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.
Tethered capsule endomicroscopy (TCE) is a new method for performing comprehensive microstructural OCT imaging of gastrointestinal (GI) tract in unsedated patients in a well-tolerated and cost-effective manner. These features of TCE bestow it with significant potential to improve the screening, surveillance and management of various upper gastrointestinal diseases. To achieve clinical adoption of this imaging technique, it is important to validate it with co-registered histology, the current diagnostic gold standard. One such method for co-registering OCT images with histology is laser cautery marking, previously demonstrated using a balloon-centering OCT catheter that operates in conjunction with sedated endoscopy. With laser marking, an OCT area of interest is identified on the screen and this target is marked in the patient by exposing adjacent tissue to laser light that is absorbed by water, creating superficial, visible marks on the mucosal surface. Endoscopy can then be performed after the device is removed and biopsies taken from the marks. In this talk, we will present the design of a tethered capsule laser marking device that uses a distal stepper motor to perform high precision (< 0.5 mm accuracy) laser targeting and high quality OCT imaging. Ex vivo animal tissue tests and pilot human clinical studies using this technology will be presented.
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.
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.
Photoimmunotherapy (PIT) is a cell-specific cancer therapy based on an armed antibody conjugate that induces rapid and highly selective cancer cell necrosis after exposure to near-infrared (NIR) light. The PIT treatment also induces the superenhanced permeability and retention effect, which allows high concentrations of nanoparticles to accumulate in the tumor bed. In our pilot studies, optical coherence tomography (OCT) reveals dramatic hemodynamic changes during PIT. We developed and applied speckle variance analysis, Doppler flow measurement, bulk motion removal, and automatic region of interest selection to quantify vessel diameter and blood velocity within tumors in vivo. OCT imaging reveals that blood velocity in peripheral tumor vessels quickly drops below the detection limit while the vessel lumen remains open (4 vessels from 3 animals). On the other hand, control tumor vessels (receive NIR illumination but no PIT drug) do not show the sustained blood velocity drop (5 vessels from 3 animals). Ultraslow blood velocity could result in a long drug circulation time in tumor. Increase of the blood pool volume within the central tumor (shown in histology) may be the leading cause of the periphery blood velocity drop and could also increase the drug pool volume in tumor vessels.
Optical coherence tomography (OCT) is a high resolution imaging technology that is rapidly being adopted as the
standard of care for medical applications such as ocular and intravascular imaging. However, clinical translation has
been hampered by the lack of standardized test methods for performance evaluation as well as consensus standards
analogous to those that have been developed for established medical imaging modalities (e.g., ultrasound). In this study,
we address low contrast detectability, specifically, the ability of systems to differentiate between regions exhibiting small
differences in scattering coefficient. Based on standard test methods for established medical imaging modalities, we
have developed layered phantoms with well-characterized scattering properties in a biologically relevant range. The
phantoms consisted of polydimethylsiloxane (PDMS) doped with varying concentrations of BaSO4 microparticles.
Microfabrication processes were used to create layered and channel schemes. Two spectral domain OCT systems - a
Fourier domain system at 855 nm and a swept-source device at 1310 nm - were then used to image the phantoms. The
detectability of regions with different scattering levels was evaluated for each system by measuring pixel intensity
differences. Confounding factors such as the inherent attenuation of the phantoms, signal intensity decay due to focusing
and system roll-off were also encountered and addressed. Significant differences between systems were noted. The
minimum differences in scattering coefficient that the Fourier domain and swept source systems could differentiate was
1.50 and 0.46 mm-1 respectively. Overall, this approach to evaluating low contrast detectability represents a key step
towards the development of standard test methods to facilitate clinical translation of novel OCT systems.
In biophotonic imaging, turbid phantoms that are low-cost, biologically-relevant, and durable are desired for
standardized performance assessment. Such phantoms often contain inclusions of varying depths and sizes in order to
quantify key image quality characteristics such as penetration depth, sensitivity and contrast detectability. The emerging
technique of rapid prototyping with three-dimensional (3D) printers provides a potentially revolutionary way to fabricate
these structures. Towards this goal, we have characterized the optical properties and morphology of phantoms fabricated
by two 3D printing approaches: thermosoftening and photopolymerization. Material optical properties were measured by
spectrophotometry while the morphology of phantoms incorporating 0.2-1.0 mm diameter channels was studied by μCT,
optical coherence tomography (OCT) and optical microscopy. A near-infrared absorbing dye and nanorods at several
concentrations were injected into channels to evaluate detectability with a near-infrared hyperspectral reflectance
imaging (HRI) system (650-1100 nm). Phantoms exhibited biologically-relevant scattering and low absorption across
visible and near-infrared wavelengths. Although limitations in resolution were noted, channels with diameters of 0.4
mm or more could be reliably fabricated. The most significant problem noted was the porosity of phantoms generated
with the thermosoftening-based printer. The aforementioned three imaging methods provided a valuable mix of insights
into phantom morphology and may also be useful for detailed structural inspection of medical devices fabricated by
rapid prototyping, such as customized implants. Overall, our findings indicate that 3D printing has significant potential
as a method for fabricating well-characterized, standard phantoms for medical imaging modalities such as HRI.
The emerging technique of three-dimensional (3D) printing provides a simple, fast, and flexible way to fabricate structures with arbitrary spatial features and may prove useful in the development of standardized, phantom-based performance test methods for biophotonic imaging. Acrylonitrile Butadiene Styrene (ABS) is commonly used in the printing process, given its low cost and strength. In this study, we evaluate 3D printing as an approach for fabricating biologically-relevant optical phantoms for hyperspectral reflectance imaging (HRI). The initial phase of this work involved characterization of absorption and scattering coefficients using spectrophotometry. The morphology of phantoms incorporating vessel-like channels with diameters on the order of hundreds of microns was examined by microscopy and OCT. A near-infrared absorbing dye was injected into channels located at a range of depths within the phantom and imaged with a near-infrared HRI system (650-1100 nm). ABS was found to have scattering coefficients comparable to biological tissue and low absorption throughout much of the visible and infrared range. Channels with dimensions on the order of the resolution limit of the 3D printer (~0.2 mm) exhibited pixelation effects as well as a degree of distortion along their edges. Furthermore, phantom porosity sometimes resulted in leakage from channel regions. Contrast-enhanced channel visualization with HRI was possible to a depth of nearly 1 mm – a level similar to that seen previously in biological tissue. Overall, our ABS phantoms demonstrated a high level of optical similarity to biological tissue. While limitations in printer resolution, matrix homogeneity and optical property tunability remain challenging, 3D printed phantoms have significant promise as samples for objective, quantitative evaluation of performance for biophotonic imaging modalities such as HRI.
We develop a novel platform based on a tele-operated robot to perform high-resolution optical coherence tomography (OCT) imaging under continuous large field-of-view magnetic resonance imaging (MRI) guidance. Intra-operative MRI (iMRI) is a promising guidance tool for high-precision surgery, but it may not have sufficient resolution or contrast to visualize certain small targets. To address these limitations, we develop an MRI-compatible OCT needle probe, which is capable of providing microscale tissue architecture in conjunction with macroscale MRI tissue morphology in real time. Coregistered MRI/OCT images on ex vivo chicken breast and human brain tissues demonstrate that the complementary imaging scales and contrast mechanisms have great potential to improve the efficiency and the accuracy of iMRI procedure.
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