Coronary arteries are covered by a thin layer of endothelial cells (ECs). Impairment of ECs is at the origin of coronary atherosclerosis and its clinical manifestations. However, the study of ECs in humans remains elusive because of a lack of an imaging tool with sufficient resolution. We have developed a light-based 1-µm-resolution microscopic imaging technology termed micro-optical coherence tomography (µOCT) that can be implemented in a coronary catheter. In this study, we investigated the capability of µOCT to visualize EC morphology. We stripped the endothelium from 36 fresh swine coronary segments with cyanoacrylate glue. Histology showed that the stripping procedure successfully removed ECs from the swine coronary arteries. Coronary segments were then imaged in 3D with µOCT, and were processed for histology and scanning electron microscopy (SEM). µOCT images of stripped vs. intact sites were volume rendered in 3D and visually compared. 3D-µOCT allowed visualization of EC pavementing on intact artery surfaces that was strongly correlated to that seen by SEM. EC pavementing disappeared, and surface roughness calculated by computed root mean squared error diminished significantly at the sites with stripped EC compared with intact sites. µOCT was also utilized in human cadaver coronary arteries, showing its capability of identifying EC morphology of human coronary plaque harboring leukocyte adhesion, EC stent strut tissue coverage, and lack of ECs at lesions with necrotic core or superficial nodular calcifications. In conclusion, µOCT enables EC visualization in coronary arteries, suggesting that it could be useful in patients with coronary artery disease to better understand the role of ECs in the pathogenesis of coronary artery disease.
In optical coherence tomography (OCT) systems, axial resolution improves with increasing light source bandwidth. However, dispersion imbalance between the sample and reference arms can degrade axial resolution and signal to noise ratio, a significant issue for ultrahigh-resolution OCT systems. In this work, we demonstrate a novel technique for estimating and compensating for OCT system dispersion, that is unique from previously reported methods in that it compensates all orders of system dispersion. Dispersion phase was estimated by first measuring the phase from of the spectrogram at two different, reference-sample arm optical path length differences (OPLD) around zero OPLD and then subtracting the two phase values to obtain the dispersion phase. This phase can be used to compensate the dispersion term in the spectrum by multiplying the interference pattern with where k is the wave-vector. This method was tested to compensate the dispersion caused by a 3-mm fused silica window in one arm of an ultrahigh spectral domain OCT system in our laboratory that utilizes a light source with a 850 nm center wavelength, 300 nm bandwidth. Using our dispersion compensation technique, the experimentally measured axial resolution of the system was fully recovered to match the theoretical resolution, improving from 10.6µm to 1.85µm in air. These results suggest that this dispersion compensation method may be useful to avoid axial resolution degradation due to dispersion effects in ultrahigh-resolution OCT systems that employ extremely broad band light sources.
Optical coherence tomography (OCT) has been a useful clinical tool for diagnosing coronary artery disease through a flexible catheter, but its full promise relies on resolving cellular and sub-cellular structures in vivo. Previously, visualizing cellular structures through an imaging catheter is not possible due to limited depth of focus (DOF) of a tightly focused Gaussian beam: typically, a Gaussian beam with 2-3 μm resolution has a DOF within 100 μm, which is not sufficient for in vivo catheter imaging. Therefore, we developed a self-imaging wavefront division optical system that generates a coaxially-focused multimode (CAFM) beam with a DOF that is approximately one order of magnitude longer than that of a Gaussian beam. In this study, we present a high-resolution, extended DOF catheter based on self-imaging wavefront division optics. The catheter generates a CAFM beam with a lateral resolution of 3 μm and a DOF close to 2 mm. To correct the aberration introduced by catheter sheath, we incorporated a cylindrical prism to compensate the sheath astigmatism. When the catheter is incorporated into a micro-resolution OCT (μOCT) system with rotational scanning mechanics, cellular-resolution cross-sectional images of the coronary artery wall can be obtained. The device serves as an important step toward characterizing cellular and sub-cellular structures in vivo for coronary artery disease diagnosis.
High resolution micro-optical coherence tomography (µOCT) technology has been demonstrated to be useful for imaging respiratory epithelial functional microanatomy relevant to the study of pulmonary diseases such as cystic fibrosis and COPD. We previously reported the use of a benchtop μOCT imaging technology to image several relevant respiratory epithelial functional microanatomy at 40 fps and at lateral and axial resolutions of 2 and 1.3μm, respectively. We now present the development of a portable μOCT imaging system with comparable optical and imaging performance, which enables the μOCT technology to be translated to the clinic for in vivo imaging of human airways.
Conventional OCT images, obtained using a focused Gaussian beam have a lateral resolution of approximately 30 μm and a depth of focus (DOF) of 2-3 mm, defined as the confocal parameter (twice of Gaussian beam Rayleigh range). Improvement of lateral resolution without sacrificing imaging range requires techniques that can extend the DOF. Previously, we described a self-imaging wavefront division optical system that provided an estimated one order of magnitude DOF extension. In this study, we further investigate the properties of the coaxially focused multi-mode (CAFM) beam created by this self-imaging wavefront division optical system and demonstrate its feasibility for real-time biological tissue imaging. Gaussian beam and CAFM beam fiber optic probes with similar numerical apertures (objective NA≈0.5) were fabricated, providing lateral resolutions of approximately 2 μm. Rigorous lateral resolution characterization over depth was performed for both probes. The CAFM beam probe was found to be able to provide a DOF that was approximately one order of magnitude greater than that of Gaussian beam probe. By incorporating the CAFM beam fiber optic probe into a μOCT system with ~1.5 μm axial resolution, we were able to acquire cross-sectional images of swine small intestine ex vivo, enabling the visualization of subcellular structures, providing high quality OCT images over more than a 300 μm depth range.
The human respiratory system is protected by a defense mechanism termed mucociliary clearance (MCC). Deficiency in MCC leads to respiratory obstruction and pulmonary infection, which often are the main causes of morbidity and mortality in diseases such as cystic fibrosis and chronic obstructive pulmonary disease (COPD). Studying key parameters that govern MCC, including ciliary beat frequency, velocity and volume of airway mucus transport, as well as periciliary liquid layer thickness are therefore of great importance in understanding human respiratory health. However, direct, in vivo visualization of ciliary function and MCC has been challenging, hindering the diagnosis of disease pathogenesis and mechanistic evaluation of novel therapeutics.
Our laboratory has previously developed a 1-µm resolution optical coherence tomography method, termed Micro-OCT, which is a unique tool for visualizing the spatiotemporal features of ciliary function and MCC. We have previously described the design of a flexible 2.5 mm Micro-OCT probe that is compatible with standard flexible bronchoscopes. This device utilizes a common-path interferometer and annular sample arm apodization to attain a sharply focused spot over an extended depth of focus.
Here, we present the most recent iteration of this probe and demonstrate its imaging performance in a mouse trachea tissue culture model. In addition, we have developed an ergonomic assembly for attaching the probe to a standard bronchoscope. The ergonomic assembly fixes the Micro-OCT probe’s within the bronchoscope and contains a means transducing linear motion through the sheath so that the Micro-OCT beam can be scanned along the trachea. We have tested the performance of these devices for Micro-OCT imaging in an anatomically correct model of the human airway. Future studies are planned to use this technology to conduct Micro-OCT in human trachea and bronchi in vivo.
Endoscopy, the current standard of care for the diagnosis of upper gastrointestinal (GI) diseases, is not ideal as a screening tool because it is costly, necessitates a team of medically trained personnel, and typically requires that the patient be sedated. Endoscopy is also a superficial macroscopic imaging modality and therefore is unable to provide detailed information on subsurface microscopic structure that is required to render a precise tissue diagnosis. We have overcome these limitations through the development of an optical coherence tomography tethered capsule endomicroscopy (OCT-TCE) imaging device. The OCT-TCE device has a pill-like form factor with an optically clear wall to allow the contained opto-mechanical components to scan the OCT beam along the circumference of the esophagus. Once swallowed, the OCT-TCE device traverses the esophagus naturally via peristalsis and multiple cross-sectional OCT images are obtained at 30-40 μm lateral resolution by 7 μm axial resolution. While this spatial resolution enables differentiation of squamous vs columnar mucosa, crucial microstructural features such as goblet cells (~10 μm), which signify intestinal metaplasia in BE, and enlarged nuclei that are indicative of dysplasia cannot be resolved with the current OCT-TCE technology.
In this work we demonstrate a novel design of a high lateral resolution OCT-TCE device with an extended depth of focus (EDOF). The EDOF is created by use of self-imaging wavefront division multiplexing that produces multiple focused modes at different depths into the sample. The overall size of the EDOF TCE is similar to that of the previous OCT-TCE device (~ 11 mm by 26 mm) but with a lateral resolution of ~ 8 μm over a depth range of ~ 2 mm. Preliminary esophageal and intestinal imaging using these EDOF optics demonstrates an improvement in the ability to resolve tissue morphology including individual glands and cells. These results suggest that the use of EDOF optics may be a promising avenue for increasing the accuracy of OCT-TCE for the diagnosis of upper GI diseases.
Using a fiber-based swept-source (SS) polarization-sensitive optical coherence tomography (PS-OCT) system, we investigate the degree of polarization (DOP) of light backscattered from the retinal nerve fiber layer (RNFL) in normal human subjects. Algorithms for processing data were developed to analyze the deviation in phase retardation and intensity of backscattered light in directions parallel and perpendicular to the nerve fiber axis (fast and slow axes of RNFL). Considering superior, inferior, and nasal quadrants, we observe the strongest degradation in the DOP with increasing RNFL depth in the temporal quadrant. Retinal ganglion cell axons in normal human subjects are known to have the smallest diameter in the temporal quadrant, and the greater degradation observed in the DOP suggests that higher polarimetric noise may be associated with neural structure in the temporal RNFL. The association between depth degradation in the DOP and RNFL structural properties may broaden the utility of PS-OCT as a functional imaging technique.
A swept-source dual-wavelength photothermal (DWP) optical coherence tomography (OCT) system is demonstrated for quantitative imaging of microvasculature oxygen saturation. DWP-OCT is capable of recording three-dimensional images of tissue and depth-resolved phase variation in response to photothermal excitation. A 1,064-nm OCT probe and 770-nm and 800-nm photothermal excitation beams are combined in a single-mode optical fiber to measure microvasculature hemoglobin oxygen saturation (SO 2 ) levels in phantom blood vessels with a range of blood flow speeds (0 to 17 mm/s ). A 50-μm-diameter blood vessel phantom is imaged, and SO 2 levels are measured using DWP-OCT and compared with values provided by a commercial oximeter at various blood oxygen concentrations. The influences of blood flow speed and mechanisms of SNR phase degradation on the accuracy of SO 2 measurement are identified and investigated.
We report design and demonstration of a dual wavelength photothermal (DWP) optical coherence tomography (OCT)
system for imaging of a phantom microvessel and measurement of hemoglobin oxygen saturation (SO2) level. The
DWP-OCT system contains a swept-source (SS) two-beam phase-sensitive (PhS) OCT system (1060 nm) and two
intensity modulated photothermal excitation lasers (770 nm and 800 nm). The PhS-OCT probe beam (1060 nm) and
photothermal excitation beams are combined into one single-mode optical fiber. A galvanometer based two-dimensional
achromatic scanning system is designed to provide 14 μm lateral resolution for the PhS-OCT probe beam (1060 nm) and
13 μm lateral resolution for photothermal excitation beams. DWP-OCT system’s sensitivity is 102 dB, axial resolution is
13 μm in tissue and uses a real-time digital dispersion compensation algorithm. Noise floor for optical pathlength
measurements is 300 pm in the signal frequency range (380-400 Hz) of photothermal modulation frequencies. Blood
SO2 level is calculated from measured optical pathlength (op) signal in a 300 μm diameter microvessel phantom
introduced by the two photothermal excitation beams. En-face and B-scan images of a phantom microvessel are recorded,
and six blood samples’ SO2 levels are measured using DWP-OCT and compared with values provided by a commercial
blood oximeter. A mathematical model indicates thermal diffusion introduces a systematic artifact that over-estimates
SO2 values and is consistent with measured data.
We report design and construction of an FPGA-based high-speed swept-source polarization-sensitive optical coherence
tomography (SS-PS-OCT) system for clinical retinal imaging. Clinical application of the SS-PS-OCT system is accurate
measurement and display of thickness, phase retardation and birefringence maps of the retinal nerve fiber layer (RNFL)
in human subjects for early detection of glaucoma. The FPGA-based SS-PS-OCT system provides three incident
polarization states on the eye and uses a bulk-optic polarization sensitive balanced detection module to record two
orthogonal interference fringe signals. Interference fringe signals and relative phase retardation between two orthogonal
polarization states are used to obtain Stokes vectors of light returning from each RNFL depth. We implement a
Levenberg-Marquardt algorithm on a Field Programmable Gate Array (FPGA) to compute accurate phase retardation
and birefringence maps. For each retinal scan, a three-state Levenberg-Marquardt nonlinear algorithm is applied to 360
clusters each consisting of 100 A-scans to determine accurate maps of phase retardation and birefringence in less than 1
second after patient measurement allowing real-time clinical imaging-a speedup of more than 300 times over previous
implementations. We report application of the FPGA-based SS-PS-OCT system for real-time clinical imaging of patients
enrolled in a clinical study at the Eye Institute of Austin and Duke Eye Center.
Segmentation of the retinal nerve fiber layer (RNFL) from swept source polarization-sensitive optical coherence
tomography (SS-PSOCT) images is required to determine RNFL thickness and calculate birefringence. Traditional
RNFL segmentation methods based on image processing and boundary detection algorithms utilize only optical
reflectivity contrast information, which is strongly affected by speckle noise. We present a novel approach to segment
the retinal nerve fiber layer (RNFL) using SS-PSOCT images including both optical reflectivity and phase retardation
information. The RNFL anterior boundary is detected based on optical reflectivity change due to refractive index
difference between the vitreous and inner limiting membrane. The posterior boundary of the RNFL is a transition zone
composed of birefringent axons extending from retinal ganglion cells and may be detected by a change in birefringence.
A posterior boundary detection method is presented that segments the RNFL by minimizing the uncertainty of RNFL
birefringence determined by a Levenberg-Marquardt nonlinear fitting algorithm. Clinical results from a healthy
volunteer show that the proposed segmentation method estimates RNFL birefringence and phase retardation with lower
uncertainty and higher continuity than traditional intensity-based approaches.
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