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HRT and GDx

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 OCTThe nerve fiber layer can be scanned with GDX and OCTmacula 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.


LAYOUTIntroduction Principle How to read print and Different parameters Limitations

INTRODUCTIONNon 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

PRINCIPLEConfocal scanning laser ophthalmoscopyUses 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

This laser, which is not powerful enough to harm the eye, is first focused on the surface of the optic nerve and captures that image. Then it is focused on the layer just below the surface and captures that image. The HRT continues to take images of deeper and deeper layers until the desired depth has been reached. Finally, the instrument takes all these pictures of the layers and puts them together to form a 3-dimentional image of the entire optic nerve.


What the HRT doesOnce the patient is positioned, HRT II automatically performs a pre-scan through the optic disc to determine the depth of the individuals 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 dataProvides 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 imageHRT draws a color-coded map. give an overview of the disc. Red cupGreen or Blue NRR tissue

Bluesloping rim

Green nonsloping rim tissue

Also gives disc size

small (sizes less than 1.6 mm2)Average (1.6 mm2.6 mm2)Large (greater than 2.6 mm2)


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 PROFILEHeight 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.


False-color image that appears similar to a photograph of the optic discDarker areas are regions of decreased overall reflectance, whereas lighter areas, such as the base of the cup, are areas of the greatest reflectanceValuable in locating and drawing the contour line around the disc marginIn the reflection image the optic nerve head is divided into 6 sectors.Depending on this patients age and overall disc size the eye is then statistically classified as.

F.MEAN CONTOUR HEIGHT GRAPHAfter 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 m below the retinal surfaceThe reference plane is used to calculate the thickness and cross-sectional area of the retinal nerve fiber layerThe 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 lineRetinal surface height profile


Any ONH that is determined to be outside normal limits is not necessarily glaucomatous but is statistically outside the normal rangesfor the group of eyes in the normative database. The decision as to whether outside normal limits represents glaucoma is a clinical judgment made by considering allclinical information together.

MRA makes use of the relationship between log neuroretinal rim area and optic disc area to define the normal ranges.Figure 3.2 illustrates the linear regression line between log neuroretinal rim area and optic disc area (marked 50%). This is the average or predicted relationship between log neuroretinal rim area and optic disc area. The lower three lines represent the lower 95.0%, 99.0%, and 99.9% prediction intervals for the same relationship. Thus, for the 95.0% prediction interval, 95.0% of normal eyes would be expected to have a neuroretinal rim area above that interval line. The same reasoning applies to the 99.0% and 99.9% prediction intervals. These intervals are calculated for the ONH as a whole and for each of the six predefined sectors.

The prediction intervals for neuroretinal rim areashould be regarded in the same way as theprobability smbols for abnormality in the reportsfrom automated perimeters. The closer the top ofthe green bar gets to the lower prediction intervals,the greater the probability that the rim area isabnormal.The MRA Report given in the HRTIIsoftwareenables a visual inspection of where the neuroretinal rim area lies in relation to thenormal ranges .22

H.Stereometric analysis

If the SD is greater than 40 m, 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


Patient informationQuality scoreC/D RatioCup shape measurementRim areaRim volumeTSNIT 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 GPSMRA is replaced by GPS.Shows the probability of damageFast, simple interpretationBased on the 3-D shape of the optic disc and RNFLUtilizes large, ethnic-selectable databasesEmploys artificial intelligence: Relevance Vector MachineNo drawing a contour line or relying on a reference planeReduced 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.

LimitationsThe 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.


INTRODUCTIONGDX evaluates the site of damage before the patients experience any vision lossGDX 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.- Doesnt 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 retardationThe 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 Henles layer in the macula as a control for measurement of corneal birefringence


GDx print out

A.Patients informationPatient data and quality score: the patients name, date of birth, gender and ethnicity are reported. An ideal quality score is from 7 to 10

B.FUNDUS IMAGEThe fundus image is useful to check for image quality:Every image has a Q score representing the overall quality of the scanThe 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 mapThe 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 mapsThe deviation map reveals the location and magnitude of RNFL defects over the entire thickness mapRNFL thickness of patient is compared to the age-matched normative databaseDark blue squares RNFL thickness is below the 5th percentile of the normative databaseLight blue squares deviation below the 2% levelYellow deviation below 1%Red deviation below 0.05%.

E.TSNIT mapTSNIT stands for Temporal-Superior-Nasal-Inferior-Temporal TSNIT displays the RNFL thickness values along the calculation circleIn 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 temporallyIn a healthy eye, the TSNIT curve will fall within the shaded area which represents the 95% normal range for that ageWhen 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 overlapin glaucoma, one eye often has more advanced RNFL loss and therefore the two curves will have less overlap

F.Parameters tableThe 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 circleSuperior Average: The average RNFL thickness in the superior 120 region of the calculation circleInferior Average: The average RNFL thickness in the inferior 120 region of the calculation circleTSNIT SDInter-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 mapCalculated using an advanced form of neural network, called a Support Vector Machine (SVM)Not colour codedOutput values range from 1 1001-30 -> low likelihood of glaucoma31-50 -> glaucoma suspect51+ -> 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 TimeSerial 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

LIMITATIONSEyes with macular pathology may show wrong RNFL values due to improper anterior segment birefringence (ASB) compensation at maculaASB may get altered after refractive surgery,so do fresh macular scan to compensate for changesLarge disc,large areas of PPA,affect RNFL,so use large scan circle

Early Glaucoma Examplemoderate Glaucoma Exampleadvanced Glaucoma Example

CONCLUSIONSThe 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 ophthalmologists capacity to evaluate all aspects of the patients diagnosis.

but they can definitely aid in the complicated decision-making process


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