OPTICAL
COHERENCE TOMOGRAPHY (OCT) IN MACULAR DEGENERATION
S
NATARAJAN, JAYASHREE BARUA, SHAHANA MOZUMDAR,ANAND BAGMAR, SONAL V LAKDAWALLA
JADHAV,ANJILI HUSSAIN, NAZIMUL HUSSAIN
Aditya Jyot Eye Hospital, Aashirwad,
168 D, Vikas Wadi, Dr. Ambedkar Road, Dadar TT, Mumbai - 400 014.
OCT is a new diagnostic imaging technique
that uses near infrared light to produce cross-sectional images of tissue. It
is a useful adjunct to thorough clinical examination and standard diagnostic
procedure in the diagnoses of macular disorders and in glaucoma.
INTRODUCTION
-New diagnostic technology
-High resolution
(Axial : 10 µm;
Transverse : 20 µm; Reproducibility; < 11 µm)
-Cross-sectional
-Quantitative digital
imaging of the retina
Optical diagnostics can be performed
without physical contact but are limited to tissues that are optically accessible.
OCT is an imaging technique which can provide tomographic or micron resolution
cross-sectional images of intraocular structures. It thus provides information
which is complementary to retinal angiography.
Swanson EA, Huang D, Fujimoto JG,
Puliafito CA, Lin CP, Schuman JS, working at Boston, Massachusetts, recognized
the importance of low-coherence interferometry for biomedical diagnosis and
obtained a patent in 1994.[1]
Salient Features
*Non-invasive
*Non-contact
*Uses near infrared, low coherent light
*Cross-sectional scan of the tissue
*Tomographic representation
of retinal layers
*High resolution, approx. 10 µ, images
*Analogous to ultrasound B-mode imaging (where resolution is
upto 150 µm)
*Complementary to fundus
photography and FFA
Limitations
* Clear
media needed
*Pupillary
dimeter of approx. 4 mm
*Costly
Agenda
*OCT Theory
*OCT Interpretation
*Clinical Applications
*OCT Theory
Key Concepts
* Light source : Superluminescent Diode (SLD) 820 nm (near infrared)
*Interferometry
*Coherence (Low coherence
= High resolution)
*Tomography
Principle
1. Low coherence interferometer
- Michaelson. Interferometer compares one optical beam with another to perform
high resolution optical measurement.[2,3]
In OCT, measurements of distance
and microstructures are performed by the reflecting light waves from the microstructures
within the eye analogous to sound waves in ultrasonography. High resolution
structural measurements to the tune of 10 µ (much higher than the 150
µ possible with B-Scan Ultrasonography) is made possible by comparing
one optical beam with another. This is achieved by causing interference of the
two beams by an interferometer.[4,5]
The interferometer measures the time delays of the echoes by comparing the beam
with the reference beam and performs high resolution measurements of the structures.
*An optical beam of short
coherence light is split into two by a beam splitter to create an examination
beam and a reference beam (Fig. 1).
*The examination beam
is directed into the patient’s eye and the reference beam is reflected from
a reference mirror at a known spatial position.
*The reflected light
from the patient’s eye consists of multiple echoes which give information
about the distance and thickness of different intraocular structures.
*The simplest measurement
is that of axial length.
*Tomographic or cross-sectional
images are constructed by performing successive axial measurements at different
transverse points.
*The data is processed
by the computer and displayed as a two-dimensional gray scale.
*False colour is then
imparted by a colour code based on reflectivity.
*The highest reflections
are represented by red and white colours and the lowest and represented by
blue and black.
2. Optical properties
of tissues : Light incident on a tissue is either i) transmitted, ii)
absorbed or iii) scattered.
Transmitted light interacts
with deeper tissues, absorbed light is converted to heat by the chromophores.
The direction of light is altered when it is scattered. In most tissues scattering
predominates over absorption. Both scattering and absorption attenuate further
light propagation and cause shadowing.
Reflectivity of a tissue : The intensity of reflections is a measure
of the discontinuity of the optical properties of the tissue. Reflectivity
is proportional to the number of cells present. Strong reflections occur from
reflective tissue as well as at boundaries between two media with different
refractive indices.
The OCT Signal : May be considered to be exclusively comprised to light
that has undergone just a single back scattering event. Hence, the OCT signal
from a particular tissue is a combination of its reflectivity and the absorption
and scattering properties of the overlying areas i.e.
OCT signal from a tissue = light reaching the tissue + tissue reflectivity
Instrument
*Super luminescent
diode light source integrated to a standard slit lamp biomicroscope with
a fibreoptic delivery system
*+78D condensing lens
used
*200 µ W of 830
nm infrared light delivered
*Each retinal scan
image is displayed on a computer monitor
*Bright colours-high
optical reflectivity (red to white)
*Dark colours-low optical
reflectivity (blue to black)
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Fig. 2: Normal
retina (Macula).
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Fig. 3
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Fig. 4: OCT Scans-
Disc Vs Macula.
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Fig. 5
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Fig. 6
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Fig. 7
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OCT Image Interpretation
1.Identify the 2 red
bands (Retinal Nerve Fibre Layer-RNFL and RPE/Choriocapillaries-RPE-CC layer).
2.Identify the broad
green/yellow band (Ganglions cell layer)
3.Identify the blue/black
band (If present) (Photoreceptor layer)
4.Consider what would
cause the result
Toth et al 1997[4]
found a good correlation between retinal morphology and macular OCT. The first
step in image interpretation is to identify the two red bands i.e. from the
RNFL and the RPE-CC which provide the orientation for further interpretation.
(False colour - imparted by a colour based on reflectivity. The highest reflections
- red and white colours and the lowest- blue and black).
*Vitreoretinal interface - demarcated
by the contrast between the non-reflective vitreous and the backscattering
surface of the retina.
*Retinal Nerve Fibre Layer - seen as a bright backscattering red layer which
thickens from macula to the disc.
*Intermediate layer - exhibits moderate backscattering from the fibrous inner
and outer plexiform layers.
*Photoreceptor layer - exhibits minimum backscattering.
*Blood vessels - may be identified
by increased backscatter and shadowing effects.
*RPE-CC complex- seen as a highly reflective red layer.
*Fovea - is identified by its characteristic depression which reaches its
maximum depth at the fovea centralis.
*Towards the disc the nerve fibre layer (NFL) increases in thickness till
it occupies nearly the entire thickness, commensurate with the presence of
superior and inferior arcuate nerve fibre bundles. The papillomacular axis
exhibits a thinner NFL. The termination of the choroid at the lamina cribrosa
is delineated. Circular tomograms best display the NFL thickness and degenerations
in the peripapillary regions.
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Fig. 8: Full
thickness macular hole.
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Fig. 11
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Fig. 12
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Fig. 13
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Fig. 14
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Fig. 15: Normal
disc.
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Fig. 16: Swollen
disc.
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Fig. 17: Atropic
disc.
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Alterations in Retinal Morphology
Retinal thickness
*Increased with accumulation
of intraretinal fluid (e.g. diabetic retinopathy, cystoid macular oedema,
retinal traction evident by the visualization of the posterior hyaloid or
an epiretinal membrane exerting traction).
*Decreased with scarring or atrophy of the retinal tissue.
Reflectivity : May be affected by amount of incident light reaching the
tissues and is decreased uniformly by abnormalities of the media. A small
pupil allows view of the parts of the scanned length.
*Increased reflectivity caused by inflammatory infiltrate, fibrosis,
exudates and haemorrhage. Highly reflective tissue, like large clumps of exudates
and haemorrhage, cause shadowing by attenuating further progression of light.
*Decreased reflectivity is seen with retinal oedema, hypopigmentation
of the RPE and as shadowing behind highly reflective regions.
Foveal changes : Include presence of a defect as in a macular hole, contour
alterations such as steepening with ERM and flattening as in impending macular
hole, foveal oedema and vitreoretinal traction.
Clinical Applications
Lesions seen on OCT may be broadly
classified as
1.Detached retinal layers-Schisis, CSR, PED
2.Oedematous, swollen retina - Diabetes, Uveitis, Vein occlusion, Post-surgical
3."Missing" retina - macular hole
4."Extra" retina/extra tissue - Epiretinal membrane, choroidal neovascular
membrane
5.Glaucoma/Optic disc
1. Detachments
* Retina - seen as optically
clear space between neurosensory retina and the RPE-CC layer.
CSCR : Neurosensory elevation over
an area of low reflectivity
Central Serous Choroiretinopathy : OCT is highly sensitive to even small
elevations of the neurosensory retina. It can easily distinguish between a
CSR and a PED. It is possible to quantify the extent and height of the elevation
and serial measurements can be used for the follow up of these patients.
Pigment Epithelium - Serous Subfoveal PED:Well-defined elevation of
the RPE layer over an area of low reflectivity with associated neurosensory
elevation.Serous PED is seen
as a dome-shaped elevation of the highly reflective RPE layer with an acute
angle of detachment.
Haemorrhagic PED is seen as an elevated RPE layer with moderate backscatter
under the RPE layer and severe attenuation of the rays.
Fibrovascular PED shows a mild to moderate reflectivity beneath the elevated
RPE layer but the choroid is visible since there is no attenuation of the
rays.
2. "Missing" Retina
Macular hole - OCT is of great
value in
•Staging : Full thickness macular holes show a clear loss of retinal
tissue at the fovea extending to the RPE. Impending macular hole (Stage I)
is identified early by evidence of perifoveal vitreous detachment and the
presence of clear space beneath the fovea suggesting foveal detachment which
obliterates the foveal pit.
•Differentiation : Lamellar holes and pseudoholes show a central loss
of tissue or a steepened foveal contour. Detachments and macular cysts show
a localized subretinal or intraretinal accumulation of fluid.
•Important anatomic information for surgical planning and post-operative follow-up.
Quantitative information such as the diameter of the hole, extent of subretinal
fluid, etc. can be obtained to help moniter progress or recovery after surgery.
•Screening and monitoring fellow eye
•Can be performed through silicon oil
•Assessment of the vitreoretinal interface. Demonstrates PVD, vitreomacular
traction and surrounding fluid cuff.
3. Choroidal Neovascular Membrane
- OCT
•Clearly delineates RPE and choriocapillaries from neurosensory retina
•Measures retinal thickness
•Characteristic appearances of dry and wet ARMD
Dry non-exudative AMD
•Drusens cause modulations in the highly reflective RPE layer; similar to
a small PED but with shallow margins milder attenuation of light waves.
•In Geographic Atrophy the neurosensory retina is thinned out and the hypopigmented
RPE allows greater penetration of the optical probe beam into the deeper choroid
thus enhancing the reflections from this layer.
Wet Exudative AMD : presents in one of three ways on OCT imaging -
•Classic CNV is seen as a localized disruption and fusiform thickening of
the RPE-CC layer. Presence of subretinal fluid or retinal oedema aids in assessing
activity.
•Fibrovascular PED displays an elevated RPE layer above an area of mild backscattering
region which corresponds to the fibrovascular proliferation. Some occult CNV
present as an area of enhanced choroidal reflectivity due to increased penetration
of the RPE. There may be associated changes in the overlying retina. It also
identifies and quantifies subretinal, intraretinal and sub-RPE fluid which
can be healed or scarred CNV are seen as highly reflective and well demarcated
lesions at the chorioretinal interface.
Fukuchi T, Takahashi K, Ida H, Sho K, Matsumura M[6] staged
idiopathic choroidal neovascularization by OCT as follows-active, intermediate,
or cicatricial, based on past history, fundus findings, and fluorescein angiography
(FAG). OCT revealed that there were characteristic tomographic images of the
choroidal neovascularization (CNV) at each stage.
•In the active stage, OCT revealed the CNV as a highly reflective, multi-layered
area protruding into the subretinal space.
•In the intermediate stage, the reflectivity of the CNV became stronger and
its margin in the subretinal space became smooth.
•With regression of the ICNV, the lesions consisted of two different areas
: a most reflective area corresponding to the fibrotic changes of the CNV
(imaged white in OCT images), and a reddish highly reflective area representing
a compound protrusion of the CNV.
•In the cicatricial stage, the ICNV was observed as a moderately high reflective
area covered by a dome-shaped highly reflective layer corresponding to the
retinal pigment epithelium. Thus OCT is useful for following the clinical
course and understanding the mechanism of the CNV regression.
•Circular tomograms best display the NFL thickness and degenerations in the
peripapillary regions.
FUTURE
•Longer wavelength to increase
penetration through retinal and sub-retinal haemorrhage
Broader bandwidth to increase resolution to single digit micron range. Establish
a normative database stratified by age for NFL thickness.
OCT-3 : OCT-3 is the next generation OCT with several added features.
It has the highest Z resolution in 3mm pupil size. It can distinguish 7-8
layers of the retina as opposed to the 4 layers distinguished by OCT 1 and
2. It causes less ‘patient sensation’ using energy less than 1mW at the cornea.
Scans are obtained at the rate of 500 axial scans per second and the acquisition
time is 0.2-1.6 secs only. Bilateral analysis is possible simultaneously and
normative data for RNFL thickness and macular thickness is available. (The
OCT-3 by Zeiss is developed by Matt Everett and Zeiss team and is pending
FDA approval in US).
CONCLUSION
1.Adjunct to thorough clinical examination and standard diagnostic examinations
such as FFA and visual fields.
2.Powerful diagnostic tool for macular
disorders and in glaucoma.
3.Final diagnostic and therapeutic
decision lie on careful clinical judgement.
R EFERENCES
1.Swanson EA, Huang D, Fujimoto JG, Puliafito CA, Lin
CP, Schuman JS. Method and apparatus for optical imaging with means for controlling
the longitudinal range of the sample. United States Patent 14 June, 1994;
#3,321,501.
2.Youngquist RC, Carr S, Davies DEN. Optical coherence domain
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3.Takada K, Yokohama I, Chida K, Noda J. New measurement system
for fault location in optical waveguide devices based on an interferometric
technique. Appl Opt 1987; 26 : 1603-6.
4.Fercher AF, Mengedoht K, Werner W. Eye length measurement
by interferometry with partially coherent light. Opt Lett 1988; 13 : 1867-9.
5.Fercher AF, Hitzenberger C, Juchem M. Measurement of intraocular
optical distances using partially coherent laser light. J Mod Opt 1991; 38
: 1327-33.
6.Staging of idiopathic choroidal neovascularization by optical
coherence tomography. Fukuchi T, Takahashi K, Ida H, Sho K, Matsumura M. Schepens
Retina Associates, Boston, MA 02114, USA. fukuchi@vision.eri.harvard.edu
