FFOCT (Full-field optical coherence tomography) is a high-resolution optical-sectioning imaging technique for biological tissues. Fractal analysis of the FFOCT image contrast is validated as a quantitative tool to measure the scattering properties of tissue. The defocus is one of the main factors that reduce FFOCT image contrast. This work applies the fractal analysis to model FFOCT image contrast. The contrast correction factor was proposed by considering tissue scattering and the system's defocused coherence transfer function. For each layer of the FFOCT image, the image is transformed to its original defocused amplitude and phase, then corrected with the proposed correction factor. The in-depth fractal dimension and EBCM (Edge-based contrast measurement) are introduced to validate the contrast enhancement quantitively. The result of the mouse organ demonstrated the correction effect quantitatively to a resolution limit, and it may guide a physics-informed interpretation of FFOCT image and augmented microscopy.
KEYWORDS: Optical coherence tomography, Point spread functions, Light sources and illumination, Picosecond phenomena, Diffraction limit, Signal attenuation, Image resolution, Fourier transforms, Signal to noise ratio, Signal intensity
Defocus in OCT can be removed by computational refocusing but with certain limitations. We theoretically investigate the limitations of the computational refocusing for standard point-scanning OCT and spatially coherence full-field OCT (SC-FFOCT). The Nyquist limit of the image sampling and the confocality define the maximum correctable defocus (MCD). The Nyquist limit becomes non-effectual limit with reasonable oversampling to the optical lateral resolution. The confocality is the main limiting factor of MCD for point-scanning OCT, while the SC-FFOCT does not have this limitation. SC-FFOCT was found to have virtually infinitely large MCD and is particularly suitable for optical coherence microscopy.
Full field optical coherence tomography (FF-OCT) utilizes low coherence gate to suppress the out-of-focus scattering light. The spatial resolution remains limited by numerical aperture of objective and bandwidth of light source. Structured illumination microscopy not only is a wide-field three-dimensional imaging modality which shift high spatial frequency components to optical transfer function of system, but also enables optical sectioning in biological tissue. Thus, we propose a structured illumination based FF-OCT(SI-FF-OCT) to realize three-dimensional sub-cellular resolution using parallel coded digital micromirror device (DMD). First, the principle of SI-FF-OCT was introduced in terms of point spread function (PSF) of the standard FF-OCT optical system. Second, a 1D illumination pattern of SI-FF-OCT for imaging random tissue was simulated for both of lateral resolution enhancement and optical sectioning capacity in the same configuration. The results of onion cells and mouse liver tissue suggested that the proposed white-light structured illumination approach in FF-OCT demonstrates tomographic imaging for thick transparent samples with a higher lateral resolution and better contrast than standard FF-OCT. As a benefit of the fringe projection and low-coherence gate filtering, SI-FF-OCT also has optical sectioning capability in free of speckle-noise in sub-pixel accuracy way.
Motion artifacts and interference residuals limit the performance of conventional full-field optical coherence tomography (FF-OCT) for in vivo imaging. It is because piezoelectric translator-based phase-shifting method bring chromatic abberation with broadband light source and is wavelegnth-dependent. Many achromatic phase-shifters were designed and introduced additional mechanical vibrations into the imaging system. In this work, an achromatic phase-shifting mechanism was proposed with dual-channel FF-OCT. Based on a Linnik interferometer, a linear polarizer and a quarter-wave plate were employed in the reference arm to replace PZT and generate the circle polarized light beam. The light field reflected from the reference arm is supposed with the unpolarized light backscattered from the sample when the path difference is within the coherence length of the light source. With one phase difference of π/2 shifted by interference between the circle polarized light and unpolarized light. Two inteference beams were received by a pair of identical CCDs. With porposed numerical phase-shifting method based on Hilbert transform, the en face tomograms of sample will be reconstructed in a single-shot. The axial and lateral resolutions of the system are around 1.4µm and 0.8µm, respectively. Tomograms of Intel microchip were displayed with clear substructures. Three-dimensional miscrostructures of fresh and isolated mouse heart were obtained in video-rate. This article aims at producing fringe-free OCT images with imaging rate of 30fps.
The measurement and visualization of transient three-dimensional (3-D) physical parameters (density and temperature) distribution of complex flow fields are critical technologies for the characteristics studies of flow fields in modern energy engineering. Among the optical computed tomography (OCT) methods, Moiré tomography has the advantages of simple optical path structure, strong anti-interference ability and wide measurement range, which is especially suitable for complex flow field measurement in noisy environments. Acquiring the transient phase information from the moiré projection is of great importance for the dynamic 3-D parameters reconstruction of complex flow fields. In this paper, the dynamic phase retrieve methods including Fourier and spatial phase-shifting in moiré tomographic are studied, respectively. In the Fourier method, an adaptive first-order spectrum extraction algorithm for Fourier transform moiré fringe and a phase calculation method are proposed. Through this, the projection phase can be obtained directly by multiplying the inverse Fourier transform of the positive first-order spectrum of deformed fringe with the inverse Fourier transform of the negative first-order spectrum of reference fringe. In spatial phase-shifting method, a spatial phase-shifting- interferometry-based moiré volume computed tomography (MVCT) method was proposed to extract the radial shearing phase distribution of grid moiré fringe. The measured results for the first-order partial derivative of the phase projection of a propane flame both by Fourier and spatial phase-shifting methods in the experimental moiré computed tomography systems are presented. The research will be valuable for monitoring the combustion state in energy engineering.
Researching the combustion process is important core contents in aviation, aerospace and energy engineering. And it has an important significance for the combustion researches to measure the physics parameters by combustion diagnosis and to acquire the 3D transient combustion distribution. Optical computerized tomography (OCT) applied in combustion diagnosis plays an important role in flow field diagnosis for its advantages of non-contact, real time, and 3D full-field measurements. The essence of CT is reconstructing the test field with projections from different directions. And the projections are independent to each other. Therefore, a multi-direction projections calibration should be performed before CT reconstruction, which can remap different projections to a unified coordinate to reduce the system errors from the installations and optical elements. Aiming at this problem, in this project, a multi-direction calibration method based on affine projection is proposed. By studying the affine projection theory, the calibration model was build based on single directional moiré tomographic system. Furthermore, the corresponding algorithm based on the calibration model is proposed. Finally, a multi-directional moiré tomographic system is built and combining with the calibration results for remapping the multi-directional projections, the 3D temperature distribution of the propane combustion flame was reconstructed.
We report rapid and high-resolution tomographic en face imaging of human liver specimens by full-field optical coherence tomography (FF-OCT). First, the arrangement of the FF-OCT system was described and the performance of the system was measured. The measured axial and lateral resolutions of the system are 0.8 and 0.9 μm, respectively. The system has a sensitivity of ∼60 dB and can achieve an imaging rate of 7 fps and a penetration depth of ∼80 μm. The histological structures of normal liver can be seen clearly in the en face tomographic images, including central veins, cords of hepatocytes separated by sinusoidal spaces, and portal area (portal vein, the hepatic arteriole, and the bile duct). A wide variety of histological subtypes of hepatocellular carcinoma was observed in en face tomographic images, revealing notable cancerous features, including the nuclear atypia (enlarged convoluted nuclei), the polygonal tumor cells with obvious resemblance to hepatocytes with enlarged nuclei. In addition, thicker fibrous bands, which make the cytoplasmic plump vesicular nuclei indistinct, were also seen in the images. Finally, comparison between the portal vein in a normal specimen versus that seen in the rare type of cholangiocarcinoma was made. The results show that the cholangiocarcinoma presents with a blurred pattern of portal vein in the lateral direction and an aggregated distribution in the axial direction; the surrounding sinusoidal spaces and nuclei of cholangiocarcinoma are absent. The findings in this work may be used as additional signs of liver cancer or cholangiocarcinoma, demonstrating capacity of FF-OCT device for early cancer diagnosis and many other tumor-related studies in biopsy.
A new full-field optical coherence tomography system with high-resolution has been developed for imaging of cells and tissues. Compared with other FF-OCT (Full-field optical coherence tomography, FF-OCT) systems illuminated with optical fiber bundle, the improved Köhler illumination arrangement with a halogen lamp was used in the proposed FF-OCT system. High numerical aperture microscopic objectives were used for imaging and a piezoelectric ceramic transducer (PZT) was used for phase-shifting. En-face tomographic images can be obtained by applying the five-step phase-shifting algorithm to a series of interferometric images which are recorded by a smart camera. Three-dimensional images can be generated from these tomographic images. Imaging of the chip of Intel Pentium 4 processor demonstrated the ultrahigh resolution of the system (lateral resolution is 0.8μm ), which approaches the theoretical resolution 0.7 μm× 0.5 μm (lateral × axial). En-face images of cells of onion show an excellent performance of the system in generating en-face images of biological tissues. Then, unstained pig stomach was imaged as a tissue and gastric pits could be easily recognized using FF-OCT system. Our study provides evidence for the potential ability of FFOCT in identifying gastric pits from pig stomach tissue. Finally, label-free and unstained exvivo human liver tissues from both normal and tumor were imaged with this FFOCT system. The results show that the setup has the potential for medical diagnosis applications such liver cancer diagnosis.
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