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I.
Optic Disc Topography-HRT II, Heidelberg Engineering, Inc.
A.
Principles
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B.
Obtaining an Image
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C.
The series begins with the focal plane located in the vitreous
Focal plane moves posteriorly through the optic nerve head until it reaches the retina
32 confocal 2-D images are obtained in 1.6 sec.
A matrix is constructed consisting of 256X256 (65,536) height measurements
(measuring points)
Each height is represented as a color
- bright = excavated areas (cup)
- dark = elevated areas (retina)
Three image series are taken and averaged
Pupil dilation not required
HRT II Display
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D.
Uses a confocal laser scanning system
670 nm diode laser
Objects are scanned sequentially in two dimensions (optic sections)
Sections are imaged layer by layer to form a three-dimensional image
Provides three-dimensional imaging of the optic nerve head
Provides objective information about macular thickening
Fundus Image
Computed average disc topography image of three captured series
Stereometric parameters of disc topography
Mooresfiled Regression Analysis display – neuroretinal rim (green) and cup (red)
Results compared against normative database of 112 Caucasians
Topography classified
a. Within Normal Limits
b. Borderline
c. Outside Normal Limits
Applications of Optic Nerve Head Topography
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Quantitative description and classification at first visit and change over time
Quantification of stereometric parameters
a. disk area
b. cup and rim area and volume
c. mean and maximum cup depth
d. cup shape
i. the more negative value the better
e. retinal nerve fiber layer (RNFL) topography
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Uses an artificial reference plane located 50 microns below the retina to
differentiate neuroretinal rim (above 50 microns) from cup (below 50 microns)
HRT II is most clinically useful for documenting change over time.
II. Optical Coherence Tomograpy (OCT3), Carl Zeiss Meditec, Inc.
A.
Principles
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B.
Infrared light (850 nm) from a diode is directed into the retina
Backscattered light is detected from several retinal layers and resolved using Low
Coherence Interferometry to produce high resolution cross sections (10 um with
OCT3)
Resulting image looks like a histological slide section of the retina
a. can differentiate 6-7 retinal layers
Pupil dilation not required
OCT Displays
1.
Optic Disc Topography
a. Cross-sectional display in 6 different merdians
b. Quantification of stereometric parameters of the disc, neuroretinal rim and cup
2. Macular Analysis
a. Thickness measurements
i. In a 1mm circular area around the fovea
ii. Four juxtafoveal sectors 3.00 mm from the fovea
iii. Four extrafoveal sectors 6.00 mm from the fovea
3. RNFL Analysis
a. RNFL thickness measurement displayed in 12 clock hours 1.73mm from the
edge of the ONH
C.
Clinical Applications of OCT3
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Increases in retinal thickness secondary to vascular disease and venous-occlusive
disease
Retinal elevations (serous retinal detachments)
Choroidal neovascular membranes
Macular hole
Vitreo-retinal traction
Juxtapapillary nerve fiber layer loss (3.4mm circular scan)
Optic nerve head topography measurements
III. Scanning laser Polarimetry (GDxVCC), Carl Zeiss Meditec, Inc.
A.
Principles
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2.
Uses principle of birefringence of layered structures in the eye (like the RNFL)
which cause retardation of polarized light as an indirect measure of the thickness
of the nerve fiber layer
Birefringent (layered) structures in eye
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cornea (lamellae)
lens
retinal (RNFL)
3. To isolate RNFL birefringence, GDx uses a corneal compensator to cancel out
corneal birerefringence; the lens of the eye has minimal birerefringence
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B.
Measures the thickness of the retinal NFL (65,536 points) in an area 15 degrees X 15
degrees around the optic nerve head
a. The thicker and more robust the NFL, the greater the retardation
GDxVCC: Screening for Retinal Nerve Fiber layer (RNFL) Loss
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Images acquired in 0.7 sec
Pupil dilation not necessary
GDxVCC Display
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Fundus image
Thickness Map
a. Bright colors (yellow and orange) = thicker areas of RNFL
i. superior and inferior to ONH
b. Dark blue colors = thinner areas of RNFL
i. nasal and temporal to ONH
ii. defects in NFL sup/inf
Thickness parameters
Deviation Map
TSNIT Curve
GDx: The NFI (Nerve Fiber Indicator)
NFL measurements are assigned a “number” between 0 and 98
The NFI is a computer-generated neural net number that represents the artificial
intelligence of a computer that has been trained to distinguish norman RNFL patterns
from glaucomatous RNFL patterns.
General Categories:
a. The number 30 correctly classifies 90% of patients into normal or abnormal
i. 0-30: Normal
ii. over 30: RNFL Loss
GDxVCC Serial Analysis
Displays up to 4 images per eye on one page-for follow-up
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IV.
Optos Retinal Exam, Optos, Inc
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V.
Principles
Uses a scanning laser ophthalmoscope to digitally image the fundus
Couples a green laser (532 nm) that scans the sensory retinal through the RPE
with a red laser (633 nm) that scans deeper structures from the RPE to the
deep into the choroid
Provides a 200 degree view
Does not require pupil dilation
Captures images in 0.25 sec per eye
Useful for detection of central and peripheral abnormalities without pupil dilation
Two lasers help differentiate retinal lesions from choroidal lesions
Pascal Dynamic Contour Tonometry (DCT), Zeimer Ophthalmics, Inc.
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B.
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Principles
A contoured tonometer tip that fits into the tip holder of a standard Goldmann
Applanation Tonometer (GAT)
Applies pressure to the cornea with a constant force
DCT measurements are not influenced by changes in corneal thickness
DCT is equal in accuracy to IOP measurements with traditional Goldman
Applications
Studies have demonstrated that DCT is more accurate that GAT for corneas
thinner than 450 nm and thicker than 550 nm
Is also useful for accurately measuring IOP in post-LASIK patients and other
thin corneas