.Deduction and synthesis

OCT INTERPRETATION

Qualitative Analysis

• Morphological studies - - Overall retinal structural changes, changes in retinal outline , retinal structural changes and morphological changes in the post layers - Anomalous structures- pre/epi/intra/sub retinal

• Reflectivity study - hyper/hypo/ shadow areas

Quantitative Analysis

• Thickness, Volumetery and shadow areas

INTERPRETATION OF RETINAL SCAN

• Vitreous anterior to retina is non reflective and is seen as a dark space.

• Vitreo retinal interface is well defined due to contrast between the non reflective vitreous and backscattering retina.

• Retinal layers are represented as below1. Anterior boundary of retina formed by highly reflective RNFL is seen as a red layer due to bright back scattering.2. Posterior boundary of retina is also seen as a red layer representing highly reflective retinal pigment epithelium(RPE) and chorio capillaries3.Outer segment of retinal photoreceptors, being minimally reflective are represented by dark layer just anterior to RPE-Choriocapillaries complex

• Different intermediate layers of neurosensory retina between the dark layer of photoreceptors and red layer of RNFL are seen as an alternating layer of moderate and low reflectivity

OS

IS/OS

ELM

RPE

IS

ILM

GCL

NFL

Choroid

IPL

INL

OPL

ONL

Cirrus HD-OCT Healthy Macula

• NFL and plexiform layers are highly reflective due to horizontal oriented axonal structure• RPE and Choriocapillaries due to high melanin and vascular content respectively• Retinal thickness is directly proportional to reflectivity• IS/OS junction is also hyper reflective and plexiform layers to some extent• Reflectivity Red-green-yellow-blue-black• Vitreous anterior to retina is non reflective and is seen as a dark space.• Vitreo retinal interface is well defined due to contrast between the non reflective vitreous and backscattering retina.

The Foveal Profile

The normal foveal profile is a slight depression in the surfaceof the retina

• Hyperreflective areas(white and red) 1.Superficial- ERM, Haemorrhage,cotton-wool spots 2.Intraretinal- hard exudates,haemorrhage 3.Deep- Drusen,SRNV,nevi,RPE Hyperplasia

• Hyporeflective areas(black and blue) 1.Intraretinal- Fluid,Cysts 2.Deep- RPE Detachments 3.Shadow areas- screened 4.Anterior-Asteroid bodies, Vitreous Haemorrhage

Macular thickness is compared to an age-matched normative database as indicated by a stop-light color code

Macular Thickness Normative data

Macular Change Analysis

ETDRS grid with thickness values is overlaid on retinal thickness maps.

Change analysis map shows variance from baseline, in micrometers, and represented in color

Advanced Visualization

The Tissue Layer image allows you to isolate and visualize a layer of the retina. The thickness and placement of the layer are adjustable. This provides an optical biopsy of the retina by extracting the layer of interest

Automatic fovea finder

Fovea center = 255, 71 Scan center = 255, 64

Macula Thickness Analysis is aligned with fovea location (left)

Resulting analysis may differ from analysis aligned on scan center (right)

Automatic fovea finder

Each high definition line is comprised of 4096 A-scans Rotation, length of lines and height of scan area can be adjusted.

Custom 5-Line Raster Scan

REGIONS

For purpose of analysis , the OCT image of the retina can be subdivided vertically into four regions

• The Pre-retina

• The Epi-retina

• The Intra-retina

• The Sub-retina

INDICATIONS FOR POSTERIOR SEGMENT SCAN

1. Anomolous structures seen in pre retinal area

• Epi retinal membrane

• Vitreo-retinal traction

• Posterior vitreous detachment

• Macular pucker

• Macular pseudo-hole

• Macular lamellar hole

• Macular cyst

• Macular hole, stage 1(no depression,cyst present)• Macular hole stage 2(partial rupture of retina,increased thickness)• Macular hole stage 3(extends to RPE,increased thickness,some fluid)• Macular hole,stage 4(complete hole,edema at margins, complete PVD)

2. Deformations in foveal profile

• Choroidal neovascular membrane• Diffuse intraretinal oedema• Cystoid macular edema• Drusen• Hard exudates• Scar tissue• Atrophic degeneration’• Sub-retinal fibrosis• RPE tear

3. Intraretinal and subretinal anomalies

Posterior Vitreous Detachment

• Syneresis of vitreous gel• liquified vitreous gains entry to the retro hyaloid space through a defect in the posterior hyaloid face• Seen as thin faint hyper reflective line above the surface of the retina

Macular Hole stage 1

• Hyperreflective Vitreo macular traction band• With fovea and foveolar detachment causing a schisis cavity

Macular stage 2

• Dehiscence of the wall of schisis cavity associated with vitreomacular traction

Macular stage 3

• Formation of operculum

Macular stage hole 4

• Full thickness macular hole with complete posterior vitreous detachment

Epi-Retinal Membrane

• Normal foveal contour is lost • HYper reflective band seen over the ILM suggestive of epi retinal membrane

Fundus Image

Thickness Map Overlay

OCT Image

ILM Layer

Thickness Map

Fly-through movie

Epiretinal Membrane HD Images

Retinal pigment epithelial detachment

• Loss of normal foveal contour• Non reflective area suggesting of serous fluid which is causing the PED

3D Volume RenderingCube can be manipulated for visualization of various aspects

Advanced Visualisation

3D Volume Rendering with RPE layer exposed

Advanced visualisation

Cystoid macular edema

• Loss of foveal contour• Increased thickness in neurosensory retina• Cystoid spaces- honey comb like

Central sereous chorioretinopathy

• Loss of normal foveal contour• Non reflective area which is separating the neurosensory retina from RPE

Choroidal neovascular membrane

• Hyperreflective band beneath the RPE causing its detachment• Posterior shadowing towards the choroid suggestive of cnvm

Sub retinal fibrosis

• Hyperreflective band seen beneath the RPE with irregularity of RPE

Drusens- seen between bruch’s membrane and RPE

• hyperreflective bumpy RPE with localised PED

NORMAL OCT OF OPTIC DISC

• TSNIT graph• Double Hump PAttern• OCT helps in detecting RNFL loss even with no VF defects in disc suspects and ocular hypertensives(in stages of undetectable and asymptomatic) before progressing to stage of functional impairment• Clinically inferior and average RNFL thickness are most commonly used as baseline measurement and follow up of glaucoma suspects• In manifest glaucoma patient, RNFL region with least measurements is followed up

• The ONH analysis depends upon automated detection of the ends of RPE by software and the distance between these ends is taken as the optic disc diameter

NORMAL OCT OF OPTIC DISC

CALCULATION CIRCLEAutoCenter™ function automatically centers the 1.73mm radius peripapillary calculation circle around the disc for precise placement and repeatable registration. The placement of the circle is not operator dependent. Accuracy, registration and reproducibility are assured.

OPTIC DISC CUBE SCANThe 6mm x 6mm cube is captured with 200 A-scans per B-scan, 200 B-scans.

Identifying and Monitoring RNFL Loss

Glaucoma - RNFL thickness analysis

DIFFICULTIES AND LIMITATIONS

• Limited by intraocular media opacities , which attenuate measurement beam and reflected light• Non cooperative patient• Expensive

ARTIFACTS

• Artifacts in the OCT scan are anomalies in the scan that are not accurate images of actual physical structures, but are rather the result of an external agent or action.  • Notice the large gap in the middle of the scan below.  This is an artifact caused by a blink during scan acquisition.  The was a high resolution scan, which takes about a second for the scan pass, which is plenty of time to record a blink.

The scan below has waves in the retinal contour.  These are not retinal folds, but rather movement of the eye during the scan pass.

Anterior Segment OCT

Two new scan patterns

Anterior Segment 5-line raster 3 mm length, adjustable rotation and spacing

Anterior Segment 512x128 cube scan. 4mmx 4mm

Cirrus HD-OCT Anterior Segment Imaging, a new indication for use, received FDA clearance in May, 2009.“…It is indicated for in-vivo viewing, axial cross-sectional,

and three-dimensional imaging and measurement of anterior and posterior ocular structures, including cornea, retina, retinal nerve fiber layer, macula, and optic disc. . .”

INDICATIONS FOR ANTERIOR SEGMENT SCAN

•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

Cirrus HD-OCT scan of normal cornea. Layers identified with colored arrows as follows: tear film (blue), epithelium (white), Bowman’s layer (red), Descemet’s/endothelium (green).

Image shows an anterior-chamber angle as viewed with gonioscopy and the OCT

Scleral spur is more reflectiveCiliary body is less reflective

Corneal ectasia

• Diffuse corneal thinnig probabaly suggestive of Post lasik ectasia

Keratoconus

• Conical cornea with central stromal thinning

Tumor of the iris

• Obscuring the angle

Tumor of ciliary body

Narrowing of angle of anterior chamber

• Scleral spur (red arrow)more reflective

• Schlemm’s canal (blue arrow)

• Schwalbe’s line (green arrow)

RECENT ADVANCES

• OPMI LUMERA 700 and RESCAN 700 from ZEISS

Now with integrated intraoperative OCT(Inbuilt OCT in microscope)A new dimension in visualizationInnovation in eye care starts with the desire to see more. With the first surgical microscope and the first commercial OCT for ophthalmic applications, two gold standards have now been fused together into one system – ushering in a new era of surgical microscopes. See more:

• during surgery• with real-time HD-OCT• for better decision making

THANK YOU