Spectral domain optical coherence tomography with
sub-micrometer sensitivity for measurement of central
corneal thickness
Lida Zhu (朱礼达)
1
, Yi Wang (王 毅)
2,
*, Yi Yuan (袁 毅)
1
, Hongxian Zhou (周红仙)
3
,
Yuqian Zhao (赵玉倩)
2
, and Zhenhe Ma (马振鹤)
2
1
School of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
2
School of Control Engineering, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China
3
Experiment Education Center, Northeastern University at Qinhuangdao, Qinhuangdao 066004, China
*Corresponding author: wangyi@neuq.edu.cn
Received October 31, 2018; accepted January 10, 2019; posted online April 1, 2019
We demonstrated a method for measurement of central corneal thickness (CCT) with a sub-micrometer sensi-
tivity using a spectral domain optical coherence tomography system without needing a super broad bandwidth
light source. By combining the frequency and phase components of Fourier transform, the method is capable of
measurement of a large dynamic range with a high sensitivity. Absolute phases are retrieved by comparing the
correlations between the detected and simulated interference fringes. The phase unwrapping ability of the
present method was quantitatively tested by measuring the displacement of a piezo linear stage. The human
CCTs of six volunteers were measured to verify its clinical application. It provides a potential tool for clinical
diagnosis and research applications in ophthalmology.
OCIS codes: 170.4500, 170.4460, 120.3180.
doi: 10.3788/COL201917.041701.
Evaluation of centra l corneal thickness (CCT) plays an
important role in clinical diagnosis and research applica-
tions in ophthalmology. CCT has been recognized as a
credible indicator of the glaucoma damage and progres-
sion of ocular hypertension in primary open-angle glau-
coma (POAG) patients
[1–3]
. It is also reported to have a
positive correlation with intraocular pressure (IOP),
which is a key risk factor for the development of glau-
coma
[4,5]
. CCT is usually about 500–600 μm in normal sub-
jects, and it statistically decreases by about 2–10 μm per
decade
[2]
. Precise measurement of CCT has become more
and more important with the booming development of re-
fractive eye surgery. Ultrasonic and optical methods are
the commonly used methods for CCT measuremen t.
The ultrasonic method (ultrasonic pachymetry, USP) uses
high-frequency sound waves (20 to 50 MHz) to detect the
time lapse of reflected sound waves from the anterior and
posterior corneal surfaces. USP has been regarded as the
gold standard for many years due to its reliability and util-
ity. However, USP has several disadvantages, including
direct contact of the probe with the cornea with topical
anesthesia, risk of infection and damage of the corneal epi-
thelium, and examiner dependence.
In recent years, optical coherence tomography (OCT)
[also referred as low-coherence interferometry (LCI),
white light interferometry (WLI), or partial coherence
reflectometry (PCR)] has been developed for CCT mea-
surement. OCT is a noninvasive modality capable of
investigating micro-structures
[6–11]
. Compared with USP,
OCT has obvious advantages, such as high resolution,
non-contact, and easy operation
[12,13]
. In laser refractive
surgery, according to Munnerlyn’s formula, the ablation
depth is approximately 3 μm per diopter for an optical
zone of 3 mm diameter to correct myopia
[6]
. Each laser
pulse results in an ablation depth of about 0.2–0.3 μm
in the corneal stroma
[7,8]
. The current OCT cannot mea-
sure the intraoperative or postoperative CCT changes
with such sensitivity. The theoretical axial resolution of
OCT is equal to 0.44λ
2
∕nΔλ, where n is the refractive
index, λ and Δλ denote the center wavelength and the
spectral bandwidth of the light source, respectively. For
example, the light source centered at 1310 or 840 nm with
a spectral bandwidth of 60 nm results in a theoretical axial
resolution of 9.2 or 3.8 μm, respectively. Recently, several
OCT systems with an ultra-high axial resolution up to
∼1 μm have been reported
[14–17]
. Among all these tech-
niques, ultra-high resolution is achieved by using a super
broad bandwidth light source. However, such a light source
is expensive, which prevents its prevalence. In traditional
Fourier domain OCT (FD-OCT), depth information
is demodulated from the frequency component by fast
Fourier transform (FFT) of detected interference fringes,
and such a demodulation method results in the limited axial
sensitivity of FD-OCT. By analyzing the phase component
of FFT, FD-OCT is able to achieve a super-high axial sen-
sitivity up to the nanometer scale
[18,19]
. However, due to
phase wrapping, phase imaging suffers from a limited dy-
namic range, which is restricted in ð−π; π, corresponding
to an optical length difference (OLD) of half a wavelength.
Phase wrapping issue occurs when the detected phase falls
outside the range of ð−π; π since the phase is periodic in
nature. Many digital phase unwrapping algorithms have
COL 17(4), 041701(2019) CHINESE OPTICS LETTERS April 10, 2019
1671-7694/2019/041701(5) 041701-1 © 2019 Chinese Optics Letters