High-res imaging with a flexible and ultrathin endoscope

New improvements in endoscopy technology, developed at South Korea’s Institute for Basic Science (IBS), allow for microscopic imaging of tissues with minimal complications.

An endoscope is an imaging device consisting of a camera and a light guide attached to a long flexible tube. It is particularly useful for acquiring images of the inside of a human body; for example, stomach and colon endoscopy are widely used for the early detection and diagnosis of diseases such as ulcers and cancers.

In general, an endoscope is manufactured by attaching a camera sensor to the end of a probe or using an optical fibre, which allows for information to be transmitted using light. In the case of an endoscope that uses a camera sensor, the thickness of the probe increases, which makes the endoscopy rather invasive. In the case of an endoscope using an optical fibre bundle, it can be manufactured in a thinner form factor, which minimises invasiveness and results in less discomfort to the patients.

The downside is that in a conventional fibre-bundle endoscope, it is difficult to perform high-resolution imaging, because the resolution of the obtained image is limited by the size of the individual fibre cores. Much of the image information is also lost due to reflection from the probe tip. Furthermore, in fibre endoscopy, it is often necessary to label the target with fluorescence, especially in biological samples with low reflectivity, due to strong back-reflection noise generated from the tip of the thin probe.

Researchers from IBS’s Center for Molecular Spectroscopy and Dynamics (CMSD) have now developed a high-resolution holographic endoscope system overcoming the previous limitation of fibre-optic endoscopy, and reconstructed high-resolution images, without attaching a lens or any equipment to the distal end of the fibre bundle. This feat was accomplished by measuring the holographic images of the light waves that are reflected from the object and captured by the fibre bundle. The breakthrough has been published in the journal Nature Communications.

The researchers first illuminated an object by focusing light onto a single core of a fibre bundle and measured holographic images that were reflected from the object at a certain distance from the optical fibre. In the process of analysing the holographic images, it was possible to reconstruct the object image with a microscopic resolution by correcting the phase retardation that occurs by each fibre core. Specifically, a unique coherent image optimisation algorithm was developed to eliminate fibre-induced phase retardations in both the illumination and detection pathways and reconstruct an object image with a microscopic resolution.

Since the developed endoscope does not attach any equipment to the end of the optical fibre, the diameter of the endoscope probe is 350 μm, which is thinner than the needle used for hypodermic injection. Using this approach, researchers were able to obtain high-resolution images with a spatial resolution of 850 nm, which is far smaller than the core size of the optical fibre bundle.

The researchers went on to test the new Fourier holographic endoscopy system to image the villi structure of rodents. It was possible to acquire a high contrast image by effectively removing the back-reflection noise of the probe, even in biological samples with very low reflectivity, such as rat villi. In addition, post-processing of the measured holographic information made it possible to reconstruct multi-depth 3D images from a single data set with a depth resolution of 14 μm.

It is believed that the practical application of this new endoscope will greatly improve the way we can image the internal structures of our body in a minimally invasive manner, with little to no discomfort for patients. It will also open the possibility of directly observing cavities as small as microvessels and the smallest airways in the lungs, which was impossible with pre-existing technologies. Beyond the medical field, the researchers suggested their new endoscope could potentially be useful for industrial inspections of semiconductors and microprocessors.

Image caption: Microscopic imaging of villi in a rat intestine, acquired using the 350 μm-diameter fibre bundle. Image shows a reconstructed image of two villi by stitching multiple images taken over a wide region of interest. Scale bar: 100 μm.

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