Fiber-optic polarization diversity detection for rotary probe optical coherence tomography

Multipath artifacts enable angular contrast in multimodal endoscopic optical coherence tomography

Multipath artifacts are inherent to double-clad fiber based optical coherence tomography (OCT), appearing as ghost images blurred in the A-line direction. They result from the excitation of higher-order inner-cladding modes in the OCT sample arm which cross-couple into the fundamental mode at discontinuities and thus are detected in single-mode fiber-based interferometers. Historically, multipath artifacts have been regarded as a drawback in single fiber endoscopic multimodal OCT systems as they degrade OCT quality. In this work, we reveal that multipath artifacts can be projected into high-quality two-dimensional en face images which encode high angle backscattering features. Using a combination of experiment and simulation, we characterize the coupling of Mie-range scatterers into the fundamental image (LP01 mode) and higher-order image (multipath artifact). This is validated experimentally through imaging of microspheres with an endoscopic multimodal OCT system. The angular dependence of the fundamental image and higher order image generated by the multipath artifact lays the basis for multipath contrast, a ratiometric measurement of differential coupling which provides information regarding the angular diversity of a sample. Multipath contrast images can be generated from OCT data where multipath artifacts are present, meaning that a wealth of clinical data can be retrospectively examined.

Novel oviduct endoscope combining optical coherence tomography with intratubal ultrasonography for fallopian tube exploration: An in vivo rabbit pilot study

BACKGROUND AND STUDY AIM

Currently, several limitations exist in the examination of the oviduct. In this study, the usefulness and feasibility of a novel ultrafine dual-modality oviduct endoscopy device for in vivo assessment of the oviduct were evaluated.

METHODS

Five Japanese white rabbits were selected to undergo oviduct probing using a combination of optical coherence tomography (OCT) and intratubal ultrasonography. The feasibility of the procedure was evaluated through 152 pairs of clear, clinically interpretable images obtained using spiral scanning via the pull-back method. OCT images were compared with the oviduct histopathology sections.

RESULTS

Visualization of the oviduct using both OCT and ultrasound revealed a differentiated three-layer tissue; however, ultrasound showed a poorer clarity than OCT. By comparing OCT images with the histological morphology of the oviduct, the inner low-reflective layer of the oviduct corresponds to the mucosal layer, the middle high-reflective layer corresponds to the fibrous muscle layer, and the outer low-reflective layer corresponds to the connective tissue layer. Postoperatively, the general condition of the animals was good.

CONCLUSION

This study demonstrated the feasibility and potential clinical value of the novel ultrafine dual-modality oviduct endoscope. Dual-modality imaging of OCT and intratubal ultrasonography can provide clearer microstructure of the oviduct wall.

Polarization-insensitive optical coherence tomography based on partly depolarized light

Polarization-related image artifacts are frequently observed in optical coherence tomography (OCT) data. As most modern OCT layouts rely on polarized light sources, only the co-polarized component of the light scattered from within a sample can be detected after interference with the reference beam. Cross-polarized sample light does not interfere with the reference beam and thus produces artifacts ranging from a reduction to the full absence of OCT signals. Here we present a simple yet effective technique to prevent polarization artifacts. By partly depolarizing the light source at the interferometer entrance, we achieve OCT signals regardless of the sample polarization state. We demonstrate the performance of our approach in a defined retarder as well as in birefringent dura mater tissue. This simple and cost-effective technique can be applied to obviate cross-polarization artifacts in virtually any OCT layout.

Birefringence-derived scleral artifacts in optical coherence tomography images of eyes with pathologic myopia

We investigated birefringence-derived scleral artifacts in optical coherence tomography (OCT) images of eyes with pathologic myopia. This study included 76 eyes of 42 patients with pathologic myopia. Five sets of OCT B-scan images of the macula were obtained using commercial swept-source OCT. A dataset of prototype swept-source polarization-diversity OCT images was used to identify polarization-dependent OCT images (i.e., complex averaging of OCT signals from two polarization channels) and polarization-independent OCT images (i.e., intensity averaging of two OCT signals). Polarization-dependent OCT images and commercial OCT images were assessed for the presence of birefringence-derived artifacts by comparison with polarization-independent OCT images. Both polarization-dependent OCT images and commercial OCT images contained scleral vessel artifacts. Scleral vessel artifacts were present in 46 of 76 eyes (60.5%) imaged by polarization-dependent OCT and 17 of 76 eyes (22.4%) imaged by commercial OCT. The proportion of images that showed scleral vessel artifacts was significantly greater among polarization-dependent OCT images than among commercial OCT images (P < 0.001). Additionally, polarization-dependent OCT images showed low-intensity band artifacts. This study demonstrated the existence of birefringence-derived scleral artifacts in commercial OCT images and indicated that polarization-diversity OCT is an effective tool to evaluate the presence of these artifacts.

Novel biliopancreatic duct endoscope combining optical coherence tomography with intraductal US for exploring the bile duct: a diagnostic study in a porcine model

BACKGROUND AND AIMS

Existing biliopancreatic duct endoscopy is deficient in the examination of early biliary and pancreatic tumors. This study aimed to evaluate the usefulness and feasibility of a novel ultrafine, separable, biliopancreatic duct endoscopy device with dual modalities of intraductal US (IDUS) and optical coherence tomography (OCT) for the in vivo assessment of the biliopancreatic duct system during ERCP.

METHODS

Five Bama miniature pigs were selected to probe their common bile duct and branches by using this novel equipment during ERCP. The feasibility of the procedure was evaluated by clear, clinically interpretable images obtained by using spiral scanning with the pull-back method. The clinical usefulness of the novel product was evaluated by postoperative choledochoscopy and assessment of the animal's general condition.

RESULTS

One hundred forty-one pairs of images from 5 Bama miniature pigs were acquired. Visualization of the bile duct using both OCT and IDUS was characterized by a differentiated 3-layer architecture, whereas IDUS had poor clarity when compared with OCT. Postoperative choledochoscopy showed no local lesion in the bile duct wall, and the general condition of animals was normal.

CONCLUSIONS

This prospective evaluation indicated the feasibility and potential clinical value of the novel, ultrafine, separable biliopancreatic duct endoscopy device. The fusion of the 2 imaging modalities can shed light on the early diagnosis of biliary and pancreatic tumors. Further studies will be carried out to establish diagnosis criteria with the dual-modality imaging using an animal pathologic model and a human clinical study.

Dual-modality optical coherence tomography and frequency-domain fluorescence lifetime imaging microscope system for intravascular imaging

SIGNIFICANCE

Detailed biochemical and morphological imaging of the plaque burdened coronary arteries holds the promise of improved understanding of atherosclerosis plaque development, ultimately leading to better diagnostics and therapies.

AIM

Development of a dual-modality intravascular catheter supporting swept-source optical coherence tomography (OCT) and frequency-domain fluorescence lifetime imaging (FD-FLIM) of endogenous fluorophores with UV excitation.

APPROACH

We instituted a refined approach to endoscope development that combines simulation in a commercial ray tracing program, fabrication, and a measurement method for optimizing ball-lens performance. With this approach, we designed and developed a dual-modality catheter endoscope based on a double-clad fiber supporting OCT through the core and fluorescence collection through the first cladding. We varied the relative percent of UV excitation launched into the core and first cladding to explore the potential resolution improvement for FD-FLIM. The developed catheter endoscope was optically characterized, including measurement of spatial resolution and fluorescent lifetimes of standard fluorophores. Finally, the system was demonstrated on fresh ex vivo human coronary arteries.

RESULTS

The developed endoscope was shown to have optical performance similar to predictions derived from the simulation approach. The FLIM resolution can be improved by over a factor of 4 by primarily illuminating through the core rather than the first cladding. However, time-dependent solarization losses need to be considered when choosing the relative percentage. We ultimately chose to illuminate with 7% of the power transmitting through the core. The resulting catheter endoscope had 40-μm lateral resolution for OCT and <100  μm lateral resolution for FD-FLIM. Images of ex vivo coronary arteries are consistent with expectations based on histopathology.

CONCLUSIONS

The results demonstrate that our approach for endoscope simulation produces reliable predictions of endoscope performance. Simulation results guided our development of a multimodal OCT/FD-FLIM catheter imaging system for investigating atherosclerosis in coronary arteries.

Steerable OCT catheter for real-time assistance during teleoperated endoscopic treatment of colorectal cancer

When detected early, colorectal cancer can be treated with minimally invasive flexible endoscopy. However, since only specialized experts can delineate margins and perform endoscopic resections of lesions, patients still often undergo colon resections. To better assist in the performance of surgical tasks, a robotized flexible interventional endoscope was previously developed, having two additional side channels for surgical instrument. We propose to enhance the imaging capabilities of this device by combining it with optical coherence tomography (OCT). For this purpose, we have developed a new steerable OCT instrument with an outer diameter of 3.5 mm. The steerable instrument is terminated with a 2 cm long transparent sheath to allow three-dimensional OCT imaging using a side-focusing optical probe with two external scanning actuators. The instrument is connected to an OCT imaging system built around the OCT Axsun engine, with a 1310 nm center wavelength swept source laser and 100 kHz A-line rate. Once inserted in one of the side channels of the robotized endoscope, bending, rotation and translation of the steerable OCT instrument can be controlled by a physician using a joystick. Ex vivo and in vivo tests show that the novel, steerable and teleoperated OCT device enhances dexterity, allowing for inspection of the surgical field without the need for changing the position of the main endoscope.

Polarization management to mitigate misalignment-induced fringe fading in fiber-based optical coherence tomography

In fiber-based optical coherence tomography (OCT), the interference fringes suffer from the fading effect due to misalignment of the light polarization states between the reference and sample arms, resulting in sensitivity degradation and image intensity variation. We theoretically and experimentally analyzed the relation between the misalignment and the fading coefficient. Assuming that the variation of the light polarization in single-mode fiber (SMF) was a random process, we statistically quantified the fading effect. Furthermore, in OCT configuration based on the Michelson interferometer, we reported an interesting observation that the polarization states of light traveling a round-trip in SMF are not evenly distributed on the Poincare sphere. Based on this observation, we demonstrated the existence of an optimal output polarization state of the reference arm to mitigate the fading effect. We demonstrated that in an optimal setup, the statistical average signal-to-noise ratio could be 3.5 dB higher than a setup without proper polarization management.

Silicon photonic integrated circuit swept-source optical coherence tomography receiver with dual polarization, dual balanced, in-phase and quadrature detection

Optical coherence tomography (OCT) is a widely used three-dimensional (3D) optical imaging method with many biomedical and non-medical applications. Miniaturization, cost reduction, and increased functionality of OCT systems will be critical for future emerging clinical applications. We present a silicon photonic integrated circuit swept-source OCT (SS-OCT) coherent receiver with dual polarization, dual balanced, in-phase and quadrature (IQ) detection. We demonstrate multiple functional capabilities of IQ polarization resolved detection including: complex-conjugate suppressed full-range OCT, polarization diversity detection, and polarization-sensitive OCT. To our knowledge, this is the first demonstration of a silicon photonic integrated receiver for OCT. The integrated coherent receiver provides a miniaturized, low-cost solution for SS-OCT, and is also a key step towards a fully integrated high speed SS-OCT system with good performance and multi-functional capabilities. With further performance improvement and cost reduction, photonic integrated technology promises to greatly increase penetration of OCT systems in existing applications and enable new applications.

Wide-field in vivo oral OCT imaging

We have built a polarization-sensitive swept source Optical Coherence Tomography (OCT) instrument capable of wide-field in vivo imaging in the oral cavity. This instrument uses a hand-held side-looking fiber-optic rotary pullback catheter that can cover two dimensional tissue imaging fields approximately 2.5 mm wide by up to 90 mm length in a single image acquisition. The catheter spins at 100 Hz with pullback speeds up to 15 mm/s allowing imaging of areas up to 225 mm(2) field-of-view in seconds. A catheter sheath and two optional catheter sheath holders have been designed to allow imaging at all locations within the oral cavity. Image quality of 2-dimensional image slices through the data can be greatly enhanced by averaging over the orthogonal dimension to reduce speckle. Initial in vivo imaging results reveal a wide-field view of features such as epithelial thickness and continuity of the basement membrane that may be useful in clinic for chair-side management of oral lesions.

Polarization sensitive optical frequency domain imaging system for endobronchial imaging

A polarization sensitive endoscopic optical frequency domain imaging (PS-OFDI) system with a motorized distal scanning catheter is demonstrated. It employs a passive polarization delay unit to multiplex two orthogonal probing polarization states in depth, and a polarization diverse detection unit to detect interference signal in two orthogonal polarization channels. Per depth location four electro-magnetic field components are measured that can be represented in a complex 2x2 field matrix. A Jones matrix of the sample is derived and the sample birefringence is extracted by eigenvalue decomposition. The condition of balanced detection and the polarization mode dispersion are quantified. A complex field averaging method based on the alignment of randomly pointing field phasors is developed to reduce speckle noise. The variation of the polarization states incident on the tissue due to the circular scanning and catheter sheath birefringence is investigated. With this system we demonstrated imaging of ex vivo chicken muscle, in vivo pig lung and ex vivo human lung specimens.