Modern medical diagnosis relies on microendoscopes to observe things we couldn’t even explain two decades ago. ICTER scientists improve probe technology.
However, fibre optic microendoscopes have physical constraints. Applications requiring a long working distance, excellent resolution, and a small probe diameter necessitate them. “Superior imaging performance of all-fiber, two-focusing-element microendoscopes” by Dr. Karol Karnowski of ICTER, Dr. Gavrielle Untracht of the Technical University of Denmark (DTU), Dr. Michael Hackmann of the University of Western Australia (UWA), Onur Cetinkaya of ICTER, and Prof. David Sampson of the University of Surrey illuminates modern microendoscopes. The authors began their research in the same UWA research group.
In it, the researchers showed that endoscopic imaging probes, particularly those for side viewing, combining fiber-optic (GRIN) and spherical lenses, perform well across all numerical apertures and enable a wider range of imaging applications. The publication compares endoscopic imaging probes to single-focusing element probes.
Micro-endoscopes, or miniature fiber-optic probes, image deep tissue microstructures. Endoscopic OCT is promising. It can image exterior tissues and organ interiors volumetrically (e.g., the upper respiratory tract, gastrointestinal tract, or lung tubules).
Three fibre optic probe ranges exist. Large, hollow organs above the upper respiratory tract require the largest imaging depth ranges (up to 15 mm or more from the probe surface), which may usually be done with low-resolution Gaussian beams (30-100 μm spot size in focus). Imaging the oesophagus, smaller airways, blood arteries, bladder, ovaries, and ear canal is easier at 10-30 μm. For animal model investigations, beams with resolutions greater than 10 μm are difficult to acquire.
Design parameter trade-offs affect imaging performance while constructing a probe. High-resolution optics have a shorter working distance (WD). As probe diameter decreases, resolution and operating distance become harder to attain. Side-viewing probes have a longer minimum operating distance than forward imaging probes, which can be problematic. Consider a catheter or needle-encased probe. This increases the minimum working distance, which often limits resolution or probe diameter.
Engineers typically minimise probe diameter to reduce sample disturbance and patient comfort. A flexible catheter and a smaller probe improve patient test tolerance. Thus, monolithic fibre optic probes, whose width is restricted by optical fibre thickness, are a good option. Fiber-optic welding technology eliminates the need to align and bond micro-optical components, making such probes easy to make.
Fiber-optic imaging probes based on GRIN fibre probes (GFP) and ball lens probes are the most used (BLP – ball lens probes). GRIN probes are easy to produce and retain their refractive power when the surrounding medium’s refractive index is near to the fiber’s. Commercial GRIN fibres limit designs. Small-core GRIN fibres are hard to resolve.
The fibre and catheter’s curved surface distorts imaging for lateral viewing probes. Spherical BLP probes do not have this issue, but to reach GFP-like resolution, they must be larger than the fibre diameter. When working near biological samples, a BLP probe’s focusing strength depends on the surrounding medium’s refractive index.
Multiple light focusing devices, including long-working-distance lenses, can improve probe performance. Multiple light-focusing elements improve imaging outcomes, according to studies. Multiple-focusing probes can produce improved resolution with smaller diameters and longer working distances without compromising resolution.
GRIN-ball-lens probes (GBLP), pioneered by Dr. Karnowski, improve monolithic fibre optic probe performance. 2018 and 2019 conferences showcased their first modelling results. GBP probes outperformed the most commonly used GFP and BLP probes, especially for applications needing longer working distances, greater resolution, and small size.
The researchers developed a novel technique to thoroughly convey simulation findings for intuitive probe performance visualisation, especially when more than two variables are involved. The effect of GRIN fibre length and spherical lens size led to two interesting conclusions: for optimal results, the range of GRIN fibre length can be kept in the field of 0.25-0.4 pitch length (so-called pitch length); even though the working distance (WD) gain is not so significant for GBLP probes with high numerical aperture, the authors showed that a search with twice the diameter achieves the same or better working distance performance. The innovative GBLP probes have higher resolution than BLP probes.