Axial eye length measurement using coherence revival and extended imaging depth

Carl Zeiss Meditec, Inc., Dublin, CA

IITE 2019 , April 27th, 2019 Abstract # 3195611

Muzammil A Arain, Simon A. Bello, Sophie Kubach, and Jochen Straub

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Axial eye length measurement using coherence revival and extended imaging depth

DisclosuresMuzammil A Arain, Ph.D, (E), Carl Zeiss Meditec, Inc., Dublin, CA Simon A. Bello, Ph.D, (E), Carl Zeiss Meditec, Inc., Dublin, CA Sophie Kubach, M.S., (E), Carl Zeiss Meditec, Inc., Dublin, CAJochen Straub, Ph.D, (E), Carl Zeiss Meditec, Inc., Dublin, CA

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Motivation of this study

Myopia (also called nearsightedness) is the most common cause of impaired vision in young people

Myopia is related to elongated eye lengths

Measuring and tracking eye length is important for:

Early detection of myopia

Understanding the progression of myopia

Obtaining the true retinal curvature from widefield optical coherence tomography (OCT) scans

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Myopia is turning into an epidemic

https://www.capitaleyes.co.nz/news/2018/7/18/myopia-epidemic

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Axial length (AL) measurements techniques

Biometry Techniques

Ultrasound

Applanation Immersion

Optical

PCI OCT

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Study Aims

Explore measurement of axial eye length using Coherence Revival (CR) property of Swept Source Optical Coherence Tomography (SS-OCT) system using External Cavity Tunable Laser (ECTL)

Evaluate axial eye length measurement accuracy and range

Use eye length to obtain true retinal curvature

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Lc

What is Coherence Revival in ECTLs?

BART JOHNSON, * WALID ATIA, MARK KUZNETSOV, BRIAN D. GOLDBERG, PETER WHITNEY, AND DALE C. FLANDERS

Axsun Technologies, Billerica, Massachusetts, USA Real OCT image

Multiple virtual OCT images due to CRat multiples of Lc

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Principle of eye length (Le) using CR property of ECTL

A. H. Dhalla, D. Nankivil, T. Bustamante, A. Kuo, J. A. Izatt, “Simultaneous swept source optical coherence tomography of the anterior segment and retina using coherence revival,” Opt. Lett. 37, 1883–1885 (2012).

LeLc < n.Le

Lc = n.Le

Lc > n.Le

Lc = Laser cavity lengthn = Assumed index of the eye=1.36Le=Eye axial length

Simultaneous imaging of cornea and retina

Separate paths for retinal and corneal imaging

Single optical path Single shot image No need to adjust focus or delay line position

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Method

Align subject Adjust vertical offset and delay line position until the retina scan and the

corneal signal are visible in the scan window Acquire data and calculate eye length during post processing

Six human subjects

Align test eye Save raw data Offline processing to produce extended 32 mm axial depth image

and calculate axial length

Test eye

Device / equipment Prototype SS-OCT system with 200 kHz A-scan rate Axial depth range from 6-16 mm Offline access to mirror image enabling 32 mm axial depth 16 mm lateral FOV line scan ECTL laser from Axsun Technologies, MA, USA

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Results on human subjects

Le=(Lc-(corneal offset + retinal offset))/nRetinal offset

Corneal offset

OSLe= 23.24 mm

OSLe= 23.44 mm

ODLe= 25.01 mm

6 m

m

16 mm

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Results

Human Subjects Summary

Limitations Suitable for 21-26 mm eye lengths only There is an error that could be due to:

Error in cavity length measurements Error in aligning cornea perpendicular to retina Error in locating fovea Error in fixation

Eye length of all six human subjects was measured successfully

The difference between our measurement and IOL Master 700 measurement is within ± 0.25 mm

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Extending range of eye length measurements using extended depth imaging

Covers 21-26 mm eye length range

6 mm Markus Kohlhaas and et al., “Effect of Central Corneal Thickness, Corneal Curvature, and Axial Length on Applanation Tonometry,” ARCH OPHTHALMOL / VOL 124, APR 2006

Youn

g ch

ildre

n

High myopic eyes

21-26 mm

16 mm extended depth

Covers 21-31 mm eye length range*

Test eye imageNote: Cavity length of the laser can be tuned to cover eye lengths > 31mm

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Use of eye length measurements to correct retinal curvature*

*Manuel A. J. Steidle; Jochen Straub , “Estimating the shape of the human eye using widefield optical coherence tomography (OCT),”, Proceedings Volume 10685, Biophotonics: Photonic Solutions for Better Health Care VI; 106851V (2018) https://doi.org/10.1117/12.2306604

Uncorrected retinal curvature

Corrected true retinal curvature

Use measured eye length to correct for retinal curvature

IITEJon Bumstead et al, Poster # PB034Jochen Straub et al, Poster # PB0143

ARVOJochen Straub et al, Poster #1599 - A0359

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Conclusions

We have demonstrated a new method of measuring eye length using a prototype SS-OCT instrument using ECTL in a single acquisition without refocusing, delay line adjustment, or multiple optical paths

The demonstrated method can be incorporated in regular work flow during routine eye exams for tracking eye length

Extended depth feature will enable measurement of eye length for a larger population range

Measured eye length can be used to obtain the true curvature of the eye from widefield OCT scans

This method is not a replacement of high precision eye length measurements using biometry instruments, but rather is a tool to obtain true retinal curvature and allow for monitoring changes in eye length

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Acknowledgements

Carl Zeiss Meditec, Inc., Dublin, CA

Allison Ferro

Tilman Schmoll

Rick Williams

Conor Leahy

Jon Bumstead

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