Hrt II

Heidelberg Retinal tomography II

Sabitri BhattaBacholer in Optometry

Institute Of Medicine

Presentation Layout

Introduction

Historical background

Principle

Different parameters

Clinical applications

Why Optic Disc Evaluation Important ?????

• Important in glaucoma diagnosis

• Important in monitoring progressive nerve damage

• It is imperative to have an objective ,quantitative and reproducible imaging technique capable of making an early diagnosis and monitoring disease

Imaging Techniques for optic Disc and RNFL evaluation in glaucoma

• 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 )

Heidelberg Retina Tomography (HRT)• Diagnostic procedure • Allows three-dimensional

topographic analysis of optic disk and retina

• Provides rapid and reproducible measurements of optic disk topography on a pixel-by-pixel basis, as well as a reproducible analysis of various optic disk parameters.

Heidelberg Retina Tomography

• HRT uses a special laser to take 3-dimensional photographs of optic nerve & surrounding retina.

• This laser, which is not powerful enough to harm eye, first focused on the surface of optic nerve and captures that image

• Then it is focused on the layer just below surface and captures point image.


• HRT continues to take images of deeper and deeper layers until desired depth has been reached

• Finally, instrument takes all these pictures of layers and puts them together to form a 3-dimentional image of the entire optic nerve.

How it works?• Confocal scanning laser

ophthalmoscopy• 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

Heidelberg Retina Tomography (HRT)

• In order to produce 2-D image ,illuminating laser beam is deflected periodically in 2-D perpendicular to optical axis using scanning mirror

• Then object is scanned point by point sequentially in 2-D

• In confocal optical system ,small diaphragm placed in front of detector at a location which is optically conjugate to focal plane of illuminating system

Heidelberg Retina Tomography (HRT)

Advantage of diaphgram in confocal laser ophthalmoscopy

• Reflected light to pinhole detected easily & light reflected from 3-D object above and below focal plane not focused &lead focal image formation

• Confocal laser scanning system enable real 3-D imaging .2-D image formation at focal plane is moved &acquire images at different depth location &series of optical section form layer -by –layer 3-D images


• For each eye, three images are obtained. Images were obtained through undilated pupils unless patient was on miotic agents.

• Uses a diode laser at 670 nm as a light source.

Heidelberg Retinal Tomography

• A series of 32 confocal images, each 256 X 256 pixels, is obtained in a duration of 1.6 seconds.

• Size of the field of view is fixed at 15° X 15° in HRT II• Digitization is performed in frames of 384 X 384 pixels.


• This ensures that with size of field of view for HRT II image being 15°,

• Spatial resolution will be same as original 10° HRT images (10 μm/pixel)

• Depth of tomographic image series ranges from 0.5 to 4.0 mm in 0.5 increments.

• Computer converts 32 confocal images to a single topographic image in approximately 90 seconds

• Topographic image height measurements are determined relative to focal point of individual eye.

• Number of image planes acquired per series depends upon required scan depth with 16 images per mm of scan depth.

• Maximum number of images taken would be 64 for a 4-mm depth

• Number of images taken and depth are reported in information section, which can be retrieved from first screen for patient


• Automatic quality control occurs during image acquisition . More than one of images cannot be used for any reason (e.g., loss of fixation or blink), additional images are automatically acquired until three useful image series are obtained.

• Three acquired images are saved on hard drive and mean topography image computed

Stereo metric Parameters Disc Area (mm2 )

Area bounded by the contour line, indicating the area of the optic disc.

( 1.63 to 2.43mm2 )

Cup Area (mm2 )• Area of optic disc cupping and is

seen as area enclosed by contour line, which is located beneath reference plane. It appears as a red overlay on topography image

( 0.11 to 0.68mm2 )

Stereo metric Parameters

Rim Area (mm2 )

Area of neuroretinal rim and is seen as area enclosed by contour line, which located above Reference plane. It appears as either blue or green on topography image (blue – sloping and green – stable NRR) (1.31 to 1.96mm2)

Cup Volume (mm3)

Volume of optic disc cupping, defined as volume enclosed by contour line and located beneath reference plane. (-0.01 to 0.18mm3)


Rim Volume (mm3)• The volume of neuroretinal rim, defined as the volume

enclosed by contour line and located above reference plane. ( 0.30 to 0.61mm3 )

Cup/Disc Area Ratio• Ratio of area of optic disc cupping to area of optic

disc. (0.07 to 0.30)


Linear Cup/Disc Ratio• Average cup/disc diameter ratio calculated as square

root of the cup/disc area ratio (0.27 to 0.55)

Mean Cup Depth (mm)• Mean depth of optic disc cup. (0.10 to 0.27mm)

Maximum Cup Depth (mm)

Maximum depth of optic disc cupping.

(0.32 to 0.76mm)


Cup Shape Measure

Measure for overall three-dimensional shape of optic cup. Determined as distribution frequency of depths inside contour line. A normal value is on negative side. ( -0.28 to -0.15 )

Height Variation Contour (mm)

Variation in height along contour line,& is difference in height between most elevated and depressed point. This parameter decreases when nerve fiber loss occurs diffusely but increases with development of a localized nerve fiber defect. ( 0.31 to 0.49 )


Mean RNFL Thickness (mm)• Mean thickness of retinal nerve fiber layer measured

along the contour line, and measured relative to reference plane. ( 0.20 to 0.30 )

RNFL Cross-Section Area (mm2 )• This is total cross-sectional area of retinal nerve fiber

layer along the contour line and is measured relative to reference plane. ( 0.99 to 1.66 mm2 )

Reference Height (mm)• Describes location of reference plane, relative to

mean height of peripapillary retinal surface ( 307 μm )


Topography Standard Deviation (SD, μm)

• This is a measure of image quality.

• Value should be under 20 μm.

• Average standard deviation of all pixels in topography image.

Formula

• HRT considered parameters are : rim volume, cup shape measure, and height variation contour.

• Normal ONH is differentiated from abnormal ones using

these parameters and patient's age, according to formula:-

• Corrected CSM ( corCSM) = CSM + (0.00981*(50 - age))

The Effect of Optic Disc Size onDiagnostic Precision with theHeidelberg Retina Tomograph

Michele Jester, MD, FrederickS. Mikelberg, MD, Stephen M. Drance, MD

• A = (RV*l.951) + (HVC*30.125) + (-28.521 *corCSM) - 10.083

• B = ( -9.039*RV) + (HVC*37.370) + ( -15.442*corCSM) - 7.4211

• These are discriminant analysis formula for two subject

• if A > B, we considered the ONH to be normal

A= control group ;B= glaucomatous group • if A <B, we considered the ONH to be glaucomatous

• Purpose: Authors evaluated the ability of a confocal scanning laser ophthalmoscope to detect glaucomatous visual field loss by using their previously described discriminant formula on a prospectively obtained cohort.

• Relationship of optic disc size to diagnostic classification was also evaluated

• Methods: One eye was chosen randomly from each of 153 subjects.

• Sixty control had intraocular pressure < 21 mmHg and normal visual fields

• 93 glaucomatous eyes had intraocular pressure >21 mmHg and abnormal visual fields.

• The optic disc status purposely was not used for classification purposes.

• All subjects were examined with HRT & Humphrey Perimeter, program 30-2 .Visual fields were considered abnormal by authors‘ previously published criteria in discriminant formula

• HRT classification used age, adjusted cup shape measure , rim volume, and height variation contour to classify optic disc as normal or glaucomatous.

• Author assessed sensitivity, specificity, and diagnostic precision for entire group, and for 3 subsets classified by disc area: disc area < 2 mm2, between 2 and 3 mm2, and > 3 mm2

• Results: Entire group had a sensitivity, specificity, and diagnostic precision of 74%, 88%, and 80%, respectively.

• The specificity was 83% when disc area was < 2 mm2 and improved to 89% when disc area was > 2 mm2

• The sensitivity tended to improve from 65% to 79%, and to 83% if disc area increased, but difference was not statistically significant.

• Conclusions: In a prospective cohort of patients, HRT discriminant analysis formula was capable of detecting glaucomatous visual field loss with good precision.

• Unusually small optic discs continue to present diagnostic difficulties.

Ophthalmology 1997; 104:545-548

Clinical background• More than a decade of research with Heidelberg Retina

Tomograph and similar instruments,showed that quantification of optic nerve head topography provides an important , tool for glaucoma detection and follow-up

• Reproducibility of local height measurements at each of 65 thousand locations of a topography image is between 10 and 20 microns [Bathija et al., J Glaucoma 1998;7:121-127].

• Coefficients of variation of the stereometric parameters are about 5% [Rohrschneider et al., Ophthalmology 1994;101:1044-1049].

Clinical background

• Most important methods for glaucoma separation from normal eye are multivariate discriminant analysis procedures [Iester et al., Ophthalmology 1997;104:545-548] that showed highest diagnostic precision in a comparative study [Nakla at al., Invest Ophthalmol Vis Sci 1999;40:S397]

Clinical background

• Regression analysis of rim area to disk area that showed very high sensitivity and specificity to detect early glaucoma [Wollstein et al., Ophthalmology 1998;105:8,1557-1563], and even allows to pick up pre-perimetric glaucoma [Kamal et al., Br J Ophthalmol 1999;83:290-294]

• Analysis of stereo metric parameter values vs. time, and computation and analysis of topographic difference images and of change probability maps

Why HRT ????

• Functional and Structural Diagnosis

• Glaucoma diagnosis in clinical practice has been traditionally based on IOP, visual fields and subjective assessment of optic disc, but these methods have many limitations

• IOP■ Large overlap between healthy and glaucomatous

eyes

■ Corneal thickness affects accuracy (thicker cornea have high IOP & thinner have LOW IOP)

■ IOP fluctuates

■ Many glaucoma patients have normal IOP

• Visual Fields■ Poor sensitivity for early detection

■ Highly variable (affetced by age , media opacity)

■ In fact, OHTS reports that 86 % of visual field abnormalities were not replicated on retesting

■ Subjective---(handicapped pt unable to perform)

Subjective Assessment of the

Optic Disc■ Poor agreement on interpretation , even among

experts

■ Progression missed in up to 50 % of time by experts

■ Time consuming

■ Subjective

• Therefore, an objective structural assessment of optic disc is necessary.

• HRT is proven to be as good or better than expert readers of optic disc photographs and provides fast, objective assessment of complete optic nerve head structure including retinal nerve fiber layer.

• In addition, HRT provides an objective validated statistical analysis to show glaucomatous progression

Early Detection: Optic Disc Changes First

Ocular Hypertension Treatment Study (OHTS), analyzed glaucoma diagnosis and treatment of ocular hypertensive patients. Study showed 55 % of glaucoma patients optic disc changes could be measured first,35 % VF changes could be detected first. Results demonstrate analysis of optic nerve head structure is a pivotal aspect of glaucoma diagnosis,

• HRT enables quantitative evaluation of all relevant anatomical structures – cup, rim and RNFL (retinal nerve fiber layer). With highest spatial resolution of any imaging device for glaucoma diagnosis, HRT provides comprehensive data for glaucoma detection and follow-up assessment

Complete ONH Assessment HRT checks all vital structure of optic nerve

head:

CUP

■ C/D Ratio

■ Shape

■ Asymmetry

RIM

■ Area & Volume

■ Asymmetry

RNFL

■ Height Variation Contour

■ Thickness

■ Asymmetry

• These stereometric parameters are compared to comprehensive, ethnic-specific databases.

Types of HRT

• HRT I /HRT classic • HRT II • HRT 3/ portable HRT

HRT II Printout

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 quality score, focus position, and whether astigmatic lenses were used during acquisition.

Topography Image • False color image • Similar to gray scale of VF printout • Provides size shape,and location of cup • Elevated area typically appear darker • Lighter color represent

depressed regions

Cont…

• Red cup• Green or Blue NRR tissue • Blue areas sloping rim • Green areas nonsloping rim tissue• If NRR lie below reference plane,it may be due to an

incorrectly drawn contour or in severe glaucomatous injury.

• Topography image and is classified as • small (disc sizes less than 1.6 mm2)

• Average (1.6 mm2–2.6 mm2)

• Large (greater than 2.6 mm2)

• This section also provides two Cup-related parameters, Cup/Disc Area Ratio and Cup Shape Measure

• Along with actual parameter measurements, a symmetry measure between eyes is also given

• This is ratio expressed as a percentage of OD/OS

Reflectance image

• Appears similar to photographs • Moorfield’s analysis symbols seen• Contour line divide optic disc into 6 sectors • Contour drawn start from global sector • Cup depth (mm) measured from global sector

Usefulness of contour line

• Allows the practitioner to trace around the disc margin (as with the HRT II)

• Used for the calculation of stereometric parameters and Moorfields analysis.

• Having accepted contour, machine will identify whether any of six segments of disc

• Suspected sector (a red cross marking), worth monitoring (yellow explanation mark) or normal (green tick

Different landmarks used to correctly place the contour lines

• Appearance of the scleral ring (a change in color going from the optic disc to the retina)

Different landmarks used to correctly place the contour lines

• bending of blood vessels at disc border

• Appearance of peripapillary atrophy

• Once disc contour line is defined, an automatic analysis occurs with computation of stereometric parameters, classification of eye, and comparison to previous examinations (if prior images exist within database).

vertical interactive analysis

• Interactive display allows horizontal and vertical profiles of disc to be analyzed with respect to slope, walls, and depth of cup.

Interactive analysis of an eye with glaucoma. depth of cup and vertical appearance of walls of cup, especially in vertical meridian.

Horizontal interactive analysis

Interactive analysis of a healthy optic nerve witha small cup and a subtle tilted disc. Note how horizontal interactive line (on the bottom) raisesas one moves nasally.

Retinal surface height variation graph

• Height variation contour line (green line) shows height along contour line, placed at edge of optic disc

• Graphically displays height of retinal tissue along contour line and provides a calculation of thickness of nerve fiber layer

• Black line on height contour graph represents mean height of peripapillary retinal surface

• Vertical or horizontal black lines mark edge of disc as defined by contour line

Retinal surface height variation graph

• Reference line red with • Height contour line in green . Green contour line should

never go below red reference plane . If it does, then contour line is likely not in proper position

• Normal retina, nerve fiber layer --- thickest supero-temporally and infero-temporally so that contour line appears as a series of hills and valley

“Double hump apperance”

• As nerve fiber layer lost in glaucoma , retinal contour flattens and draws closer to red reference plane

• Mean height contour line, drawn clockwise for right eye and counterclockwise for LE starts at temporal region & moves superior, nasal, and inferior to finish back at temporal location. Contour may be diminished locally (focal damage) or diffusely with an area of contour line or entire line falling close to reference plane

Other analysis tools available on computer screen

Three-dimensional (3-D) image

3-D view of a person with optic nerve drusen

3-D image from a person with advancedglaucoma. Depth of the cup, steepness of walls, and reduced rim tissue.

Other analysis tools available on computer screen

Digital movie• Movie plays back image series in a rapid sequence,• Appears similar to live ophthalmoscopy with ability to

stop image at any depth• Increasing depth or a focal notch may appear as focal

plane moves deeper into cup

An image of the movie, in which images are played back sequentially.

Stereometric analysis of ONH area

• Stereometric parameters are calculated once

contour line accepted

• Quantify size, area, and volume measurements for the optic nerve head and surrounding area

• This range represents ± 1 standard deviation from mean

Stereometric analysis of ONH area

• Wide range of overlap in many parameters between normal and affected individuals made parameter difficult to analysis

. • Mikelberg developed a discriminant function analysis

that takes into account several parameters, and classifies patients being either normal or glaucomatous

• This measure was found to have 87% sensitivity and 84% specificity.

TABLE 1.Normative Stereometric Parameters

PARAMETER NORMAL EARLY MODERATE ADVANCED

Disc Area (mm2)

2.257 ± 0.563 2.345 ± 0.569 2.310 ± 0.554 2.261 ± 0.461

Cup Area (mm2)

0.768 ± 0.505 0.953 ± 0.594 1.051 ± 0.647 1.445 ± 0.562

Rim Area (mm2)

1.489 ± 0.291 1.3 93 ±0.340 1.260 ± 0.415 0.817 ± 0.334

Cup Volume (mm3)

0.240 ± 0.245 0.294 ± 0.270 0.334 ± 0.318 0.543 ± 0.425

Rim Volume (mm3)

0.362 ± 0.124 0.323 ± 0.156 0.262 ± 0.139 0.128 ± 0.096

Cup/Disc Area Ratio

0.314 ± 0.152 0.380 ± 0.179 0.430 ± 0.203 0.621 ± 0.189

Burk R. Laser Scanning Tomographié: Interpretation der Ausdrucke des Heidelberg Retina Tomographen HRT II.Z prakt Augenheilkd [Laser scanning tomography: interpretation of the HRT II printout]. 2001;22:183-190.

TABLE 1. Normative Stereometric ParametersPARAMETER NORMAL EARLY MODERATE ADVANCED

Mean Cup Depth (mm)

0.262 ± 0.118 0.279 ± 0.115 0.289 ± 0.130 0.366 ± 0.182

Maximum Cup Depth (mm)

0.679 ± 0.223 0.680 ± 0.210 0.674 ± 0.249 0.720 ± 0.276

Cup Shape Measure

-0.181 ± 0.092 -0.147 ± 0.098

-0.122 ± 0.095 -0.036 ± 0.096

Height VariationContour (mm)

0.384 ± 0.087 0.364 ± 0.100 0.330 ± 0.108 0.256 ± 0.090

Mean RNFLThickness (mm)

0.384 ± 0.063 0.217 ± 0.076 0.182 ± 0.086 0.130 ± 0.061

RNFL Cross-SectionalArea (mm2)

1.282 ± 0.328 1.155 ± 0.396 0.957 ± 0.440 0.679 ± 0.302

Burk R. Laser Scanning Tomographié: Interpretation der Ausdrucke des Heidelberg Retina Tomographen HRT II.Z prakt Augenheilkd [Laser scanning tomography: interpretation of the HRT II printout]. 2001;22:183-190.

Accuracy of Standard Deviation

• Low standard deviation image quality high.

• Standard deviation values 10 & below are labeled

“excellent”• Values between 11-20 are “very good,”• Values 21-30 are “good” • 31-40 are “acceptable”• 41-50 are “poor” • Values above 50 are “very poor” &should be

interpreted with caution

MRA explained with reference to Histogram

• Include 7 histogram• Each histogram split into 2

colors• Red --- cup & green –rim• 4 lines are drawn from top

to bottom • 4 lines represent predicted

rim area for a disc of that size Lower 95%prediction

interval Lower 99% PI

Lower 99.9% PI

In this given Histogram

• Temp/sup sector--WNL –means as the measured area fall above the lower 95% prediction interval

• Temp sector—borderline—measured rim area fall below the lower 95% but above 99.9%prediction interval

• Nasal sector—ONL –rim area fall below 95% &99.9% prediction interval

Reference plane

• Plane to quantify stereo metric rim and cup parameters in OHN topography (Burk et al.1990)

• Parallel to retinal surface

• Needs to be stable in HRT II reference plane lies 50 μm posterior to temporal disc margin

• 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

Reference plane • 320 reference plane and

standard reference plane were compared with an individually determined reference level fixed on ring of Esching (Tuulonen et al,1994) and a reference level fixed on mean height of the 1° disc margin above the horizontal meridian (Airaksinen 1994) & give most reliable results of diagnosis of glaucoma

Moorfields Regression Analysis(MRA)

• MRA analyses regression of logarithm of the global and six sectoral rim areas to the matching disc areas and compares results to normative database (Wollstein et al.1998)

• Defines these areas as Normal , Borderline ,Out side normal limits based on the 95% and 99.9% confidence intervals

• Method accurately discriminates between healthy controls and early glaucomatous patients using stereoscopic ONH photography (Wollstein et al.1998) or visual fields (Ford et al.2003, Miglior et al.2003)

Moorfields Regression Analysis(MRA)

• Method based on knowledge of physiological relationships as the dependence of NRR area on optic disc size

• Diagnostic specificity of this method is decreased by increasing size of the ONH in a non-glaucomatous population (Hawker et al 2006)

• As a screening method for glaucomatous change ,it has been reported superior to standarized automated perimetry and frequency doubling perimetry (Robin et al,2005)

Glaucoma Probability Score (GPS)

• GPS provide an automatized interpretation of ONH topography

• Model constructed by combining horizontal and vertical curvature of RNFL with steepness , size,and depth of cup ( Swindale et al.2000)

• Shape of optic nerve head changes as a normal eye converts to glaucoma. Using advanced artificial intelligence, software employs a new 3-D shape analysis, combining measures of optic nerve head and peripapillary retina into one sophisticated model.

Glaucoma Probability Score (GPS)

• Outcome determined by sector with highest probability score for glaucoma (Coops et al.2006)

• Value range from 0 to 1 represent the probability of glaucomatous damage .

• Values between 0.28 to 0.64 considered as borderline (Coops et al.2006)

• Method independent of contour line and reference plane ,which reduce source of variability

• High reproducibility ,but increase variability seen related to age ,diminished image quality and diagnosis of glaucoma ( Taibbi et al.20090

Topographic change analysis

• Method for detecting glaucomatous progression

• Estimate probability that a difference in surface height

occurs between baseline and follow up images by chance alone

• Performed in 4 x 4 pixelscluster called superpixels

• Method take account into local varibility

• PRACTICAL ASPECTS OF IMAGE ACQUISITION

Patient factor

• Obtaining sharp, high-quality image with HRT II dependent upon several variables, including pupil size, media clarity, patient fixation, and focus.

• Each of these aspects of image acquisition should be assessed prior to examination and addressed when possible.

• Patient should be instructed to keep his/her forehead against headrest at all times and look into lens of the camera as it is brought close to eye

• Lens should be near but not touch eyelashes, approximately 10 mm.

• Pupil may not need to be dilated if it is 3 to 4 mm in diameter, and majority of patients can be imaged without dilation

• A poor-quality tear film will also reduce image quality. In this latter situation, image quality degrades as eyes remain open, show ring type artifacts

Figure A blurred image due to tear film evapouration

• Figure

same pt immediately after a tear has been placed into eye

ACQUIRING THE IMAGE

• First step during image acquisition with HRT II is to enter patient’s name into computer and to select “acquisition.”

• Patient should be instructed to look straight ahead into camera until a red box in center of field of view is visualized

• For RE , pt should look to left of the box (toward the nose) and for LE, to right (toward the nose) until a green flashing light is seen which serves as fixation point

• With proper fixation optic nerve should appear centered in monitor. Approximate power of eye’s refraction is dialed into lens of camera as the technician views image on monitor

• Adjustable lens on camera is moved one click at a time, with image brightening as retinal surface is brought into focus

• A bracketing technique is used to obtain correct focus with an extra click in either direction darkening image and reducing sensitivity

Figure 1 well-focused image. Blue bars, justabove sensitivity value, are extended to right witha sensitivity value of 78.

Figure 2Same eye in Figure 1 with image not properlyfocused. Number of blue bars has decreased and sensitivity value has increased to 94

• Camera switch is depressed, camera performs an automatic pre-scan using a 4- to 6-mm depth

• From image obtained in pre-scan, software automatically sets correct location of focal plane (fine focus), required scan depth (depth), and proper sensitivity to obtain images with correct brightness (sensitivity).

• HRT II automatically acquires three three-dimensional images with predetermined acquisition settings

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

• In height contour line graph, initial green and baseline grey line superimposed for comparison

OU QUICKVIEW New Print Report

• All parameter values automatically adjusted for age-related changes, and also for their correlation with optic disc size.

• Results in greatly reduced normative range for each parameter , making comparisons to normative database using ethnic-specific which more sensitive for detecting abnormalities.


• Classification symbol also based on the p value• If the parameter within the 95% normal range

(p>.05), Green√ -- within normal range• Between 5th &0.1 percentile of normal distribution

(p<.05 &>0 .001), yellow ! point -- borderline• p value < 0.1 percentile of normal distribution, red

X -- outside normal limits--means that < 0.1% (1 out of 1,000) of all normal from the database have values this low, indicate high probability of abnormality


• Contour height graph presented with 95% normative range superimposed in green

• Lightly colored solid line gives average value for specific age , optic disc size & ethnicity

• Yellow area represents values between 5th and 0.1 percentile of normal distribution (p< .05 and greater than .001) indicating a borderline classification

• Red area represents < 0.1 percentile of normal distribution outside normal limits.


• Mean RNFL thickness, &inter-eye symmetry • Inter-eye symmetry is r value of Pearson Product

Correlation coefficient obtained by correlating right and left eyes point by point along this graph.

• Two contour height graphs plotted together .Solid black line gives OD profile & dashed line gives OS profile.

Clinical applications of HRT II• glaucoma diagnosis

• evaluation of macular holes , oedema

• detection and quantification of Nerve Fiber layer defects

HRT Has Been Shown To Be a Predictor of Glaucoma

• Topographic optic disc

measurements, when combined with other known predictive factors such as age, lOP, and central corneal thickness, could assist eye care professional in assessing likelihood of developing POAG.

HRT 3

• Neural network analysis for classifying eye

• Able to analyze HRT I images ,but types of imaging heads not comparable as such

• Slight scaling error corrected than HRT II

• Drawing of contour line automatized (Strouthidis & Garway –Health 2008)

• Ethnicity –specific

HRT 3

• Normative database & improvements in the image alignment algorithms introduced

• The software itself delineates the optic disc margin

• 6 nerve parameters and glaucoma probability score(GPS)

• Lateral resolution --- 10 μm/ pixel • Longitudinal resolution --- 62 μm/plane • Image acquisition time --- 1.0 sec

HRT HRT IIField of View:- Transverse

Longitudinal

10° x 10°, 15° x 15°, or 20° x 20° 0.5 to 4.0 mm

15° x 15°

1.0 to 4.0 mm

Digital Image size 2-D image

3-D image

256 x 256 pixels

256 x 256 x 32 voxels

384 x 384 pixels

384 x 384 x 16 to 384 x 384 x 64 voxels

Acquisition Time 2-D image3-D image

0.032 sec1.4 sec

0.025 sec1.0 sec typical (2-nm depth)

HRT HRT II

Focus Range -12 to +12 diopters -12 to +12 diopters

Optical Resolution Transverse/ Lateral(limited by the eye) Longitudinal

10 μm

300 μm

10 μm

300 μm

Digital Resolution Transverse

Longitudinal

10 to 20 μm/pixel

62 to 128 μm/plane

10 μm/pixel

62 μm/plane

HRT HRT II

Laser Source Diode laser, 675 nm Diode laser, 670 nm

Reference plane 320μm posterior to a reference ring located in the image periphery

50 μm posterior to temporal disc margin

Thank You

Thank You !!!!!!!!!!!!!!!!!!!!!!!!

Hrt II

Health & Medicine

Transcript of Hrt II