Endoscopic optical coherence tomography with a flexible fiber bundle

Improved resolution in fiber bundle inline holographic microscopy using multiple illumination sources

Recent work has shown that high-quality inline holographic microscopy images can be captured through fiber imaging bundles. Speckle patterns arising from modal interference within the bundle cores can be minimized by use of a partially-coherent optical source such as an LED delivered via a multimode fiber. This allows numerical refocusing of holograms from samples at working distances of up to approximately 1 mm from the fiber bundle before the finite coherence begins to degrade the lateral resolution. However, at short working distances the lateral resolution is limited not by coherence, but by sampling effects due to core-to-core spacing in the bundle. In this article we demonstrate that multiple shifted holograms can be combined to improve the resolution by a factor of two. The shifted holograms can be rapidly acquired by sequentially firing LEDs, which are each coupled to their own, mutually offset, illumination fiber. Following a one-time calibration, resolution-enhanced images are created in real-time at an equivalent net frame rate of up to 7.5 Hz. The resolution improvement is demonstrated quantitatively using a resolution target and qualitatively using mounted biological slides. At longer working distances, beyond 0.6 mm, the improvement is reduced as resolution becomes limited by the source spatial and temporal coherence.

Large-field-of-view optical-resolution optoacoustic microscopy using a stationary silicon-photonics acoustic detector

Significance

Optical-resolution optoacoustic microscopy (OR-OAM) enables label-free imaging of the microvasculature by using optical pulse excitation and acoustic detection, commonly performed by a focused optical beam and an ultrasound transducer. One of the main challenges of OR-OAM is the need to combine the excitation and detection in a coaxial configuration, often leading to a bulky setup that requires physically scanning the ultrasound transducer to achieve a large field of view.

Aim

The aim of this work is to develop an OR-OAM configuration that does not require physically scanning the ultrasound transducer or the acoustic beam path.

Approach

Our OR-OAM system is based on a non-coaxial configuration in which the detection is performed by a silicon-photonics acoustic detector (SPADE) with a semi-isotropic sensitivity. The system is demonstrated in both epi- and trans-illumination configurations, where in both configurations SPADE remains stationary during the imaging procedure and only the optical excitation beam is scanned.

Results

The system is showcased for imaging resolution targets and for the in vivo visualization of the microvasculature in a mouse ear. Optoacoustic imaging with focal spots down to 1.3    μ m , lateral resolution of 4    μ m , and a field of view higher than 4 mm in both lateral dimensions were demonstrated.

Conclusions

We showcase a new OR-OAM design, compatible with epi-illumination configuration. This setup enables relatively large fields of view without scanning the acoustic detector or acoustic beam path. Furthermore, it offers the potential for high-speed imaging within compact, miniature probe and could potentially facilitate the clinical translation of OR-OAM technology.

Real-timing processing of fiber bundle endomicroscopy images in Python using PyFibreBundle

Fiber imaging bundles allow the transfer of optical images from place-to-place along narrow and flexible conduits. Traditionally used extensively in medical endoscopy, bundles are now finding new applications in endoscopic microscopy and other emerging techniques. PyFibreBundle is an open-source Python package for fast processing of images acquired through imaging bundles. This includes detection and removal of the fiber core pattern by filtering or interpolation, and application of background and flat-field corrections. It also allows images to be stitched together to create mosaics and resolution to be improved by combining multiple shifted images. This paper describes the technical implementation of PyFibreBundle and provides example results from three endomicroscopy imaging systems: color transmission, monochrome transmission, and confocal fluorescence. This allows various processing options to be compared quantitatively and qualitatively, and benchmarking demonstrates that PyFibreBundle can achieve state-of-the-art performance in an open-source package. The paper demonstrates core removal by interpolation and mosaicing at over 100 fps, real-time multi-frame resolution enhancement and the first demonstration of real-time endomicroscopy image processing, including core removal, on a Raspberry Pi single board computer. This demonstrates that PyFibreBundle is potentially a valuable tool for the development of low-cost, high-performance fiber bundle imaging systems.

Needle guidance with Doppler-tracked polarization-sensitive optical coherence tomography

Significance

Optical coherence tomography (OCT) can be integrated into needle probes to provide real-time navigational guidance. However, unscanned implementations, which are the simplest to build, often struggle to discriminate the relevant tissues.

Aim

We explore the use of polarization-sensitive (PS) methods as a means to enhance signal interpretability within unscanned coherence tomography probes.

Approach

Broadband light from a laser centered at 1310 nm was sent through a fiber that was embedded into a needle. The polarization signal from OCT fringes was combined with Doppler-based tracking to create visualizations of the birefringence properties of the tissue. Experiments were performed in (i) well-understood structured tissues (salmon and shrimp) and (ii) ex vivo porcine spine. The porcine experiments were selected to illustrate an epidural guidance use case.

Results

In the porcine spine, unscanned and Doppler-tracked PS OCT imaging data successfully identified the skin, subcutaneous tissue, ligament, and epidural spaces during needle insertion.

Conclusions

PS imaging within a needle probe improves signal interpretability relative to structural OCT methods and may advance the clinical utility of unscanned OCT needle probes in a variety of applications.

Large field-of-view short-wave infrared metalens for scanning fiber endoscopy

SIGNIFICANCE

The scanning fiber endoscope (SFE), an ultrasmall optical imaging device with a large field-of-view (FOV) for having a clear forward view into the interior of blood vessels, has great potential in the cardiovascular disease diagnosis and surgery assistance, which is one of the key applications for short-wave infrared biomedical imaging. The state-of-the-art SFE system uses a miniaturized refractive spherical lens doublet for beam projection. A metalens is a promising alternative that can be made much thinner and has fewer off-axis aberrations than its refractive counterpart.

AIM

We demonstrate a transmissive metalens working at 1310 nm for a forward viewing endoscope to achieve a shorter device length and better resolution at large field angles.

APPROACH

We optimize the metalens of the SFE system using Zemax, fabricate it using e-beam lithography, characterize its optical performances, and compare them with the simulations.

RESULTS

The SFE system has a resolution of ∼ 140    μ m at the center of field (imaging distance 15 mm), an FOV of ∼ 70    deg , and a depth-of-focus of ∼ 15    mm , which are comparable with a state-of-the-art refractive lens SFE. The use of the metalens reduces the length of the optical track from 1.2 to 0.86 mm. The resolution of our metalens-based SFE drops by less than a factor of 2 at the edge of the FOV, whereas the refractive lens counterpart has a ∼ 3 times resolution degradation.

CONCLUSIONS

These results show the promise of integrating a metalens into an endoscope for device minimization and optical performance improvement.

Fourier holographic endoscopy for imaging continuously moving objects

Coherent fiber bundles are widely used for endoscopy, but conventional approaches require distal optics to form an object image and acquire pixelated information owing to the geometry of the fiber cores. Recently, holographic recording of a reflection matrix enables a bare fiber bundle to perform pixelation-free microscopic imaging as well as allows a flexible mode operation, because the random core-to-core phase retardations due to any fiber bending and twisting could be removed in situ from the recorded matrix. Despite its flexibility, the method is not suitable for a moving object because the fiber probe should remain stationary during the matrix recording to avoid the alteration of the phase retardations. Here, we acquire a reflection matrix of a Fourier holographic endoscope equipped with a fiber bundle and explore the effect of fiber bending on the recorded matrix. By removing the motion effect, we develop a method that can resolve the perturbation of the reflection matrix caused by a continuously moving fiber bundle. Thus, we demonstrate high-resolution endoscopic imaging through a fiber bundle, even when the fiber probe changes its shape along with the moving objects. The proposed method can be used for minimally invasive monitoring of behaving animals.

Endoscopic image-guided laser treatment system based on fiber bundle laser steering

A miniaturized endoscopic laser system with laser steering has great potential to expand the application of minimally invasive laser treatment for micro-lesions inside narrow organs. The conventional systems require separate optical paths for endoscopic imaging and laser steering, which limits their application inside narrower organs. Herein, we present a novel endoscopic image-guided laser treatment system with a thin tip that can access inside narrow organs. The system uses a single fiber bundle to simultaneously acquire endoscopic images and modulate the laser-irradiated area. The insertion and operation of the system in a narrow space were demonstrated using an artificial vascular model. Repeated laser steering along set targets demonstrated accurate laser irradiation within a root-mean-square error of 28 [Formula: see text]m, and static repeatability such that the laser irradiation position was controlled within a 12 [Formula: see text]m radius of dispersion about the mean trajectory. Unexpected irradiation on the distal irradiated plane due to fiber bundle crosstalk was reduced by selecting the appropriate laser input diameter. The laser steering trajectory spatially controlled the photothermal effects, vaporization, and coagulation of chicken liver tissue. This novel system achieves minimally invasive endoscopic laser treatment with high lesion-selectivity in narrow organs, such as the peripheral lung and coronary arteries.

Depixelation of coherent fiber bundle imaging by fiber-core-targeted scanning

A novel fast proximal scanning method, to the best of our knowledge, termed fiber-core-targeted scanning (FCTS), is proposed for illuminating individual fiber cores sequentially to remove the pixelation effect in fiber bundle (FB) imaging. FCTS is based on a galvanometer scanning system. Through a dynamic control of the scan trajectory and speed using the prior knowledge of fiber core positions, FCTS experimentally verifies a precise sequential delivery of laser pulses into fiber cores at a maximal speed of 45,000 cores per second. By applying FCTS on a FB-based photoacoustic forward-imaging probe, the results demonstrate that FCTS eliminates the pixelation effect and improves the imaging quality.

Inline holographic microscopy through fiber imaging bundles

Fiber imaging bundles are widely used as thin, passive image conduits for miniaturized and endoscopic microscopy, particularly for confocal fluorescence imaging. Holographic microscopy through fiber bundles is more challenging; phase conjugation approaches are complex and require extensive calibration. This paper describes how simple inline holographic microscopy can be performed through an imaging bundle using a partially coherent illumination source from a multimode fiber. The sample is imaged in transmission, with the intensity hologram sampled by the bundle and transmitted to a remote camera. The hologram can then be numerically refocused for volumetric imaging, achieving a resolution of approximately 6 µm over a depth range of 1 mm. The scheme does not require any complex prior calibration and hence is insensitive to bending.

A Silicon Optical Bench-Based Forward-View Two-Axis Scanner for Microendoscopy Applications

Optical microendoscopy enabled by a microelectromechanical system (MEMS) scanning mirror offers great potential for in vivo diagnosis of early cancer inside the human body. However, an additional beam folding mirror is needed for a MEMS mirror to perform forward-view scanning, which drastically increases the diameter of the resultant MEMS endoscopic probe. This paper presents a new monolithic two-axis forward-view optical scanner that is composed of an electrothermally driven MEMS mirror and a beam folding mirror both vertically standing and integrated on a silicon substrate. The mirror plates of the two mirrors are parallel to each other with a small distance of 0.6 mm. The laser beam can be incident first on the MEMS mirror and then on the beam folding mirror, both at 45°. The MEMS scanner has been successfully fabricated. The measured optical scan angles of the MEMS mirror were 10.3° for the x axis and 10.2° for the y axis operated under only 3 V. The measured tip-tilt resonant frequencies of the MEMS mirror were 1590 Hz and 1850 Hz, respectively. With this compact MEMS design, a forward-view scanning endoscopic probe with an outer diameter as small as 2.5 mm can be made, which will enable such imaging probes to enter the subsegmental bronchi of an adult patient.

Detection of laser-induced bulk damage in optical crystals by swept-source optical coherence tomography

An approach to characterize laser-induced bulk damage in optical crystal materials was demonstrated. With a homemade swept-source optical coherence tomography (SS-OCT) system, we obtained three-dimensional images of the bulk damage produced by laser pulses with wavelength of 351 nm, pulse width of 5 ns, beam diameter of 5.5 mm and fluences from 4.56 J/cm² to 9.95 J/cm² in Potassium Dihydrogen Phosphate (KDP) crystal. Algorithms based on three-dimensional OCT images were specially designed to count and locate bulk damage pinpoints in KDP crystal, obtaining their equivalent diameter distribution and pinpoint density caused by different fluences. It is demonstrated that the characteristics of bulk damage detected by SS-OCT are similar to those obtained by available approaches. The rapid three-dimensional imaging by SS-OCT provides a new approach of detecting laser-induced bulk damage.