Jiefeng Xi

Scanning all-fiber-optic endomicroscopy system for 3D nonlinear optical imaging of biological tissues.

An extremely compact all-fiber-optic scanning endomicroscopy system was developed for two-photon fluorescence (TPF) and second harmonic generation (SHG) imaging of biological samples. A conventional double-clad fiber (DCF) was employed in the endomicroscope for single-mode femtosecond pulse delivery, multimode nonlinear optical signals collection and fast two-dimensional scanning. A single photonic bandgap fiber (PBF) with negative group velocity dispersion at two-photon excitation wavelength (i.e. approximately 810 nm) was used for pulse prechirping in replacement of a bulky grating/lens-based pulse stretcher. The combined use of DCF and PBF in the endomicroscopy system made the endomicroscope basically a plug-and-play unit. The excellent imaging ability of the extremely compact all-fiber-optic nonlinear optical endomicroscopy system was demonstrated by SHG imaging of rat tail tendon and depth-resolved TPF imaging of epithelial tissues stained with acridine orange. The preliminary results suggested the promising potential of this extremely compact all-fiber-optic endomicroscopy system for real-time assessment of both epithelial and stromal structures in luminal organs.

A compact fiber-optic SHG scanning endomicroscope and its application to visualize cervical remodeling during pregnancy

We report the development of an all-fiber-optic scanning endomicroscope capable of high-resolution second harmonic generation (SHG) imaging of biological tissues and demonstrate its utility for monitoring the remodeling of cervical collagen during gestation in mice. The endomicroscope has an overall 2.0 mm diameter and consists of a single customized double-clad fiber, a compact rapid two-dimensional beam scanner, and a miniature compound objective lens for excitation beam delivery, scanning, focusing, and efficient SHG signal collection. Endomicroscopic SHG images of murine cervical tissue sections at different stages of normal pregnancy reveal progressive, quantifiable changes in cervical collagen morphology with resolution similar to that of bench-top SHG microscopy. SHG endomicroscopic imaging of ex vivo murine and human cervical tissues through intact epithelium has also been performed. Our findings demonstrate the feasibility of SHG endomicroscopy technology for staging normal pregnancy, and suggest its potential application as a minimally invasive tool for clinical assessment of abnormal cervical remodeling associated with preterm birth.

Scanning fiber-optic nonlinear endomicroscopy with miniature aspherical compound lens and multimode fiber collector

A flexible scanning fiber-optic endomicroscope using a miniature compound lens and a multimode-fiber (MMF) collector was developed for two-photon fluorescence (TPF) and second-harmonic generation (SHG) imaging. The compound lens consisted of a pair of aspherical lenses and exhibited reduced chromatic aberration compared with gradient-index lenses, thus increasing the TPF/SHG collection efficiency. The introduction of a short MMF collector at the distal end of the double-clad fiber of the endomicroscope further mitigated the adverse influence of chromatic aberration of the lens and enhanced the TPF/SHG collection efficiency. Both ray-tracing simulations and experiments on TPF imaging of fluorescent beads and SHG imaging of rattail tendon demonstrated approximately nine (approximately four) times improved collection efficiency for TPF (SHG) with the new endomicroscope design that utilized a compound lens and an MMF collector.

Detection of human brain cancer infiltration ex vivo and in vivo using quantitative optical coherence tomography

Optical coherence tomography (OCT) can distinguish cancer from noncancer tissue in vivo in rodent models of human brain cancer and ex vivo in fresh human brain cancer specimens with high sensitivity and specificity. Observing cancer with OCT The label-free imaging technology optical coherence tomography (OCT) is used routinely in the clinic for detecting abnormalities in certain tissues, such as in the eye. Now, Kut et al. demonstrate that OCT can differentiate different grades of human brain cancer from noncancer in rodents and in patient tissue samples. Fresh human brain cancer and noncancer tissues that had been surgically removed were used to generate “optimal attenuation thresholds” for high- and low-grade cancers. These thresholds were then used by blinded pathologists to diagnose cancer in a separate set of tissues, showing 100% specificity and 80% sensitivity for high-grade brain cancer (and 80% specificity and 100% sensitivity for low-grade brain cancer). To demonstrate the potential of this technology during neurosurgery, mice had their human tumors removed with the guidance of OCT “maps” that displayed color-coded optical properties of the tissue. Thus, surgeons were able to remove only the cancerous areas, as confirmed by histology. Such real-time, intraoperative imaging to ensure total cancer resection will markedly improve patient survival. More complete brain cancer resection can prolong survival and delay recurrence. However, it is challenging to distinguish cancer from noncancer tissues intraoperatively, especially at the transitional, infiltrative zones. This is especially critical in eloquent regions (for example, speech and motor areas). This study tested the feasibility of label-free, quantitative optical coherence tomography (OCT) for differentiating cancer from noncancer in human brain tissues. Fresh ex vivo human brain tissues were obtained from 32 patients with grade II to IV brain cancer and 5 patients with noncancer brain pathologies. On the basis of volumetric OCT imaging data, pathologically confirmed brain cancer tissues (both high- and low-grade) had significantly lower optical attenuation values at both cancer core and infiltrated zones when compared with noncancer white matter, and OCT achieved high sensitivity and specificity at an attenuation threshold of 5.5 mm−1 for brain cancer patients. We also used this attenuation threshold to confirm the intraoperative feasibility of performing in vivo OCT-guided surgery using a murine model harboring human brain cancer. Our OCT system was capable of processing and displaying a color-coded optical property map in real time at a rate of 110 to 215 frames per second, or 1.2 to 2.4 s for an 8- to 16-mm3 tissue volume, thus providing direct visual cues for cancer versus noncancer areas. Our study demonstrates the translational and practical potential of OCT in differentiating cancer from noncancer tissue. Its intraoperative use may facilitate safe and extensive resection of infiltrative brain cancers and consequently lead to improved outcomes when compared with current clinical standards.

Forward-viewing resonant fiber-optic scanning endoscope of appropriate scanning speed for 3D OCT imaging.

A forward-viewing resonant fiber-optic endoscope of a scanning speed appropriate for a high-speed Fourier-domain optical coherence tomography (FD-OCT) system was developed to enable real-time, three-dimensional endoscopic OCT imaging. A new method was explored to conveniently tune the scanning frequency of a resonant fiber-optic scanner, by properly selecting the fiber-optic cantilever length, partially changing the mechanical property of the cantilever, and adding a weight to the cantilever tip. Systematic analyses indicated the resonant scanning frequency can be tuned over two orders of magnitude spanning from ~10Hz to ~kHz. Such a flexible scanning frequency range makes it possible to set an appropriate scanning speed of the endoscope to match the different A-scan rates of a variety of FD-OCT systems. A 2.4-mm diameter, 62.5-Hz scanning endoscope appropriate to work with a 40-kHz swept-source OCT (SS-OCT) system was developed and demonstrated for 3D OCT imaging of biological tissues.

Integrated multimodal endomicroscopy platform for simultaneous en face optical coherence and two-photon fluorescence imaging

We report an all-fiber-optic scanning, multimodal endomicroscope capable of simultaneous optical coherence tomography (OCT) and two-photon fluorescence (TPF) imaging. Both imaging modalities share the same miniature fiber-optic scanning endomicroscope, which consists of a double-clad fiber with a core operating in single mode at both the OCT (1310 nm) and two-photon excitation (1550 nm) wavelengths, a piezoelectric two-dimensional fiber-optic beam scanner, and a miniature aspherical compound lens suitable for simultaneous acquisition of en face OCT and TPF images. A fiber-optic wavelength division multiplexer was employed in the integrated platform to combine the low coherence OCT light source and the femtosecond two-photon excitation laser into the same optical path. Preliminary imaging results of cell cultures and mouse tissue ex vivo demonstrate the feasibility of simultaneous real-time OCT and TPF imaging in a scanning endomicroscopy setting for the first time.

Generic real-time uniform K-space sampling method for high-speed swept-Source optical coherence tomography

We developed a universal, real-time uniform K-space sampling (Rt-UKSS) method for high-speed swept-source optical coherence tomography (SS-OCT). An external clock uniform in K-space was generated. The clock was synchronized with the zero-crossing time of an interferometric calibration signal and used as triggers for a high-speed data acquisition system in a point-by-point fashion, hence enabling uniform data sampling in K-space. Different from the numerical calibration algorithm commonly used in an SS-OCT system, the method reported here does not require over-sampling, thus greatly reducing the demand for digitization, data processing and storage speed. The Rt-UKSS method is adaptive and applicable to a generic SS-OCT system of a wide range of A-scan rates without special adjustment. We successfully implemented the Rt-UKSS method in an SS-OCT system based on a Fourier-domain mode-locked laser (FDML) of a 40-kHz scanning rate. Real-time imaging of biological tissues using such a system was demonstrated with a measured axial resolution of 9.3 μm and detection sensitivity greater than 120dB.

High-resolution OCT balloon imaging catheter with astigmatism correction

We report new optics designs for an optical coherence tomography (OCT) balloon imaging catheter to achieve diffraction-limited high resolution at a large working distance and enable the correction of severe astigmatism in the catheter. The designs employed a 1 mm diameter gradient-index lens of a properly chosen pitch number and a glass rod spacer to fully utilize the available NA of the miniature optics. Astigmatism caused by the balloon tubing was analyzed, and a method based on a cylindrical reflector was proposed and demonstrated to compensate the astigmatism. A catheter based on the new designs was successfully developed with a measured diffraction-limited lateral resolution of approximately 21 microm, a working distance of approximately 11-12 mm, and a round-shape beam profile. The performance of the OCT balloon catheter was demonstrated by 3D full-circumferential imaging of a swine esophagus in vivo along with a high-speed, Fourier-domain, mode-locked swept-source OCT system.

Robust High-Resolution Fine OCT Needle for Side-Viewing Interstitial Tissue Imaging

Fine optical coherence tomography (OCT) imaging needles that can be integrated with a standard biopsy needle have been developed with a new optics design to improve the optical quality and mechanical robustness, where a fiber-optic lens (that is spliced to a single-mode fiber) and a microreflector are encased within a microglass tube. The design also minimizes the cylindrical lens effect induced by the glass tube and eases the needle assembly process. Real-time cross-sectional OCT imaging of various tissue samples were performed ex vivo using the miniature-imaging needle along with a 1300-nm swept-source OCT system. The preliminary results demonstrate the improved mechanical and optical performance and suggest the potential of the fine OCT needle for minimally invasive interstitial imaging and image-guided biopsy.

An all-fiber-optic endoscopy platform for simultaneous OCT and fluorescence imaging

We present an all-fiber-optically based endoscope platform for simultaneous optical coherence tomography (OCT) and fluorescence imaging. This design entails the use of double-clad fiber (DCF) in the endoscope for delivery of OCT source and fluorescence excitation light while collecting the backscattered OCT signal through the single-mode core and fluorescence emission through the large inner cladding of the DCF. Circumferential beam scanning was performed by rotating a 45° reflector using a miniature DC motor at the distal end of the endoscope. Additionally, a custom DCF coupler and a wavelength division multiplexer (WDM) were utilized to seamlessly integrate both imaging modalities to achieve an entirely fiber-optically based dual-modality imaging system. We demonstrated simultaneous intraluminal 3D OCT and 2D (surface) fluorescence imaging in ex vivo rabbit esophagus using the dual-modal endomicroscopy system. Structural morphologies (provided by OCT) and fluorophore distribution (provided by the fluorescence module) could be clearly visualized, suggesting the potential of the dual-modality system for future in vivo and clinical applications.