Jessica Mavadia-Shukla

Optimal operational conditions for supercontinuum-based ultrahigh-resolution endoscopic OCT imaging.

We investigated the optimal operational conditions for utilizing a broadband supercontinuum (SC) source in a portable 800 nm spectral-domain (SD) endoscopic OCT system to enable high resolution, high-sensitivity, and high-speed imaging in vivo. A SC source with a 3-dB bandwidth of ∼246  nm was employed to obtain an axial resolution of ∼2.7  μm (in air) and an optimal detection sensitivity of ∼-107  dB with an imaging speed up to 35 frames/s (at 70 k A-scans/s). The performance of the SC-based SD-OCT endoscopy system was demonstrated by imaging guinea pig esophagus in vivo, achieving image quality comparable to that acquired with a broadband home-built Ti:sapphire laser.

Optical clearing for luminal organ imaging with ultrahigh-resolution optical coherence tomography

Abstract. The imaging depth of optical coherence tomography (OCT) in highly scattering biological tissues (such as luminal organs) is limited, particularly for OCT operating at shorter wavelength regions (such as around 800 nm). For the first time, the optical clearing effect of the mixture of liquid paraffin and glycerol on luminal organs was explored with ultrahigh-resolution spectral domain OCT at 800 nm. Ex vivo studies were performed on pig esophagus and bronchus, and guinea pig esophagus with different volume ratios of the mixture. We found that the mixture of 40% liquid paraffin had the best optical clearing effect on esophageal tissues with a short effective time of ∼10  min, which means the clearing effect occurs about 10 min after the application of the clearing agent. In contrast, no obvious optical clearing effect was identified on bronchus tissues.

Super-achromatic Optical Coherence Tomography Capsule for Ultrahigh-resolution Imaging of Esophagus

Endoscopic optical coherence tomography (OCT) is a noninvasive technology allowing for imaging of tissue microanatomies of luminal organs in real time. Conventional endoscopic OCT operates at 1300 nm wavelength region with a suboptimal axial resolution limited to 8-20 μm. In this paper, we present the first ultrahigh-resolution tethered OCT capsule operating at 800 nm and offering about 3- to 4-fold improvement of axial resolution (plus enhanced imaging contrast). The capsule uses diffractive optics to manage chromatic aberration over a full ~200 nm spectral bandwidth centering around 830 nm, enabling to achieve super-achromaticity and an axial resolution of ~2.6 μm in air. The performance of the OCT capsule is demonstrated by volumetric imaging of swine esophagus ex vivo and sheep esophagus in vivo, where fine anatomic structures including the sub-epithelial layers are clearly identified. The ultrahigh resolution and excellent imaging contrast at 800 nm of the tethered capsule suggest the potential of the technology as an enabling tool for surveillance of early esophageal diseases on awake patients without the need for sedation.

High-speed, ultrahigh-resolution distal scanning OCT endoscopy at 800 nm for in vivo imaging of colon tumorigenesis on murine models

We present the first, most compact, ultrahigh-resolution, high-speed, distal scanning optical coherence tomography (OCT) endoscope operating at 800 nm. Achieving high speed imaging while maintaining an ultrahigh axial resolution is one of the most significant challenges with endoscopic OCT at 800 nm. Maintaining an ultrahigh axial resolution requires preservation of the broad spectral bandwidth of the light source throughout the OCT system. To overcome this critical limitation we implemented a distal scanning endoscope with diffractive optics to minimize loss in spectral throughput. In this paper, we employed a customized miniature 900 µm diameter DC micromotor fitted with a micro reflector to scan the imaging beam. We integrated a customized diffractive microlens into the imaging optics to reduce chromatic focal shift over the broad spectral bandwidth of the Ti:Sapphire laser of an approximately 150 nm 3dB bandwidth, affording a measured axial resolution of 2.4 µm (in air). The imaging capability of this high-speed, ultrahigh-resolution distal scanning endoscope was validated by performing 3D volumetric imaging of mouse colon in vivo at 50 frames-per-second (limited only by the A-scan rate of linear CCD array in the spectral-domain OCT system and sampling requirements). The results demonstrated that fine microstructures of colon could be clearly visualized, including the boundary between the absorptive cell layer and colonic mucosa as well the crypt patterns. Furthermore, this endoscope was employed to visualize morphological changes in an enterotoxigenic Bacteriodes fragilis (ETBF) induced colon tumor model. We present the results of our feasibility studies and suggest the potential of this system for visualizing time dependent morphological changes associated with tumorigenesis on murine models in vivo.

Ultrahigh-resolution tethered OCT endoscopic capsule at 800 nm (Conference Presentation)

We present an ultrahigh-resolution, distal-scanning OCT capsule operating at 800 nm targeted towards gastrointestinal tract imaging. Among many others, one significant challenge in the 800-nm OCT capsule technology is the severe chromatic aberration in the imaging optics of the capsule. By combining commercial miniature lenses and customized diffractive lens, the achromatic focal shift was essentially eliminated (i.e. down to ~1 µm) over a 3dB spectral bandwidth of ~150 nm centered around 825 nm. We have achieved an axial resolution ~2.7 µm. Initial proof-of-concept experiments with ex vivo pig esophagus demonstrated the excellent imaging performance of this 800-nm OCT capsule.

Electro-thermal MEMS fiber scanner for endoscopic optical coherence tomography(Conference Presentation)

This work report an electro-thermal micro-electro-mechanical system (MEMS) fiber scanner for endoscopic optical coherence tomography (OCT) imaging. The electro-thermal MEMS actuator is composed of a micro-platform, a group of bimorph actuators and a substrate. At first, a 40 mm long bare fiber was fixed on the actuator while keeping the distal end tip free. The micro-platform was then, attached with the fiber at 20 mm apart from the fixed end. Electro-thermal bimorph MEMS actuator with large vertical displacement realizes 1-D forward optical scanning up to 3 mm of scanning range with only 5 VACp-p and 2 VDC operation voltages. The electro-thermal MEMS fiber scanner was combined with the high speed FDML-based swept-source OCT (SS-OCT) system and demonstrated its capability of performing cross-sectional imaging. The FDML laser source has a central wavelength of 1310 nm and a full wavelength sweeping range of ~ 150 nm, which provided an axial resolution of ~ 9.3 to 9.5 µm in air. The FDML sweeping frequency was 220 kHz, and the OCT imaging frame rate was synchronized with the resonant frequency of the MEMS fiber scanner (~88 frames per second). Due to the high actuation force of the electro-thermal actuation, proposed MEMS fibers canner can scan the fiber tip to a millimeter range with low actuation voltages and thus may have potential of performing raster scan with non-resonant fiber cantilevers directly.

Broadband rotary joint for high speed ultrahigh resolution endoscopic OCT imaging (Conference Presentation)

Endoscopic OCT is a promising technology enabling noninvasive in vivo imaging of internal organs, such as the gastrointestinal tract and airways. The past few years have witnessed continued efforts to achieve ultrahigh resolution and speed. It is well-known that the axial resolution in OCT imaging has a quadratic dependence on the central wavelength. While conventional OCT endoscopes operate in 1300 nm wavelength, the second-generation endoscopes are designed for operation around 800 nm where turn-key, broadband sources are becoming readily available. Traditionally 1300 nm OCT endoscopes are scanned at the proximal end, and a broadband fiber-optic rotary joint as a key component in scanning endoscopic OCT is commercially available. Bandwidths in commercial 800 nm rotary joints are unfortunately compromised due to severe chromatic aberration, which limits the resolution afforded by the broadband light source. In the past we remedied this limitation by using a home-made capillary-tube-based rotary joint where the maximum reliable speed is ~10 revolutions/second. In this submission we report our second-generation, home-built high-speed and broadband rotary joint for 800 nm wavelength, which uses achromatic doublets in order achieve broadband achromatic operation. The measured one-way throughput of the rotary joint is >67 % while the fluctuation of the double-pass coupling efficiency during 360° rotation is less than +/-5 % at a speed of 70 revolutions/second. We demonstrate the operation of this rotary joint in conjunction with our ultrahigh-resolution (2.4 µm in air) diffractive catheter by three-dimensional full-circumferential endoscopic imaging of guinea pig esophagus at 70 frames per second in vivo.

Multi-modal endoscopy: OCT and fluorescence

Optical coherence tomography (OCT) is a biomedical imaging modality based on the technique of low-coherence interferometry. Although it is usually analogous to ultrasound imaging, the millimeter range echo time delay is too small to be directly measured due to the celerity of light. One uses a broadband light source and the interference between two arms in a low-coherence interferometer, which provides a coherence gate and depth-resolved information. The depth (axial) resolution is inversely proportional to the spectral bandwidth of the light source and ranges from 1 to 15 mm in most OCT systems. The reference arm in an OCT system also provides heterodyne (or homodyne) gain to the weak backscattered light from biological sample, offering OCT the most significant detection sensitivity (i.e., 100–120 dB). The imaging penetration depth of OCT can often reach 2–3 mm in most highly scattering biological tissues when using a near-infrared (NIR) wavelength from 800 to 1,300 nm. In short, OCT fills a gap between the scales of high-frequency ultrasound and confocal microscopy, providing better resolution than the former as well as deeper penetration than the latter. Recent development in Fourier-domain optical coherence tomography (FD-OCT) technology has significantly improved the imaging speed. It has been reported that an