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![OCT image of inflated rat lung, showing several layers of alveoli in cross-section. Scale bar is 500μm [17]. © Materials Research Society 2009—reprinted with permission.](publication/224920279/figure/fig1/AS:11431281252842338@1718809872899/OCT-image-of-inflated-rat-lung-showing-several-layers-of-alveoli-in-cross-section-Scale.jpg)
OCT image of inflated rat lung, showing several layers of alveoli in cross-section. Scale bar is 500μm [17]. © Materials Research Society 2009—reprinted with permission.
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Visualization and correct assessment of alveolar volume via intact lung imaging is important to study and assess respiratory mechanics. Optical Coherence Tomography (OCT), a real-time imaging technique based on near-infrared interferometry, can image several layers of distal alveoli in intact, ex vivo lung tissue. However optical effects associated...
Citations
... It can be seen that the scatterer appears distorted. Experiments with bubble raft phantoms and recorded OCT images 66,67 show similar distortions. At a greater depth position behind the cylinder, at x = 63.75 μm, the signal from multiple reflections inside the cylinder (transmission, 3 × reflection, transmission) can be seen clearly. ...
An algorithm for the simulation of two-dimensional spectral domain optical coherence tomography images based on Maxwell’s equations is presented. A recently developed and modified time-harmonic numerical solution of Maxwell’s equations is used to obtain scattered far fields for many wave numbers contained in the calculated spectrum. The interferometer setup with its lenses is included rigorously with Fresnel integrals and the Debye-Wolf integral. The implemented model is validated with an existing FDTD algorithm by comparing simulated tomograms of single and multiple cylindrical scatterers for perpendicular and parallel polarisation of the incident light. Tomograms are presented for different realisations of multiple cylindrical scatterers. Furthermore, simulated tomograms of a ziggurat-shaped scatterer and of dentin slabs, with varying scatterer concentrations, are investigated. It is shown that the tomograms do not represent the physical structures present within the sample.
... The inflation pressure was set at 10 cmH2O. Owing to the large difference in refractive index between the air and lung tissue, the shapes of the alveoli changed due to total or multiple reflections at the alveolar wall [55]. One or two alveoli just beneath the visceral pleura were observed. ...
Optical coherence tomography (OCT) is a non-invasive cross-sectional imaging technique with micrometer resolution. The theoretical axial resolution is determined by the center wavelength and bandwidth of the light source, and the wider the bandwidth is, the higher the axial resolution is. The characteristics of OCT imaging depend on the optical wavelength used. In this paper, we investigated the wavelength dependence of ultrahigh-resolution (UHR) OCT using a supercontinuum for biomedical imaging. Wideband, high-power, low-noise supercontinua (SC) were generated at $\lambda$ = 0.8, 1.1, 1.3, and 1.7 μm based on ultrashort pulses and nonlinear fibers. The wavelength dependence of OCT imaging was examined quantitatively using biological phantoms. Ultrahigh-resolution imaging of a rat lung was demonstrated with $\lambda$ = 0.8-1.0 μm UHR-OCT. The variation of alveolar volume was estimated using 3D image analysis. Finally, UHR-spectral domain-OCT and optical coherence microscopy (OCM) at 1.7 μm were developed, and high-resolution and high-penetration imaging of turbid tissue, especially mouse brain, was demonstrated.
... It is also important to note that the lung is inherently a very porous organ because of the distal airways and the alveoli. While that is not represented in this phantom, the optical effects of similar structures have been observed using a Bragg-Nye bubble raft for optical coherence tomography 21 , air bubbles in olive oil 42 , and shaving cream or dish detergent for nuclear magnetic resonance imaging 43 . Creating polymer foams with reproducible characteristics may be able to reconcile this difference between the solid phantoms presented here and the lung microstructure 44 . ...
The rapid development of new optical imaging techniques is dependent on the availability of low-cost, customizable, and easily reproducible standards. By replicating the imaging environment, costly animal experiments to validate a technique may be circumvented. Predicting and optimizing the performance of in vivo and ex vivo imaging techniques requires testing on samples that are optically similar to tissues of interest. Tissue-mimicking optical phantoms provide a standard for evaluation, characterization, or calibration of an optical system. Homogenous polymer optical tissue phantoms are widely used to mimic the optical properties of a specific tissue type within a narrow spectral range. Layered tissues, such as the epidermis and dermis, can be mimicked by simply stacking these homogenous slab phantoms. However, many in vivo imaging techniques are applied to more spatially complex tissue where three dimensional structures, such as blood vessels, airways, or tissue defects, can affect the performance of the imaging system. This protocol describes the fabrication of a tissue-mimicking phantom that incorporates three-dimensional structural complexity using material with optical properties of tissue. Look-up tables provide India ink and titanium dioxide recipes for optical absorption and scattering targets. Methods to characterize and tune the material optical properties are described. The phantom fabrication detailed in this article has an internal branching mock airway void; however, the technique can be broadly applied to other tissue or organ structures.
... These goals are the successful development of (1) a methodology to derive the sample RI distribution; (2) an accurate but practical PSF model that accounts for specimen RI variability, which we reported elsewhere; 32 (3) a computationally tractable SV forward imaging model; 23,33 and (4) a practical restoration algorithm based on the SV model. The determination of the sample RI map is a challenging problem that has been tackled by several groups [34][35][36][37][38] including ours, 39,40 and it is beyond the scope of this paper. In this paper, we focus on the mathematical formulation, simulation, and experimental validation in the development of the third and fourth goals, in which we integrate our new PSF model from the second goal. ...
Development of a block-based restoration (BBR) method that addresses spatially-variant (SV) imaging in wide-field fluorescence microscopy of thick samples is presented. The BBR method is based on a block-based imaging model, which approximates SV imaging using an efficient orthonormal basis decomposition of multiple SV point-spread functions computed at block vertices. The effect of reducing the number of blocks needed to account for SV imaging on the restoration accuracy was investigated with simulations using a numerical lung phantom relevant to biological studies. Results show that reducing the number of blocks by 82% and 98% resulted in a 19% and 27% reduction in restoration accuracy, respectively, thereby establishing a reasonable tradeoff between computational resources and accuracy. Comparison of the BBR method to existing methods (deconvolution) that do not account for SV imaging demonstrates a 90% improvement in restoration accuracy. BBR results from synthetic and experimental images of a controlled test sample with SV refractive index (RI) show consistency, providing a validation of the BBR approach. In this study, information from DIC and fluorescence images was combined to identify regions with changing RI within the imaging volume. The BBR method provides a first step towards computationally tractable reconstruction of images from thick samples.
... The OPL is the product of RI and the physical distance that light travels when it propagates through the sample. The determination of the sample RI map is a challenging problem that has been tackled by several groups 5,7,27 including ours. 14 Our overarching method described in Sec. 1 utilizes an automated multimodal microscope capable of 3-D wide-field fluorescence microscopy and differential interference contrast (DIC) microscopy, which facilitates the determination of sample RI and thickness from quantitative phase derived from DIC images. ...
A three-dimensional (3-D) point spread function (PSF) model for wide-field fluorescence microscopy, suitable for imaging samples with variable refractive index (RI) in multilayered media, is presented. This PSF model is a key component for accurate 3-D image restoration of thick biological samples, such as lung tissue. Microscope- and specimen-derived parameters are combined with a rigorous vectorial formulation to obtain a new PSF model that accounts for additional aberrations due to specimen RI variability. Experimental evaluation and verification of the PSF model was accomplished using images from 175-nm fluorescent beads in a controlled test sample. Fundamental experimental validation of the advantage of using improved PSFs in depth-variant restoration was accomplished by restoring experimental data from beads (6 μm in diameter) mounted in a sample with RI variation. In the investigated study, improvement in restoration accuracy in the range of 18 to 35% was observed when PSFs from the proposed model were used over restoration using PSFs from an existing model. The new PSF model was further validated by showing that its prediction compares to an experimental PSF (determined from 175-nm beads located below a thick rat lung slice) with a 42% improved accuracy over the current PSF model prediction.
... Through this, confocal, multi-photon, and second-harmonic microscopy can image a few micrometers with sub-septal resolution, and optical coherence tomography (OCT; sometimes referred to as the "optical analog of ultrasound") can image collapse and re-inflation of one layer of sub-pleural alveoli in situ, and has provided the first in-vivo 3D images of individual alveoli (10). These images can suffer distortions such as underestimation of alveolar volume, so algorithms are being developed to correct them (11), but validation of the results against ex-vivo "ground truth" is also difficult because the lung collapses under biopsy and fixation or freezing of inflated lung can alter its micro-structure. Liquid-filled lung has limited physiological relevance, but is easier to image and may facilitate validation. ...
... Liquid-filled lung has limited physiological relevance, but is easier to image and may facilitate validation. Computational models (12) and lung phantoms (11) partially compensate for the lack of validation in physiological specimens. ...
In recent years, the fledgling field of small animal fluorescence molecular imaging has enabled great advances in pre-clinical research, driven by the development of both new optical imaging modalities at the macroscopic and microscopic scales (1), and novel classes of targeted imaging probes that report on the molecular status of cells and target tissues with exquisite specificity (2).
... 10 However, artifacts have been reported in the comparison of air-filled and fluid-filled peripheral lung tissue that greatly reduce the imaging depth and create the appearance of double walls. 11,12 It has been hypothesized that these artifacts are caused by the refraction and total internal reflection of light at the tissue-air interfaces and could result in inaccurate representations of alveolar shapes and sizes in OCT images. Thus far, there have been limited quantitative studies on individual alveolar areas, 13,14 and the validity of alveolar volume measures within OCT images has not been shown. ...
... The ray-trace modeling results predict a 22% underestimation for cross-sectional area and 5% underestimation for the perimeter of a superellipse with equal width and height and a 7), polynomial coefficients in Table 2, and n ratio ¼ 1.53. Journal of Biomedical Optics 126015-5 December 2012 • Vol. 17 (12) refractive index ratio between the tissue and alveolar filling n ratio ¼ 1.53. The model further predicts an underestimation of 40% for the volume and 25% underestimation for the surface area of a circularly symmetric superellipsoid with equal width and height. ...
Optical coherence tomography (OCT) has been increasingly used for imaging pulmonary alveoli. Only a few studies, however, have quantified individual alveolar areas, and the validity of alveolar volumes represented within OCT images has not been shown. To validate quantitative measurements of alveoli from OCT images, we compared the cross-sectional area, perimeter, volume, and surface area of matched subpleural alveoli from microcomputed tomography (micro-CT) and OCT images of fixed air-filled swine samples. The relative change in size between different alveoli was extremely well correlated (r>0.9, P<0.0001), but OCT images underestimated absolute sizes compared to micro-CT by 27% (area), 7% (perimeter), 46% (volume), and 25% (surface area) on average. We hypothesized that the differences resulted from refraction at the tissue-air interfaces and developed a ray-tracing model that approximates the reconstructed alveolar size within OCT images. Using this model and OCT measurements of the refractive index for lung tissue (1.41 for fresh, 1.53 for fixed), we derived equations to obtain absolute size measurements of superellipse and circular alveoli with the use of predictive correction factors. These methods and results should enable the quantification of alveolar sizes from OCT images in vivo.
Optical coherence tomography (OCT) relies on the reflection of light from structures in different layers to interferometrically reconstruct the volumetric image of the sample. However, light returned from multiple layers suffers from imbalanced attenuation owing to the optical path difference and inhomogeneous tissue absorption. We report an optimization algorithm to improve signal strength in deep tissue for swept-source (SS)-OCT imaging. This algorithm utilizes the attenuation coefficient of consecutive layers within the sample and combines them to compensate for the signal intensity loss from deep tissue. We stacked 170-µm thick cover slides as a standard sample for benchmark testing. The optimized OCT image provides a 30% increase in signal intensity in the deep structure compared with the conventional images. We applied this method for pearl inspection, whose layered structure demonstrates a great application for our optimized OCT imaging. In contrast to X-ray micro-CT scan and scanning electron microscope (SEM) imaging modalities, the optimized OCT imaging provides great potential for pearl quality inspection. The proposed improvement algorithm for SS-OCT could also be applied to diverse biomedical imaging scenarios, including label-free tissue imaging.
Accurate measurement of alveolar volume by lung imaging is crucial to study and understand respiratory mechanics. Optical Coherence Tomography (OCT), which generates cross-sectional microscopic image of tissue structure in real time based on near-infrared interferometry, can image several layers of distal alveoli in intact, ex vivo lung tissue. However, optical effects associated with heterogeneity of lung tissue results in incorrect alveoli shape imaging and inaccurate assessment of lung volume. Due to the irregular cellular shape of lung tissue and the lack of ground truth, these errors are difficult to analyze, as well as remove. To address this, the Bragg-Nye bubble raft, a two-dimensional crystalline arrangement of elements similar in geometry to alveoli has been adapted as an inflated lung phantom to study and analyze these errors.
We have been investigating ultrahigh resolution optical coherence tomography (UHR-OCT) imaging of lung tissues
using fiber based super continuum (SC) sources. The high power, low-noise, Gaussian shaped SC generated with
ultrashort pulses and optical fibers at several wavelength regions were used as the broadband light sources for UHROCT. Since the lung consists of tiny alveoli which are separeted by thin wall, the UHR-OCT is supposed to be effective for lung imaging. The normal and diseased lung tissues were observed without invasive procedures to the lung itself. The clear images of alveoli were observed with index matching effect by saline. In this work, we investigated the three-dimensional UHR-OCT imaging of lung structure. The lungs of rats inflated with 10% formalin at 5 cmH2O, 15 cmH2O, and 20 cmH2O pressure were prepared as the sample for investigation of size and shape of the lung structure. These samples were fixed with 10% formalin. The interalveolar septa, thin walls separating the alveoli, were clearly observed. The difference of size and shape of alveoli and thier three-dimensional network was clearly observed from the UHR-OCT images. The clear images of alveoli were observed with index matching effect of 10% formalin. We investigated the wavelength dependence of 3D UHR-OCT image of lung structure at 800 nm, 1060 nm, and 1700 nm wavelength regions. The 3D UHR-OCT images of structure of rat lung were clearly observed in all wavelength regions and wavelength dependence of imaging was discussed.
