The new probe design promises many imaging applications in biological and biomedical research

The new probe design promises many imaging applications in biological and biomedical research

Microendoscopes are the cornerstone of modern medical diagnostics - they allow us to see what we couldn't even describe two decades ago. The technology is constantly improving, with ICTER scientists contributing to the development of the probes.

Fiber optic microendoscopes are becoming increasingly important imaging tools, but they have physical limitations. They are essential for applications that require a long working distance, high resolution and a minimum probe diameter. The research work entitled “Superior imaging performance of all-fiber, two-focusing-element microendoscopes” by Dr. Karol Karnowski from ICTER, Dr. Gavrielle Untracht from the Technical University of Denmark (DTU), Dr. Michael Hackmann from the University of Western Australia (UWA), Onur Cetinkaya from ICTER and Prof. David Sampson from the University of Surrey shed new light on modern microendoscopes. It is noteworthy that the research began while the authors were working in the same research group at UWA.

In it, the researchers showed that endoscopic imaging probes, particularly those for so-called side viewing, which combine fiber optic (GRIN) and spherical lenses, offer excellent performance across the entire range of numerical apertures, opening the way to a wider range of imaging applications. In the paper, the performance of endoscopic imaging probes is comparable to commonly used single focusing element probes.

What are microendoscopes?

Miniature fiber optic probes or microendoscopes enable imaging of tissue microstructures deep within the sample or patient. Endoscopic optical coherence tomography (OCT) is particularly promising. It is suitable for volumetric imaging of external tissues and the interior of organs (e.g. the upper respiratory tract, the gastrointestinal tract or the pulmonary tubules).

Three main areas of fiber optic probes can be distinguished. Studies of large hollow organs (e.g. above the upper respiratory tract) require the largest imaging depth ranges (up to 15 mm or more from the probe surface), usually using low-resolution Gaussian beams (spot size in focus in the range of 30-100 μm). The medium resolution range (10-30 μm) is useful for broader applications such as: B. imaging the esophagus, small airways, blood vessels, bladder, ovaries or the ear canal. The main challenge is to obtain beams with a resolution better than 10 μm, which is potentially useful for animal model studies.

When developing a probe, the trade-offs between design parameters and their impact on imaging performance must be considered. Large numerical aperture (high resolution) optical systems tend to have a shorter working distance (WD). Additionally, better resolution and longer working distance are more difficult to achieve when the probe diameter is reduced. This can be particularly problematic for side-viewing probes – a greater minimum working distance is required compared to their forward imaging counterparts. Suppose the probe is enclosed in a catheter or needle. In this case, the required minimum working distance increases - in many cases this is the limiting factor for the minimum achievable resolution or probe diameter.

It is worth noting that engineers are usually interested in minimizing the probe diameter to reduce sample disturbance and patient comfort. A smaller probe means a more flexible catheter and therefore better patient tolerance of the test. Therefore, one of the best solutions is to use monolithic fiber optic probes whose diameter is limited by the thickness of the optical fibers. Such probes are characterized by ease of manufacture thanks to fiber-optic welding technology, which avoids the laborious alignment and connection (usually gluing) of individual micro-optical components.

Different types of microendoscopes

The most popular fiber optic imaging probe designs are based on two types of focusing elements: GRIN fiber probes (GFP - GRIN fiber probes) and ball lens probes (BLP - ball lens probes). GRIN probes are easy to fabricate and their GRIN refractive power is not lost when the refractive index of the surrounding medium is close to that of the fiber used. Commercially available GRIN fibers limit the designs that can be achieved. High resolution is difficult to achieve with GRIN fibers with small core diameters.

For side-view probes, the curved surface of the fiber (and possibly the catheter) introduces distortions that can affect image quality. Spherical BLP probes do not have this problem, but a sphere larger than the fiber diameter is often required to achieve resolution comparable to GFP probes. The focusing performance of a BLP probe depends on the refractive index of the surrounding medium, which is an important point when working in a medium with close or nearby biological samples.

One solution to improve the performance of probes is to use multiple light focusing elements, similar to the design of long working distance lenses. Studies have shown that combining numerous light-focusing elements produces better results for many imaging purposes. Probes with multiple focusing elements can achieve better resolution with smaller diameters while offering greater working distances without sacrificing resolution.

How do we improve the probes?

In their latest work, researchers led by Dr. Karnowski showed that probes with two focusing elements that use both GRIN segments and spherical lenses - so-called GRIN ball lens probes (GBLP) - significantly improve the performance of monolithic fiber optic probes. Their first modeling results were already shown at conferences in 2018 and 2019. GBP probes were compared to the most commonly used GFP and BLP probes and demonstrated performance advantages, particularly for applications requiring longer operating distances, better resolution and small size.

To intuitively visualize probe performance, researchers introduced a novel method for comprehensively displaying simulation results, which is particularly useful when more than two variables are used. The analysis of the effect of GRIN fiber length and spherical lens size led to two interesting conclusions: for optimal results, the range of GRIN fiber length can be kept in the range of 0.25–0.4 pitch length (so-called pitch length); Although the working distance (WD) gain is not as significant for high numerical aperture GBLP probes, the authors showed that the same or better performance in terms of working distance is achieved for a double diameter search. In addition, the novel GBLP probes offer higher resolution compared to BLP probes.

The conclusion of the paper is:

We have demonstrated the potential of GBLP probe design for increased working distance applications, which is important for lateral imaging probes, with a greatly reduced influence of the refractive index of the probe surroundings and a significantly smaller size compared to BLP or GFP probes. These advantages make GBLP probes a tool worth considering for many imaging applications in biological and biomedical research, particularly for projects requiring microendoscopes.

Source:

Institute of Physical Chemistry of the Polish Academy of Sciences

Reference:

Karnowski, K., et al. (2022) Superior imaging performance of full-fiber microendoscopes with two focusing elements. IEEE Photonics Journal. doi.org/10.1109/JPHOT.2022.3203219.

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