This document provides an overview of various imaging modalities used in glaucoma, including OCT, HRT, and GDx. It describes the principles, clinical parameters measured, and interpretation of printouts for each technique. OCT uses interferometry to measure retinal nerve fiber layer thickness. HRT uses confocal laser scanning to obtain optic disc topography. GDx uses scanning laser polarimetry to measure RNFL thickness based on birefringence. Each modality provides quantitative measurements and maps to detect glaucomatous changes in the optic nerve head and retinal nerve fiber layer.

1. IMAGING IN GLAUCOMA
Dr. Anujeet Paul
Moderator: Dr. A.R.Rajalakshmi

2. OVERVIEW
• Anterior segment
1. AS-OCT
2. UBM
• Posterior segment
1. OCT
2. HRT
3. GDx VCC
4. ONH imaging
• Importance of Imaging in Glaucoma

3. POSTERIOR SEGMENT

4. PRINCIPLE CLINICAL PARAMETER
MEASURED
OCT Interferometry Retinal Nerve Fibre Layer
Thickness
HRT Confocal Scanning Laser
Ophthalmoscopy
Optic Disc Tomography
GDx Scanning Laser Polarimetry Retinal Nerve Fibre Layer
Thickness

5. OPTICAL COHERENCE
TOMOGRAPHY

6. INTRODUCTION
• New diagnostic imaging tool
• Non invasive, real-time, high resolution
• Principle – light interferometry
• Uses low coherence light in the near-infrared range (820nm) to
perform cross-sectional images of biological tissues
• Axial resolution – 10µ
• Transverse resolution – 20µ

7. PRINCIPLE OF OCT
• Near infra-red beam (820m) split
into two components:
• Probe beam – to the tissue of
interest (retina)
• Reference beam – travels to a
reference mirror at a known
variable position

8. PRINCIPLE OF OCT
• Light reflected back from the
boundaries of the microstructures
• Light is also scattered differently
by tissues of different optical
properties
• Echo time delay of this light
(reflected from the retina) is
compared with the same of the
reference mirror and the
interference pattern is noted

9. PRINCIPLE OF OCT
• A positive interference is
produced when light reflected
from the retina and reference
mirror arrive simultaneously or
within short coherence length of
each other
• Interference measured by a
photodetector

10. PRINCIPLE OF OCT
• Interferometer integrates several
data points over 2mm of depth to
construct a tomogram of retinal
structures
• Real time tomogram using a false
colour scale

11. PRINCIPLE OF OCT

12. TIME DOMAIN OCT FOURIER DOMAIN OCT
A scans generated sequentially one
pixel at a time in depth
Entire A scan generated at once based
on Fourier transformation of
spectrometer analysis
Moving reference mirror Stationary reference mirror
400 A scans per second 26000 A scans per second
10 micron depth resolution 5 micron depth resolution
B scan (512 A scans) in 1.28 seconds B scan (1024 A scans) in 0.04 seconds
Slower than eye movement Faster than eye movement

14. TIME DOMAIN OCT
SLD
Detector
Data Acquisition
Processing
Combines light
from reference
with reflected
lightfrom retina
Distancedetermines
depth in A scan
Reference mirror moves
back and forth
Lens
Scanning mirror
directs SLD
beam on retina
Interferometer
Broadband
Light Source
Creates
A-scan 1
pixel ata
time
Final A-scan
Process
repeated many
times tocreate
B-scan

15. FOURIER DOMAIN OCT
SLD
Spectrometer
analyzes signal
bywavelength
FFT
Grating splits
signal by
wavelength
Broadband
Light Source
Reference mirror
stationary
Combines light
from reference
with reflected
light from retina
Interferometer
Spectral
interferogram
Fouriertransform
converts signal to
typical A-scan
Entire A-scan
created ata
single time
Process
repeated many
times tocreate
B-scan

16. SPECTRAL DOMAIN OCT AND SWEPT SOURCE OCT
• Both instruments use Fourier domain detection techniques
• SD-OCT instruments use a broadband near-infrared superluminescent diode as
a light source, currently with a center wavelength of approximately 840 nm,
with a spectrometer as the detector
• SS-OCT instruments use a tunable swept laser, currently with a center
wavelength of approximately 1050 nm, with a single photodiode detector

17. ANALYSIS FROM OCT
• Optic nerve head
• Peripapillary retinal nerve fibre
layer (RNFL)
• Macular ganglion cell complex
(GCC)

18. 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

19. OPTIC NERVE HEAD ANALYSIS
• Radial line scans through optic disc
provide cross-sectional information
on cupping and neuroretinal rim area
• Disc margins are objectively identified
using signal from end of RPE
• Parameters:
1. Disc
2. cup and rim area
3. horizontal and vertical cup-to-disc
ratio
4. vertical integrated rim area
5. horizontal integrated rim width

20. INTERPRETATION OF OCT
PRINT-OUT

21. 1. SIGNAL STRENGTH
• Determines the reliability
• Cirrus OCT: >= 6/10
• Optovue: 40 and above
• Poor signal strength –
underestimation of thickness of
optical fibres

22. 2. RNFL THICKNESS MAP
Normal – hour-glass or butterfly
wing appearance, yellow and red
Abnormal –
1. Asymmetry between superior
and inferior sectors
2. Early signs of a peripheral
defect -blue notch in yellow red
sectors

23. 3. RNFL THICKNESS
DEVIATION MAP
• Detect abnormalities situated
outside the classical peripapillary
RNFL B- scan
• 3.46mm in diameter, used as a
reference zone for evaluation of
RNFL thickness.
• Outside normal limits – yellow
and red

24. HEALTHY GLAUCOMA SUSPECT MODERATE GLAUCOMA SEVERE GLAUCOMA

25. 4. TSNIT GRAPH
• Thickness of the optical fibre layer
in different regions (temporal,
superior, nasal, inferior, temporal)
at 3.4 mm from centre of the
optic nerve
• RE – continuous line
• LE – dotted line
• Normal zones – in green
• Abnormal zones in yellow and
then red

26. 5. NRR THICKNESS MAP
• Thickness of Neuro-retinal Rim
(NRR) in different regions
• RE – continuous line
• LE – sotted lines
• Normal zones - In green
• Abnormal zones - In yellow and
red

27. 6. ENLARGED VERTICAL AND
HORIZONTAL B SCAN
• To check the correct localization
of the Bruch’s membrane (black),
which define the limits of the
ONH and on the projections on
to the surface of the internal
limiting membrane (red) to
determine the edge of the NRR

28. 7. RNFL THICKNESS
VALUES
• RNFL thickness – with a colour
coded representation compared
to normative database
• Inferior quadrant has the best
discriminating ability for early
detection of glaucoma

29. 8. CIRCULAR B SCAN
• Extracted from the optic disc
cube, as well as the internal and
external limits of the RNFL

31. ASSESSMENT OF NRR

32. CUP DISC RATIOS AND CUP VOLUMES
Disc size:
• By measuring the distance between
the terminal ends of the choroid at the
level of the pigment epithelium (green
line)
Cup size:
• Determined by drawing a line b/w
both sides of the cup at a point 150µ
(reference plane) above the green
line.
• Area below the line is cup and

33. OCT IMAGING OF MACULAR GANGLION CELLS
• The macula is densely
populated by RGCs,
containing 30% of the total
number of these cells while
occupying only 2% of the
retina’s area.
• Loss of RGCs in early
glaucoma is more likely to
occur in macula

35. OCT IMAGING OF MACULAR GANGLION CELLS
• The macula is densely
populated by RGCs,
containing 30% of the total
number of these cells while
occupying only 2% of the
retina’s area.
• Loss of RGCs in early
glaucoma is more likely to
occur in macula

37. ASSESSMENT OF MACULAR GANGION CELL
COMPLEX
GCC scan data is displayed as
thickness map of GCC layer
Thicker regions –
hot colors
(yellow and orange )
Thinner regions –
cooler colors
(blue and green).
GCC map for a normal eye shows a
bright circular band surrounding the
macula representing thick GCC

38. ASSESSMENT OF MACULAR GANGION CELL
COMPLEX
GCC scan data is displayed as
thickness map of GCC layer
Thicker regions –
hot colors
(yellow and orange )
Thinner regions –
cooler colors
(blue and green).
GCC map for a normal eye shows a
bright circular band surrounding the
macula representing thick GCC

43. LIMITATIONS OF OCT
• Limited applications in cases of poor media clarity – corneal edema,
dense cataracts, vitreous hemorrhage and asteroid hyalosis
• High astigmatism and decentered IOL may compromise the quality
• Limited transverse sampling

44. HEIDELBERG RETINA
TOMOGRAPHY

46. INTRODUCTION
• Rapid, reproducible measurement of optic disc topography on a
pixel by pixel basis, as well as analysis of various optic disc
parameters
• Non contact, non invasive imaging of optic disc in three-dimensions
• HRT I – HRT Classic – research purpose
• HRT II – used commonly, user friendly but operator dependant
• HRT III – operator independent (portable)

47. CONFOCAL SCANNING LASER
• Confocal – conjugate + focal
• Describes that the locations of
focal plane of retina and focal
plane in the image sensor are
located at conjugate positions
• This confocality is achieved by
placing a pinhole in front of the
detector, which is conjugate to
laser focus

48. CONFOCAL SCANNING LASER
• Size of pinhole determines the
degree of confocality
(small pinhole aperture highly
confocal image)

49. PRINCIPLE
• Principle – Confocal scanning laser
ophthalmoscopy
• Laser source – helium-neon diode
laser
• Wavelength – 670nm
• The laser raster scans the x-y plane
to obtain confocal optical sections
of the retina
• Once one plane has been scanned,
the laser changes focus to scan a
slightly deeper plane of retina

50. PRINCIPLE
• Range of scan depth - 1-4mm
• Each successive plane is set to
measure 0.0625mm deeper
• Automatically obtains three scans
for analysis
• Aligns and averages the scans to
create the mean topography
image

51. PROCEDURE
• A series of 32 confocal
images, each 256x256 pixels
is obtained in a duration of
1.6 seconds
• Size of field of view – 15° X
15°(HRT II)
• Axial resolution of the scan –
62 µm planes

52. INTERPRETATION OF HRT
PRINT-OUT

53. 1. PATIENT INFORMATION
AND IMAGE QUALITY
• Clear representation from
excellent to very poor
• Score below 30 represents a good
quality image

54. 2. TOPOGRAPHY IMAGE
• Color coded map – overview of
disc
• Red  cup
• Green / blue  NRR tissue
• Blue – sloping rim
• Green - non sloping rim
• Elevated area – darker
• Lighter colour – depressed
regions

55. 3. REFLECTION IMAGE
• Darker areas – decreased
reflection
• Lighter areas (base of cup) –
greatest reflectance

56. 4. VERTICAL AND HORIZONTAL
HEIGHT PROFILE
• Allows analysis of horizontal and
vertical profile of disc with respect
to slope, walls and depth of cup

57. 5. RETINAL SURFACE HEIGHT
VARIATION GRAPH
• Variation height contour (green)
line shows height along contour
line placed at edge of the disc
• Graphically displays height of
retinal tissue along contour line
and provides a calculation of
thickness of NFL
• Black line – represents mean
height of peripapillary retinal
surface
• Red – reference line
• Green – height contour line

58. 5. RETINAL SURFACE HEIGHT VARIATION GRAPH
• Reference plane – 50 micron below
retinal surface
• Cup – area of image that falls
below the reference plane
• NRR – above the reference plane
• Glaucoma – loss of RNFL , retinal
contour flattens and draws closer
to red reference plane

59. 5. RETINAL SURFACE HEIGHT
VARIATION GRAPH
• NFL is thickest supero-temporally
and infero-temporally so that the
contour line appears as a series of
hills and valleys – ‘Double Hump’
appearance
• As NFL is lost in glaucoma, retinal
contour flattens and draws closer
to the red reference plane

60. 6. MOORFIELDS REGRESSION ANALYSIS (MRA)
• MRA analyses regression of logarithm of the global and six sectoral
rim areas to the matching disc areas and compares the results to
normative database
• Defines these areas as: normal, borderline, outside normal limits
based on 95% and 99% confidence intervals
• Method accurately discriminates between healthy controls and early
glaucomatous patients using stereoscopic ONH photography –
detects diffuse focal changes in NRR area
• method is based on knowledge of physiological relationships as the
dependence of NRR area on optic disc size

61. 6. MRA
• 7 histograms
• Each split into 2 colours
• Red – cup ; Green – rim
• 4 lines drawn from top to bottom
– each represent the predicted
area for a disc of that size

62. 7. GLAUCOMA
PROBABILITY SCORE
• Automated interpretation of ONH
topography
• Constructed amalgamating
horizontal and vertical curvature
of RNFL with steepness, size and
depth of cup
• Value range from 0 to 1 represent
the probability of glaucomatous
damage
• Borderline – 0.28-0.64

63. GLAUCOMA DIAGNOSIS (GDX)

64. INTRODUCTION
• Principle - Scanning Laser
Polarimetry
• Uses near infra red
wavelength (780nm)
• Measurement time – 0.7
seconds
• Undilated pupils

65. PRINCIPLE
• To measure peripapillary RNFL
thickness; based on the principle
of birefringence (double
refraction)
• Due to this property part of the
laser beam is phase shifted
(retarded).
• The phase shift directly correlates
with the thickness of the RNFL

66. PRINCIPLE
• Birefringence also occurs when
the laser passes through the
cornea.
• The VCC (variable corneal
compensator) feature of the
newer gdx machines measures
the corneal birefringence and
optically cancels it with each
measurement of RNFL

67. INTERPRETATION OF GDX
PRINT-OUT

68. 1. FUNDUS IMAGE
• Reflectance image of the
posterior pole depicting a 20x20
degree image of the posterior
pole
• Utilizes 16000 data points from
the scan area to produce and
display the fundus image

69. 2. RNFL THICKNESS MAP
• Color coded format from blue to red
• Hot colours – red and yellow – high
retardation or thicker RNFL
• Cool colours – blue and green – low
retardation or thinner RNFL
• Healthy eye – yellow and red in
superior and inferior regions; blue
and green in nasal and temporal
regions
• Glaucoma: RNFL – uniform blue
appearance

70. 3. DEVIATION MAPS
• Reveals the location & magnitude of
RNFL
• Defects over the entire thickness map
• Each square represents a “super pixel”
• RNFL thickness at each super pixel is
compared to 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 0.5%.

71. 4. TSNIT MAP
• RNFL thickness values along the
calculation circle
• Double hump pattern
• RE – green; LE - purple
• Healthy eye – TSNIT curve fall
within shaded area – 95%
normal range for that age &
good symmetry between two
eyes and two curves overlap
• In glaucoma – less overlap

73. OPTIC NERVE HEAD IMAGING

74. ONH IMAGING
• Morphological glaucomatous changes that can be assessed
qualitatively are represented by ONH and RNFL changes
• Changes of the optic nerve head are represented by a focal and/or
generalized narrowing of the neuroretinal rim that determines a
focal or generalized enlargement of the optic disc cup
• Changes of the RNFL are represented by the onset or the
enlargement of wedge shaped RNFL defects.

75. ONH IMAGING
• Appreciated either directly at the slit lamp, by high magnification
ophthalmoscopy, or by means of consecutive monoscopic or
stereoscopic photographs.
• Disadvantage - qualitative assessment of fundus photographs
suffers from the problem of subjectivity

78. ANTERIOR SEGMENT

79. ULTRASOUND BIOMICROSCOPY (UBM)
• High frequency ultrasound at 50–100 MHz for anterior segment
imaging
• A computer program then converts these sound waves into a high-
resolution B scan image
• The probe provides a scan rate of 8 Hz, with a lateral resolution of
50 µm and an axial resolution of 25 µm

80. APPLICATIONS OF UBM
• Good agreement with gonioscopy in its ability to evaluate angle-
closure
• Possible to determine the mechanism of elevated intraocular
pressure by analyzing the relationship between the structures of the
angle
• Imaging of the anterior segment structures is possible in eyes with
corneal edema or corneal opacification
• Evaluate the iris insertion and its relationship with the trabecular
meshwork
• It is possible to identify ciliary processes pushing the lens and iris
forward in angle closure glaucoma or iris cysts pushing the iris

90. AS-OCT
• Biometric parameters that can
be measured by the AS-OCT
include:
• Iris thickness
• Iris curvature
• AC depth
• AC width
• Lens vault

95. LIMITATIONS OF AS-OCT
• Cannot penetrate through pigmented tissue
• Cannot image beyond posterior pigmented epithelium of iris

96. ADVANTAGES OF UBM
• Unlike AS-OCT, UBM can achieve visualization of structures posterior
to the iris pigment epithelium (sound penetrates the pigment
epithelium but light does not)
• UBM is better for visualizing the posterior chamber structures,
including the lens zonules, ciliary body, and even the anterior
choroid.
• Unlike AS-OCT, UBM can also be performed with the subject lying
down, and thus it is useful in the operating room when an
examination needs to be performed under anesthesia.

97. LIMITATIONS OF UBM
• Poor inter-observer reproducibility had more variability
• Narrower field of view compared to the AS-OCT

99. IMPORTANCE OF IMAGING IN
GLAUCOMA

101. APPLICATIONS OF IMAGING IN GLAUCOMA
• Detection of pre-perimetric changes
• Structural changes (ONH and RNFL) precede functional changes
• Potential value in delaying and avoiding progression of the disease
• Functional changes show up after approximately 50% of the
ganglion cells have been destroyed.
• Structural changes can be seen almost 3-5 years before functional
changes

105. REFERENCES
• Maslin JS, Barkana Y, Dorairaj SK. Anterior segment imaging in
glaucoma: An updated review. Indian J Ophthalmol [serial online]
2015 [cited 2021 Apr 11];63:630-40.
• Stein DM, Wollstein G, Schuman JS. Imaging in
glaucoma. Ophthalmol Clin North Am. 2004;17(1):33-52.
doi:10.1016/S0896-1549(03)00102-0
• Glaucoma Imaging – Antonio Ferraras (Springer)
• Diagnosis and Therapy of Glaucomas – Becker and Shaffer
• Textbook of Glaucoma – Shields’

106. THANK YOU