Measurement of ocular axial length using full-range
spectral-domain low-coherence interferometry
Yi Wang (王 毅)
1
, Liang Feng (冯 亮)
1
, Lida Zhu (朱礼达)
2
, Hongxian Zhou (周红仙)
1
,
and Zhenhe Ma (马振鹤)
1,
*
1
School of Control Engineering, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China
2
School of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
*Corresponding author: wangyi@neuq.edu.cn
Received October 25, 2017; accepted January 4, 2018; posted online March 2, 2018
We demonstrate a system for measuring the ocular axial length (AL) with high sensitivity and high speed using
spectral-domain low-coherence interferometry (SD-LCI). To address the limit in measuring such a large range by
using SD-LCI, we propose a full-range method to recognize the positive and negative depths. The reference arm
length is changed synchronously with the shift of the focal point of the probing beam. The system provides a
composite depth range that is sufficient to cover the whole eye. We demonstrate the performance of the pre-
sented system by measuring the ALs of five volunteers. This system can provide the A-scan ocular biometric
assessment of the corneal thickness and AL in 0.1 s.
OCIS codes: 170.4500, 170.4460, 120.3180.
doi: 10.3788/COL201816.031701.
A cataract remains the world’ s leading cause of blind-
ness. Cataract surgery is the most common procedure
performed by ophthalmic surgeons
[1,2]
. Accurate measure-
ment of the ocular axial length (OAL) is essential for pre-
cisely calculating the intraocular lens (IOL) power
[3,4]
.
Ultrasound-based axial length (AL) measurement has
been the gold standard for many years. However, the
ultrasound-based method is not a preferred clinical solu-
tion due to its numerous limitations, such as moderate
resolution (approximately 200 μm), examiner dependence,
patient’s discomfort, and the risk of corneal infection
[5,6]
.
In recent years, optical methods based on low-coherence
interferometry (LCI) [also referred as low-coherence
reflectometry (LCR), optical coherence tomography
(OCT), or partial coherence reflectometry (PCR)] have
found their applications booming in ophthalmology
[7–10]
.
The commercial optical instruments IOL Master 500
(Carl Zeiss Meditec Inc.) and Lenstar LS900 (Haag-Streit
AG)
[11–14]
were based on time-domain interferometry.
Then, Fourier-domain methods, including spectral-
domain LCI (SD-LCI) and swept-source LCI (SS-LCI),
were demonstrated by using a broadband light source
and a spectrometer or by using a frequency swept light
source and a high-speed detector
[15–18]
. Fourier-domain
optical biometry has more advantages compared with
time-domain optica l biometry
[19–21]
, such as higher sensitiv-
ity and higher speed. The more recently developed
OA-2000 (Tomey GmbH) and IOL Master 700 (Carl Zeiss
Meditec Inc.) are based on SS-LCI. One of the advantage s
of SS-LCI over SD-LCI is its reduced sensitivity drop-off,
resulting in an enhanced depth range sufficient to cover
the human AL. However, the SS-LCI-based instrument
is expensive, which prevents its prevalence. In recent
years, various SD-LCI-based methods for AL measure-
ment have been reported
[22–24]
. The major advantage of
SD-LCI is its relatively low cost. The average human
AL is ∼23.09 mm and ranges from ∼18.13 mm to
∼29.09 mm
[25]
. The limited depth range and complex con-
jugate artifact of SD-LCI hinder its application in meas-
uring such a large range.
The depth range of standard SD-LCI is generally a
few millimeters. The full-range OCT can double the
depth range
[26–31]
. However, the technique increases the
depth range by only a limited amount. Grajciar et al. used
two separate reference arms to measure the human axial
eye length
[32]
. Ruggeri et al. performed imaging and
biometric measurement of the human eye by using an
SD-OCT system with three reference arms
[33]
. Theoreti-
cally, the multi-reference-arm scheme can extend the
depth range of standard SD-LCI , and one can measure
a large range by separating it into multiple short
segments. However, the multi-reference-arm scheme is im-
practical because ambiguous depth interpretation to each
short segment could occur due to the complex conjugate
artifact inherent to SD-LCI. A reconstructed A-scan
image of SD-LCI is symmetrical about the zero delay line.
This is because the Fourier transform of a real valued
function is Hermitian symmetric, and it produces a com-
plex conjugate artifact that mirrors the desired true image
about the zero delay line. Consequently, the negative and
positive depths cannot be distinguished from each other.
Therefore, one must limit the sample under test entirely
within the positive or negative depth range to avoid the
ambiguous depth interpretation. The reported multi-
reference-arm schemes for AL measurement are also lim-
ited by this problem. In full-range OCT, the negative and
positive depths can be corrected by introducing a constant
modulation frequency into each B-scan
[26–29]
. However, this
method is not suitable for AL measurement, in which the
A-scan is implemented.
COL 16(3), 031701(2018) CHINESE OPTICS LETTERS March 10, 2018
1671-7694/2018/031701(5) 031701-1 © 2018 Chinese Optics Letters