HRT and GDxSIVATEJA CHALLA
INTRODUCTIONNormal methods of detecting glaucoma:1. IOP measurement2. Optic disc observation3. Functional assessment : Visual field assessment4. Structural assessment : Assess the structure of optic nerve and/or RNFL: By Imaging : Confocal scanning laser ophthalmoscopy ( HRT ; Heidelberg
Retinal Tomography ;Heidelberg Engineering, Heidelberg, Germany ) Scanning Laser polarimetry(GDx ; Carl Zeiss Meditec , Dublin ,
California , USA ) Optical Coherence Tomography ( OCT ; Carl Zeiss Meditec)
Changes in RNFL and optic nerve head may precede the VFD. ONH can be scanned with HRT and OCT The nerve fiber layer can be scanned with GDX and OCT macula can be scanned with OCT
In advanced glaucoma:1- Scanning computerized ophthalmic diagnostic imaging play a least prominent role.2– VF testing is more appropriate to assess disease progression.
HRT
LAYOUT
Introduction Principle How to read print and Different parameters Limitations
INTRODUCTION
Non quantitative methods like disc photography, measurement of CDR require subjective physician interpretation and can be difficult and time-consuming in a busy clinical practice.and also observer dependent
Have to provide a more objective method to detect changes and progression
The culmination of these efforts has resulted in the development of confocal scanning laser ophthalmoscopy, which provides rapid, noninvasive, non contact imaging of the disc.
Provides three-dimensional topographic analysis of optic disk
PRINCIPLE Confocal scanning laser ophthalmoscopy Uses laser light instead of a bright flash of
white light to illuminate the retina (670 nm diode laser)
Laser is used as light source & beam focused to one point of examined object
Reflected light go same way back through optics & separated from incident laser beam by beam splitter &deflected to detector
This allow to measure reflected light only at one individual point of object
What the HRT does
Once the patient is positioned, HRT II automatically performs a pre-scan through the optic disc to determine the depth of the individual’s optic nerve.
Next, it determines the number of imaging planes to use (range of scan depth 1-4mm)
Each successive scan plane is set to measure 0.0625 mm deeper
Automatically obtains three scans for analysis. Aligns and averages the scans to create the mean topography
image
A series of 32 confocal images, each 256 X 256 pixels, is obtained in a duration of 1.6 seconds.
Computer converts 32 confocal images to a single topographic image in approximately 90 seconds
Print out
A. PATIENT DATA
B.TOPOGRAPHY
C.HORIZONTALHEIGHT PROFILE
D.VERICALHEIGHT PROFILE
E.REFELCTION IMAGE
H.TOP FIVE PARAMETERS
F.MEAN HEIGHT CONTOUR GRAPH
G.MOORFIELDS REGRESSION ANALYSIS
A.Patient data
Provides information on exam type (baseline or follow-up), patient demographic information (patient name , age, gender, ethnicity, etc.), and basic image information including image focus position, and whether astigmatic lenses were used during acquisition.
B.Topography image
HRT draws a color-coded map. give an overview of the disc. Red cup Green or Blue NRR tissue
Bluesloping rim
Green nonsloping rim tissue
Also gives disc size
small (sizes less than 1.6 mm2)
Average (1.6 mm–2.6 mm2) Large (greater than 2.6 mm2)
C.HORIZONTAL HEIGHT PROFILE Height profile along the white horizontal line in the topography
image. The subjacent reference line (red) indicates the location of the
reference plane (separation between cup and neuroretinal rim). The two black lines perpendicular to the height profile denote the borders of the disc as defined by the contour line.
D.VERTICAL HEIGHT PROFILE
Height profile along the white vertical line in the topography image.
The subjacent reference line (red) indicates the location of the reference plane (separation between cup and neuroretinal rim).
The two black lines perpendicular to the height profile denote the borders of the disc as defined by the contour line.
E.REFELCTION IMAGE False-color image that appears similar to a
photograph of the optic disc Darker areas are regions of decreased overall
reflectance, whereas lighter areas, such as the base of the cup, are areas of the greatest reflectance
Valuable in locating and drawing the contour line around the disc margin
In the reflection image the optic nerve head is divided into 6 sectors.
Depending on this patient’s age and overall disc size the eye is then statistically classified as.
F.MEAN CONTOUR HEIGHT GRAPH After the contour line is drawn around the border of the optic
disc, the software automatically places a reference plane parallel to the peripapillary retinal surface located 50 μm below the retinal surface
The reference plane is used to calculate the thickness and cross-sectional area of the retinal nerve fiber layer
The parameters of area and volume of the neuroretinal rim and optic cup are also calculated based on the location of the reference plane. cup area of the image that falls below the reference plane, neuroretinal rim above the reference plane
Green contour line should never go below red reference plane . If it does, then contour line is likely not in proper position
The graph depicts, from left to right: the thicknesses of the temporal (T); temporal-superior (TS); nasal-superior (NS); nasal (N); nasal-inferior (NI); temporalinferior(TI); and temporal (T) sectors.
the thickness of the normal retina is irregular, the contour line will appear as what is known as the ‘double-hump.’ The hills or ‘humps’ correspond to the superior and inferior nerve fiber layer, which are normally thicker than the rest of the areas.
Reference line
Retinal surface height profile
G.MOORFIELDS REGRESSION ANALYSIS
H.Stereometric analysis
If the SD is greater than 40 µm, the test should be repeated to improve reproducibility or the results should be interpreted with caution.
Difference in follow up report
Normal values of the HRT II stereometric parameters
So…
Patient informationQuality score
C/D RatioCup shape measurement
Rim areaRim volume
TSNIT graph
Follow-Up Report Baseline exam, and length of time in
months between reports compared Topography image red indicate worse
area and green indicate improved area
Glaucoma Probability Score (GPS) new software included in the HRT 3 generation allows calculation
of the GPS MRA is replaced by GPS. Shows the probability of damage Fast, simple interpretation Based on the 3-D shape of the optic disc and RNFL Utilizes large, ethnic-selectable databases Employs artificial intelligence: Relevance Vector Machine No drawing a contour line or relying on a reference plane Reduced dependency on operator skill
unlike the MRA, the GPS utilizes the whole topographic image of the optic disc, including the cup size, cup depth, rim steepness, and horizontal/vertical RNFL curvature whereas the MRA uses only a logarithmic relationship between the neuroretinal rim and optic disc areas.
Limitations
The contour line (which is a subjective determination of the edge of the disc) and the reference plane set by the device to delineate cup from rim, are the two main sources of error in this technology.
Because these determinations may be incorrect, this makes the HRT II not a good on-the-spot diagnostic device. However, in sequential analyses, these sources of error remain constant and the device is good to measure change over time.
Moorfields Regression Analysis Can Discriminate Glaucomatous Nerves From Normals With 84.3% Sensitivity And 96.3% Specificity.
How Ever These Problems Were Solved In Hrt3 Where Gpa Software Is Used.
The HRT Will Occasionally Call A Severely Damaged Optic Nerve Normal Or A Normal Optic Nerve Abnormal.
Heidelberg Retina Tomography Tends To Overestimate Rim Area In Small Optic Nerves And To Underestimate Rim Area In Large Nerves. So On Either Extreme Of Disc Size Range, Care Should Be Taken When Analyzing These Scans.
GDx VCC
INTRODUCTION GDX evaluates the site of damage before the patients experience
any vision loss GDX is:- Simple to use and easy for both the patient and operator.- Near infra-red wavelength(780 nm)- Measurement time is 0.7 seconds.- Total chair time less than 3 minutes for both eyes.- Undilated pupils work best.- Painless procedure.- Doesn’t require any drops.- Completely safe.
The GDx : - 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%.
GDx VCC should be added to the standard clinical examination to compliment the information from these other methods
PRINCIPLE - scanning laser polarimetry
Scanning laser polarimetry is an imaging technology that is utilized to measure peripapillary RNFL thickness
based on the principle of birefringence main birefringent intraocular tissues are the cornea, lens and the retina In the retina, the parallel arrangement of the microtubules in retinal
ganglion cell axons causes a change in the polarization of light passing through them.
The change in the polarization of light is called retardation The retardation value is proportionate to the thickness of the RNFL
Light polarized in one plane travels more slowly through the birefringent RNFL than light polarized perpendicularly to it.
This difference in speed causes a phase shift (retardation) between the perpendicular light beams.
VCC stands for variable corneal compensator, which was created to account for the variable corneal birefringence in patients
Uses the birefringence of Henle’s layer in the macula as a control for measurement of corneal birefringence
GDx VCC
GDx print out
A.Patients information Patient data and quality score: the patient’s
name, date of birth, gender and ethnicity are reported. An ideal quality score is from 7 to 10
B.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.
The Operator Centers The Ellipse Over The ONH In This Image
The Ellipse Size Is Defaulted To A Small Setting But Manipulating The Calculation Circle Can Change The Size Of The Ellipse
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 The Calculation Circle And Ellipse, The Operator Is Able To Measure Beyond A Large Peripapillary Atrophy Area
C.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
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
D.Deviation maps The deviation map reveals the location and magnitude of RNFL
defects over the entire thickness map RNFL thickness of patient is compared to the age-matched
normative database 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 0.05%.
E.TSNIT map
TSNIT stands for Temporal-Superior-Nasal-Inferior-Temporal TSNIT displays the RNFL thickness values along the calculation
circle In a normal eye the TSNIT plot follows the typical ‘double hump’
pattern, with thick RNFL measures superiorly and inferiorly and thin RNFL values nasally and temporally
In a healthy eye, the TSNIT curve will fall within the shaded area which represents the 95% normal range for that age
When there is RNFL loss, the TSNIT curve will fall below this shaded area, especially in the superior and inferior regions
In the center of the printout at the bottom, the TSNIT graphs for both eyes are displayed together.
healthy eye there is good symmetry between the TSNIT graphs of the two eyes and the two curves will overlap
in glaucoma, one eye often has more advanced RNFL loss and therefore the two curves will have less overlap
F.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.
TSNIT Average: The average RNFL thickness around the entire calculation circle
Superior Average: The average RNFL thickness in the superior 120° region of the calculation circle
Inferior Average: The average RNFL thickness in the inferior 120° region of the calculation circle
TSNIT SD Inter-eye Symmetry Values range from –1 to 1, Normal eyes have good
symmetry with values around 0.9
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
Serial AnalysisDetecting RNFL Change Over Time
Serial Analysis can compare up to four exams
The Deviation from Reference Map displays the RNFL difference, pixel by pixel, of the followup exam compared to the baseline exam
LIMITATIONS
Eyes with macular pathology may show wrong RNFL values due to improper anterior segment birefringence (ASB) compensation at macula
ASB may get altered after refractive surgery,so do fresh macular scan to compensate for changes
Large disc,large areas of PPA,affect RNFL,so use large scan circle
Early Glaucoma Example moderate Glaucoma Example advanced Glaucoma Example
CONCLUSIONS The ability to detect early glaucomatous structural changes has great
potential value in delaying and avoiding progression of the disease
the most difficult optic discs to interpret in terms of glaucomatous changes– specifically highly myopic and tilted optic discs – are also those discs which optic nerve imaging devices have the greatest limitations in discriminating abnormality from pathology
should not be regarded as replacing the skilled ophthalmologist’s capacity to evaluate all aspects of the patient’s diagnosis.
but they can definitely aid in the complicated decision-making process
THANK YOU
Topographic Change Analysis (TCA) Statistically-based progression algorithm that accurately detects
structural change over time by comparing variability between examinations and providing a statistical indicator of change.
Aligns subsequent images with the baseline examination, providing a point-by-point analysis of the optic disc and peripapillary RNFL
