10. INTRODUCTION
✘ GDX measures thickness of RNFL based on birefringent property
of the tissue.
✘ GDX is:
- Simple to use and easy for both the patient and operator.
- Near Infra-red wavelength(780 nm).
- Total chair time less than 3 minutes for both eyes.
- Undilated pupils work best.
- Painless & safe procedure.
11. ✘ Maps the RNFL and compares them to a database of healthy,
glaucoma-free patients.
✘ Analyses the RNFL thickness around the optic disc.
✘ Sensitivity of 89% and a Specificity of 98%.
19. ✘ Main birefringent intraocular tissues are the cornea, lens and the
retina.
RNFL - highly ordered parallel axon bundles
Axons - microtubules, cylindrical intracellular organelles with diameters
smaller than the wavelength of light.
Highly ordered structure of microtubules are the source of RNFL
birefringence
20. ✘ Anterior segment birefringence - cornea and lens.
✘ The total retardation is the sum of the cornea, lens and the retina.
✘ Compensation of anterior segment birefringence is needed to
isolate RNFL birefringence.
✘ GDx VCC (Variable Corneal Compensation) measures and
individually compensates for each eye.
✘ GDx ECC (Enhanced Corneal Compensation)
24. Patients information
✘ Patient data and quality score:
- Patient’s name
- Date of birth
- Gender
- Ethnicity
- An ideal quality score is from 8
to 10
25. FUNDUS IMAGE
✘ The fundus image is useful to check for
image quality.
✘ Every image has a Q Score representing
the overall quality of the scan
✘ The Q ranges from 1-10, with values 8-10
representing acceptable quality.
✘ This score is based on a number of
factors including :
-Well focused,
- Evenly illuminated,
- Optic disc is well centered,
- Ellipse is properly placed around the ONH.
27. ✘ The fixed sized default calculation circle is a centred on
ONH.
✘ Inner diameter of the band is 2.4mm, outer diameter of
the band is 3.2mm & the band is 0.4mm wide.
✘ The calculation circle is the area found between the two
concentric circles, which measure the temporal-
superiornasal-inferior-temporal (TSNIT) and nerve
fiber indicator (NFI) parameters.
✘ By resizing ellipse, the operator is able to measure
beyond a large peri papillary atrophy area.
28. RNFL Thickness Map
✘ The thickness map shows the RNFL thickness
in a color-coded format from blue to red.
✘ Hot colors like red and yellow mean high
retardation or thicker RNFL
✘ Cool colors like blue and green mean low
retardation / thinner RNFL
29. Healthy RNFL Glaucomatous RNFL
• A healthy eye has yellow and red colors in the superior and inferior regions
representing thick RNFL regions and blue and green areas nasally and temporally
representing thinner RNFL areas.
• In glaucoma, RNFL loss will result in a more uniform blue appearance
30. Deviation Maps
✘ Reveals the location & magnitude of
RNFL defects over the entire thickness
map
✘ RNFL thickness of patient is compared
to the age-matched normative database
✘ Each square represents a “Super Pixel”
✘ RNFL thickness at each super pixel is
compared to age matched normative
database
✘ Super pixel that fall below normal range
is flagged by color squares based on
probability of normality
31. ✘ Dark blue squares RNFL thickness is below the 5th
percentile of the normative database.
✘ Light blue squares deviation below the 2% level.
✘ Yellow deviation below 1%.
✘ Red deviation below 0.5%.
32.
33. TSNIT Map
✘ Displays the RNFL thickness values along
the calculation circle.
✘ In a normal eye the TSNIT plot follows the
typical ‘Double Hump’ pattern.
✘ In a healthy eye, curve will fall within the
shaded area (95% normal range for age)
✘ When there is RNFL loss, the TSNIT curve
will fall below this shaded area
(superior and inferior regions)
34. Parameters Table
✘ The TSNIT parameters are summary
measures based on RNFL thickness
values within the calculation circle.
✘ Normal parameter values are displayed
in green.
✘ Abnormal values are color-coded based
on their probability of normality.
Colours are similar to deviation maps.
35. Parameters Table
✘ The TSNIT parameters are summary
measures based on RNFL thickness
values within the calculation circle.
✘ Normal parameter values are displayed
in green.
✘ Abnormal values are color-coded based
on their probability of normality.
Colours are similar to deviation maps.
36.
37. ✘ TSNIT SD : Captures the amount of modulation (peak to trough
difference) in TSNIT graph.
In normal eye there is high modulation.
In glaucoma eye there is RNFL loss in Sup
& Inf region which results in low
modulation & a low TSNIT SD value.
39. The Nerve Fiber Indicator (NFI)
✘ Global measure based on the entire RNFL thickness map
✘ Calculated using an advanced form of neural network, called a
Support Vector Machine (SVM)
✘ Not colour coded
✘ Output values range from 1 –100
○ 1-30 -> low likelihood of glaucoma
○ 31-50 -> glaucoma suspect
○ 51+ -> high likelihood of glaucoma
Clinical research has shown that the
NFI is the best parameter for
discriminating normal from glaucoma
44. Glaucoma is Detected Early with GDx VCC
Normal HFA
GDx VCC Abnormal OS
Normal HFA
45. Advantages
Easy to operate
Does not require pupillary dilation
Comparison with age matched normative database
Good reproducibility
Does not require a reference plane
Limitations
Affected by anterior and posterior segment pathologies.
Does not measure actual RNFL thickness.
Limited use in moderate/advanced glaucoma.
Difficult in nystagmus, very small pupil and media opacities.
Requires wider database for Indian population.
Young patients database not available.
47. ✘ k/a Scanning Laser Tomography (SLT)
✘ Three generations: HRT, HRT II and HRT 3
✘ a diagnostic procedure allows 3-dimensional topographic analysis of
optic disc and retina
✘ provides rapid and reproducible measurements of optic disc
topography on a pixel-by-pixel basis, as well as an analysis of
various optic disc parameters
48. Laser Source
✘ Helium-Neon Diode Laser of wavelength 670 nm.
✘ Laser not powerful enough to harm the eye.
49. Principle: Confocal Scanning Laser System
✘ Confocal : Conjugate + Focal
Describes that the locations of focal plane in retina & focal plane in the
image sensor are located in conjugate positions.
50. Principle: Confocal Scanning Laser System
✘ Confocal : Conjugate + Focal
Describes that the locations of focal plane in retina & focal plane in the
image sensor are located in conjugate positions.
Photodetector
✘ Confocality of system is achieved by placing a pinhole
52. ✘ Laser raster scans x-y plane to obtain confocal scans of Retina.
✘ Subsequently, laser changes focus to scan a slightly deeper plane of retina.
✘ Continues until a series of confocal sections through the depth of fundus are obtained.
✘ Instrument takes all these pictures of layers and puts them together to form a 3-dimentional
image of the optic nerve.
53. ✘ A series of 32 confocal images, each 256 X 256 pixels
✘ Depth of tomographic image series ranges from 0.5 to 4.0
mm in 0.5 increments.
56. ✘ The latest software available, HRT3, can provide ONH stereometric
analysis without manual delineation of the disc margin by the operator.
✘ After a three-dimensional model of the ONH is constructed, optic
nerve parameters are calculated and then analyzed with an artificial
intelligence classifier, the Relevance Vector Machine (RVM).
✘ From this analysis, a Glaucoma Probability Score (GPS) is created.
57. Reference Plane
✘ Plane to quantify stereometric rim and cup parameters in OHN topography (Burk et
al.1990)
✘ Reference plane is defined parallel to the peripapillary retinal surface & is located 50μ
posteriorly to the retinal surface at the papillomacular bundle.
✘ Fixed upon most stable part of disc margin, which is on the papillomacular bundle 4° to 10°
below the horizontal meridian (Bruk et al.2000) – Standard Reference Plane.
58. ✘ Patient Data
- Provides information on exam type (baseline or
follow-up)
- Patient demographic information (patient name ,
age, gender, ethnicity, etc.)
✘ Topographic Image
- HRT draws color coded maps, overview of the disc
Red – Cup
Blue or Green – NRR tissue
Blue – sloping rim, Green - Non sloping rim
- Also gives disc size- Small <1.6
Average 1.6-2.6
Large >2.6
59. ✘ Two cross-sectional images
- amount of cupping in vertical &
horizontal planes.
- two lines represent edge of optic
disc(Contour line) and the single red line
represents reference plane.
60. ✘ Reflection Image
- False color image that appears similar to a photograph of
optic disc.
Within normal limits √
Borderline !
Outside normal limits X
- Darker areas are regions of decreased over all
reflectance, where as the lighter areas such as base
of cup are areas of greater reflectance.
- ONH is divided in to six sectors
- Depending on the patients age and over all disc size eye is
then statistically classified as
61. ✘ Mean height contour graph
- Variation of the retinal surface height
along the contour line (green)
- Reference line (red)
- Height contour graph begins temporally at
0 degree.
-Plotted clockwise for RE &
counter-clockwise for LE.
62. ✘ Moorfields Regression Analysis
✘ Predicted: 50% of ONH in normal data base have larger
rim area than this limit.
✘ Low: 95.0/99.0/99.9% of ONH in normal data base have
larger rim area than this limit.
63. ✘ If the percentage of rim is larger than or equal to the 95% limit, the respective sector is
classified as ‘within normal limits’. (√)
✘ If the percentage of rim is between the 95.0% and the 99.9% limit, the respective sector is
classified as ‘borderline’. (!)
✘ If the percentage of rim is lower than 99.9% limit, the respective sector is classified as
‘borderline’. (X)
64. Stereometric Analysis
- If the SD is greater than 40 microns, the test
should be repeated to improve reproducablility or
the results to be interpreted with caution.
65. ✘ Glaucoma Probability Score (GPS).
✘ New software included in the HRT 3
generation allows calculation of GPS
✘ MRA is replaced by GPS.
✘ Unlike the MRA, the GPS utilizes the
whole topographic image of the optic disc
including cup size, cup depth, rim
steepness and vertical/horizontal RNFL
curvature where as the MRA uses the
logarithmic relationship between NRR and
Optic disc area
66.
67.
68.
69.
70.
71. ✘ Limitations
-The contour line (which is a subjective determination of the disc edge) and the
reference plane set by the device to delineate cup from the rim, are the two
main source of error in this technology.
✘ MRA can discriminate glaucomatous nerve from normal with 84% sensitivity
96% specificity
✘ However these problems were solved in HRT3 where GPA software is used
✘ HRT tends to over estimate the rim area in small ONH and the under estimate
the rim area in large ONH
