By By DIPAK KUMAR SAH.DIPAK KUMAR SAH.

B.Optom 3B.Optom 3rdrd year student. year student.

BPKLCOS,Maharajgunj Medical campus.BPKLCOS,Maharajgunj Medical campus.

Optical Coherence Tomography (OCT)

What is OCT?What is OCT?

Diagnostic imaging technique that examines Diagnostic imaging technique that examines living tissue non-invasively. It is based on living tissue non-invasively. It is based on a complex analysis of the reflection of low a complex analysis of the reflection of low coherence radiation from the tissue under coherence radiation from the tissue under examination.examination.

Real time cross sectional analysisReal time cross sectional analysis

OCT allows both qualitative and quantitative OCT allows both qualitative and quantitative analysis of the retina analysis of the retina

Qualitative analysis includes description by Qualitative analysis includes description by location, a description of form and location, a description of form and structure, identification of anomalous structure, identification of anomalous structures, and observation of the structures, and observation of the reflective qualities of the retina reflective qualities of the retina

Quantitative analysis involves Quantitative analysis involves measurements of the retina, specifically measurements of the retina, specifically retinal thickness and volume, and nerve retinal thickness and volume, and nerve fiber layer thickness.  This is possible fiber layer thickness.  This is possible because the OCT software is able to because the OCT software is able to identify and "trace" two key layers of the identify and "trace" two key layers of the retina, the NFL and RPE retina, the NFL and RPE

How does it work?How does it work?

128 to 768 axial samples (A-scans) in a 128 to 768 axial samples (A-scans) in a single "scan pass“single "scan pass“

Each A-scan has 1024 data points and is Each A-scan has 1024 data points and is 2mm long (deep).  2mm long (deep). 

OCT: Basic PrinciplesOCT: Basic Principles

Three-dimensional imaging technique with high Three-dimensional imaging technique with high spatial resolution and large penetration depth spatial resolution and large penetration depth even in highly scattering mediaeven in highly scattering media

Based on measurements of the reflected light Based on measurements of the reflected light from tissue discontinuitiesfrom tissue discontinuities e.g. the epidermis-dermis junction.e.g. the epidermis-dermis junction.

Based on interferometryBased on interferometry interference between the reflected light and the interference between the reflected light and the

reference beam is used as a coherence gate to reference beam is used as a coherence gate to isolate light from specific depth.isolate light from specific depth.

1 mm 1 cm 10 cm

Penetration depth (log)

1 m

10 m

100 m

1 mm

Resolution (log)

OCTConfocalmicroscopy

Ultrasound

Standardclinical

Highfrequency

OCT vs. standard imaging

ResolutionResolution

When all of the A-scans are combined into When all of the A-scans are combined into one image, the image has a resolving one image, the image has a resolving power of about 10 microns vertically and power of about 10 microns vertically and 20 microns horizontally 20 microns horizontally

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

OCTOCT Ultrasound Ultrasound

                                      

                                     

Scan Protocol TypesScan Protocol Types

LineLineCircleCircleRadial LinesRadial Lines

The "line" scan simply scans in a single, The "line" scan simply scans in a single, straight line.  The length of the line can straight line.  The length of the line can be changed as well as the scan angle. be changed as well as the scan angle.

The "circle" scans in a circle instead of a The "circle" scans in a circle instead of a line. line.

The "radial lines" scans 6 consecutive line The "radial lines" scans 6 consecutive line scans in a star pattern scans in a star pattern

The Cross Hair ScanThe Cross Hair Scan Cross Hair Scan performs a high resolution Cross Hair Scan performs a high resolution

horizontal line scan and then automatically horizontal line scan and then automatically flips to a vertical line scan without having flips to a vertical line scan without having to exit the protocol to exit the protocol

This is a common technique used in B-scan This is a common technique used in B-scan ultrasonography ultrasonography

Not All OCT Scans Are Created Not All OCT Scans Are Created Equally Equally

The "fast" scan protocols of the OCT 3 reduce The "fast" scan protocols of the OCT 3 reduce the time needed for multiple scans the time needed for multiple scans

The scan time reduction is intended to minimize The scan time reduction is intended to minimize the error created by patient movement the error created by patient movement

Fast scans grab fewer A-scans in the 6 mm Fast scans grab fewer A-scans in the 6 mm length of the scan.  The normal 6 mm scan length of the scan.  The normal 6 mm scan contains 512 A-scans, whereas the fast 6 mm contains 512 A-scans, whereas the fast 6 mm scan contains only 128 A-scans, resulting in a scan contains only 128 A-scans, resulting in a lower resolution image lower resolution image

Fast OCT 3 scan

The same eye scanned with maximum resolution

Retinal Anatomy Compared to OCTRetinal Anatomy Compared to OCT

The vitreous is the black space on the top The vitreous is the black space on the top of the imageof the image

We can identify the fovea by the normal We can identify the fovea by the normal depression depression

The nerve fiber layer (NFL) and the retinal The nerve fiber layer (NFL) and the retinal pigment epithelium (RPE) are easily pigment epithelium (RPE) are easily identifiable layers as they are more highly identifiable layers as they are more highly reflective than the other layers of the retina reflective than the other layers of the retina

This higher reflectivity is represented by This higher reflectivity is represented by the "hotter" colors (red, yellow, orange, the "hotter" colors (red, yellow, orange, white) in the false color representation of white) in the false color representation of the OCT 3. the OCT 3.

The middle layers of the retina, between The middle layers of the retina, between the NFL and RPE, are much less easily the NFL and RPE, are much less easily identifiable in the scan. identifiable in the scan.

Optical Coherence Tomography

This is what we wanted…

Optical Coherence Tomography

…this is what we got…

internal limiting membrane

nerve fiber layer

ganglion cell layer

inner plexiform layer

inner nuclear layer

outer plexiform layer

outer nuclear layer

outer limiting membrane

photoreceptor inner segments

photoreceptor outer segments

retinal pigment epithelium

choriocapillaris

Interpretation of OCT images

Layers of the retina

Nervefiberlayer

Ganglioncell layer

Innerplexiformlayer

Innernuclearlayer

RPE and choriocapillaris Outer and inner External limiting Outer Outerphotoreceptor membrane nuclear plexiformsegments layer layer

Interpretation of OCT images

Layers of the retina

Nerve fiber layer

Ganglion cell layer

Inner plexiform layer

Inner Nuclear layer

Outer plexiform layer

Outer nuclear layer

External limiting membrane

Inner/outer segment junction

RPE

Larger choroidal vessels

300 !m____

High resolution spectral OCT

Image of good quality

Out of focus

Vignetted image

Fixation error

The pre-retinal profileThe pre-retinal profile

A normal pre-retinal profile is black space A normal pre-retinal profile is black space Normal vitreous space is translucent Normal vitreous space is translucent The small, faint, bluish dots in the pre-The small, faint, bluish dots in the pre-

retinal space is "noise" retinal space is "noise" This is an electronic aberration created by This is an electronic aberration created by

increasing the sensitivity of the instrument increasing the sensitivity of the instrument to better visualize low reflective structures. to better visualize low reflective structures.

Anomalous structures Anomalous structures

pre-retinal membranepre-retinal membraneepi-retinal membraneepi-retinal membranevitreo-retinal strandsvitreo-retinal strandsvitreo-retinal tractionvitreo-retinal tractionpre-retinal neovascular membranepre-retinal neovascular membranepre-papillary neovascular membranepre-papillary neovascular membrane

A pre-retinal membrane with traction on the foveaA pre-retinal membrane with traction on the fovea

a pigment epithelial detachment is causing the convexity

Aside from the retinal detachment,

notice the underlying concave curvature of the retina, suggesting the long eye of a significant myope

Deformations in the foveal profile Deformations in the foveal profile

macular puckermacular pucker macular pseudo-holemacular pseudo-hole macular lamellar holemacular lamellar hole macular cystmacular cyst macular hole, stage 1 (no depression, cyst macular hole, stage 1 (no depression, cyst

present)present) macular hole, stage 2 (partial rupture of retina, macular hole, stage 2 (partial rupture of retina,

increased thickness)increased thickness) macular hole, stage 3 (hole extends to RPE, macular hole, stage 3 (hole extends to RPE,

increased thickness, some fluid)increased thickness, some fluid) macular hole, stage 4 (complete hole, edema at macular hole, stage 4 (complete hole, edema at

margins, complete PVD)margins, complete PVD)

Macular cyst Macular cyst

Macular hole, stage 2 Macular hole, stage 2

Macular hole, stage 3 Macular hole, stage 3

Macular hole, stage 4, operculum Macular hole, stage 4, operculum

suspended by the hyaloid membranesuspended by the hyaloid membrane

The macular profileThe macular profile

The macular profile can, and often does,  The macular profile can, and often does,  include the fovea as it's center include the fovea as it's center

Deformations in the macular profile Deformations in the macular profile

Serous retinal detachment (RD)Serous retinal detachment (RD)Serous retinal pigment epithelial Serous retinal pigment epithelial

detachment (PED)detachment (PED)Hemorrhagic pigment epithelial Hemorrhagic pigment epithelial

detachmentdetachment

Serous retinal pigment epithelial Serous retinal pigment epithelial detachment (PED) detachment (PED)

Intra-retinal anomalies in the Intra-retinal anomalies in the macular profile macular profile

Choroidal neovascular membraneChoroidal neovascular membrane Diffuse intra-retinal edemaDiffuse intra-retinal edema Cystoid macular edemaCystoid macular edema DrusenDrusen Hard exudatesHard exudates Scar tissueScar tissue Atrophic degenerationAtrophic degeneration Sub-retinal fibrosisSub-retinal fibrosis RPE tearRPE tear

Choroidal neovascular membrane Choroidal neovascular membrane

Cystoid macular edema cause by Cystoid macular edema cause by diabetic maculopathy diabetic maculopathy

Sub-retinal fibrosis Sub-retinal fibrosis

Diabetic Macular EdemaDiabetic Macular Edema

Sponse like retinal thickening.Sponse like retinal thickening.Cystoid macular edema.Cystoid macular edema.Serous retinal detachment.Serous retinal detachment.Foveal tractional retinal detachment.Foveal tractional retinal detachment.Taut posterior hyaloid membrane.Taut posterior hyaloid membrane.

Retinal TraumaRetinal Trauma

Retinal DetachmentRetinal Detachment

Followed by scleral buckling Followed by scleral buckling and PPVand PPV

Retinal pigment epithelium irregularities.Retinal pigment epithelium irregularities.Neurosensory atrophy.Neurosensory atrophy.CMECMERetinal pigment epithelium hyperplasia.Retinal pigment epithelium hyperplasia.

Intraocular TumorIntraocular Tumor

Retinal leukemic Retinal leukemic infiltrate.infiltrate.

Degenerative MyopiaDegenerative Myopia

Postoperative EndophthalmitisPostoperative Endophthalmitis

GlaucomaGlaucoma

Structural Damage precedes Functional Structural Damage precedes Functional loss.loss.

Pre-perimetric Glaucoma.Pre-perimetric Glaucoma.RNFL measurement.RNFL measurement.Macular Thickness.Macular Thickness.ONH Analysis.ONH Analysis.

Normal Vs Glaucomatous optic Normal Vs Glaucomatous optic cup.cup.

OCT and Fluorescein OCT and Fluorescein Angiography in retinal diagnosisAngiography in retinal diagnosis

FAs provide excellent characterization of FAs provide excellent characterization of retinal blood flow over time, as well as size retinal blood flow over time, as well as size and extent information on the x and y axis and extent information on the x and y axis (north-south, east-west) (north-south, east-west)

The OCT gives us information in the z The OCT gives us information in the z (depth) axis, telling us what layers of the (depth) axis, telling us what layers of the retina are affected retina are affected

Scan analysis protocols for Scan analysis protocols for qualitative analysisqualitative analysis

Line scans can be viewed with a variety of Line scans can be viewed with a variety of analysis tools (see the OCT manual).  I analysis tools (see the OCT manual).  I have found the "Align process" to be the have found the "Align process" to be the most useful, with the "Proportional" most useful, with the "Proportional" analysis a good choice if the align process analysis a good choice if the align process is not needed is not needed

The Align ProcessThe Align Process

This tool "straightens" motions artifacts This tool "straightens" motions artifacts

Proportional analysis Proportional analysis Proportional analysis produces an image Proportional analysis produces an image

with its true horizontal and vertical with its true horizontal and vertical proportions proportions

Retinal Thickness AnalysisRetinal Thickness Analysis

Using the retinal thickness analysis tool, the Using the retinal thickness analysis tool, the software then traces a line along the NFL software then traces a line along the NFL layer and a line along the RPE layer. layer and a line along the RPE layer.

The software then measures the distance The software then measures the distance between the two lines and a graph is between the two lines and a graph is produced which compares the measured produced which compares the measured thickness to the thickness of a normal thickness to the thickness of a normal retina retina

Each of the six scans can be reviewed by Each of the six scans can be reviewed by clicking on the slider bar to the left, and any clicking on the slider bar to the left, and any

or all of them can be printed out for the or all of them can be printed out for the patient's record patient's record

Retinal thickness analysis does not measure Retinal thickness analysis does not measure retinal retinal elevationelevation

for example this eye with a pigment epithelial for example this eye with a pigment epithelial detachment (PED) pictured below.  The arrow detachment (PED) pictured below.  The arrow on the left would represent retinal elevation, on the left would represent retinal elevation, from the choroid, through the fluid space of the from the choroid, through the fluid space of the PED, to the nerve fiber level.  The arrow on the PED, to the nerve fiber level.  The arrow on the right shows what the analysis measures, defined right shows what the analysis measures, defined by the distance from the RPE (which is by the distance from the RPE (which is detached) to the NFL detached) to the NFL

Retinal Thickness/Volume Retinal Thickness/Volume Change AnalysisChange Analysis

Two FMT scans on the same eye, but taken on Two FMT scans on the same eye, but taken on different dates, can be selected at the same time different dates, can be selected at the same time while holding down the "ctrl" key.  "Retinal while holding down the "ctrl" key.  "Retinal Thickness/Vol Change is chosen from the Thickness/Vol Change is chosen from the analysis tab. analysis tab.

The analysis will give you a "change map", The analysis will give you a "change map", showing the difference between the two showing the difference between the two scans scans

Glaucoma ScansGlaucoma Scans

When evaluating the glaucoma suspect When evaluating the glaucoma suspect or the glaucoma patient, two or the glaucoma patient, two parameters that the ophthalmologist is parameters that the ophthalmologist is interested in are the characteristics of interested in are the characteristics of the optic nerve cup and the thickness the optic nerve cup and the thickness of the nerve fiber layer surrounding the of the nerve fiber layer surrounding the optic nerve headoptic nerve head

The Fast Optic Disc scanThe Fast Optic Disc scan The Fast RNFL Thickness scanThe Fast RNFL Thickness scan

The Fast Optic Disc scanThe Fast Optic Disc scan   

The optic cup profile can be evaluated by The optic cup profile can be evaluated by capturing a "Fast Optic Disc" scan capturing a "Fast Optic Disc" scan

The patient fixes on the target, which is The patient fixes on the target, which is automatically placed at the edge of the scan automatically placed at the edge of the scan window so that the optic nerve is viewed toward window so that the optic nerve is viewed toward the center of the video window.  The operator the center of the video window.  The operator then moves the scan so that the star pattern is then moves the scan so that the star pattern is centered on the optic nerve head.  Centering centered on the optic nerve head.  Centering can be aided by clicking on the scan window to can be aided by clicking on the scan window to view the white centering lines. view the white centering lines.

The optic nerve scan can be analyzed with The optic nerve scan can be analyzed with the "optic nerve head analysis" protocol the "optic nerve head analysis" protocol

The Fast RNFL Thickness scanThe Fast RNFL Thickness scan

Nerve fiber layer thickness can be evaluated with Nerve fiber layer thickness can be evaluated with the "Fast RNFL Thickness" scan.  This is a the "Fast RNFL Thickness" scan.  This is a circular scan that requires the operator to place circular scan that requires the operator to place the circle so that the center of the circle is the circle so that the center of the circle is centered on the optic nerve head. centered on the optic nerve head.

The analysis software places lines on the top and The analysis software places lines on the top and bottom of the nerve fiber layer and the distance bottom of the nerve fiber layer and the distance between the two lines is interpreted to be the between the two lines is interpreted to be the thickness of the nerve fiber layer thickness of the nerve fiber layer

Care must be take to make sure that the image is Care must be take to make sure that the image is captured with the circle centered on the optic captured with the circle centered on the optic nervenerve

The placement of the circle can make a big The placement of the circle can make a big difference in the analysis of the nerve fiber layer difference in the analysis of the nerve fiber layer thickness thickness

These two scans (OD) are of a normal eye.  The These two scans (OD) are of a normal eye.  The scan in the first analysis is well centered and the scan in the first analysis is well centered and the RNFL thickness falls within the normal range.  RNFL thickness falls within the normal range.  The scan in the second analysis is of the same The scan in the second analysis is of the same eye (OD), but the scan is not well centered.  The eye (OD), but the scan is not well centered.  The analysis is abnormal (black arrows). analysis is abnormal (black arrows).

Reflectivity may be further enhanced by Reflectivity may be further enhanced by moving the focus knob on the side of the moving the focus knob on the side of the OCT unit.  OCT unit. 

Is it Perfect?Is it Perfect?

Scanning with the OCT suffers from a lack Scanning with the OCT suffers from a lack of registration and questionable of registration and questionable repeatability.  Until improvements in the repeatability.  Until improvements in the hardware and software improve or hardware and software improve or eliminate these problems, eliminate these problems, operator skilloperator skill will play a major roll in the quality of OCT will play a major roll in the quality of OCT scanning. scanning.

What makes a good OCT scan?What makes a good OCT scan?

A good quality OCT scan has good A good quality OCT scan has good reflectivity from edge to edge.reflectivity from edge to edge.

    The "hotter" colors (orange, red, white, The "hotter" colors (orange, red, white, yellow) are maximized yellow) are maximized

Generally, the retina should be in the Generally, the retina should be in the lower portion of the scan window so that lower portion of the scan window so that the vitreous can be images as well. the vitreous can be images as well.

Scanning TipsScanning Tips Communicate with the doctor regarding the size Communicate with the doctor regarding the size

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

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

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

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

Scanning TipsScanning Tips Use the two buttons near the joystick for Use the two buttons near the joystick for

freezing and saving scans.  This saves you freezing and saving scans.  This saves you from having to juggle the joystick and the from having to juggle the joystick and the mouse. mouse.

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

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

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

What’s NewWhat’s New

OCT better understanding (FA and ICG)OCT better understanding (FA and ICG) Increase in resolution to 5 micronsIncrease in resolution to 5 micronsOverlays, 3D imagingOverlays, 3D imaging

Questions?Questions?

References:References:  

Brancato R. and Lumbroso B. Guide to Brancato R. and Lumbroso B. Guide to Optical Coherence Tomography Optical Coherence Tomography

Interpretation. Rome: Innovation-News-Interpretation. Rome: Innovation-News-Communication, 2004.Communication, 2004.

  Schuman J., Puliafito C., and Fujimoto J. Schuman J., Puliafito C., and Fujimoto J. Ocular Coherence Tomography of Ocular Ocular Coherence Tomography of Ocular Diseases. Thorofare NJ: Slack Inc., 2004.Diseases. Thorofare NJ: Slack Inc., 2004.