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Retinal Imaging Update
A. Scanning Laser Ophthalmoscope (SLO) – (Optos - Panoramic200)
a. How it works
i. Spinning polygon produces a rapid scan of the laser lights at
30,000 RPM. Raster
ii. Mirrors reduce the scan raster from approximately a 1 meter square
raster to a 1 mm square raster. They also produce a virtual image
of the raster at a fixed point in front of the instrument. The image
of the raster must be placed in the patient’s pupil and thus, head
positioning is critical.
iii. There is a red and green laser that are produced and scanned
simultaneously. The green laser penetrates less (most of the
information being gathered from the sensory retina to the RPE) and
the red wavelength light penetrates deeper (most of the information
being gathered from the RPE to the choroid). The image produced
is like having to 2 color transparencies laid on top of each other to
form the final image.
iv. Color light detectors accept the reflected laser light (red & green)
and transform it into electronic signals to the computer. Very little
laser light is reflected back from the fundus and therefore, the
detector are set on high sensitivity (sometimes this causes the optic
disc to be overexposed).
v. The computer with the propriety software transforms the electronic
signals into programmed image.
vi. Computer monitor produces final visible image.
b. Characteristics of the Panoramic200 image
i. It is able to capture 80% of the retina in one shot.
ii. Steering (gaze positions) can allow a field of view further into a
desired direction.
iii. Pupillary dilation doesn’t increase the field of view but does may
brighten the image a little.
iv. Digital image that can be magnified, lightened, darkened, and
emailed.
c. Retinal hole
d. Operculated retinal tear
i. Pathophysiology
ii. Clinical appearance
1. A tear is red in the base due to being able to see the
choriocapillaris more easily. The margins are more likely to
be ragged. The torn retina is seen as a plug above the break.
iii. SLO imaging
1. The operculum can be seen adjacent to the round tear and
the underlying shadow can also be seen.
iv. Clinical significance
1. Operculated tears are associated with retinal detachment
and again due mostly to vitreous traction.
e. Flap (horseshoe) retinal tear
i. Pathophysiology
1. A flap tear is caused by vitreous traction.
ii. Clinical appearance
1. A tear is red in the base due to being able to see the
choriocapillaris more easily. The margins are more likely to
be ragged. The torn retina is seen as a plug above the break.
iii. SLO imaging
iv. Clinical significance
1. Flap tears are associated with retinal detachment and again
due mostly to vitreous traction. Flap tears are almost
always treated.
f. Retinal detachment
i. Pathophysiology
1. The sensory retina separates from the pigment epithelium.
ii. Clinical appearance
1. Fresh detachment look whitish and longstanding are mostly
clear. The choroidal detail is fuzzy under the detachment.
Posterior border is almost always convex to the posterior
pole.
iii. Ultrasound imaging
iv. SLO imaging
1. Typical appearance as seen with a BIO just much more is
seen in one view. Because fresh RDs are white due to
ischemic edema, it tends to saturate the green color
detectors and therefore, often looks green.
v. Clinical significance
1. Loss of visual field and blindness.
g. Retinoschisis
i. Pathophysiology
1. Vitreous traction on the peripheral thin retina causes it to
split in the middle layers of the retina. Progression is
usually the result of increase or continued vitreous traction.
ii. Clinical appearance
1. Thin smooth bullous membrane that doesn’t move on eye
movements. Red or white vessels are sometimes seen
traversing the inner detached layer. The choroidal detail is
fuzzy under the detachment. Posterior border convex to the
posterior pole.
iii. Ultrasound imaging.
iv. SLO imaging
1. Smooth thin blister in the far periphery with an outer layer
break and convex posterior border.
v. Clinical significance
1. Retinoshises can involve the macula but is unusual for that
to happen.
h. Diabetic retinopathy
i. Age-related macular degeneration
j. Glaucoma – glaucomatous optic atrophy
i. Pathophysiology
ii. Clinical appearance
iii. SLO imaging
1. The detectors need to be on lower contrast settings in order
to view the optic disc. In most cases, the physiologic optic
disc cup is the brightest structure of the fundus. High
settings can saturate (over exposed or bleached out) the cup
and cause it to looking larger and haze or obliterate the
neural rim to cup margins. By lowering the sensitivity, the
discs will have a good appearance and cupping can be more
accurately assessed..
B. Optical Coherence Tomography (OCT)
a. Noninvasive, noncontact transpupillary imaging technology
b. Analogous to ultrasound B-wave imaging or radar except light is used
instead of acoustic or radio waves
c. Can image retinal structures in vivo with a resolution of 10 μ
d. The retinal detail provided is liken to an "optical biopsy" to provide 2- and
3-dimensional cross-sectional images of tissue microstructure, by
collecting backscattering of light reflected from the fundus and related
structures
e. How It works:
i. Cross-sectional images of the retinal are produced using the
optical backscattering of light
ii. The anatomic layers within the retina can be differentiated and
retinal thickness can be measured
iii. Utilizes spatially uniform, low-coherent light that is generated
by a superluminescent diode laser
1. Low coherent light allows for a propagation speed
nearly 1 million times faster than sound
2. This allows for the high resolution images
iv. Transmitted to the eye via fiber optic delivery
1. Similar to using 78D lens
2. Mounted on a slit lamp delivery system
v. 2 Galvanometer-driven mirrors are used allows for scanning
light across the retina in approx 2.5 seconds
1. Can be done with visible or IR Videoscopy
vi. The temporal information contained in the resulting
interference pattern is the basis for constructing the images
vii. Received light is converted into electric signal by a photodiode
and then processed by a computer
f. Provides cross-sectional images of retinal structures
i. Allows for clinical correlation
ii. Better anatomic perspective
iii. Supplements other diagnostic testing
g. Provides better understanding of vitreomacular interactions and related
diseases
i. Has redefined our understanding of the pathogenesis of full
thickness macular holes.
1. It is now understood that 'perifoveal' vitreous
detachment and not tangential traction leads to the
development of stages of macular hole
ii. Has helped understand the expanding spectrum of
vitreomacular syndrome b/c of better visualization of
vitreoretinal interactions
h. Instrumental in understanding in new retinal diseases.
i. Has help define the disease Retinal Angiomatous Proliferation
(RAP)
i. Provides important diagnostic and management information post treatment
j. Emerging technology for optic nerve evaluation
i. ON head and NFL evaluated via circular scans around the
nerve or radial scan through the nerve
ii. Cross sectional circle around the ON is produced
1. Provides “cylinder” of information
2. Cylinder is unfolded -> looked at in cross-section
iii. Especially good for glaucoma suspects and OHT
iv. Average NFL thickness maybe most useful in monitoring
glaucoma
v. Multiple studies show that OCT has the ability to detect early
glaucoma change by measuring NFL thickness
1. Particularly inferior quadrant
2. Often before VF loss
vi. Studies also show that nasal side of ON is affected earlier and
more than what is usually considered
vii. The Ave & quadrant NFL thickness have good correlation with
MD an HVF
C. Retinal Thickness Analyzer (RTA)
a. Produces a color-coded 2D and 3D thickness and topography map of the
retina, allowing accurate measurement of retinal thickness.
b. Can also provide deviation probability maps from a normative database
and quantitative numerical values.
c. Uses a computerized slit lamp to measure retinal thickness at the central
20 degrees of the macula and overlays a map of measurements on the
patient’s retinal image.
d. In 3-5 minutes 5-13 scans are acquired, up to 208 optical cross sections are
analyzed by a thickness algorithm at the posterior pole and peripapillary
area, and topography algorithm for optic disc is measured.
e. Can detect as little as 34m of macular edema
f. Serial RTA studies can be used to monitor progression macular thickening
as DME in response to treatment.
g. It can be used to monitor progression of NFL thinning in glaucoma
h. Technique
i. A thin laser slit beam is projected obliquely through a dilated pupil
onto the retina
ii. Viewed at an angle in a similar manner to that of slit-lamp
biomicroscopy.
iii. The reflected images are recorded by a video camera and digitized.
iv. Laser slit intersects with the interface between the vitreous and the
retina before intersecting with the interface between the retina and
the choroid.
v. Because the laser is projected at an angle, this results in two
separate reflections.
vi. The images are analyzed by an automated, operator-free software
algorithm, which detects the different reflections and uses this to
produce a map showing the distance between the interfaces.
vii. Sensitive mapping allows quantitation of height and area of
thickening.
viii. The high-speed cross-sectional imaging of the retina may be useful
for identifying, monitoring and quantitatively assessing macular
diseases, and detecting macular edema and subretinal fluid in
common retinal diseases such as diabetic retinopathy and AMD
ix. Can also be used for optic nerve evaluation and to follow
glaucoma