Optical coherence tomography (OCT) is a noninvasive imaging modality which can provide cross-sectional imaging of the tissues in high-resolution. Especially in retina imaging, the OCT becomes one of the most valuable imaging tools for the diagnostics of the eye diseases. Considering the scattering and absorption properties of the eye, the 1000 nm OCT system is preferred for the retina image. In this study, we describe an akinetic swept source OCT system based on pulse-modulated active mode locking (AML) fiber laser at the 1080 nm wavelength region for in-vivo human retina imaging. The akinetic AML wavelength swept fiber laser is constructed with polarization maintaining fiber which has average linewidth of 0.625 nm, a spectral bandwidth of 81.15 nm and a duty ratio of 90 % without buffering method. We successfully obtained in-vivo human retina images using proposed OCT system without the additional k-clock and the frequency shifter providing wide field of view of 43.1º. The main retina layers such as RPE can be distinguished through the OCT image with axial resolution of 6.3 m.
We propose an image reconstruction method of digital holography for getting more accurate reconstruction. Digital holography provides both the light amplitude and the phase of a specimen through recording the interferogram. Since the Fresenl diffraction can be efficiently implemented by the Fourier transform, zero padding technique can be applied to obtain more accurate information. In this work, we report the method of frequency domain zero padding (FDZP). Both in computer-simulation and in experiment made with a USAF 1951 resolution chart and target, the FDZD gave the more accurate rconstruction images. Even though, the FDZD asks more processing time, with the help of graphics processing unit (GPU), it can find good applications in digital holography for 3-D profile imaging.
We present the coherent anti-Stokes Raman scattering (CARS) microscopy system that has been implemented by using a photonic crystal polarization maintaining optical fiber. Free space CARS system is hard in alignment and unstable in harsh environment. To overcome this problem the femto-second laser pulses of pump and the Stokes beams were delivered through the optical fiber, so that the system became less complex and robust to the surrounding environment. In order to confirm the feasibility of the fiber-based CARS system, the CARS images of polystyrene beads and zinc oxide (ZnO) are presented.
KEYWORDS: In vivo imaging, 3D image processing, Nerve, Optical coherence tomography, Cornea, Eye, Visualization, Data acquisition, Surgery, Image processing
We have employed Fourier-domain optical coherence tomography (FD-OCT) to achieve corneal nerve imaging, which could be useful in surgical planning and refractive surgery. Because the three-dimensional (3-D) images of the corneal nerves were acquired in vivo, unintentional movement of the subject during the measurement led to imaging artifacts. These artifacts were compensated for with a series of signal processing techniques, namely realigning A-scan images to flatten the boundary and cross-correlating adjacent B-scan images. To overcome the undesirably large signal from scattering at the corneal surface and iris, volume rendering and maximum intensity projections were performed with only the data taken in the stromal region of the cornea, which is located between 200 and 500 μm from the corneal surface. The 3-D volume imaging of a 10×10 mm2 area took 9.8 s, which is slightly shorter than the normal tear breakup time. This allowed us to image the branched and threadlike corneal nerve bundles within the human eye. The experimental results show that FD-OCT systems have the potential to be useful in clinical investigations of corneal nerves and by minimizing nerve injury during clinical or surgical procedures.
We present an all optical fiber combined-imaging system that integrates non-contact photoacoustic tomography (NPAT) and optical coherence tomography (OCT) to simultaneously provide PA and OCT images. The fiber-based PAT system utilizing a Mach-Zehnder interferometer with a fiber laser of 1550 nm measures the photoacoustic signal at the sample surface. For the generation of a PA signal, a pulse train from a bulk type Nd:YAG laser illuminates the sample via a large core multimode optical fiber. The fiber-based OCT operating at a center wavelength of 1310 nm allowed is combined with the fiber-based PAT system by sharing the same optical fiber probe. The two lights from the fiber laser and the OCT source are guided into the probe through each port of a 2 by 2 optical fiber coupler. The back-reflected lights from the sample are guided to respective imaging systems by the same coupler. With these both NPAT and OCT images could be co-registered without physical contact with the sample. To demonstrate the feasibility of the proposed system, a phantom experiment has been carried out with a phantom composed of a black PET fiber and a fishing wire. The proposed all fiber-optic combined-imaging system has the potential for minimally invasive and improved diagnosis.
We present a photoacoustic microscopy (PAM) system based on a Fabry-Perot Interferometer (FPI) consisting of a transparent Polydimethylsiloxane (PDMS) thin film. Most of the PAM systems have limitations with the system alignment because the ultrasound transducers for detection are not transparent. Therefore, the excitation laser source should avoid the opaque transducer to illuminate the sample, which makes the system difficult to build-up. Especially, the system volume is highly limited to be compact. In our experiment, to solve these difficulties, a FPI based on the PDMS film has been implemented and applied to measure the acoustic wave signal. The system uses a FPI as an acoustic wave detector instead of a conventional ultrasound transducer. A tunable laser was used to choose the quadrature-point at which the signal has the highly sensitve and linear response to the acoustic wave. Also a 20Hz pulsed Nd:YAG laser was used to generate acoustic waves from a sample. When the acoustic waves arrive at the PDMS film, one of the surfaces of the film is modulated at the detecting point, which gives the tuned FPI interference signal. From the signal arriving time, the depth location of the sample is calculated. As a primary experiment using the PDMS thin film as an ultrasound transducer, a couple of narrow black friction tapes located in a water container were used as the samples. This proposed imaging method can be used in various applications for the detection and measurement of acoustic waves.
We have imaged human corneal nerve bundles by using real-time Fourier-domain OCT (FD-OCT). Corneal nerves contribute to the maintenance of healthy ocular surface owing to their trophic influences on the corneal epithelium. The FD-OCT system was based on a swept laser of a 50 kHz sweeping rate and 1.31 μm center wavelength. At the area including sclera, limbus, and cornea, we could successfully get the in-vivo tomograms of the corneal nerve bundles. The scan range was 5 x 5mm. In this study, the A-scan images in each B-scan were realigned to have a flat air-surface boundary in the final B-scan image. With this effort, we could align corneal nerve bundle in a same depth and get the 3D image showing the branched and threadlike corneal nerve bundles.
We demonstrate a highly linear wavenumber- swept active mode locking (AML) fiber laser for optical sensing and imaging without any wavenumber-space resampling process. In this all-electric AML wavenumber-swept mechanism, a conventional wavelength selection filter is eliminated and, instead, the suitable programmed electric modulation signal is directly applied to the gain medium. Various types of wavenumber (or wavelength) tunings can be implemented because of the filter-less cavity configuration. Therefore, we successfully demonstrate a linearly wavenumber-swept AML fiber laser with 26.5 mW of output power to obtain an in-vivo OCT image at the 100 kHz swept rate.
In this study, polarization-sensitive optical coherence tomography (PS-OCT) capable of providing polarization contrasts such as phase retardation and degree of polarization uniformity (DOPU) was used for visualizing human meibomian glands (MGs) and investigating morphological characteristics of them. Especially, with the help of the DOPU contrast, MGs were exclusively extracted from the volumetric OCT image. In vivo PS-OCT measurements were performed on the upper eyelids of different age groups. From these measurements, different age-dependent aspects of the MG structure were also observed. Based on these observations, it can be inferred that the PS-OCT system has the potential for clinical diagnosis and investigation of MG-related dry eye diseases like MG dysfunction (MGD) and acinar atrophy.
We demonstrate segmentation of human MGs based on several image processing technic. 3D volumetric data of upper
eyelid was acquired from real-time FD-OCT, and its acini area of MGs was segmented. Three dimensional volume
informations of meibomian glands should be helpful to diagnose meibomian gland related disease. In order to reveal
boundary between tarsal plate and acini, each B-scan images were obtained before averaged three times. Imaging area
was 10x10mm and 700x1000x500 voxels. The acquisition time was 60ms for B-scan and 30sec for C-scan. The 3D data
was flattened to remove curvature and axial vibration, and resized to reduce computational costs, and filtered to
minimize speckles, and segmented. Marker based watershed transform was employed to segment each acini area of
meibomian gland.
We propose a dual-channel fiber scanning probe for simultaneous measurement of swept source optical coherence tomography (SS OCT) and fluorescence spectroscopy (FS) signals. For the purpose, SS OCT and FS system were combined by adopting the specially fabricated double cladding fiber (DCF) and wavelength division multiplexer (WDM) coupler, and DCF fiber was directly connected to sample arm of DCF coupler for fiber-based probe. Moreover, for sample scanning, the fiber was driven by piezoelectric bender. Since DCF has dual-channel configuration consists of core and inner cladding, both OCT and FS signals propagate through the two channels at the same time. Therefore, the suggested system enables multifunctional imaging that would make it possible to determine a more specific diagnosis. To demonstrate the feasibility of the probe, a photosensitizer injected in-vivo mice were imaged with scanning speed of 16 Hz and scanning range of 2 mm.
We present a birefringence analysis method based on polarization-sensitive swept-source optical coherence tomography (PS-SS-OCT) for distinguishing pearls. To cope with the round shape of general pearls, a rotation stage was used for the sample scanning. With the system, the birefringence of several cultured pearls including south sea, Akoya, freshwater cultured pearls, and imitation pearls are analyzed and compared. Interestingly, PS-SS-OCT surely shows well developed birefringence patterns of phase retardation and fast axis orientation with the cultured pearls, whereas the pattern does not appear in the imitation pearls. In addition, the intensity image can help to distinguish the cultured pearls. Therefore, PSSS-OCT enables a more accurate interpretation for identifying the cultured pearls from imitation pearls.
We propose full-range spectral domain optical coherence tomography equipped with a fiber-based sample scanner,
which is used for sample scanning and phase shifting for full-range image at the same time. For a fiber-based sample
scanner, since the fiber tip oscillates as a free standing cantilever in general, unintentional phase shift occurs inevitably.
The unintentional phase shift was used for eliminating the bothersome complex conjugate ghost image of OCT. In
addition, fiber was tilted a few degree to give proper phase shift. In this scheme, moreover, image can be obtained
without any physical modification of the scanner. To realize this technique, we constructed the SD-OCT system and
fabricated a magnetically actuated single-body lensed fiber scanner due to advantages of simple design, low operating
voltage, cost-effectiveness and low insertion loss. The scanner was made of lensed fiber loaded with an iron-based bead
and a solenoid which is placed perpendicular to the lensed fiber. When a sinusoidal current is applied into the solenoid,
the lensed fiber oscillated due to magnetic force between the iron-based bead and the solenoid. With the suggested full
range method, we obtained contrast enhanced full-range SD OCT images of pearl and tooth. This simple and effective
method can be applied to any fiber-based scanner and it has great potential as a handheld probe/endoscopic probe in
biomedical imaging field.
We propose a single piece optical fiber-based two-dimensional scanning hand-held probe suitable for three-dimensional
optical coherence tomography. The probe consists of only a single piece of optical fiber loaded with a bead
of ferromagnetic material, which acts as a vibrating cantilever. The fiber cantilever is two dimensionally actuated with a
single miniaturized solenoid. For effective beam focusing, a fiber lens is formed at the end of the fiber. The inductance
and input current of the solenoid were 100 μH and 216 mA, respectively. The iron-bead on the fiber is located at the off-axis
of solenoid for two-dimensional scanning. Then, by modulating the input current to the solenoid, it was possible to
mechanically oscillate the fiber cantilever in an elliptically spiral pattern. With the proposed probe, 2-dimensional
scanning could be experimentally achieved in a rate of 4 s/vortex across a scanning area of approximately 30 mm2,
which could be controlled with the length of the fiber or/and the weight of the iron-bead. Three-dimensional tomographic
image of a coin was successfully obtained with the spectral domain optical coherence tomography equipped with the
proposed scanner. It is expected that the scheme of 2-dimentional scanning with a single actuator might be useful in
various real-time imaging applications including OCT owing to the advantages of low cost, low power consumption,
simple fabrication process and versatile design.
An optical integrating system composed of optical coherence tomography (OCT) and fluorescence spectroscopy (FS) has
been designed and utilized to distinguish pearls by determining their mother oysters used in pearl culturing as well as
discriminate and evaluate the pearls, nondestructively. By adopting a wavelength division multiplexing (WDM) and a
double clad fiber (DCF) coupler, a FS system could be successfully combined with a fiber-based swept source OCT (SSOCT)
system. Applying a common-path configuration, furthermore, the integrating system could be implemented in a
simple and effective way with highly minimized group velocity dispersion (GVD) and polarization mismatch problems.
The internal structure measurement and the fluorescence spectrum measurement, which were previously performed by
two independent apparatus, were concurrently made with the proposed system. From the OCT measurement, we could
measure the thickness of the nacre layer, observe the fine sub-structure of the nacre, and inspect the nucleus through the
nacre of a pearl. With the fluorescence spectrum measurement, we could categorize the pearls by determining their
mother oysters.
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