OCT

OPTICAL COHERENCE TOMOGRAPHY
BY: Dr Vaibhav Khanna
Dept Of ophthalmology
KIMS, Hubli

PHYSICS
• WAVELENGTH – The distance over which the
wave’s shape repeats

PHYSICS
• FREQUENCY – It is the number of occurrences
of a repeating event per unit time.
• Wavelength is inversely proportional to
frequency

non contact
non invasive
micron resolution
cross-sectional study of retina
correlates very well with the retinal histology
Principle – Low coherence interferometry
(Michelson interferometer)

INTERFERENCE
In physics , interference is a phenomenon in
which two waves superimpose to form a
resultant wave of greater or lower amplitude

• In physics two waves are coherent if they have
a constant phase difference and same
frequency and are non coherent if there is a
constant changing phase difference
COHERENCE

TOMOGRAPHY
• Tomogram – It’s a two – dimensional image
representing a slice or section through a three
– dimensional object i.e. cross-sectional image
• Combining these tomograms we get a three-
dimensional structure of the object which is
being analyzed

PRINCIPLE
• Effectively ‘optical ultrasound’ with the following differences :
a. Uses near red infrared (830-850nm) light coupled to a fibre
optic system
b. Does not require any contact
• OCT images obtained by measuring
– echo time delay
– intensity of reflected light
• Optical properties of ocular tissues, not a true histological section

The process is similar to that of ultrasonography, except
that invisible light is used instead of sound waves.
Analog to
ultrasound

COHERENCE
• Coherence length
• Coherence time
• Coherence length is dependent on :
a)Wavelength
b)Bandwidth / spectrum (broad or narrow )of light
source
• Any light which remains in the tissue and bounces
back is no more coherent.
That is why depth of reach of OCT is few mm’s
Whereas the confocal microscopy it is 50 microns

• Low coherence near infra-red(monochromatic)
light coupled to a fibreoptic travels through a
beam splitter and is directed through the ocular
media to the retina and a reference mirror
• The distance between the beam splitter and
reference mirror is continuously varied
• When the distance between light source & retinal
tissue = distance between light source and
reference mirror , the reflected light and the
reference mirror interacts to produce an
interference pattern

• The interference is measured by a photo detector
and processes in to a signal. A 2D image is built as
the light source moves along the retina , which
resembles a histology section.
The small faint bluish dots in the pre-retinal space is
noise
NOTE -This is an electronic aberration created by
increasing the sensitivity of the instrument to better
visualize low reflective structures and is corrected by
Digital smoothening technique

• The interferometer integrates several data points over
2mm depth to construct a real time tomogram of
retinal structures with false colours.
• Highly reflective structures are shown in bright colours
(white and red) .
• Those with low reflectivity are represented by dark
colours (black and blue).
• Intermediate reflectivity is shown Green.

CONFOCAL MICROSCOPY –Spatial rejection only
whereas OCT has spatial + coherent rejection
Confocal
pinhole
ANY LIGHT ABOVE AND BELOW THE FOCAL PLANE IS GONNA BE REJECTED BY THE
CONFOCAL PINHOLE –SPATIAL REJECTION

ADVANTAGES
• Its noncontact unlike USG, and noninvasive, unlike FFA,ICG.
• Children easily tolerate it.
• Very helpful for quantitative information about macular
thickness.
• Valuable teaching tool for the ophthalmologist as well as
patient.
DISADVANTAGES
• Media opacity.
• Scan quality depends on the skill of OCT operator.
• Not possible with uncooperative patients.
• Measurement of Fovea Thickness not accurate if scan not
through the center of fovea.

•When all of the A-scans are combined into one image, the
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, or 1/10th of a millimeter.
•The image on the right has 1/10th of the pixels per inch that
the image on the left does. The image on the right would
represent the resolution of the ultrasound as compared to
the resolution of the OCT on the left.

NO MOVING
PARTS
MOVING PARTS
PRESENT

SPECTERAL
DOMAIN OCT
TIME DOMAIN OCT BENEFIT OF
SPECTERAL
DOMAIN
LIGHT SOURCE 840 nm
Broader Bandwidth
820 nm Provides higher
resolution
DETECTOR Spectrometer Single detector No moving parts –
faster acquisition
less motion
artifacts
AXIAL RESOLUTION 6-7 microns 10 microns Better visualization
of retinal layers and
pathology
TRANSVERSE
RESOLUTION
10 microns 20 microns
SCAN DEPTH 2mm 2mm Slightly better
penetration of light
SCAN SPEED About 28,000 A-
scans per second
400 A-scans per
second
Better registration ,
3- D scanning and
analysis

Resolution of an OCT
• Resolution – Is the capability of the
sensor to observe or measure the
smallest object clearly with distinct
boundaries.
• Image resolution is an important
parameter that determines the size
of the smallest feature that can be
visualized
• Axial resolution (is governed by
coherence length )
-Wavelength and
-Bandwidth of the light source
Long wavelength - visualisation of
choroid, laminar pores, etc

 Transverse resolution -
Based on spacing of A-
scans i.e. spot size
Since there is a trade-off
between spot size and
depth of focus , most
commercial OCT systems
use a 20 micron
transverse resolution in
order to have a sufficient
depth of focus.

Procedure
• Machine is activated
• Patients pupils are dilated (however 4mm is
the limit for pupil size during the scan )
• Patient seated comfortably
• Asked to look into the target light in the
ocular lens
• Discouraged to blink
• Protocol selected as per case requirement -A
protocol is simply a pre-determined procedure
or method

• Obtaining a scan
1)Scan acquisition protocol
2)Scan analysis protocol
a)Image processing
b)Quantitative analysis

SCAN ANALYSIS PROTOCOL
IMAGE PROCESSING PROTOCOL
• Normalize: this eliminates background noise and improves signal
strength
• Align: this protocols corrects the errors that have resulted from the
patient movements in the axial direction
• Normal and align : performs both above functions
• Gaussian smoothing: this protocol balances the background noise
and improves colour of the scan image
• Median smoothing: similar to gaussian without removing minor
details.
• Proportional : gives an image that is true in its horizontal and vertical
proportions so that images are not stretched/compressed to fit in the

The Align Process
This tool "straightens" motions artifacts

Proportional analysis
Proportional analysis produces an image with its
true horizontal and vertical proportions

SPECIFIC SCANNING PROTOCOLS
• Macular scans
• Retinal nerve fibre layer analysis
• Optic nerve head analysis

Line scan
• acquire multiple line scans
• Default angle is 0 degree
• Scan line length is usually 5mm
(can be varied)
• But on increasing the length the resolution
decreases

Radial lines-
• Default number is 6 lines
separated by 30degrees
angle- can be altered up to 24
lines
• Default length of each line is
6mm which can be altered but
only after saving the first study.
• Used to determine
entire macular scan and macular
thickness/ volume scan

Macular thickness map-
• same as radial lines except that aiming
circle has a fixed number of lines and
diameter of 6mm.
• Fast macular thickness map: similar to
macular thickness map, but is quick takes
only 1.92 sec to acquire and can be used
for comparative retinal thickness

The Fast Macular Thickness Scan
• The Fast Macular Thickness
Scan consists of 6 radial line
scans in a spoke pattern. It is a
low resolution scan that was
designed for quantitative
analysis (thickness and
volume)
• When scanning the macula,
the patient simply looks at the
fixation target. The center of
the FMT scan lines up with the
fixation target by default

Each of the 6 scans can be viewed individually
by clicking on the thumbnails on the left of
the scan selection screen

Normal macular thickness map appears green (210-270microns)
foveal depression appears blue. normal thickness ranges from
190 –210 microns

• The thickness of the macula is depicted by
colour codes
• Blue150-210 microns
• Green210-270 microns
• Yellow270-320microns
• Orange320-350microns
• Red350-470 microns
• White>470 microns

Raster line
• This protocol provides an option of acquiring
series of lines scans that are parallel, equally
spaced and are in 6-24 in numbers
• Multiple line scans are placed over a rectangular
region , the area of which can be adjusted so as
to cover the entire area of pathology
• This is useful where one wishes to obtain scans at
multiple levels
• Default setting has 3mm square with 6 lines

• 5 Line Raster – 5 six mm lines closely arranged 1 mm
vertically
• Each line consists of 4096 -A scans.
• This scan gives the greatest resolution
• Each Raster lines setis done superior to inferior and each
line moves from nasal to temporal
• Raster line scan is important in CNVM lesion where one
wishes to obtain scans at multiple levels

• Macular cube 200x200 combo
• 6mm square grid ,acquire 200 horizontal scans
lines each composed of 200 A-scans
• Macular cube 512 x 128 combo
6mm square grid acquire 128 horizontal scans each
composed of 512 A scans
Greater resolution in each line from left to right(512 A
scans) but lines are separated further apart giving less
resolution from top to bottom.

En Face scans
• Scans adapted to the natural concavity of the
posterior pole for example :
a) CSR – They allow the study of dimensions and
shape of detachment also giving details of the
shape , thickness , smoothness of the walls
b) Diabetic retinopathy and CME –It gives the
details about the type and extension of retinal
edema with their evolution pattern and stage.

REPEAT
• Enables one to repeat any of the previously
saved protocols using the same set of
parameters that include scan size , angle ,
placement of fixation LED and landmark.
• Landmark – Is a pulsating point of light that
can act as a reference point for telling the
location of the lesion.

The Cross Hair Scan
Cross Hair Scan performs a high resolution
horizontal line scan and then automatically
flips to a vertical line scan without having to
exit the protocol
This is a common technique used in B-scan
ultrasonography

• Section 1: Patient related data, examination date, list and signal strength
• Section 2: Indicates whether the scan is related to macula with its pixel strength (as in
this
• picture) or optic disc cube (It also displays the laterality of the eye: OD
• (right eye), OS (left eye).
• Section 3: Fundus image with scan cube overlay. 3A: Color code for thickness overlays.
• Section 4: OCT fundus image in grey shade.
• Section 5: The circular map shows overall average thickness in nine sectors. It has three
• concentric circles representing diameters of 1 mm, 3 mm and 6 mm, and except for the
• central circle, is divided into superior, nasal, inferior and temporal quadrants. The
central circle has a radius of 500 micrometers.
• Section 6: Slice through cube front. Temporal – nasal (left to right).
• Section 7: Slice through cube side. Inferior – superior (left to right).
• Section 8: Thickness between Internal limiting membrane (ILM) to retinal pigment
epithelium (RPE) thickness map. 8A: Anterior layer (ILM). 8B: Posterior layer (RPE). All
these are 3-D surface maps.
• Section 9: Normative database uses color code to indicate normal distribution
percentiles.
• Section 10: Numerical average thickness and volume measurements.

Scan Quantitative analysis protocol for
retina
1) Retinal thickness (single eye)
Obtained by measuring the distance between
the first highly reflective band corresponds to
vitreoretinal surface & second to the RPE
• Displacement between anterior surfaces of
these interfaces gives the retinal/macular
thickness

2) Retinal Map: 2 maps of Retinal thickness
a) color code
b) numerical values in 9 maps sectors.(table form)
Map diameter default adjusted to 1 ,3 and 6 mm
centered on the macula.

Retinal thickness/volume tabular output
• Gives data table that displays thickness
,volume quadrant average , ratios and
differences among various quadrants

ONLY OCT..????
PATIENT HISTORY
CLINICAL EXAMINATION
(including MOBiRET)
FFA / ICG STUDY
(Assuming it’s a retina case)
OCT
DIAGNOSIS

Regions
For purposes 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

• THICKNESS – 2D
• VOLUME -3D
• Always move from VR interface to Choroid to
study the OCT SCAN
• Comment about the contour of the Fovea if
included in the scan
• Grey and White scans ??
Allows me to assess slight variation in the
intensities of grey and white and make out details
in the scan which is rather difficult by pseudo
colors (high contrast )
QUALITATIVE ANALYSIS

The over-all retinal profile
NOTE : The NFL increases in thickness towards the optic disc

RPE BRUCH’S AND PHOTORECEPTOR
• RPE -Is defined as in a good scans as three parallel
strips of which 2 are hyper reflective sandwiching a thin
hypo reflective layer
• RPE-Bruch’s complex - Normally it is impossible to
distinguish Bruch’s membrane from RPE but becomes
evident in detachments , drusens etc.
• BRUCH’s Membrane – Studied under these – Integrity
, Discontinuity , Reflectivity , Anomalous Structures
• Above RPE – Strong Reflective line that represents
junction b/w inner and outer segment of
photoreceptors , these follow RPE hyper reflective lines
and takes the shape of circumflex accent which is the
great vertical dimension of the CONES

The pre-retinal profile
• A 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 aberration created by
increasing the sensitivity of the instrument to
better visualize low reflective structures.

Anomalous structures
• pre-retinal membrane
• epi-retinal membrane
• vitreo-retinal strands
• vitreo-retinal traction
• pre-retinal neovascular membrane
• pre-papillary neovascular membrane

EPIRETINAL MEMBRANE
Highly reflective diaphanous membrane over the surface
of retina
According to the degree of distortion classified as:
a.Cellophane Maculopathy
b.Crinkled cellophane Maculopathy Globally Adherent
c.Macular Pucker:

• A epi-retinal membrane with traction on the fovea
• These membranes may be associated with true or
pseudo macular holes Clearly seperable

SHADOWING
• It produces a screen like effect that masks the
deeper outer structures.
• Blood in the cavity produces shadow effect , blood
cells form a moderate hyper reflective band
beneath the elevated space/ cavity.
• Photoreceptor lesion – IS/OS is blurred / fuzzy /
interrupted.
• Sup Hemorrhages & cotton wool spots produce
shadow cone only if very dense or very thick.

Shadowing vs photoreceptor lesions

HARD EXUDATE
Dense hyper reflective nodular structure deep in the retina casting a shadow cone
on the posterior layers of the retina.

ARMD ( Age related Macular Degeneration)
• Degenerative disorder affecting Macula
• Presence of specific clinical findings including
Drusen and RPE changes , choriocapillaris and
Bruch’s membrane changes as early feature with no
evidence the signs are secondary to some another
disorder.
• Conventionally divided in to two :
a. Dry/non-exudative/Non-neovascular – Drusen
,Geographical Atrophy
b. Wet/exudative/Neovascular – CNV ,PED

Dry/non-exudative/Non-neovascular
DRUSEN
• Seen as areas of focal
elevation of RPE
• Low modulations in the RPE
associated with shallow
borders with no shadowing
underneath
• No shadowing underneath
differentiates it from PED
GEOGRAPHICAL ATROPHY
• Well demarcated pigment
epithelial / choriocapillaris
atrophy
• OCT shows Increased
optical reflectivity from the
choroid due to increased
penetration of the light
through the overlying
atrophic retina

SOFT DRUSEN
Replacement of three hyper reflective bands with a single irregular hyper reflective
band with decreased photoreceptor thickness

In a case of Retinal atrophy OCT shows a highly reflective choroid signal ,
which allows greater beam penetration in to the choroid and greater reflectivity. The
retinal map allows the quantification of the decreased retinal thickness

Disease progression will show replacement of the
three hyper reflective bands with a single
irregular hyper reflective band with decreased
photoreceptor layer thickness.

CNVM
• Integrity of RPE – Bruch complex is lost , with
abnormal growth of blood vessels complex from
the choriocapillaris. OCCURS IN OLDER AGE
GROUP (ABOVE 60 YRS )
• (FFA CLASSIFICATION)
• Two types : a) classic
• b)occult
• Major use of OCT in the management of CNVM is
in monitoring the response to treatment for which
it provides an accurate quantitative assessment

Classic cnvm -The normal RPE-Choriocapillaris complex forms a Highly Reflective
continuous band .Thickening of this band with thickened edges demarcating the
boundaries of CNVM

The normal RPE-Choriocapillaris complex forms a Highly Reflective continuous band
Thickening of this band with thickened edges demarcating the boundaries of CNVM

How OCT is helpful in CNVM…?
• Diagnosing – Even soft confluent drusen occult
CNVM can often be missed on FFA.
• Response to Treatment :Monitor response to
thermal laser , photocoagulation , Photodynamic
Therapy (PDT) and VEGF inhibitors. Following PDT a
staging has been described to check for the
progression or regression of the disease by using
the REPEAT protocol.

DETACHMENTS
• Serous detachment appears as a hypo reflective ,
shallow separation of the neurosensory retina
from the RPE
• Retinal pigment detachments appears as well
defined , dome shaped hypo reflective elevation
of the RPE
• Subretinal fibrinoid deposits appear as moderate
high reflectivity collection within neurosensory
serous detachments

PED (Pigmentary epithelial detachment )
• Basically due to the reduction of the hydraulic
conductivity of a thickened and dysfunctional
Bruch’s membrane , thus impending the
movement of fluid from the RPE to choroid.
• Types :
a. Serous
b. Fibro vascular
c. Drusenoid
d. Haemorrhagic

SEROUS PED
Elevation of the RPE with an optically clear space underneath. The underlying
choroid shows minor optical shadowing.

Fibrovascular PED
Elevation of the RPE with a demarcation b/w RPE and underlying
structures . Optical shadowing

FIBROVASCULAR PED
Elevation of the RPE with a demarcation b/w RPE and
underlying structures . Optical shadowing from the underlying
choroid is absent

HAEMORRHAGIC PED
Same as serous PED except backscattering from the RPE attenuates towards the
outer retina with absent choroidal reflections.

CSR
PATHOLOGY : Localized serous detachment of the sensory retina at the macula
secondary to leakage from choriocapillaris through focal , or less commonly diffuse hyper
permeable RPE defects. OCCURS IN YOUNG PATIENTS 25-45 YRS

RPE TEAR
PATHOLOGY : Occurs at the junction of attached and detached RPE if Tangential stress
becomes sufficient. Tears may occur spontaneously following laser photocoagulation , or
after intravitreal injection etc

CRVO
• Macula in venous occlusions shows
1. intraretinal fluid accumulation
2. serous retinal detachment
3. Cystoid macular edema
• Also an excellent modality to study the
response of the macula to any intervention

Diabetic retinopathy
• Macular edema: 5 distinct pattern of macular edema are
defined on Oct.
Type 1: focal macular thickening
Type 2: Diffuse thickening without cysts
Type 3: diffuse cystoid macular edema
Type 4: Tractional macular edema
4A: posterior hyaloid traction
4B: epiretinal membrane
4C: both posterior hyaloid and epiretinal membrane
Type 5: DME from one of the previous types associated to
a macular serous retinal detachment
• Useful in monitoring response to any intervention
• Helps in quantifying retinal thickness

• How has it helped ..????
• Foveal TRD and TPHM are indications of Pars
plana Vitrectomy.
• Both the above lead to Recalcitrant Macular
Edema
• CSME secondary to foveal TRD and TPHM are
non-responsive to laser photocoagulation and
thus an indication for pars plana vitrectomy.

Sponge like retinal thickening

CSME with cystoid macular edema

CSME with tractional foveal detachment

CSME with taut posterior hyaloid
membrane

Deformations in the foveal profile
• 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 (hole extends to RPE, increased
thickness, some fluid)
• macular hole, stage 4 (complete hole, edema at
margins, complete PVD)

Macular hole
• Differentiating from other vitreo-retinal lesions
• Diagnosing vitreofoveal traction
• Distinguishing true from lamellar macular hole
• Staging and diameter of macular hole
• Evaluation for surgical intervention
• To study progression
• Visual outcome prediction
• GASS CLASSIFICATION

Macular cyst stage 1a (no
depression, cyst present)
Pathology : Inner retinal layers (Muller cell cone) detach from the underlying
photoreceptor layer , with the formation of schisis cavity

satge1B: Occult macular hole
Pathology : Loss of the structural support causes the photoreceptor layer to
undergo centrifugal displacement.

Macular hole, stage 2 (partial rupture of
retina, increased thickness(edema)
Pathology : Dehiscence develops in the roof of schitic cavity , often with
persistent vitreofoveolar adhesion

Macular hole, stage 3 (hole extends to
RPE, increased thickness, some fluid)
Pathology : Avulsion of the roof of the cyst with an operculum and persistent
parafoveal attachment of the vitreous cortex

Macular hole, stage 4 (complete hole, edema
at margins, complete PVD)

OCT and Fluorescein Angiography in
retinal diagnosis
FAs provide excellent characterization of retinal
blood flow over time, as well as size and
extent information on the x and y axis (north-
south, east-west)
The OCT gives us information in the z (depth)
axis, telling us what layers of the retina are
affected

OCT for glaucoma
• Analysis of optic nerve head
• Analysis of RNFL

• Glaucoma is characterized by irreversible
damage to the retinal ganglion cells, resulting in
retinal nerve fibre layer loss
• Structural damage precedes functional loss:
RNFL loss precedes visual field defects.
• 40% axonal loss occur prior to any detectable
change in visual function
• The recognition of RNFL loss inpatients with
normal visual fields has led to the concept of
pre- perimetric glaucoma

Optic nerve head evaluation
– Optical disc scan
– Fast optical disc scan protocol

1. Optical disc scan consists of equally placed 6
(minimum)Axial scans 4mm(fixed) in length at 30
degree intervals, centred on the optic disc.
• The number of lines can be adjusted between 6-24
lines until the first scan in the series is saved with
each radial spoke composed of 128 sampling points.
2. Fast optical disc scan compresses 6 optical disc
scan into one scan in short time of 1.92sec

The various optic nerve head parameters
1.Disc area ,
2.Cup area
3.Rim area
4.rim volume
5.cup volume
6.avg cup-disc ratio and
7.horizontal and vertical cup-disc ratio

RNFL SCANNING PROTOCOL
Scan ACQUISATION protocol
• RNFL thickness(3.4): this protocol consists of
three circle scan of 3.4 mm diameter around the
optic disc.Average of three scans taken.
• RNFL thickness (2.27xdisc):this protocol enables
one to acquire a single circular scans around the
optic disc which is 2.27 times the radius of the
aiming circle (size of the patient optic disc- 1.5
mm approx)

• Fast RNFL thickness (3.4) this protocol compresses
the three RNFL (3.4) scans into one scan in a time
period of 1.92 seconds
• RNFL map: this consists of a set of six concentric
circle scans of predetermined radius increasing
from the centre. It is designed to thoroughly assess
RNFL thickness at increasing distances from the
disc margin

RNFL analysis protocol
1. RNFL thickness: is used to obtain graphs of
RNFL thickness along circle scans made
around the optic disc
• Normal RNFL graph appears as a double
hump because to the increased RNFL
thickness at the superior and inferior poles of
disc

2.RFNL thickness average:
• Creates two maps of retinal nerve fibre thickness
around the optic disc
• One map shows RNFL Thickness using a colour code (A)
the other shows average RNFL thickness in microns(B)
• Summary parameters (C) – Gives maxm avg thickness
in each quadrant and their ratios.
3.RFNL thickness serial analysis :
• Comparison of RNFL thickness over time.
• It utilizes the circle scans around the optic disc to analyse
the changes in RNFL thickness from one visits to the next.
• The RNFL thickness graphs of up to 4 visits can be
superimposed on the same chart and each visit is colour
coded.

Circle Scan
Differences betweeen average
thickness in sectors
(along the calculation circle) in each
eye
OCT 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 with
Compressed Color Scale in
8x8 Analysis Grid
Mean Thickness
Hemisphere Analysis with
Asymmetry Gray Scale
OCT scan of macular region

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.

• Best discriminating parameter for diagnosis
of glaucoma was the average inferior RNFL
thickness and the C/d area ratio of the ONH
analysis

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
• These artifacts can be operator induced (focussing ,
depolarizing & out of range images) ; can be patient
induced (off center fixation leading to incorrectly
centered retinal thickness maps , blink and motion
artifacts) ; may be due to limitations of imaging
techniques( Td-OCT has lower imaging speed and
thus frequent blink and motion artifacts)

• 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 inner retinal layer:
Occurs due to software breakdown, mostly in eyes
with epiretinal membrane vitreomacular traction
or macular hole.

• Misidentification of outer retinal layers:
Commonly occurs in outer retinal diseases such as
central serous retinopathy ,ARMD, 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 quality image acquisition.
• 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 measure
Inner 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

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