IMAGING TECHNIQUES IN GLAUCOMA

Presenter: Dr. RujutaModerator: Dr. Rita Dhamankar

Various imaging techniques

Anterior Segment:AS-OCTUBM

Posterior Segment:OCTHRTGDx

Stereoscopic Optic Disc Photography

Used to document structural abnormalities and longitudinal changes in glaucomatous eyes

Highly reproducible and records a natural color image of the retina

Conventional ONH evaluation includes estimation of the ONH dimensions by observing the image pair with a stereo viewer

In the stereo image pair, depth is inversely proportional to the disparity between the two matching points from the left and right images

Quantitative Imaging

Principles Clinical Parameters Measured

OCT Interferometry Retinal Nerve Fiber Layer Thickness

HRT Confocal Scanning LaserOphthalmoscopy

Optic Disc Tomography

GDx Scanning Laser Polarimetry

Retinal Nerve Fiber Layer Thickness

Optical Coherence Tomography (OCT)

Optical Coherence Tomography (OCT)

Non-invasive, real-time, high-resolution imaging Transverse resolution -20 μm Axial resolution - 8–10 μm Software uses interpolation to fill in the gaps

Optical Coherence Tomography (OCT)

OCT uses a principle called low coherence interferometry to derive depth information of various retinal structures

This is performed by comparing the time difference in reflected light from the retina at various depths with a reference ‘standard’

Differences between the reflected light and the reference standard provide structural information in the form of an ‘A’ scan

Time Domain OCT

SLD

Lens

Detector

Data Acquisition

Processing

Combines light from reference

with reflected light from

retina

Distance determines depth

in A scan

Reference mirror moves back and

forth

Scanning mirror directs SLD beam on

retina

Interferometer

Broadband Light Source

Creates A-scan 1 pixel

at a time

Final A-scan

Process repeated

many times to create B-

scan

Fourier Domain OCT

SLD

Spectrometer analyzes signal by

wavelength FFT

Grating splits signal by

wavelength

Broadband Light Source

Reference mirror stationary

Combines light from reference

with reflected light from

retina

Interferometer

Spectral interferogra

m

Fourier transform converts signal to

typical A-scan

Entire A-scan created at a single

time

Process repeated

many times to create B-

scan

Principles of OCT Technology

An A-scan is the intensity of reflected light at various retinal depths at a single retinal location

Combining many A-scans produces a B-scan

A-scan A-scan

+ +. . . =

B-scanA-scans

Retin

al D

ep

th

Reflectance Intensity

RNFL Analysis

Analysis of RNFL aids in identification of early glaucomatous loss

Circular scans of 3.4 mm diameter in the peripapillary region (cylindrical retinal cross-section)

RNFL thickness measurement is graphed in a TSNIT orientation

Compared to age-matched normative data

Optic Nerve Head Analysis

Radial line scans through optic disc provide crosssectional information on cupping and neuroretinal rim area

Disc margins are objectively identified using signal from

end of RPEParameters:

Disccup and rim area horizontal and vertical cup-to-disc ratiovertical integrated rim areahorizontal integrated rim width

Signal Strength

Signal Strength

Effect of Decentration

Heidelberg Retinal Tomogram (HRT)

Heidelberg Retinal Tomogram (HRT)

Confocal scanning laser ophthalmoscope that is capable of acquiring and analysing three-dimensional images of the optic nerve head and peripapillary retina

Confocal Scanning Laser Ophthalmoscopy

Uses laser light instead of a bright flash of white light to illuminate the retina

Confocal imaging is the process of scanning an object point by point by a focused laser beam and then capturing the reflected light through a small aperture (a confocal pinhole)

The confocal pinhole suppresses light reflected or scattered from outside of the focal plane, which otherwise would blur the image. The result is a sharp, high contrast image of the object layer located at the focal plane

Advantages over Fundus PhotographyImproved image qualitySmall depth of focusSuppression of scattered lightPatient comfort through less bright light3D imaging capabilityVideo capabilityEffective imaging of patients who do not dilate well

Confocal Scanning Laser Ophthalmoscopy

Principle

Rapid scanning 670-nm diode laserEmitted beam is redirected in the x and y-axis Along a plane of focus perpendicular to z-axis

using two oscillating mirrors Two-dimensional image reflected from the surface

of the retina and optic discThe confocal aperture limits the depth from which

reflected light reaches the detectorConfocal aperture is shifted to acquire multiple

optical sections through the tissue of interest in order to create a layered three-dimensional image

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.

Using information from this pre-scan, the fine focus and scan depth are automatically adjusted to ensure that the entire optic disc is included on the imaging cross-sections.

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

HRT Images

Reflectance 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

HRT Images

Topographic ImageRelays information concerning

the height of the surface contour of the optic disc and retina

False-color codedPixels that appear bright in the

topographic image are deeper, and dark pixels are elevated

Thus, the neuroretinal rim should appear darker than the surrounding retina and the base of the cup usually appears lightest

Analysis

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. The cup is considered to be the area of the image that falls below the reference plane, whereas areas that are of greater height than the reference plane are considered the neuroretinal rim

HRT can differentiate between normal & early glaucomatous eyes with a sensitivity of 79% to 87% & specificity of 84 to 90%

Moorfields Regression Analysis (MRA)

MRA differentiates between glaucomatous and healthy ONHs by detecting diffuse and focal changes of the neuroretinal rim area

Encorporates ONH size, and the effect of age

Classifies the eye using normative data, for both global and sectoral analyses, the latter using six sectors

Results are indicated as color-coded symbols: A green checkmark when “inside normal limits”; a yellow exclamation mark when “borderline”; and a red cross when “outside normal limits”.

Glaucoma Probability Score (GPS)

Shows the probability of damage

Fast, 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

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

GDx VCC

GDx VCC

Provides highly reproducible, objective measurements of the RNFL, to detect structural changes early

Compares these measurements to an age-stratified, multi-ethnic normative database, providing a unique visual representation

Scanning laser polarimetry

Use of polarised light to measure the thickness of the retinal nerve fiber layer

Measures the phase shift (retardation) of polarized laser light passing through the eye

The phase of the light is changed by the arrangement and density of retinal nerve fiber layer (RNFL)

Scanning laser polarimetry Principle

The polarised laser scans the fundus, building a monochromatic image

The state of polarisation of the light is changed (retardation) as it passes through birefringent tissue (cornea and RNFL)

Corneal birefringence is eliminated (in part) by a proprietary 'corneal compensator‘

The amount of retardation of light reflected from the fundus is converted to RFNL thickness

GDx VCC

Provides quantitative RNFL evaluationKey elements:

Thickness MapDeviation Map TSNIT graphParameter Table

Key Features of the Printout

Fundus ImageUseful for checking image quality

Well focusedEvenly illuminatedOptic disc well centered

Key Features of the Printout

Thickness MapShows the RNFL thickness in a color-coded

formatThick RNFL values are coloured yellow, orange,

redThin RNFL values are coloured dark blue, light

blue, green

Key Features of the Printout

Deviation MapReveals the location and magnitude of RNFL

defects over the entire thickness mapAnalyzes a region 20° x 20° centered on the

optic discFor each scan, the RNFL thickness at each pixel

is compared to the age-matched normative database, and the pixels that fall below the normal range are flagged by coloured squares based on the probability

Deviation Map continued…

Dark blue squares represent areas where the RNFL thickness is below the 5th percentile of the normative database

Light blue squares represent deviation below the 2% level

Yellow represents deviation below 1%

Red represents deviation below 0.05%

Uses a grayscale fundus image of the eye as a background

Deviation Maps for eyes at different stages of disease

Key Features of the Printout

TSNIT MapDisplayed at the bottom of

the printoutIn a normal eye the TSNIT

plot follows the typical ‘double hump’ pattern

When there is RNFL loss, the TSNIT curve will fall below this shaded area, especially in the superior and inferior regions

Also, a dip in the curve of one eye relative to another is indicative of RNFL loss

Key Features of the Printout

Parameter TableThe TSNIT parameters are summary measures

based on RNFL thickness values within the calculation circle

Parameters continued…

Inter-eye Symmetry: Measures the degree of symmetry between the right and left eyes by correlating the TSNIT functions from the two eyes

Values range from –1 to 1, where values near one represent good symmetry

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)

Output values range from 1 –1001-30 -> normal31-50 -> borderline51+ -> abnormal

Normal printout

Early Glaucoma Example

Advanced Glaucoma example

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

Summary

The imaging techniques provide comprehensive RNFL assessment to aid the clinician in the diagnosis of glaucoma

However, they do not replace a careful clinical evaluation or visual field testing

THANKYOU!