3. Optical coherence tomography
noncontact, noninvasive imaging technique
used to obtain high resolution cross-
sectional images of the retina and anterior
segment.
OCT of the retina is like doing a vertical
biopsy section of the retina. Instead of a
knife, light is used. Instead of viewing a
stained section under a microscope, we are
presented with a "false-color" view with
micron level resolution.
4. Applications of OCT
Ophthalmology
Dermatology (skin diseases - early detection
of skin cancers)
Cardio-vascular diseases
Endoscopy (fiber-optic devices)
Functional imaging (Doppler OCT)
Guided surgery
brain surgery
knee surgery
Spinal cord surgery
5. How is Doing
Measures reflected light from tissue
discontinuities
Based on interferometry
involves interference between reflected
light and a reference beam.
process is similar to that of
ultrasonography, except that light is
used instead of sound waves.
6. Resolution
microscope resolution is the shortest
distance between two separate points in a
microscope’s field of view that can still be
distinguished as distinct entities.
There is difference between resolution
and pixel. Pixel is actually a unit of digital
image. Resolution depends upon the size of
pixel. Smaller the size of pixel, higher will
be the resolution and more clear will the
object in image.
7. OCT vs Ultrasound
Both methods create a cross-sectional image
by measuring the echo time delay and intensity
of the reflected light or sound
OCT has a much higher axial resolution than
ultrasound, ~10 μm for TDOCT and ~5 to 7 μm
for SD-OCT vs ~150 μm for ultrasound at a
frequency of 10 MHz.
Higher axial resolutions can be achieved with
higher frequencies of ultrasound.
Ultrasound at 50 to 80 MHz has axial
resolutions of ~50 to 20 μm, but with a
penetration depth of only 4 to 3 mm.
8.
9. The use of light as the medium in OCT gives
it the advantage of being noncontact for the
patient
These physical differences make ultrasound
useful for measuring intraocular distances
and overall structure of the anterior
eye,with greater penetration of signal into
the angle
near-infrared light is blocked by the sclera
OCT is far more useful for detecting
detailed structures in the retina and
anterior segment.
11. OCT images use this information to depict
variations in optical reflectance through the
depth of the tissue along a point, creating
what is known as an A-scan. These single axial
scans through the tissue can be gathered
linearly across the tissue, making one cross-
sectional image, known as a B-scan, and a
collection of parallel B-scans can be used to
gather a 3D data set.
12. Based on interferometry involves interference
between reflected light and a reference beam
•Because light is so much faster than sound,
the time delays between reflections from
different layers cannot be measured
directly, since differences would be on the
order of femtoseconds.
•Optical coherence tomography uses low-
coherence interferometry to see the time
difference corresponding to the distances
between structures.
13.
14. • The process starts with a broadbandwidth laser ,
the beam from which travels to a beam splitter.
• One half of the light goes to a mirror at a known
position on a reference arm, and the other goes
to the sample arm, where it is scattered and
reflects off of tissue structures.
• Light from the reference and sample arms
travels back to the beam splitter and recombines
to form an interference pattern, which is sensed
by a photodetector.
• The light beams combine constructively only if
the light from the tissue and the light from the
reference mirror are at almost exactly the same
distance.
15.
16. SDOCT Vs DOCT3
Time domain OCT3
3D SPECTRAL
DOMAIN OCT
•750 uW power
•400 A-lines per sec
•1.28 seconds per image
•SLD (820 nm)
•10 microns
•600 uW power
•29,326 A-lines per sec
•1/29 seconds per image
•SLD (840 or 875 nm)
•2-3 microns
17. A word to the OCT technicians
• Miotic pupil, though not a deterrent to macular imaging, induces pupillary
block of the incident light and hence affects two ends of the scan permits
artifact free and a happy OCT procedure. A 3mm pupil would leave both you
and your patient smiling.
• Wheel less chair adds to the stability of the patient.
• Encourage patient to blink frequently during the procedure. Wet the cornea
if required.
• Instruct the patient to look at the center of the green target, not at the
moving red light of the scanning beam.
• Attempt to start with fast, low resolution, and then switch to high resolution
scans quickly through areas of interest.
• Remind the ophthalmologist to have prior digital fundus photos and
fluorescein angiography (FA) images for correlation with OCT scans later on.
• Ensure that the signal strength is five and above.
18. The indications of OCT
Optical coherence tomography provides both qualitative
(morphology and reflectivity) and quantitative (thickness,
mapping and volume) analyses of the examined tissues.
The indications of OCT include:
Posterior segment lesions like detection of fluid within
the retinal layers or under the retina which may not be
visible clinically, macular holes , pseudoholes,epiretinal
membranes (ERMs), vitreo-macular traction, retinoschisis,
retinal detachment, diabetic retinopathy (DR), age-related
macular degeneration.
Retinal nerve fiber layer thickness (RNFLT), optic disc
parameters.
Assessment and analysis of anterior segment structures.
19. • Light rays from OCT can remarkably
penetrate ocular media opacities like mild
cataract,mild posterior capsular
opacification, mild vitreous hemorrhage,
asteroid hyalosis and mild vitritis.
• It is possible to image retina in a silicone
oil filled eye, however, in gas filled eyes, it
is possible to scan only after gas bubble
has receded to 45% of the fill, when
inferior meniscus has reached above the
foveal level.
Notes
1
20. • The strength of the signal
reflected by a specific tissue
depends on properties like
tissue reflectivity, the amount of light
absorbed by the overlying tissues, and the
amount of reflected light that reaches the
Sensor after it has been further attenuated
by the interposed tissue.
• So when the strength of the reflected signal
is strong, the scanned tissue has high
reflectivity and vice-versa.
2
21. • The shadow effect represents
an area of dense, highly refractile
tissue that produces a screening effect,
which may be complete or incomplete,
thereby casting a shadow on an OCT scan
that hides the elements behind it.
• Vertical structures like PRs are less
reflective than horizontal structures like
RNFL or RPE layer.
3
22. pseudocolor
Red and White - High reflectivity
Black and Blue – low reflectivity
Green - intermediate reflectivity.
23. Normal ocular tissues which show high
reflectivity (represented by red color
on OCT scans and printouts) are:
Retinal nerve fiber layer.
Internal limiting membrane.
Junction between inner and outer
segments of PRs
Retinal pigment epithelium-Bruch’s
membrane - choriocapillaries complex.
24. Normal ocular tissues which show low
reflectivity (represented by black
color on OCT scans and printouts)
are:
inner nuclear layer
outer nuclear layer
ganglion cell layers
PRs
25. Normal ocular tissues which show
intermediate reflectivity
(represented by green color on OCT
scans and printouts) are:
inner plexiform layers
outer plexiform layers
external limiting membrane.
27. High reflectivity
Superficial lesions
Epiretinal and vitreal membranes.
Exudates and hemorrhages which produce an
underlying shadow effect, if dense.
Cotton wool spots .
Intraretinal lesions
Hemorrhages.
Hard exudates.
Retinal fibrosis and disciform degenerative
scars.
31. Lesions causing hypo or low
reflectivity
Atrophic RPE (loss of pigment).
Cystic or pseudocystic areas containing
serous fluid.
Cystoid edema, serous neural retinal
detachment and RPE detachment. Such
lesions appear as black,optically empty
spaces.
35. 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.
36. 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.
37. The scan below has waves in the retinal
contour. These are not retinal folds,
but rather movement of the eye during
the scan pass.
38. To refresh our knowledge, the normal values :
• Distance between vitreoretinal interface and
anterior surface of retinal pigment epithelium
(RPE): 200 - 275 microns.
• Mean thickness in the foveal region: 170 – 190
microns.
• Mean thickness in peripheral retina: 220 – 280
microns.
• Mean thickness of retinal nerve fiber layer
(RNFL): 270 microns (1000 microns from fovea
where nerve fibers form a slight arcuate
thickening).
• Normal retinal volume: 6-7 cubic mm.
39. • The OCT scan printouts bear
pseudocolor imaging and retinal
mapping is based on different color
codes (white, red, orange, yellow,
green, blue, and black in order),
White being the thickest scanned
retina (>470 microns)
Black the thinnest scanned retina (<150
microns).
• However, such color imaging or color
maps may vary for different models of
OCT equipment.
41. Exam Date:
Exam Time:
Technician:
Signal Strength:
It is ideal to have
Minimum signal
strength of 5.
If it is less, look for
media opacities, dry
cornea or a very small
pupil.
42. 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).
43. • Fundus image with
scan cube overlay.
• Color code for
thickness overlays.
44. OCT fundus image in
grey shade. It shows
the surface of the
area over which the
measurements were
made.
45. 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.
ILM-RPE Thickness (µm)
46. • Slice through cube
front. Temporal –
nasal (left to right).
T N
48. • Thickness between Internal
limiting membrane (ILM) to
retinal pigment epithelium
(RPE) thickness map.
• Anterior layer (ILM).
• Posterior layer (RPE).
• All these are 3-D surface
maps.
49. Normative database uses color
code to indicate normal
distribution percentiles.
Of the normal population :
5 % fall within the white band
5 to 95% fall within the green
band
1 to 5 % fall within the yellow
band.
1 % fall within the red band
(Outside normal limits)
50. Numerical average
thickness and volume
measurements.
The central subfield
thickness refers to
the central circle of
the circular map .
The total volume and
average thickness
refer to the ILM-
RPE tissue layer over
the entire 6 x 6 mm
square scanned area.
Cube
average
thickness
Cube volume
(mm³)
central
subfield
thickness
51. Optical coherence tomography
and its applications in glaucoma
• OCT offers comprehensive glaucoma
evaluation by providing assessment of
the RNFL thickness, optic disc
morphology, and the ganglion cell
complex thickness. All three of these
structures are affected in glaucoma.
• OCT provides both qualitative and
quantitative information
55. RNFL thickness maps.
The maps report
thickness using GDx
(glaucoma diagnostics)
color pattern, where
warm colors (reds,
yellows) represent
thicker areas and cool
colors (blues and
greens) represent
thinner areas.
56. • RNFL thickness deviation maps
• These maps report statistical
comparison against normal
thickness range, overlaid on the
fundus OCT image.
• These maps apply only yellow
and red colors.
• The green color is not applied
because it may obscure the
anatomical details in the
underlying OCT fundus image, as
most of the superpixels would
be green for normal patients.
• Any region that is not red or
yellow means it falls within
normal limits.
• One can have a gross clue of
the cup-disc ratio and position
of the vessels in the cup.
57. • Displays average RNFL thickness
along the whole calculation circle
(squares in the print out, as well
as quadrant and clock hour
measurements).
• These measures are represented
in pseudocolor coded programs by
comparing the measured RNFL
thickness to age-matched data in
the normative database of the
OCT machine.
• Green and white colors indicate
normal RNFLT (white color means
thickest).
• Red indicates reduced average
thickness of the RNFL.
58. • The RNFL normative database uses a
white-green-yellow-red color code to
indicate the normal distribution
percentiles.
• The color code applies to the quadrant,
clock hour, graphs.
• The percentiles apply as follows (among
same-age individuals in the normal
population):
• Red represents thinnest 1% of
measurements and is considered outside
normal limits.
• Yellow represents thinnest 5% of
measurements and is considered
suspect.
• Green represents 90% of measurements
and is considered normal.
• White area represents thickest 5% of
all measurements.
59. • Symmetry: Indicates the extent
of symmetry of distribution of
RNFL thickness in TSNIT
(temporal-superior-nasal-
inferior-temporal) quadrants
between two eyes
• RNFL-TSNIT thickness graph:
OU (Both eyes):
• This section plots RNFL
thickness in Y axis (vertical) and
retinal quadrants in X axis
(horizontal).
• This normally has a “double
hump” appearance owing to
the thicker RNFL
measurements in the superior
and inferior quadrants
compared with the nasal and
temporal quadrants.
60. • Separate RNFL-TSNIT
normative data graph
for right and left eyes
respectively.
• The graph is
superimposed against
the color codes.
• If the graph dips into
red color in any
quadrant, the RNFL
thickness in that
quadrant is not normal
61. • Extracted RNFL
tomograms.
• Display the reflectivity
of the RNFL.
• Not of much clinical
significance in taking
clinical decisions.
• Some models of OCT
can display optic disc
modules including
parameters
62. the ganglion cell complex
thickness
• ganglion cell complex (GCC),
encompass three layers in the retina
1) the retinal nerve fiber layer (NFL)
2) the ganglion cell layer (GCL)
3) the inner‐plexiform layer (IPL)
63. • The macula region contains over 50%
of all retinal ganglion cells and is
likely an ideal region to detect early
cell loss and changes over time
because of the high density of cells.
• the ganglion cell complex (GCC)
become thinner as the ganglion cells
die from glaucoma.
64. GCC Thickness Map
Color coded where
brighter
colors (red and orange)
represent thicker areas
and
cooler colors (blue and
green)
represent thinner
areas. Fovea
has no ganglion cells and
so is
very thin (black spot).
65. Deviation Map
Color coded to reflect
the percent loss from
normal.
Green represents no
GCC loss.
Yellow and red are
above average GCC (no
loss).
Blue is around 20%
GCC loss and black is
50% loss or greater.
66. Significance Map
• A probability map
indicating statistical
significance of GCC loss.
• Color coded where
green is normal GCC
thickness, yellow is
borderline, and red is
outside normal limits.
• Fovea is masked due to
lack of ganglion cells.
67. GCC Parameters
• Average GCC
parameters color-
coded based on
comparison to
normative database.
• FLV parameter is a
measure of focal
GCC loss, similar to
PSD on visual fields.
• FLV detects local
patterns of loss
68. OCT in anterior segment
imaging
• Anterior Segment Optical Coherence
Tomography (AS-OCT) is a non-contact,
non-invasive light-based imaging
modality of diagnostic technique that
provides image resolution higher than
that of UBM (axial resolution of 18 μm
in Visante OCT versus 50 μm in UBM) of
the anterior segment in cross section in
vivo.
69. Cornea
• Based on the 4 mm x
4 mm data cube
captured by the
Anterior Segment
Cube 512x 128 scan.
• This analysis provides
qualitative and
quantitative
evaluation of the
cornea, including
visualization of
pathology and
measurement of
central corneal
thickness.
