Transcript Optical Coherence Tomography OCT

Optical Coherence
Tomography
OCT
Dennis M. West, CRA
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
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
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
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).
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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
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OCT
Ultrasound
Protocols
The Zeiss OCT 3 has several built-in
protocols for scanning the retina and the
optic nerve head.
 A protocol is simply a pre-determined
procedure or method
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Scan Protocol Types
Line
 Circle
 Radial Lines
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The "line" scan simply scans in a single,
straight line. The length of the line can
be changed as well as the scan angle.
The "circle" scans in a circle instead of a
line.
The "radial lines" scans 6 consecutive line
scans in a star pattern
Not All OCT Scans Are Created
Equally
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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
Fast OCT 3 scan
The same eye scanned with maximum resolution
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
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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.
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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
The pre-retinal profile
A normal pre-retinal profile is black space
 Normal vitreous space is translucent
 The small, faint, bluish dots in the preretinal space is "noise"
 This is an electronic aberration created by
increasing the sensitivity of the instrument
to better visualize low reflective structures.
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Anomalous structures
pre-retinal membrane
 epi-retinal membrane
 vitreo-retinal strands
 vitreo-retinal traction
 pre-retinal neovascular membrane
 pre-papillary neovascular membrane
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The over-all retinal profile
A pre-retinal membrane with traction on the fovea
a pigment epithelial detachment is causing the convexity
Aside from the retinal detachment,
notice the underlying concave curvature of the retina,
suggesting the long eye of a significant myope
The foveal profile
The normal foveal profile is a slight
depression in the surface of the retina
Deformations in the foveal profile
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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 cyst
Macular hole, stage 2
Macular hole, stage 3
Macular hole, stage 4, operculum
suspended by the hyaloid membrane
The macular profile
The macular profile can, and often
does, include the fovea as it's center
Deformations in the macular profile
Serous retinal detachment (RD)
 Serous retinal pigment epithelial
detachment (PED)
 Hemorrhagic pigment epithelial
detachment
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Serous retinal pigment epithelial
detachment (PED)
Intra-retinal anomalies in the
macular profile
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Choroidal neovascular membrane
Diffuse intra-retinal edema
Cystoid macular edema
Drusen
Hard exudates
Scar tissue
Atrophic degeneration
Sub-retinal fibrosis
RPE tear
Choroidal neovascular membrane
Cystoid macular edema cause by
diabetic maculopathy
Sub-retinal fibrosis
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 scans for qualitative
analysis of the retina
The Fast Macular Thickness Scan (FMTS,
FMTM, or FMT scan)
 The Line Scan
 The Cross Hair Scan (3mm and 6mm)
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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)
The FMT scan is placed over the area of
interest, which is usually the
macula. 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
A scan is saved and then reviewed with any
of the retina analysis tools
Each of the 6 scans can be viewed
individually by clicking on the thumbnails
on the left of the scan selection screen
The Line Scan
The line scan is particularly useful because
of it's flexibility. The length of the line can
be changed, the angle of the line can be
changed, and the line can be dragged with
the mouse to any position or angle on the
video screen
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
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
The Align Process
This tool "straightens" motions artifacts
Proportional analysis
Proportional analysis produces an image
with its true horizontal and vertical
proportions
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
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
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
Serial FMT scans over time
One of the most useful functions of the OCT
is the ability to take volume measurement
over time. For example, a FMT scan
before treatment for AMD, and FMT scans
at various intervals after
treatment. Successful treatment should
be followed by a decrease in retinal
thickness and volume
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.
The analysis will give you a "change map",
showing the difference between the two
scans
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
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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.
The optic nerve scan can be analyzed with
the "optic nerve head analysis" protocol
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.
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
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
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).
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.
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.
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Scanning Tips
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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
Scanning Tips
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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
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Reflectivity may be further enhanced by
moving the focus knob on the side of the
OCT unit.
Alignment and focus
Alignment and focus are used to maximize
the quality of the scan. Alignment begins
with centering and zooming in on the
"football" shaped reflex in the video
image. The initial lens-to-subject distance
is achieved when the retinal image fills the
video screen. This is similar to the image
you see when doing retina
photography. At this point, your attention
should shift to the scan window
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If an image is not visible in the scan
window, you should click on the "z-offset
optimize" button on the "scan parameter"
tab. Once the scan image is visible, it can
be move with the z-offset arrows.
At this point, the "optimize polarize" button
should be clicked.
This should automatically refine the focus on
the retina, and you should see an increase
in the "hot" colors in the scan, as
illustrated below. The left image is before
optimization, the right is after.
At this point, the "scan mode" button is clicked so that you
have a full resolution image of the scan(s) in the scan
window. From this point on, maintaining a good scan
image is a matter of adjusting the unit with the joystick if
necessary to compensate for movement by the patient
that may degrade alignment. Encourage the patient to
blink until you are ready to freeze the image
Freezing and selecting the scan
The image can be frozen with a video
image with a flash or without a flash
 When ready to freeze the image, tell the
patient not blink and let the scan go
through several passes before freezing the
image
 The software saves the last 8 scan passes
for review. Always use the review button
(on the bottom row of buttons).
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When you click on the thumbnail at the bottom, the
current set of scans is displayed in the windows
above. The software also tells you what the signal
strength was for that particular scan, on a scale from 1
to 10, with 10 being maximum signal strength. You
can only save one scan from the group
Noise
The OCT scan can sometimes be improved by
changing the "noise" and "range" settings on the
"OCT Image" tab. The default settings are
indicated by blue markers on the scale
Noise
Most instruments produce the best scans when set
with the default values. Increasing the noise
level will produce "hotter' colors in the scan, but
the noise artifacts will also increase. Noise
artifacts are those "snowflakes" that you see in
the dark areas of the scan (e.g. the vitreous)
What’s New
OCT with SLO
 OCT with HRA (FA and ICG)
 Increase in resolution to 5 microns
 Overlays, 3D imaging
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Questions?
References:
Brancato R. and Lumbroso B. Guide to
Optical Coherence Tomography
Interpretation. Rome: Innovation-NewsCommunication, 2004.
Schuman J., Puliafito C., and Fujimoto J.
Ocular Coherence Tomography of Ocular
Diseases. Thorofare NJ: Slack Inc., 2004.