2. LAYOUT OF PRESENTATION:
INTRODUCTION
HISTORY
PRINCIPLE
TYPES
APPLICATIONS
ADVANTAGES AND LIMITATIONS
2
3. INTRODUCTION
Optical coherence tomography (OCT)
A non-invasive
Non-contact
2-Dimensional imaging system
Provides high resolution (10-15 micrometer)
Cross-sectional images
Imaging of anterior and posterior segment structures
“Optical” – meaning light.
“Coherence” – constant phase difference (interference)
“Tomography” – imaging in cross section
Handbook of OCT Brett E. Bouma Guillermo J. Tearney 3
4. HISTORY:
The use of OCT in ocular tissue imaging was first
reported by Huang et al in 1991.
1st and 2nd generation OCT -- axial (depth) resolution of
approx. 12 – 15 micro meter.
3rd generation OCT (OCT -3) uses 500 axial scans taken
in 1 second and has increased the resolution to 7 – 8
micrometer.
New ultra-high resolution OCT achieves a resolution of 2
– 3 micrometer but is not yet commercially available
4
6. The first demonstration of corneal and anterior segment
OCT imaging was described by Joseph Izatt and others in
Prof. Fujimoto’s laboratory in 1994.
6
7. 7
Radhakrishnan S. Rollins AM, Roth JE, et al. Real-time optical coherence tomography of
the anterior segment at 1310 nm. Arch Ophthalmology 2001 19:1179-85
8. PRINCIPLE
Based on the principle of Michelson
interferometer.
Utilizes interferometry and Low
coherence radiation in near – infrared
range.
OCT images obtained by measuring-
•Echo time
•Intensity of reflected light
8
9. 9
A broadband light of wavelength near – infrared (1310nm) is
projected by a diode source axially to a beam splitter (tilted at 45
degree).
Among the splitted beam, 1st beam is projected by 78D lens to the
tissue of interest e.g. cornea ( probe/object beam ).
Another beam travels to a reference mirror at known reference
distance ( reference beam )
10. Both the light beams are back - reflected and are projected back to
the “beam splitter”.
From here, both the beams are reflected towards the “detector” .
However , light reflected are scattered differently from tissue with
different optical properties.
The time delay of the light reflected from various structures is
compared with the time delay of light reflected from reference
mirror.
Resulting in a signal generation and is displayed on the OCT
monitor. 10
14. TIME- DOMAIN OCT
Uses a single photo detector, and an A-
scan is created by moving a reference
mirror to change the optical path of
the reference beam in order to match
different axial depths in the target tissue.
This setup limited the scanning speed to
a few thousand A-scans per second and
has narrow range of depth.
14
15. SPECTRAL- DOMAIN OCT
Is able to acquire an entire A-scan by using an
array of detectors and the reference mirror is kept
stationary.
The spectral pattern of interference between target
and reference reflections is measured.
Thus reflections from all layers are detected
simultaneously.
SD-OCT can provide20000-52000 A-scans per 15
16. 16
S.
N.
TIME- DOMAIN OCT SPECTRAL- DOMAIN
OCT
1. Captures 1 pixel at a
time.
Can capture 2000 pixels
simultaneously
2. More time consuming Less time consuming.
3. Axial resolution 10
microns
Axial resolution 6-7 microns
4. Transverse resolution
20 microns
Transverse resolution 10
microns
5. Motion artifact may be No motion artifact
17. SPECTRAL-DOMAIN OCT: –
Spectralis (Heidelberg)
Cirrus (Zeiss)
RTVue (Optovue)
Optovue and Cirrus : Anterior Segment imaging
capabilities in addition to posterior segment.
Spectralis : Require special lens and anterior segment
module for anterior segment imaging.
17
18. The development of newer technology in AS-OCT has
following features:-
Longer wavelength (1310nm)
Telocentric transverse scanning
Very high speed axial scanning with a grating based rapid
scanning optical delay (RSOD)
This system provides:
A speed of 4000 A-scan/sec,
17-um axial resolution ( in tissue) and
scan dimension of up to 15mm(width) X 8mm ( depth)
18
19. Advantages of Longer Wavelength
Less than 7% of the 1310 nm light incident on the cornea reaches
the retina compared to 93% transmission for 830 nm light – that
means higher power level can be used safely.
Higher exposure limit : 15mW for the 1310 nm wavelength
compared to 0.7 mw for 830 nm.
Reduced scattering in opaque tissues such as the limbus, sclera and
iris.
Penetration is six times deeper in highly scattering tissue such as
the sclera.
Allows deeper penetration of the limbus for visualization of the
scleral spur and angle recess, important landmarks for narrow angle
glaucoma diagnosis and anterior chamber biometry.
19
21. USES OF AS-OCT:-
Mapping of corneal thickness on patients with keratoconus, corneal
scars and corneal dystrophies, glaucoma.
Measurement of LASIK flap and stromal bed thickness pre and
postoperative.
Visualizing and measuring the result of corneal implants and
lamellar procedures.
Imaging through corneal opacity to see internal eye structure.
21
22. Images of the angle are performed to quantify the angle for angle
closure glaucoma and attempt to identify the angle structures.
Detection of pathological processes such as dry eye syndrome,
ocular surface conditions, tumors, and infections
Measuring the dimensions of anterior chamber and assessing the
fitness of intraocular lens implants
Imaging the lens informs Ophthalmologists about the location of
intraocular lenses (IOLs)
22
23. AS-OCT is an excellent preoperative and postoperative
tool to evaluate and manage patients with:
Blebs,
Intrastromal corneal rings,
Full-thickness penetrating Keratoplasty (PK),
Descemet-Stripping Endothelial Keratoplasty (DSEK),
Deep Lamellar Endothelial Keratoplasty (DLEK),
IOLs and laser-assisted in situ Keratomileusis
(LASIK).
23
27. ASOCT AND KERATOCONUS
AS-OCT demonstrates alterations in corneal epithelial
thickness and distribution in keratoconus.
The depth of the demarcation line following corneal
collagen cross-linking.
27
31. 32
AS-OCT can be implemented to evaluate changes
in the geometric properties of keratoconic corneas
after the insertion of intracorneal ring segments and
also assess their position and depth in the cornea.
32. AS-OCT is useful to evaluate the Descemet’s membrane tear,
dimensions of intrastromal clefts, and corneal thickness in acute
corneal hydrops of keratoconus].
AS-OCT is also useful in assessing the response of treatment
following interventions such as injection of sulfur hexafluoride
(SF6)/perfluoropropane (C3F8) gas into the anterior chamber.
33
33. AS-OCT AND DRY EYE SYNDROME
OCT can be a quick, noninvasive method to measure the tear
meniscus height (TMH) and treatment response in dry eye patients.
Studies show that lower tear meniscus parameters measured with
SD-OCT correlate well with the Schirmer test, break-up time, and
subjective symptoms.
Czajkowski G, Kaluzny BJ, Laudencka A. Tear meniscus measurement by spectral optical coherence tomography.
Optom Vis Sci. 2012;89(3):336-42.
35
34. AS-OCT AND OCULAR SURFACE LESIONS
OCT provides a noninvasive technique to document the
extent and depth of corneal/conjunctival intraepithelial
neoplasia and pterygium.
A. Slit lamp photograph of a pterygium. B. AS-OCT image of the pterygium
shows a dense,hyper-reflective, fibrillary subepithelial lesion that is between the
corneal epithelium and Bowman’s layer. 36
35. 37
Pterygium
AS- OCT with subepithelial thickening, shadowing,and hyperreflectivity. The epithelium is
preserved without any thickening or hyperreflectivity and lacks a transition point as would be
seen with ocular surface neoplasias.
Large nodular ocular surface squamous
neoplasia
36. 38
(A) Slit lamp photo with corresponding AS- OCT of an irregular ocular surface squamous
neoplasia with significant shadowing.
(B) Slit lamp photo graph of the same patient after four cycles of 5- fluorouracil with
normalization of corneal epithelium and subepithelial scarring .
37. AS-OCT AND CORNEAL INFECTIONS
39
(A)Slit lamp photo of a pigmented corneal scar from fungal keratitis.
(B) Corresponding AS- OCT with subepithelial stromal hyperreflectivity (arrow) with one- third
thickness of the anterior stroma with residual scarring inferiorly, less hyper reflective in nature in
the remaining two- thirds of the stroma.
38. AS-OCT serves as a powerful tool for the non-invasive diagnosis of
OSSN and can be used to determine the need for treatment initiation as
well as monitoring of the disease course.
Other lesions that can be characterized by AS-OCT include
conjunctival nevus, melanomas, lymphomas and amyloidosis.
40
(a) Slit lamp photograph displaying a cystic nevus .
(b) On AS-OCT, this lesion is a well-circumscribed subepithelial lesion containing cystic spaces
39. AS-OCT can be used to image several dystrophic and degenerative
conditions of the cornea.
41
(A) Slit lamp photo of lattice corneal dystrophy.
(B) Corresponding AS- OCT with the arrow highlighting the amorphous hyperreflective
material in the central stroma.
40. 42
(a) Slit lamp photograph of a central Salzmann’s nodule.
(b) On AS-OCT, the nodule is seen as a localized area of hyperreflective material that has replaced the anterior stroma and
Bowman’s layer underneath normal epithelium.
(c) Slit lamp photograph of band keratopathy in the peripheral cornea.
(d) AS-OCT imaging shows a thin band of hyperreflectivity along Bowman’s layer with underlying shadowing.
41. AS-OCT AND CORNEAL DEPOSITS
AS-OCT is useful to see early drug deposits and early
KayserFleischer rings of Wilson disease which is missed by
slit-lamp biomicroscope.
Kayser–Fleischer ring is seen as hyperreflectivity at the level of
Descemet’s membrane in the peripheral cornea
Amiodarone-induced keratopathy is observed as highly reflective
and bright intracellular inclusions in the epithelial basal layer.
43
42. AS-OCT AND REFRACTIVE SURGERY
Visualization of flap thickness, flap interface.
Look for any flap displacement .
Measuring residual stromal thickness following LASIK surgery.
AS-OCT has been used as a rescue tool for difficult lenticular
extraction in SMILE surgery to identify the cause for retained
lenticle.
44
LASIK FLAP
43. Tool to monitor the success and complications of several anterior segment
surgical procedures including Descemet stripping automated endothelial
keratoplasty (DSAEK), Descemet membrane endothelial keratoplasty
(DMEK),Penetrating Keratoplasty, Laser-assisted in situ keratomileusis
(LASIK).
Detecting early graft detachment after DMEK surgery and also to find
interface fluid between the host cornea and the graft.
Excellent intra-operative adjunct for the anterior segment surgeon
particularly during lamellar keratoplasty.
Post-operatively, high quality AS-OCT images can allow clinicians to
assess graft adherence, graft centration, graft thickness and even epithelial
remodeling after DSAEK surgery, all of which can affect the optical
quality of corneas post-operatively.
45
45. AS-OCT allows to evaluate the anterior lens capsule in
pseudoexfoliation patients and predict postoperative (IOL) tilt, and
assist in IOL power calculations to improve visual outcome after
cataract surgery.
AS-OCT is useful to evaluate the wound characteristics of clear
corneal incisions of cataract surgery, analyzing incision angle,
incision length, corneal thickness, epithelial side closure of the
incision or epithelial gaps, endothelial side closure or endothelial
gaps and Descemet’s membrane detachment.
• Lee H, Kim EK, Kim HS, Kim TI. Fourier-domain optical coherence
tomography evaluation of clear corneal incision structure according to blade material. J
Cataract Refract Surg 2014;40:1615-24.
49
46. AS-OCT AND AQUEOUS OUTFLOW SYSTEM
The first OCT image of SC and the TM was presented by Sarunic
and co-workers in 2008 using a SS OCT system operating at 1310
nm and proving an axial resolution of approximately 9mm.
Visualizing aqueous outflow pathway also help in guiding glaucoma
surgery and evaluating treatment success.
50
47. AS-OCT demonstrated to be useful in the evaluation :
•Trabeculectomies
•Bleb morphology
•Patency/position of tubes of
glaucoma drainage devices
51
48. Qualitative and quantitative assessment of anterior chamber angle
(ACA), anterior chamber, iris and lens are accomplished with AS-
OCT.
Angle closure with AS OCT is determined by any contact between
the iris and the angle wall anterior to the scleral spur.
•The identification of the scleral spur is an important landmark to be
assessed, although the Schwalbe's line (SL) has been proposed as
another possible landmark since it has better identification in FD
OCT devices. (Cheung et al.,2011)
52
49. •A study comparing AS-OCT with goniscopy
AS-OCT detected more closed angles than gonioscopy
•Disparity to attributed:
Possible distortion of the anterior segment by contact gonioscopy
Differences in illumination
• Nolan W, See JL, (‘hew PT, et al. Detection of primary angle- closure using
anterior segment optical coherence tomography in Asian eyes. Ophthalmology
2007:114:33-9 53
51. ADVANTAGES OF AS- OCT
Best axial resolution available so far
Scans various ocular structures
Tissue sections comparable to histopathology
sections
Easy to operate and non-invasive.
Short scanning time
56
52. LIMITATIONS OF AS-OCT
Inability to visualize structures posterior to the iris
and ciliary body due to blockage of wavelength by
pigment.
Each scan much be taken in range and in focus
must be examined for blinks and motion artifacts
Axial motion is corrected with computer correlation
software
transverse motion cannot be corrected
57
53. KERATOMETRY
Also called as Ophthalmometry.
Basically comprises: Kerato- Cornea and Metry- Measurement
A technique used to measure the curvature of the anterior surface of
the cornea across a fixed chord length, usually 2-4 mm, which lies
within the optical spherical zone of cornea.
59
54. HISTORY
In 1980, the development of autokeratometer
1619, Scheiner - Glass sphere of known
radii to estimate corneal curvature
1796, Ramsden – Inventor of keratometer
1854, Helmholtz improved Ramsden’s
design for laboratory use
1881, Javal & Schiotz modified Helmholtz’s
instrument for clinical use
55. CLINICAL USES:
Determines curvature of the cornea
Estimates the amount and direction of corneal astigmatism
IOL power calculation (Pre-op cataract surgery workup)
Monitors pre and post-op astigmatism
Helps to diagnose and monitor keratoconus and other corneal
diseases
Contact lens fitting
61
57. PRINCIPLE OF KERATOMETER
Keratometry is based on the fact that the anterior surface of cornea
acts as a convex mirror and the size of the image formed varies with
its curvature.
Greater the curvature of cornea lesser is the image size
•Therefore, from the size of the image formed by the anterior surface
of cornea (1st Purkinje image) the radius of curvature calculated as:
•Where,
• r is the radius of curvature of the reflective cornea,
• u is the distance from the object to the cornea,
• I is the size of the image, and
• O is the size of the object
64
r = 2u(I/O)
58. Optical principle involved is the relationship between the size of an
object and size of the image of that object reflected from the surface
Radius of curvature determined by the apparent size of the image
of bright object (mires) viewed by the reflection from anterior
corneal surface which acts as a convex mirror
65
AB is the object and A' B' is the image. By measuring the size of the object and
image, curvature of the convex surface can be calculated.
59. The final step is to convert the radius of curvature into an estimate of
the cornea’s dioptric refractive power;
P is the refractive power of the cornea,
n’ is the refractive index of the cornea,
n is the refractive index of air (which is close to 1.0), and
r is the measured radius of curvature of the cornea
An “averaged” corneal refractive index of 1.3375 is used
66
P = (n’ – n)/r
61. Measurement of image height
Doubling device - Plano prism
Lateral displacement of doubled image = IMAGE HEIGHT
Prism is moved along the optical axis until two images are just
touching
At this point, the prismatic displacement is exactly equal to the size
of the image
The larger the image size is, the greater the amount of doubling
must be to achieve contact of two image
TYPES
•Fixed doubling
•Variable doubling
68
63. Basically there are two types of keratometer on the basis of
operation;
1. Manual Keratometer
a. Bausch and Lomb Keratometer
b. Javal- schiotz keratometer
c. Zeiss ophthalmometer
d. Haag streit ophthalmometer
e. Topcon OM-4 keratometer
2. Auto keratometer
a. IOL Master
b. Pentacam
c. Humphrey autokeratometer
d. Corneal topographer 70
66. AUTOMATED KERATOMETRY
Focuses the reflected corneal image on to an electronic
photosensitive device
No doubling device is needed
Computes angle as well as power in many meridians
Absence of annoying glaze of the brightly illuminated mires
76
68. Limitations of keratometry
Measurement by keratometer is based on false assumption
that cornea is a symmetrical spherical or sphero-cylindrical
structure, with 2 principal meridia separated from each other
by 900
Measures refractive status of small central cornea (3-4 mm)
Loses accuracy when measuring very flat or very steep
cornea
78
69. SOURCES OF ERRORS IN
KERATOMETRY:
79
Improper
calibration Faulty
positioning
of patient
Improper
fixation by
patient
Accomodat-
ive
fluctuation
by examiner
Localized
corneal
distortion
Excessive
tearing
Abnormal
lid position
Improper
focusing of
corneal
image
70. REFERENCES:
American Academy of Ophthalmology, Clinical Optics, 2020-2021
Yanoff And Duker Ophthalmology , 5th edition
Principles of Ocular Imaging by Daniel Gologorsky and Richard B.
Rosen, 2021
Anterior and Posterior Segment OCT: Current Technology and Future
Applications, First Edition: 2014
Copeland and Afshari’s principles and practice of cornea volume-1 1st
edition- 2013
A K Khurana,’Theory and practice of optics and refraction’ fourth
edition
Kanski’s clinical ophthalmology, a systematic approach- eighth edition
Various internet sources and journals
80
Non-probability purposive sampling was taken as a technique for sample collection.
( According to the annual report of BPKLCOS, the number of HLA-B27 positive uveitic patients visiting BPKLCOS is around 30 per year so the sample size of HLA-B27 positive uveitis was 30)
(Difference in power between two principal meridians )
FOCUSING OF MIRES AND MEASUREMENT OF CORNEAL CURVATURE.
