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
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.
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.
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
58.
59.
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
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
66.
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.
71.
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
81.
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.