COL 9(12), 121001(2011) CHINESE OPTICS LETTERS December 10, 2011
Resolution enhancement with improved range Doppler
algorithm in high numerical aperture OCT
Xiaodong Chen (¡¡¡ÁÁÁ)
1,2∗
, Yong Lei (XXX nnn)
1,2
, Yi Wang ( ÀÀÀ)
1,2
, and Daoyin Yu (ÕÕÕ)
1,2
1
College of Precision Instrument and Opto-Electronic Engineering, Tianjin University, Tianjin 300072, China
2
Opto-Electronic Information Science and Technical Lab, Ministry of Education, Tianjin University, Tianjin 300072, China
∗
Corresp onding author: xdchen@tju.edu.cn
Received June 22, 2011; accepted September 15, 2011; posted online October 31, 2011
The improved range Doppler algorithm is proposed to abate the trade-off between resolutions and depth
of focus in spectral domain optical coherence tomography. By considering the finite beam width and
the shape of the wavefronts produced by the Gaussian beam, a physical diffraction model is presented to
simulate the light propagation process in the sample. The two-dimensional processing of the spectrum data
is decomposed into two one-dimensional pro cessings of Stolt transform and matched filter. Experimental
results show that image reconstruction can be achieved. The transverse and axial resolutions are both
improved significantly, esp ecially in the out-of-focus range, and the resolutions are almost equivalent
throughout the entire region of interest.
OCIS codes: 100.3010, 100.2000, 100.2980.
doi: 10.3788/COL201109.121001.
Optical coherence tomography (OCT) is a cross-sectional
optical imaging technique with high resolution of 1–15
µm. Since its invention, it has been used in diverse areas
of medical imaging
[1]
. In standard time-domain OCT
(TDOCT), special hardware designs for the scanning of
the reference are needed, which complicates the instru-
ment and reduces the imaging speed. Spectral domain
OCT (SDOCT) measures depth-resolved reflectance of
tissues by resampling and Fourier transforming the spec-
trum data. SDOCT has attracted considerable attention
due to its high sensitivity and imaging speed.
In the traditional reconstruction, the resolution in the
axial direction and the transverse resolution are deter-
mined by the bandwidth of the light source and the
numerical aperture (NA), respectively. However, the
spot diameter becomes large, and the curvature of wave-
fronts become significant as NA increases; thus, the
transverse and axial resolutions are both related to NA
during scanning. Only scatterers in the depth of fo-
cus (DOF) range exhibit high resolution and those in
the out-of-focus (OOF) range are seriously blurred in
both transverse and axial directions. Several methods
have been introduced to improve transverse resolution,
such as dispersion comp ensation
[2]
and wavenumber do-
main algorithms
[3]
, whereas the light source with non-
Gaussian spectral shape has been introduced to improve
axial resolution
[4]
. However, most of these methods are
one-dimensional (1D), making them limited. Others have
taken account of the phase information of the OCT im-
age and extended for two-dimensional (2D) studies
[5,6]
;
however, the curvature produced by the wavefronts of
the light source and transverse scan have not been taken
into account.
In this letter, we propose a novel modality that achieves
depth-independent resolution, wherein the focus is fixed
at one depth and alleviates the compromise between res-
olutions and DOF in OCT. A 2D scalar diffraction model
is derived, and the inverse scattering problem is solved.
The matched filter is applied to correct image curvature
and axial range cell migration (RCM). In addition, Stolt
transform is used to realize the fo cusing in transverse
direction. The numerical diffraction algorithm is similar
to the range Doppler algorithm (RDA) of synthetic aper-
ture radar (SAR)
[7]
. A few limitations of this method
employed in the improved RDA, along with some possi-
ble extensions of this method, are also discussed.
The OCT system is based on a fiber Michelson inter-
ferometer, as illustrated in Fig. 1. The beam from a
superluminescent-diode (SLD) source is focused onto a
sample while it is scanned transversely over the sample.
The probe beam contains components propagating in
different directions, as shown in Fig. 2. As the beam
is translated over the imaging regions (from the dashed
line profile of probe beam to the solid line one), different
parts of the probe beam become incident on the scat-
terer. Therefore, it can be interpreted that a single
detection receives contributions from multiple scatterers,
and different detections contain contributions of mul-
tiple directions from the same scatterer as the probe
beam is translating over the sample. Fortunately, the
principle of synthetic aperture also meets the physi-
cal model of diffraction
[8]
. Hence, the scalar diffraction
Fig. 1. Schematic of the SDOCT system. RM is reference
mirror, PC is personal computer, and CCD is charge-coupled
device.
1671-7694/2011/121001(4) 121001-1
c
° 2011 Chinese Optics Letters