Many medical scenarios require observation equipment to have the characteristics of long working distance, high resolution requirements and small probe diameter. At this time, fiber optic microendoscopes are indispensable. Some things that could not be seen clearly with the technical means of 20 years ago can now be fully observed and described with the help of the basic medical diagnostic equipment of the microendoscope. As an increasingly important imaging tool, endoscopy has some limitations.
Recent advances in microendoscopy research are reported by researchers from ICTER, the Technical University of Denmark, the University of Western Australia and the University of Surrey. The results show that endoscopic imaging probes, especially side-looking imaging probes, combined with GRIN fibers and spherical lenses, can achieve high-performance imaging across the entire range of numerical apertures. This advance opens the way to a wider range of imaging applications and enables endoscopic imaging probes to perform even comparable to commonly used single-focus element probes.
Paper Screenshot | Reference [2]
What is Microendoscopy?
Microendoscopes, also known as miniature fiber optic probes, are capable of reaching deep inside specimens or inside patients to image tissue microstructure. The most promising of these is endoscopic optical coherence tomography (OCT), which is suitable for volumetric imaging of external tissues and the interior of organs such as the upper respiratory tract, gastrointestinal tract, or pulmonary tubules.
Fiber optic probes can be roughly divided into three categories based on resolution. The low-resolution Gaussian beam with a focused spot size in the range of 30-100 μm has a deep imaging depth, and the distance from the probe surface can reach more than 15 mm, and is mostly used to study large hollow organs such as the upper respiratory tract; while the medium resolution of 10-30 μm Beams are widely used in esophagus, small airways, blood vessels, bladder, ovaries, or ear canal imaging; however, it is difficult to obtain beams with a resolution higher than 10 μm, and its potential applications are mainly animal model research.
When developing probes, researchers must make trade-offs between design parameters and imaging performance. High-resolution optical systems with large numerical apertures (NA) often require short working distances (WD), and it is difficult to obtain high resolution and longer working distances if the probe diameter is reduced. This is especially critical for side-looking probes – which require a longer minimum working distance than front-looking probes. If the probe is wrapped in a conduit, the minimum working distance increases, limiting the minimum resolution and probe diameter design.
Designers minimize probe diameter to reduce sample disturbance and improve patient comfort. Smaller probes mean more flexible catheters and better patient compliance with the endoscope during testing. Therefore, one of the best solutions is to use a monolithic fiber optic probe whose diameter is limited by the thickness of the fiber. Thanks to the fiber-optic welding technique, the probe is easy to manufacture and can avoid the tedious process of aligning and connecting micro-optical components.
How can we improve detectors?
Currently the most popular fiber optic imaging probe designs are based on two focusing elements: GRIN fiber probes (GFP—GRIN fiber probes) and ball lens probes (ball lens probes). Researchers led by Dr. Karnowski have found that using both a GRIN and a spherical lens GRIN-Ball Lens Probe (GBLP) with two focusing elements can significantly improve the performance of a monolithic fiber optic probe. Their first modeling results have been presented at conferences in 2018 and 2019.
Comparing GBP probes with the most commonly used GFP and BLP probes, GBP probes especially have performance advantages in applications that require longer working distances, better resolution, and smaller size. In order to visually visualize the probe performance, the researchers introduced a new way to comprehensively present the simulation results. Through the analysis of the effect of GRIN fiber length and spherical lens size, two interesting conclusions were drawn: for the best results, The range of GRIN fiber length can be kept within 0.25 ~ 0.4 pitch length; even for GBLP probes with high numerical aperture, the gain in working distance is not significant. But the authors also show that GBLP achieves equal or better results in terms of working distance if the observation diameter is doubled. In addition, the new GBLP probe has higher resolution compared to the BLP probe.
Schematic of monolithic design of common side-looking optical probe and hybrid GRIN fiber and ball lens design | Reference [2]
In the conclusion of their paper, they also wrote: “We have demonstrated that the GBLP probe has a significant increase in working distance and side-view imaging, while also greatly reducing the influence of the refractive index of the probe environment. And compared with BLP or GFP probes Compared with GBLP, the size of GBLP is significantly smaller. These advantages make the GBLP probe a very promising imaging tool in biological and medical research.”
references
[1] https://ift.tt/fdasbP1
[2] https://ift.tt/lrjqLD4
Compilation: Oasis
Editor: Jin Xiaoming
Typesetting: Yin Ningliu
Title image source: Wikimedia Commons, melvil / CC BY-SA 4.0 (https://ift.tt/AtRiyL4)
research team
First/corresponding author Karol Karnowsk
Research group homepage https://ift.tt/t9pBEQg
Paper information
Publish journal IEEE Photonics Journal
Release time August 31, 2022
Paper title Superior Imaging Performance of All-Fiber, Two-Focusing-Element Microendoscopes
(DOI: https://ift.tt/NefPQxX)
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