Understanding and Comparing
Confocal Scanning Lasers to
Optical Coherence Tomography for
Optic Nerve Head Analysis (ONH)
A presentation courtesy of Zeiss
Third Generation Products
• LDT abandoned
TopSS in 1999 to
continue development of GDx Access
• HDT released their
HRT II instrument,
which is smaller,
faster, easier and
cheaper
How they Work
• A 670-780nm LASER light source plus (2) Pinhole (20-25
micron) apertures detect light reflected off the retina
LASER
FOCAL
POINT
PINHOLE
APERTURE
DETECTOR
Signal
• Confocal Scanning Lasers sense the shape of
the surface of the Hyloid Face or the Vitreous
to Internal Limiting Membrane interface.
• At this point the peak reflection is recorded
• Difficulties with some common conditions as
PVD can cause artifacts.
• Have excellent “X-Y” resolution, but coarse
“Z” resolution.
Scanning Lasers make Slices
• Typically 32
“Slices” are
acquired over a
4mm depth.
• Each slice has a
depth of 25
microns.
• A non sampled
gap of 100
microns is
between slices.
Z
Y
X
Scanning
• Typically a 6mm
by 6mm area of
retinal surface is
scanned.
• Each slice is about
20 –25 microns
thick.
• Each gap between
slices is about 100
microns thick.
• Only a few slices
fall on the ONH.
Examples of Data
• Two of the 32 slices
are displayed at the
right.
• Lets plot the paths of
light of the 3 datapoints.
• At different areas on
the disk in slice 14 of
32 notice:
– Yellow is on a blood
Vessel
– Green is on the rim
– Purple is in the cup
Slice
14 of
32
Slice
18 of
32
How the slices determine topography
60
Reflection Intensity
• A graph illustrates
REFLECTANCE
for the same
location at each of
the 32 slices.
Since the Yellow
curve peaked
before the Green
or Purple curve, a
RELATIVE
difference in
Topography can
be calculated.
50
40
Vessel
Rim
Cup
30
20
10
0
1
5
9
13
17
Slice
21
25
29
Establishing a Reference Plane
• A Publication by
Weinreb et al shows
the Papilomacular
bundle as an area
adjacent to the rim
showing the least
amount of change. So
these instruments use
this area and other
areas as a peripheral
band concentric around
the disk to manufacture
“ZERO” or a reference
plane.
Volumetric Analysis
• The operator must
draw the outline of
the disk.
• This is not easy as the
image is not taken in
white light.
• Notice how two
different operators
outline the same disk
with different shapes.
• Volumetric
parameters are
operator dependent.
Optical Coherence Tomography
• To scan the ONH,
Optical Coherence
Tomography acquires (6)
4mm long by 2mm deep
“Slices” in the X-Z axis.
• Datapoints are sampled
every 2 microns
providing excellent
resolution in the Z axis.
• Confocal Scanning
Lasers typically sample
every 100 microns.
Y
X
Z
Image Analysis
• Using a proprietary algorithm, OCT identifies the
Retina Pigment Epithelium and places a marker.
• The markers show the boundaries of the disk in each
individual slice. The next slide shows all six slices.
ONH Tomography Examples
Slice 1
Slice 4
Slice 2
Slice 5
Slice 3
Slice 6
ONH Analysis
• Using the algorithm
OCT objectively finds
the margin of the disk.
• OCT objectively finds
the cup by using an
offset value of 150
microns up from the
RPE.
• No reference plane is
required.
OCT Printout
• Details volumetric analysis of cup and Disk.
• Provides Direct Cross Section of anatomy.
“Z” Dimension Datapoints
Confocal
Scanning
Lasers acquire
32 datapoints
over 4mm
Optical
Coherence
Tomography
acquire 1024
datapoints
over 2mm
Datapoint Comparrison
100 Micron Gap
150 Microns
• Confocal
Scanning Lasers
have 100 micron
gaps between
datapoints in “Z”
dimension.
• Optical Coherence
Tomography has
only 2 micron
gap.
Conclusion
• Confocal Scanning Laser Technology is not capable
of seeing small changes in the ONH.
• Glaucomatous change occurs very slowly.
• Progression may not be observed with sampling
points with 100 micron gaps.
• OCT with higher “Z” resolution is more likely to see
glaucomatous progression.
• Objective placement of Cup and Disk insures
accuracy between operators.
• Viewing the anatomy confirms ONH Analysis from
artifacts seen in confocal scanning lasers.