Handout - STERIS University

CORNEA-ANTERIOR SEGMENT 

the MOST COMPREHENSIVE OCT 

Anterior Segment 

Imaging…. 

AM Option and RTVue FD-OCT 

Tear Film 

Bag and Lens 

Cornea Transplant 

Soft contact lens 

CK scar 

sule 

IOL 

Vitreous margin 

3D Crystalline Lens w/ SUM of C-scan display 

What Would We 

Like to See? 

Disclosures 

Financial interest: 

Alcon Laboratories, Inc. 

Abbot Medical Optics, Inc. 

Zeimer, Inc. 

MP Weikert, MD ✦ ASORN Regional Meeting ✦ San Antonio, TX ✦ August 24, 2012 

Part No. 43721 Rev. A 

Advances in 

Corneal Imaging 

Anatomy 

- Macroscopic Optical Coherence 

Tomography 

Scanning 

Reference 

Mirror 

Beam 

Splitter 

Scanning 

Mirror 

Low 

Coherence 

Source 

Interfering 

Beams 

“Partial Coherence” 

(multiple wavelengths) 

maintains interference 

over short distances 

Detector 

Target 

ASI - What Would 

We Like to See? 



Interferometry 

Optical Coherence 

Tomography 

Scanning 

Reference 

Mirror 

Beam 

Splitter 

Scanning 

Mirror 


Tomography 

Time vs. Frequency Domain: 

Scanning vs. Fixed 

Reference Mirror 

Beam 

Splitter 

Scanning 

Mirror 

Low 

Coherence 

Source 

Interfering 

Beams 

Low 

Coherence 

Source 

Interfering 

Beams 

Relative difference in pathlengths of 

reference and measurement arms 

determines interference pattern 

Detector 

Target 

“Partial Coherence” 

(multiple wavelengths) 

maintains interference 

over short distances 

Detector 

Target 



Interferometry 



Time Domain vs. Frequency Domain


Tomography 



“Optical B-Scan” Multiple optical A-scans 

combined to produce 

cross-sectional image @ 

different meridians 

Optical Coherence Tomography (OCT) 



Parameter 

Wavelength 

Penetration Depth 

Axial Resolution 

Transverse Resolution 

A-Scans/Line 

A-Scans/Second 

Acquisition Time 

Scan Diameter 

Scan Alignment 

System Design 

Zeiss Visante ® (Time) 

1310 nm 

3 - 6 mm 

18 µm 

60 µm 

128 - 512 

2048 

0.25 - 0.5 sec 

Approx. 12-14 mm 

V-Trac system aligns images on 

corneal vertex 

Dedicated Anterior Segment 

Zeiss Visante 

Zeiss Visante 

LASIK Flap - Mechanical µKeratome 

LASIK Flap - Femtosecond Laser 





Zeiss Visante 

Optical Coherence Tomography (OCT) 

Parameter 

Wavelength 




A-Scans/Line 

A-Scans/Second 


Scan Diameter 

Scan Alignment 

System Design 

Zeiss Visante ® (Time) 

1310 nm 

3 - 6 mm 

18 µm 

60 µm 

128 - 512 

2048 

0.25 - 0.5 sec (1 scan per merid) 

Approx. 12-14 mm 

V-Trac system aligns images on 

corneal vertex 

Dedicated Anterior Segment 

Optovue RTVue ® (Frequency) 

830 nm 

2.3 mm 

5 µm 

5 - 10 µm 

1024 

26,000 

0.31 sec (5 scans per meridian) 

4 - 6 mm 

Corneal vertex 

Hybrid Anterior/Posterior Segment 




We Like to See?


Only with the CAM Option and RTVue FD-OCT 

Tear Film 


Optovue RTVue ® 

CK scar 

Cataract Wound Changes 

Bag and Lens 


Early Descemet’s 

Detachment 

Early Posterior 

Wound Gape 

Late Posterior 

Wound Retracton 

Iris 

LASIK Flap 

Corneal Transplant 

IOL 


Posterior capsule 


AC Angle 

LASIK Interface Fluid 



Pterygium 



Part No. 43721 Rev. A 

Cataract Wound Changes 

% Eyes 

Descemet’s Membrane Detachment 

Posterior Wound Gape 

40 

100 

30 

75 

20 

50 

10 

25 

0 

0 

1 Day 1 Wk 2-3 Wk 1-3 Mo 3-12 Mo 1-2 Yr 3-15 Yr 


Time After Surgery 


Posterior Wound Retraction 

% Eyes 

% Eyes 

100 

75 

50 

25 

0 



VHF Ultrasound 

U/S 

Emitter/ 

Detector 

Saline Bath/Coupling Medium 

Reflection of the incident 

U/S wave occurs at 

acoustical interfaces 



Huang D, et al. (Unpublished Data) 



Very High Frequency Ultrasound 

ArcScan Artemis ® 

Parameter ArcScan Artemis ® 

Frequency 




Meridians Scanned 


Scan Diameter 

Other 

50 MHz 

3 - 6 mm 

1 µm 

200 µm 

4-12 

2-3 min 

Up to 15 mm 

ONLY system that can penetrate 

iris to image ciliary sulcus 

Off-axis imaging possible 




We Like to See?



Pre-Op Post-Op Change Layers 





Anatomy - Microscopic 

Confocal Microscopy 

Illumination 

Pinhole 

Detection 

↑Axial Resolution 






Superficial 

Epithelium 

Anterior 

Stroma 


Fuch’s Diffuse Dystrophy Lamellar LASIK Keratoconus Interface 

Polymegathism 

Acanthamoeba 

Keratitis 

Keratitis 

Basal 

Epithelium 

Superficial Nerve 

Plexus 

Posterior 

Stroma 

Endothelial 

Mosaic 




We Like to See?

Corneal Power/Curvature 

How Do We Measure Curvature? 

Directly - reflection principle: 

Keratometer 

Placido disc 





How Do We Measure Curvature? 

Directly - reflection principle: 

Keratometer 

Placido disc 

Indirectly - elevation measured: 

Scanning slit 

Scheimpflug 

Optical coherence tomography 

Very high frequency ultrasound 

Proven expertise 

for anterior segment care. 

Placido Imaging 

Illuminated rings 

(mires) projected 

onto corneal 

surface 

Reflected rings 

imaged by 

digital camera 

With revolutionary instruments such as the Stratus OCT 

Anterior segment imaging 

system, the unique technological expertise of Carl Zeiss 

The anterior segment can be evaluated and measured pre- 

Meditec has long proved invaluable in the diagnosis and 

and postoperatively after image acquisition using the ana- 

monitoring of retinal disease. Now, with the introduction of 

lysis mode of the Visante OCT system’s software. Practical 



Narrow Angle 

the Visante OCT system, this same expertise is being 

applied to high-resolution, non-contact optical coherence 

tomography customized for the anterior segment. 

Exceptional design, greater confidence 

The Visante OCT system is the first to provide clear, highly 

tools enable planning and measurement of anterior segment 

ocular structures, including anterior chamber depth 

(ACD), anterior chamber angles and anterior chamber diameter 

(commonly referred to as angle-to-angle distance). 

Anterior chamber angle measurement results provide you 

with quick and reliable data for narrow-angle evaluation. 



detailed, in-depth images of the anterior chamber - includ- 

Anterior segment images can be printed with or without 

ing the angle - without the need for ocular anesthesia or a 

measurement tools and results. 

Analysis Results 

messy, time-consuming water bath. As a result, the Visante 

OCT system will provide unique images and measure-ments 

that will dramatically expand the potential for diagnostic 

confidence and therapeutic precision. And, just as importantly, 

Visante OCT is so easy to use and efficient to operate 

that it will seamlessly take its place in your daily workflow, 

offering new clinical insights and practice opportunities 

from day one. 

Corneal imaging and pachymetry 

Visante OCT provides high-resolution corneal images and 

documentation for the anterior segment specialist to support 

the evaluation of ocular health. Rapid acquisition 

during the pachymetry scan ensures an accurate and repeatable 

pachymetry map result for application in refractive 

and glaucoma care. 

Versatile in application 

The unique, versatile Visante OCT system provides superb, 

highly detailed results, consistently supporting surgical 

Corneal Ring Segments 

planning and postoperative care across a range of anterior 

segment applications. 

Placido Imaging 

Placido Curvature Measurement 

Reflected rings 

imaged by 

Image processing analyzes ring digital camera 

spacing to determine curvature 

Advantages: 

Lots of data points 

Accurate curvature measurement 

Information on surface quality 

Disadvantages: 

Mires reflected off of tear film 

No true measurement of central 

cornea 

Anterior surface only 

(adjusted η = 1.3375) 




We Like to See?

Irregular Mires 

Decentered 

Placido Rings 

Pupil Center 





Axial Maps 

Measurement tied to optical 

axis 

Spherical bias (all radii share 

the same axis/origin) 

Better for central/paracentral 

cornea 

Accuracy decreased in the 

periphery 

r 

Optical 

Axis 

r 

Tangential/Instantaneous Map 

Optical 

Axis 

Not tied to optical axis 

Calculate curvature from 

neighboring points 

More accurate shape 

(especially in periphery) 

More detail to detect 

irregular astigmatism 





Axial vs Tangential/Instantaneous 

= 

Scheimpflug & 

Placido Combo 

Note: Sim K’s calculated from Axial Map 



Axial vs. Tangential

Map Comparison 

Comparing corneal 

topographic maps can be 

challenging when subtle 

differences are present 

Make sure maps have the 

same scale when comparing 

them 





Elevation Data 

Obtained with Scheimpflug 

imaging, OCT, or scanning slit 

systems, 

Raw elevation data: 

Difficult to interpret 

Must be compared to a 

reference surface 

Best-fit sphere, ellipse, toroidal 

ellipse 





Image Plane 

Camera Film 

Lens & Image 

Plane are 

Typically Parallel 

Image Plane 

Camera Film 

Lens Plane 

Lens Plane 

Plane of Focus 

Parallel to Both 

Lens & Image Planes 

Scheimpflug 

Subject 


“Coronal Image” 

If planar subject is 

parallel to Image 

Plane, it can 

coincide with the 

Plane of Focus & 

the entire subject 

will be in focus 


= 

Subject Plane 




We Like to See?

Scheimpflug Intersection 

Image Plane 

Camera Film 

Image Plane 

Camera Film 

Lens Plane 

If the Subject Plane is 

not parallel to the 

Image Plane, it will be 

in focus only along a 

line that intersects the 


Subject Plane 


Lens Plane 

Subject Plane 

If the lens is 

positioned such that 

the Image Plane, 

Lens Plane, & 

Subject Plane all 

intersect, the subject 

(not parallel to the 

Image Plane) can be 

completely in focus 





Image Plane 

CCD Camera 

Sagital Cross-Section 

Lens Plane 

Subject 






Scheimpflug Imaging 

Advantages: 

Lots of elevation data 

Data from central cornea 

Images anterior & posterior 

corneal surface 

Disadvantages: 

Unable to image ciliary sulcus 

Must compare to reference 

Need extremely high resolution 

to detect curvature differences 

Reference Surface 

Shape? - Best-fit sphere, ellipse, toric ellipsoid 

Different days → different reference spheres 

r = 8.21 mm 

r = 8.41 mm 





Difference = 1 D

Elevation Data & 

Curvature Measurement* 

h 

d 

45D vs. 45.25D Curvatures: 

7.5 mm (R2) vs. 7.99 mm (R1) 

R1 

R2 

Diameter (D) Height Diff. (H) 

1 mm 0.1 µm 

2 mm 0.4 µm 

3 mm 0.9 µm 

4 mm 1.6 µm 

5 mm 2.5 µm 



*Roberts C. Corneal Topography in Refractive Surgery, 2nd edition. 

Dimitri Azar (ed.). Stanford, CT: Appleton & Lange. 



Galilei Dual 

Scheimpflug Analyzer 

2 Scheimpflug cameras (instead of 1): 

Elevation data averaged from each camera 

Eliminates measurement errors due to 

angle of orientation & ocular decentration 

Placido Rings (20): 

Scheimpflug cameras occupy gaps in 

Placido rings 

2 Placido images recorded w/ cameras 

oriented vertically & horizontally (fills in gaps) 




We Like to See?

Study Results: 

SimK repeatability (20 eyes) = 0.19 D 







SimK accuracy compared w/ Humphrey Atlas, IOL Master & 

Manual Keratometry (20 eyes): 

Mean astigmatism ranged from 0.50 D to 0.60 D 

Mean difference b/w Galilei & IOL Master = 0.10 ± 0.07D 

Mean difference b/w Galilei & Atlas = 0.11 ± 0.12D 



SimK accuracy compared w/ Humphrey Atlas, IOL Master & 

Manual Keratometry (20 eyes): 

Mean astigmatism ranged from 0.50 D to 0.60 D 

Mean difference b/w Galilei & IOL Master = 0.10 ± 0.07D 

Mean difference b/w Galilei & Atlas = 0.11 ± 0.12D 

Corneal thickness compared w/ ultrasonic pachymetry (USP-GP): 

Virgin corneas (77 eyes) - mean difference = -0.7 ± 7.1 µm 

Post-LASIK/PRK (39 eyes) - mean difference = -6.2 ± 9.9 µm 





IOL Calculations & Corneal Power 

Corneal power serves 2 functions in IOL calculations: 

IOL power estimation 

Effective lens position (ELP) 

Rx 

Obj @ -∞ 

ηair 

ACD AL-ACD 

K 

ηeye 

PIOL 

Vtx AL 

R 

Holladay 1 (Based on Vergence Formula) 

PIOL = 

1336 

AL - - 1336 

ELP 

1336 

1000 

]- 

1000 

- Vtx (mm) 

Rx 

ELP = SF + (T + 0.3375 - 

K 

[ ] ( ) 

√ 

( ) 2 

0.3375 

K 

+ K 

ELP 

- 0.071 * AL 2 




We Like to See?

IOL Calculations in Normal Eyes 

MAE (D) 

0.8 

0.6 

0.4 

0.2 

0 

IOL MAE vs Corneal Power Method 

Auto K SimKp SimKp+ds TCPm TCPc 

Auto K - IOLMaster 

SimKp - Atlas 

SimKp+ds - Galilei 

TCPm - Galilei 

TCPc - Galilei 

% Eyes 

% Eyes 

100 

80 

60 

40 

20 

0 

100 

80 

60 

40 

20 

0 

% Eyes within ± 0.5D 


% Eyes within ± 1.0D 


*IOL calculations optimized (SF) for each method of corneal power measurement 

ASCRS IOL Calculator 

“Double-K” 



*Shirayama M, et al. Cornea 2010; 29:1136-1138 



http://www.ascrs.org 

IOL Calculation after LASIK/PRK 

Refractive Mean Absolute Error (MAE) 

MAE (D) 

n=17 

1.20 

1.00 

0.80 

0.60 

0.40 

0.20 

0 

1.00 

Clin History 

1.05 

Feiz-Mannis 

1.00 

K Bypass 

0.63 

Adj EffRP 

0.48 

Adj Atlas 0-3 

0.42 

Masket 

0.44 

Mod Masket 

0.59 

W-K-M 

0.60 

Shammas 

0.54 

Haigis-L 

0.65 

TCP-2mm 

0.62 

TCP-3mm 

0.60 

TCP-4mm 

0.59 

TCP-5mm 

Sample Case - Post-RK 

59-y/o woman: 

AL = 23.91 mm 

IOLMaster K = 38.16 D 

EyeSys EffRP = 38.19 D 

Atlas Zone 0-3 = 38.79 D 

Galilei TCP Zone 0-4 = 36.53 D 

Galilei TCP Annuli 1-4 = 36.79 D 



Significantly greater MAE in methods w/ pre-LASIK K’s & ΔMR (all P

OCT Net Corneal Power 

Parabolic fit to central 3-mm zone 

Average 3 sets per meridian 

1.5mm 

R p 

D 

1.5mm 

R a 

n 0 = 1 

n 1 = 1.376 

n 2 = 1.336 

Net Corneal Power - combines anterior 

& posterior curvature measurements 

from 8 meridional scans 

IOL Calculations in Post-LASIK Eyes 

MAE (D) 

2.0 

1.5 

1.0 

0.5 

0 

MAE - OCT vs Other K Methods 

n = 14 

IOLM CL OR Clin Hx Haigis Orbscan OCT 

SD of PE (D) 

2.5 

2.0 

1.5 

1.0 

0.5 

0 

SD of PE - OCT vs Other K Methods 

n = 14 

IOLM CL OR Clin Hx Haigis Orbscan OCT 

OCT Net Corneal Power: 

Converted to keratometry equivalent 

prior to use in IOL formulas (+1.21 D) 

IOLM 

CL OR 

Clin Hx 

Haigis 

Orbscan 

OCT 







Posterior Corneal Astigmatism 

Spherical Aberration 

Traditional measurement of 

corneal astigmatism: 

Based on anterior surface 

Assumes fixed anterior-posterior 

curvature ratio 

Ignores contribution of posterior 

surface 

Retrospective study of 

posterior corneal astigmatism: 

Galilei DSA measurements 

Cornea exhibits (+) spherical 

aberration (SA): 

Mean +0.28 µm* 

Balanced by lens in youth 

Additive with lens in cataracts 

Cataract surgery: 

SA of crystalline lens removed 

SA of cornea remains 

Spherical IOLs have (+) SA 

(typically +0.18 µm) 

⊕ 

⊕ 



*Wang L, et al. ARVO 2011 



Spherical Aberration 

Corneal Ectasia & Ectasia Risk 

SA = 0.01 0.58 0.24 µm 

Reduction in SA: 

Improved contrast sensitivity 

Improved visual performance 

(e.g. night driving) 

Measurement of corneal SA may 

influence choice of IOL: 

Alcon AcrySof IQ (SA = -0.20 µm) 

AMO Tecnis (SA = -0.27 µm) 

B&L SofPort AO (SA = 0 µm) 

Standard Spherical (SA = +18 µm) 




We Like to See?

Abnormal Corneal Shape 

Determining ectasia risk is a 

very important part of refractive 

surgery screening 

Several criteria developed from 

the anterior surface 

Rabinowitz-McDonnell: 

Central K > 47.2 D 

Astigmatism > 1.5 D 

I - S > 1.4 D 

SRAX > 21° 

KISA% > 100 (Suspect 60 to 100) 

SRAX 





CCT: 539 µm 

Thinnest: 516 µm 

KPI: 43.4% 

Posterior Elevation: 35 µm 





OCT KCN Pachymetry Index (KPI) 

Parameter 

Min 

Min - Med 

Y Min 

S - I 

SN - IT 

KPI 

Definition 

KCN Cutoff 

(1 percentile) 

KCN 

Accuracy* 

FF KCN 

Accuracy* 

Minimum corneal thickness < 464 µm 0.94 µm 0.61 µm 

Minimum corneal thickness minus 

median corneal thickness 

Y coordinate of minimum corneal 

thickness 

Avg thickness of superior octant 

minus inferior octant 

Avg thickness of superonasal 

octant minus inferotemporal octant 

KPI = 0.41Min + 0.23(Min-Med) - 

0.14[(S-I) + (SN-IT)] + 0.22Ymin 

< -33 µm 0.93 µm 0.62 µm 

< -1.1 mm 0.82 mm 0.67 mm 

> 54 µm 0.84 µm 0.68 µm 

> 52 µm 0.90 µm 0.60 µm 

0.97 0.69 

OCT KCN Pachymetric Score (Example) 

23-y/o Male: 

-0.25 + 1.75 x 165 (20/15) 

SimK - 40.8/4.9 @ 16 

Topography 

OCT 

KISA % 96 Min (µm) 518 0 

K (D) 40.8 Min - Med (µm) -38 3 

I-S (µm) 5.7 Y Min (mm) -1.23 1 

AST (D) 1.1 S - I (µm) 62 2 

SRAX (deg) 46 SN - IT (µm) 50 1 

Total Pachy Score 7 

*Area under receiver operating curve 






Huang D, et al. (Unpublished Data)

Conclusions 

Still need multiple devices to see all that we want to see 

Ability to image posterior cornea (accurately) is only 

increasing in importance 

“Go-to” topographer - Still Placido-based 

Can’t do without other devices, now that I’ve got them 

Probably use “imaging” features the least in current 

practice (lower priority) 

It sure is nice that we have all of this technology! 

Acknowledgements 

Douglas D. Koch, M.D. 

Li Wang, M.D./Ph.D. 

Mariko Shirayama, M.D. 

Shazia Ali, B.S 

Richard Jenkins, M.D. 

Zaina Al-Mohtaseb, M.D. 

David Huang, M.D./Ph.D. 

Advances in 

Corneal Imaging 

CORNEA-ANTERIOR SEGMENT 


AM Option and RTVue FD-OCT 

Tear Film 

Anterior Segment 

Imaging…. 

Bag and Lens 



CK scar 

sule 

IOL 



What Would We 

Like to See? 

MP Weikert, MD ✦ ASORN Regional Meeting ✦ San Antonio, TX ✦ August 24, 2012 

Part No. 43721 Rev. A

Handout - STERIS University

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