Optical coherence tomography (OCT) is a non-invasive imaging technique that examines living tissue using low coherence radiation. OCT allows for both qualitative and quantitative analysis of the retina. Qualitatively, OCT can describe retinal structures and anomalies. Quantitatively, OCT can measure retinal thickness and nerve fiber layer thickness. OCT works by capturing 128 to 768 axial scans in a single pass, with each scan containing 1024 data points over a 2mm depth. OCT has higher resolution and penetration depth than standard clinical imaging.

1. Optical Coherence
Tomography (OCT)
By
DIPAK KUMAR SAH.
B.Optom 3rd year student.
BPKLCOS,Maharajgunj Medical campus.

2. What is OCT?
Diagnostic imaging technique that examines
living tissue non-invasively. It is based on
a complex analysis of the reflection of low
coherence radiation from the tissue under
examination.
Real time cross sectional analysis

3. OCT allows both qualitative and quantitative
analysis of the retina
Qualitative analysis includes description by
location, a description of form and
structure, identification of anomalous
structures, and observation of the
reflective qualities of the retina

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

5. How does it work?
 128 to 768 axial samples (A-scans) in a
single "scan pass“
 Each A-scan has 1024 data points and is
2mm long (deep).

6. OCT: Basic Principles
 Three-dimensional imaging technique with high
spatial resolution and large penetration depth
even in highly scattering media
 Based on measurements of the reflected light
from tissue discontinuities
 e.g. the epidermis-dermis junction.
 Based on interferometry
 interference between the reflected light and the
reference beam is used as a coherence gate to
isolate light from specific depth.

7. OCT vs. standard
imaging
Resolution (log)
1 mm Standard
clinical
Ultrasound
High
100 µm frequency
10 µm
Confocal
microscopy OCT
1 µm
Penetration depth (log)
1 mm 1 cm 10 cm

8. Resolution
 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

9. OCT Ultrasound

11. Scan Protocol Types
 Line
 Circle
 Radial Lines

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

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

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

15. 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

16. Not All OCT Scans Are Created
Equally
 The "fast" scan protocols of the OCT 3 reduce
the time needed for multiple scans
 The scan time reduction is intended to minimize
the error created by patient movement
 Fast scans grab fewer A-scans in the 6 mm
length of the scan. The normal 6 mm scan
contains 512 A-scans, whereas the fast 6 mm
scan contains only 128 A-scans, resulting in a
lower resolution image

17. Fast OCT 3 scan
The same eye scanned with maximum resolution

18. Retinal Anatomy Compared to OCT
 The vitreous is the black space on the top
of the image
 We can identify the fovea by the normal
depression
 The nerve fiber layer (NFL) and the retinal
pigment epithelium (RPE) are easily
identifiable layers as they are more highly
reflective than the other layers of the
retina

19.  This higher reflectivity is represented by
the "hotter" colors (red, yellow, orange,
white) in the false color representation of
the OCT 3.
 The middle layers of the retina, between
the NFL and RPE, are much less easily
identifiable in the scan.

20. Optical Coherence Tomography
This is what we wanted…

21. Optical Coherence Tomography
…this is what we got…

22. 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

23. Interpretation of OCT images
Layers of the retina
Nerve
fiber
layer
Ganglion
cell layer
Inner
plexiform
layer
Inner
nuclear
layer
RPE and choriocapillaris Outer and inner External limiting Outer Outer
photoreceptor membrane nuclear plexiform
segments layer layer

24. 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

25. Image of good quality
Out of focus
Vignetted image
Fixation error

26. 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.

27. Anomalous structures
 pre-retinal membrane
 epi-retinal membrane
 vitreo-retinal strands
 vitreo-retinal traction
 pre-retinal neovascular membrane
 pre-papillary neovascular membrane

28. A pre-retinal membrane with traction on the fovea

29. a pigment epithelial detachment is causing the convexity

30. Aside from the retinal detachment,
notice the underlying concave curvature of the retina,
suggesting the long eye of a significant myope

31. 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)

32. Macular cyst

33. Macular hole, stage 2

34. Macular hole, stage 3

35. Macular hole, stage 4, operculum
suspended by the hyaloid membrane

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

37. Deformations in the macular profile
 Serous retinal detachment (RD)
 Serous retinal pigment epithelial
detachment (PED)
 Hemorrhagic pigment epithelial
detachment

38. Serous retinal pigment epithelial
detachment (PED)

39. Intra-retinal anomalies in the
macular profile
 Choroidal neovascular membrane
 Diffuse intra-retinal edema
 Cystoid macular edema
 Drusen
 Hard exudates
 Scar tissue
 Atrophic degeneration
 Sub-retinal fibrosis
 RPE tear

40. Choroidal neovascular membrane

41. Cystoid macular edema cause by
diabetic maculopathy

42. Sub-retinal fibrosis

43. Diabetic Macular Edema
 Sponse like retinal thickening.
 Cystoid macular edema.
 Serous retinal detachment.
 Foveal tractional retinal detachment.
 Taut posterior hyaloid membrane.

46. Retinal Trauma

48. Retinal Detachment

50. Followed by scleral buckling
and PPV
 Retinal pigment epithelium irregularities.
 Neurosensory atrophy.
 CME
 Retinal pigment epithelium hyperplasia.

51. Intraocular Tumor
 Retinal leukemic
infiltrate.

52. Degenerative Myopia

53. Postoperative Endophthalmitis

54. Glaucoma
 Structural Damage precedes Functional
loss.
 Pre-perimetric Glaucoma.
 RNFL measurement.
 Macular Thickness.
 ONH Analysis.

55. Normal Vs Glaucomatous optic
cup.

57. 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

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

61. The Align Process
This tool "straightens" motions artifacts

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

63. Retinal Thickness Analysis
Using the retinal thickness analysis tool, the
software then traces a line along the NFL
layer and a line along the RPE layer.
The software then measures the distance
between the two lines and a graph is
produced which compares the measured
thickness to the thickness of a normal
retina

64. Each of the six scans can be reviewed by
clicking on the slider bar to the left, and any
or all of them can be printed out for the
patient's record

65. Retinal thickness analysis does not measure
retinal elevation
for example this eye with a pigment epithelial
detachment (PED) pictured below. The arrow
on the left would represent retinal elevation,
from the choroid, through the fluid space of the
PED, to the nerve fiber level. The arrow on the
right shows what the analysis measures, defined
by the distance from the RPE (which is
detached) to the NFL

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

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

69. Glaucoma Scans
When evaluating the glaucoma suspect
or the glaucoma patient, two
parameters that the ophthalmologist is
interested in are the characteristics of
the optic nerve cup and the thickness
of the nerve fiber layer surrounding the
optic nerve head
 The Fast Optic Disc scan
 The Fast RNFL Thickness scan

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

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

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

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

75. Care must be take to make sure that the image is
captured with the circle centered on the optic
nerve
The placement of the circle can make a big
difference in the analysis of the nerve fiber layer
thickness

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

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

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

79. What makes a good OCT scan?
A good quality OCT scan has good
reflectivity from edge to edge.
 The "hotter" colors (orange, red, white,
yellow) are maximized
 Generally, the retina should be in the
lower portion of the scan window so that
the vitreous can be images as well.

80. Scanning Tips
 Communicate with the doctor regarding the size
and location of the pathology of interest.
 Refer to other images of the pathology, e.g.
color photos and FA.
 Review past OCT exams and repeat scan types
used before.
 Dilate the eye well??????
 The patient must keep the forehead against the
bar and the chin in the chinrest, with teeth
together. Use the marker on the headrest to
align the patient vertically. The outer canthus
should be even with the line

82. Scanning Tips
 Use the two buttons near the joystick for
freezing and saving scans. This saves you
from having to juggle the joystick and the
mouse.
 Minimize patient fatigue by keeping scan time
to a minimum. Never scan an eye for more
than 10 minutes (FDA regulation).
 Keep the cornea lubricated. Use artificial tears
and have the patient blink when you are not
saving a scan pass.
 Move the instrument on the x and y axis (using
the joystick) to work around opacities

83. What’s New
 OCT better understanding (FA and ICG)
 Increase in resolution to 5 microns
 Overlays, 3D imaging

84. Questions?
References:
Brancato R. and Lumbroso B. Guide to
Optical Coherence Tomography
Interpretation. Rome: Innovation-News-
Communication, 2004.
Schuman J., Puliafito C., and Fujimoto J.
Ocular Coherence Tomography of Ocular
Diseases. Thorofare NJ: Slack Inc., 2004.