Nanshuo Wang

Depth of focus extension in optical coherence tomography via multiple aperture synthesis

In this paper, we report what we believe is a novel technique to overcome the depth-of-focus (DOF) limitation in optical coherence tomography (OCT). Using confocal optics on a sample arm, we scanned the illumination beam across the under-filled objective lens pupil plane by steering the beam at the pinhole using a microcylindrical lens. The detected interferometric signals from multiple distinctive apertures were digitally refocused, which is analogous to synthetic aperture radar (SAR). Using numerical simulations and imaging experiments, we verified that this technique can maintain a diffraction-limited transverse resolution along a DOF that is ∼10 times larger than the confocal parameter. The ability to extend the DOF without signal loss and sidelobe artifacts may ultimately overcome the DOF limitation in high-resolution OCT.

Contrast Enhanced Subsurface Fingerprint Detection Using High-Speed Optical Coherence Tomography

Spectral-domain optical coherence tomography (SD-OCT) has been demonstrated to be a viable tool in forensic science for fingerprint detection, yet it still suffers from certain practical issues, e.g., the limited scanning speed and low image contrast. In this letter, we report a high-speed SD-OCT together with an image contrast enhancement mechanism for reliable subsurface fingerprint detection. The constructed SD-OCT system achieves a scanning rate up to 60k A-lines/s, and thus, both 3D volumetric images, reaching up to 20 mm × 20 mm × 1.2 mm, and en face internal fingerprint furrow pattern images could be obtained. Based upon the analyses of the papillary layer anatomical structures, the contrast enhancement technique not only suppresses image artifacts, but also is effective in detecting fingerprint spoofing. Experiments on healthy subjects have also been conducted to verify the system imaging capability and the effectiveness of the contrast-enhancement technique.

Visualizing Micro-anatomical Structures of the Posterior Cornea with Micro-optical Coherence Tomography

Diagnosis of corneal disease and challenges in corneal transplantation require comprehensive understanding of corneal anatomy, particularly that of the posterior cornea. Micro-optical coherence tomography (µOCT) is a potentially suitable tool to meet this need, owing to its ultrahigh isotropic spatial resolution, high image acquisition rate and depth priority scanning mode. In this study, we explored the ability of µOCT to visualize micro-anatomical structures of the posterior cornea ex vivo and in vivo using small and large animals. µOCT clearly delineated cornea layers and revealed micro-anatomical structures, including not only polygonal endothelial cells, stellate keratocytes, collagen fibres and corneal nerve fibres but also new structures such as the dome-shaped basolateral side of endothelial cells and lattice structures at the interface between endothelium and Descemet’s membrane. Based on these observations, a short post-harvest longitudinal study was conducted on rat cornea to test the feasibility of using µOCT to monitor the quality of endothelial cells. This study successfully reveals a series of morphological features and pathological changes in the posterior cornea at the cellular level in situ and in real time with µOCT. These findings enrich knowledge of corneal anatomy and suggest that µOCT may be a promising imaging tool in corneal transplantation.

Endomicroscopic optical coherence tomography for cellular resolution imaging of gastrointestinal tracts

Our ability to detect neoplastic changes in gastrointestinal (GI) tracts is limited by the lack of an endomicroscopic imaging tool that provides cellular-level structural details of GI mucosa over a large tissue area. In this article, we report a fiber-optic-based micro-optical coherence tomography (μOCT) system and demonstrate its capability to acquire cellular-level details of GI tissue through circumferential scanning. The system achieves an axial resolution of 2.48 μm in air and a transverse resolution of 4.8 μm with a depth-of-focus (DOF) of ~150 μm. To mitigate the issue of limited DOF, we used a rigid sheath to maintain a circular lumen and center the distal-end optics. The sensitivity is tested to be 98.8 dB with an illumination power of 15.6 mW on the sample. With fresh swine colon tissues imaged ex vivo, detailed structures such as crypt lumens and goblet cells can be clearly resolved, demonstrating that this fiber-optic μOCT system is capable of visualizing cellular-level morphological features. We also demonstrate that time-lapsed frame averaging and imaging speckle reduction are essential for clearly visualizing cellular-level details. Further development of a clinically viable μOCT endomicroscope is likely to improve the diagnostic outcome of GI cancers.

A two-step iteration mechanism for speckle reduction in optical coherence tomography

Abstract Optical coherence tomography (OCT) is an imaging tool that has been widely utilized for various disease diagnoses for its noninvasive and high-resolution properties. Due to the dual role of speckles in the imaging process, however, OCT images suffer from the unavoidable speckle noise, which is usually regarded to be multiplicative in nature and reduces image contrast and resolution. In this study, we propose to categorize OCT speckle noise into additive portion and multiplicative portion, and present a simple two-step iteration (TSI) mechanism to suppress such noises separately. With the augmented Lagrange minimization (ALM) method adopted to recover a low-rank image, the first step of TSI is to remove the additive Gaussian noise, while the second step of TSI is to suppress the multiplicative noise by employing a split Bregman method (SBM) to solve the total-variation (TV) de-noising problem. Extensive experiments with OCT images of the swine eye, human and rabbit retina are conducted to verify the effectiveness of the proposed method. Results show that the proposed TSI method outperforms the existing methods in different cases. Specifically, TSI helps improve the peak signal-to-noise ratio (PSNR) and structure similarity (SSIM) of the swine eye images from 17.19 dB to 33 dB and 0.12 to 0.92, respectively, with the important structural details well preserved. The clearer boundaries and higher image contrast obtained with the TSI method would largely facilitate image analyses and interpretations for the clinical applications of OCT systems.

Reliable internal fingerprint detection using micro-optical coherence tomography

Fingerprint recognition is a method commonly utilized for both personal identification and security control in practice. Based mainly on 2D surface topography analysis, however, most of the existing methods are prone to both spoofing attacks and fingertip surface conditions. To address such issues, we report on an internal fingerprint detection method based on the papillary layer anatomical structures using a lab-built optical coherence tomography (OCT) system, which achieves a resolution of 2.3 μm and ∼7.81 μm in axial and lateral directions, respectively. Specifically, to facilitate the fingerprint identification process, a simple image processing method based on the papillary layer anatomical analysis for OCT image contrast enhancement is also proposed. In vivo experiments are carried out to verify the system imaging capability and the effectiveness of the proposed contrast-enhancement mechanism in different cases. Results show that the system together with the image processing method is capable of providing reliable internal fingerprint imaging for identification purpose. The main factors that limit the practical applications of the OCT system for fingerprint identification are also discussed.

Speckle reduction in optical coherence tomography using two-step iteration method (Conference Presentation)

Optical coherence tomography (OCT) provides high resolution and cross-sectional images of biological tissue and is widely used for diagnosis of ocular diseases. However, OCT images suffer from speckle noise, which typically considered as multiplicative noise in nature, reducing the image resolution and contrast. In this study, we propose a two-step iteration (TSI) method to suppress those noises. We first utilize augmented Lagrange method to recover a low-rank OCT image and remove additive Gaussian noise, and then employ the simple and efficient split Bregman method to solve the Total-Variation Denoising model. We validated such proposed method using images of swine, rabbit and human retina. Results demonstrate that our TSI method outperforms the other popular methods in achieving higher peak signal-to-noise ratio (PSNR) and structure similarity (SSIM) while preserving important structural details, such as tiny capillaries and thin layers in retinal OCT images. In addition, the results of our TSI method show clearer boundaries and maintains high image contrast, which facilitates better image interpretations and analyses.

Polarization management to mitigate misalignment-induced fringe fading in fiber-based optical coherence tomography

In fiber-based optical coherence tomography (OCT), the interference fringes suffer from the fading effect due to misalignment of the light polarization states between the reference and sample arms, resulting in sensitivity degradation and image intensity variation. We theoretically and experimentally analyzed the relation between the misalignment and the fading coefficient. Assuming that the variation of the light polarization in single-mode fiber (SMF) was a random process, we statistically quantified the fading effect. Furthermore, in OCT configuration based on the Michelson interferometer, we reported an interesting observation that the polarization states of light traveling a round-trip in SMF are not evenly distributed on the Poincare sphere. Based on this observation, we demonstrated the existence of an optimal output polarization state of the reference arm to mitigate the fading effect. We demonstrated that in an optimal setup, the statistical average signal-to-noise ratio could be 3.5 dB higher than a setup without proper polarization management.

Spectral domain optical coherence tomography with dual-balanced detection

We developed a spectral domain optical coherence tomography (SD-OCT) system employing dual-balanced detection (DBD) for direct current term suppression and SNR enhancement, especially for auto-autocorrelation artifacts reduction. The DBD was achieved by using a beam splitter to building a free-space Michelson interferometer, which generated two interferometric spectra with a phase difference of π. These two phase-opposed spectra were guided to the spectrometer through two single mode fibers of the 8 fiber v-groove array and acquired by ultizing the upper two lines of a three-line CCD camera. We rotated this fiber v-groove array by 1.35 degrees to focus two spectra onto the first and second line of the CCD camera. Two spectra were aligned by optimum spectrum matching algorithm. By subtracting one spectrum from the other, this dual-balanced detection system achieved a direct current term suppression of ~30 dB, SNR enhancement of ~3 dB, and auto-autocorrelation artifacts reduction of ~10 dB experimentally. Finally we respectively validated the feasibility and performance of dual-balanced detection by imaging a glass plate and swine corneal tissue ex vivo. The quality of images obtained using dual-balanced detection was significantly improved with regard to the conventional single-detection (SD) images.

Spectral estimation optical coherence tomography for axial super-resolution(Conference Presentation)

The sample depth reflectivity profile of Fourier domain optical coherence tomography (FD-OCT) is estimated from the inverse Fourier transform of the spectral interference signals (interferograms). As a result, the axial resolution is fundamentally limited by the coherence length of the light source. We demonstrate an axial resolution improvement method by using the autoregressive spectral estimation technique to instead of the inverse Fourier transform to analyze the spectral interferograms, which is named as spectral estimation OCT (SE-OCT). SE-OCT improves the axial resolution by a factor of up to 4.7 compared with the corresponding FD-OCT. Furthermore, SE-OCT provides a complete sidelobe suppression in the point-spread function. Using phantoms such as an air wedge and micro particles, we prove the ability of resolution improvement. To test SE-OCT for real biological tissue, we image the rat cornea and demonstrate that SE-OCT enables clear identification of corneal endothelium anatomical details ex vivo. We also find that the performance of SE-OCT is depended on SNR of the feature object. To evaluate the potential usage and define the application scope of SE-OCT, we further investigate the property of SNR dependence and the artifacts that may be caused. We find SE-OCT may be uniquely suited for viewing high SNR layer structures, such as the epithelium and endothelium in cornea, retina and aorta. Given that SE-OCT can be implemented in the FD-OCT devices easily, the new capabilities provided by SE-OCT are likely to offer immediate improvements to the diagnosis and management of diseases based on OCT imaging.