The document discusses Anterior Segment Optical Coherence Tomography (AS-OCT), a non-invasive imaging technique for detailed visualization of the anterior segment of the eye, presenting its history, clinical applications, and technical specifications. Key applications include corneal thickness mapping, angle measurement for glaucoma, and evaluation for LASIK surgery, along with potential limitations like inability to visualize certain posterior structures. Overall, AS-OCT offers high-resolution imaging that aids in the diagnosis and management of various corneal conditions.

1. ANTERIOR SEGMENT OCT
PRESENTED BY
AFSANA ANSARI
B.OPTOMETRY
MMC, IOM

2. PRESENTATION Layout
INTRODUCTION
HISTORY
SPECTRALIS
PACHYMETRY
PACHYMETRY WIDE
CROSS LINE
CONEAL ANGLE
3D-CORNEA
SCLERA
CLINICAL IMPLICATIONS
LIMITATIONS

3. Introduction
OCT is a non-invasive, non-contact imaging technology that provides detailed cross
sectional images ( tomography) of internal structure in biological tissue.
OCT is cross sectional, three dimensional ,high resolution imaging modality that uses
low coherence interferometry to achieve high axial resolution ranging from 10-
20Um.

4. HISTORY
First demonstration of AS-OCT imaging was described by Joseph Jzatt and other in
professor Fujimoto’s laboratory in 1994.
Firstly investigators adapted retinal OCT scanner for corneal and anterior segment
imaging .
Commercial retinal scanner used 830nm wavelength with image acquisition time of 1-5
sec
Using wavelength higher than 830nm in posterior segment had more dissipation in
vitreous and so discarded for diagnosis of retinal disorder

5. Wavelength of 1310nm allows deeper penetration and cross sectional imaging of anterior
chamber including visualization of angle.
ASOCT is machine which permit high resolution imaging very much like an optical biopsy.

6. Optimal Wavelength
OCT at 1310nm wavelength is better for anterior segment imaging due to significant
properties;
Amount of scattering in tissue is lowered at this wavelength
Strongly absorbed by water in ocular media and so only 10% of light incident on cornea
reaches retina

8. Specifications and Resolutions
 Scanning speed is 4000 axial scans per image giving an acquisition rate of 8 frames per
second.
 With high resolution corneal software axial resolution of ASOCT can reach upto 8-
10Um.
 Scan geometry is telecentric allowing wide field capability essential for corneal and
anterior chamber studies .

9.  SLOCT has slit lamp attached to it and uses charge couple device camera to visualize the scan
area in real time.
 Scan area is 15x7mm and image to depth of 7mm which is up to posterior lens capsule in
case of transparent lens .

10. Indications
• Mapping of corneal thickness and keratoconus evaluation.
• Measurement of lASIK flap and stromal bed thickness.
• Visualization and measurement of anterior chamber angle and diagnosis of
narrow angle glaucoma.
• Measuring the dimensions of anterior chamber .
• Visualizing and measuring the results of corneal implants and lamellar
procedures.

11. Cornea
 Corneal thickness measurements can be performed for the entire cornea or any
lamellar segments.
 Interactive pachymetry option is present where any position can be selected for
measurement .
 Distance can be measured within cornea also like flap and stromal bed thickness in
refractive surgery.
 For pachymetry high resolution (10x3mm) scan option in visante OCT can be used for
further precision as in refractive surgeries and keratoplasty cases .

12. • Pachymetry map scans consists of 10mm radial lines on 8 meridian centered on
vertex reflection.
• Entire scan pattern of 1024 A-scans can be acquired in 0.5 sec.
• Measurement of anterior and posterior curvature of cornea and allows total corneal
power measurement and its changes after corneal refractive surgery.

13. Scan Pattern
Select cornea
Select scan pattern from pop up list and mouse click to activate scan acquisition (a list of
scan patterns and specifications is provided in section .

14. Scan acquisition:
The following is a general procedure to acquire Cornea OCT images:
1. Turn off the exam room light (recommended)
2. Attach the CAM lens
3. Use the two red external illumination LED on headrest to illuminate the cornea (one
for each eye).
4. Instruct patient to fixate on the center of light blue internal fixation target.

15. 5. Use the yellow external fixation LED on headrest to guide patient fixation if required. For
corneal power scan, use internal fixation target
Operator should center the scan on the pupil. If the misalignment exceeds 1 mm (pupil
center exceeds the boundary of the smallest alignment circle), the scan should be excluded.
 Operator should make sure that the eyelid or eyelashes are not blocking or shadowing a
significant portion of the image in vertical meridians concentric circle on the screen. If there
is blocking or shadowing, the scan should be excluded.
 Operator should observe the measurement reliability index status on the report screen (for
cornea power). A measurement with poor measurement reliability indicates increased risk
of measurement variability. Measurements with poor reliability should be replaced if
possible.

16. 6. Align on the desired area to scan
7. Move forward until the iris is in focus in the live IR image (the image of the desired external
scanned region should be within the target zone (two dashed red horizontal lines).
8. Adjust scan beam to target zone and orientation with joystick.
9. Adjust image quality/scan strength (P-Motor adjustment)
10. Capture scan using joystick button or capture button on screen
11. Review and process (averaging) the OCT images
12. Save the scan

17. Scan alignment OCT scan window:
Place the cornea B-scan image in between the two red guide lines to optimize the cornea
scan images. Pachymetry and cross line scans will have two OCT windows, one for vertical
b- scan, and one for horizontal b- scan.

18. Live video window-alignment of scan pattern will depend on scan type chosen:
1. Pachymetry scan:
Align the aiming circle (inner circle is 4mm diameter and outer circle is 6mm diameter) to the
center of the pupil.

19. 2. Angle scan:
 Use the external fixation (yellow light on gooseneck cable) to guide the patients fixation
(of the fellow eye) until the cornea/sclera edge is parallel and located in the red guided
lines region.
Place center of scan line pattern on the limbus and cornea/scleral OCT image parallel to
the red horizontal guidelines
3. line, cross line, 3-D cornea:
These scans be centered on the pupil or particular area of interest.

20. 3D Cornea
The Cornea 3D scan has scan density of 513 A-scans and 101 B-scans.

21. Pachymetry
The pachymetry scan is a set of 8 radial meridians 6mm in length and centered on the
pupil. Illustration of Pachymetry scan pattern consists of 8 median scans 6mm in length.

22. The pachymetry report is a comprehensive collection of maps, tables, and images that
provide qualitative and quantitative assessment of the cornea.
Cornea thickness results are presented as a color-coded map (6mm) and a color scale
provides reference values for colors.
Thicker values are hot colors like red and orange, while thinner values are cool colors like
blue and black.
 Individual B-scans are displayed in the presentation window above the map.
 Different B-scans can be displayed by clicking on the thickness map (making it interactive)
and moving the cursor around slowly to the white lines (scan location indicators).

26. Pachymetry wide

27. Epithelial Thickness Mapping
Corneal epithelium serves as multiple functions;
Provides transparent and smooth refractive surface.
Acts as barrier to noxious stimuli.
Contributes to the refractive power of eye.

28. Epithelium thickness varies from 50-90um.
Thickness varies in different regions even in normal corneas that is thickest in the
center ,thinnest superiorly and temporally.

29. There is a clinical need to measure epithelial cell layer separately from the pachymetry
of the cornea.
The thickness distribution of the layer is useful in the evaluation and follow up of
patients for irregularities and/or changes due to pathologies, contact lens, or refractive
surgeries.
Epithelium Thickness measures the thickness from the epithelial cell surface to
Bowman’s membrane.
The Epithelium Thickness Mapping feature is an upgrade to the pachymetry scan – a
sample pachymetry report with epithelium thickness mapping is shown below

32. Anterior Chamber Angle
Due to low scattering loss at 1310nm highly detailed AC angle is possible and angle
structures including iris root , angle recess ,anterior ciliary body ,scleral spur and can
be visualized.
Scleral spur is highly reflective and can be easily identified on ASOCT.
Digital gonioscopy allows objective anterior chamber angle assessment which is non-
contact ,accurate and rapid method for angle assessment .
Can be used to assess angle width.

33. Images obtained are processed using computer to correct image distortion arising from two
sources.
First the fan shaped scanning geometry of OCT beam and second the effect of refraction at
cornea-air interface.
The angle can be measured manually or automatically with software .

36. Corneal Angle

37. Clinical Implications
LASIK Patients
In the evaluation of a patient for LASIK (laser-assisted in-situ keratomileusis) eye surgery,
the Visante is able to provide preoperative pachymetry mapping of the cornea, evaluation
of the angles, and any corneal pathology that is present.
In evaluation of post-LASIK surgery patients, detailed anatomical data m ay be obtained to
further guide in clinical decision making.

38. Flap Evaluation
In the evaluation of postoperative LASIK patients whom may be considering enhancement,
the Visante can provide detailed information that would be essential to the safety of the
procedure.
The Visante can provide details of corneal flap shape and regularity.
The LASIK flaps are created using either a mechanical microkeratome or a femtosecond
laser microkeratome.

40. Dry eye evaluation
Tear Volume measurement
The tear meniscus is a wedge-shaped area measured between the posterior margin of the
lower lid and the bulbar conjunctiva.
The AS-OCT is a non-invasive technique to evaluate this.
The parameters calculated are tear meniscus height (TMH) , tear meniscus depth (TMD) and
tear meniscus area (TMA).
 parameters may help in differentiating a healthy ocular surface from that of a patient with dry
eyes. parameters may also be affected in patients with ectropion, lower lid mass, lid defects.

41. Figure 1: AS-OCT image of a normal eye. Infrared image showing the tear meniscus
height (TMH) (A). Line scan depicting TMH calculated between the posterior margin of
the lower lid and the bulbar conjunctiva

42. Congenital corneal opacities
Congenital corneal opacity is a common feature of various conditions associated with
anterior-segment dysgeneses like Peters’ anomaly, sclerocornea, corneal plana, and
others.
 AS-OCT helps us to understand the involvement of the iris and the crystalline lens and
determine the presence of irido-lenticular and iridocorneal adhesions .

43. Keratoconus and other ectatic corneal diseases:
Diagnosis of keratoconus:
AS-OCT has been known to serve as an adjunct tool in the diagnosis and management
of corneal ectasias.
Features suggestive of keratoconus;
Focal, eccentric corneal thinning
Asymmetric thinning between superior and inferior quadrants
Corneal epithelial thickness maps have been useful in the early diagnosis of
keratoconus.
 Apical epithelial thinning in the typical inferotemporal cone has a corresponding
epithelial thickening in the superio nasal area.

44. In the non-ectatic cornea, the epithelium is thinner superiorly and thicker inferiorly.
A decrease in the epithelial thickness over a period of time is also noted in overtime
when the patients are followed up serially.
Changes in these parameters over time may also help in the diagnosis of progression.

45. Keratoconus Evaluation
An OCT keratoconus classification based on structural corneal changes occurring at the conus as
follows;
Stage 1: Thinning of epithelial and stromal layers at the conus. Corneal layers have a norm al
aspect.
Stage 2: Hyperreflective anomalies occurring at the Bow m an layer level (varying from a barely
visible hyperreflective line to a hypertrophic scar) and epithelial thickening at the conus (3a, clear
stroma; b, stromal opacities)
Stage 3: Posterior displacement of the hyperreflective structures occurring at the Bow m an layer
level with increased epithelial thickening and stromal thinning (a, clear stroma; b, stromal
opacities).

47. Stage 4: Pan-stromal scar.
In stage 4, when the residual stroma is thin, it acquires an hourglass-shaped scar with
increased epithelial thickening.
Stage 5. Represents the acute form of keratoconus (hydrops):
a Acute onset, characterized by the rupture of the Descemet mem brane with dilacerations
of collagen lamellae, large fluid-filled intrastromal cysts, and the formation of epithelial
edema.
b Healing stage, pan-stromal scarring with a remaining aspect of Descemet mem brane
rupture.

49. LIMITATIONS OF ASOCT
 Since the posterior layer of the iris (pigment epithelium) is not transparent for
infrared light, the posterior chamber is not visualized in most cases.
 The infrared light is absorbed on it´s way through the sclera and the entire thickness
of the sclera is not visible.
Angle closure due to a anterior placed ciliary body (plateau iris) or any other cause
related to the ciliary body (swelling, tumor) cannot be imaged.
 Preoperative evaluation in eyes with traumatic cataracts, pseudo exfoliation,
subluxated lenses to identify amount of zonular damage is not possible.

50. References