Abstract
We have designed and fabricated a 4 mm diameter rigid endoscopic probe to obtain high resolution micro-optical coherence tomography (µOCT) images from the tracheal epithelium of living swine. Our common-path fiber-optic probe used gradient-index focusing optics, a selectively coated prism reflector to implement a circular-obscuration apodization for depth-of-focus enhancement, and a common-path reference arm and an ultra-broadbrand supercontinuum laser to achieve high axial resolution. Benchtop characterization demonstrated lateral and axial resolutions of 3.4 μm and 1.7 μm, respectively (in tissue). Mechanical standoff rails flanking the imaging window allowed the epithelial surface to be maintained in focus without disrupting mucus flow. During in vivo imaging, relative motion was mitigated by inflating an airway balloon to hold the standoff rails on the epithelium. Software implemented image stabilization was also implemented during post-processing. The resulting image sequences yielded co-registered quantitative outputs of airway surface liquid and periciliary liquid layer thicknesses, ciliary beat frequency, and mucociliary transport rate, metrics that directly indicate airway epithelial function that have dominated in vitro research in diseases such as cystic fibrosis, but have not been available in vivo.
Highlights
Optical coherence tomography (OCT) [1] has become a mainstay biomedical imaging technology due to its ability to non-invasively acquire cross-sectional images with high resolution compared to ultrasound
Polarization diversity is difficult to achieve in SD-OCT due to the need for two detectors, which in the case of spectrometers, requires extremely precise alignment [22], given the wide bandwidth utilized in μOCT
By sampling 5 spaced positions along the epithelium, we found airway surface liquid (ASL) and periciliary liquid (PCL) thickness to be 19.5 ± 1.7 μm and 6.0 ± 0.2 μm respectively, as shown in Fig. 8
Summary
Optical coherence tomography (OCT) [1] has become a mainstay biomedical imaging technology due to its ability to non-invasively acquire cross-sectional images with high resolution compared to ultrasound. Depth of focus determines instead the axial field of view, while axial resolution is derived independently from the bandwidth of the light source This creates conflicting pressures for the choice of NA: high NA yields superior resolution, while low NA allows cross-sectional OCT imaging over greater depths. Our laboratory has previously developed a spectral-domain implementation of OCT that combines a new generation of ultra-broadband supercontinuum light sources with an engineered beam shape to produce 2-μm lateral resolution, 1-μm axial resolution, and 300-μm depth of focus performance. We have termed this technique micro-optical coherence tomography (μOCT). This firstgeneration design employed a rigid design for reasons of simplicity; a flexible probe would likely be subject to much greater scanning hysteresis as the driveshaft transitions from compression to tension
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