Lensless Endoscope Captures 3D Images

Researchers have developed a new self-calibrating endoscope that produces 3D images of objects smaller than a single cell. (Source: J. Czarske, TU Dresden)

Researchers have developed a new self-calibrating endoscope that produces 3D images of objects smaller than a single cell. Without a lens or any optical, electrical or mechanical components, the tip of the endoscope measures just 200 microns across, about the width of a few human hairs twisted together. As a minimally invasive tool for imaging features inside living tissues, the extremely thin endoscope could enable a variety of research and medical appli­cations.

According to Juergen W. Czarske, TU Dresden, Germany: “The lensless fiber endoscope is approxi­mately the size of a needle, allowing it to have minimally invasive access and high-contrast imaging as well as stimulation with a robust cali­bration against bending or twisting of the fiber.” The endoscope is likely to be especially useful for opto­genetics – research approaches that use light to stimulate cellular activity. It also could prove useful for monitoring cells and tissues during medical procedures as well as for technical inspections.

Conven­tional endoscopes use cameras and lights to capture images inside the body. In recent years researchers have developed alternative ways to capture images through optical fibers, eliminating the need for bulky cameras and other bulky components, allowing for signi­ficantly thinner endoscopes. Despite their promise, however, these technologies suffer from limi­tations such as an inability to tolerate temperature fluc­tuations or bending and twisting of the fiber. A major hurdle to making these technologies practical is that they require complicated cali­bration processes, in many cases while the fiber is collecting images. To address this, the researchers added a thin glass plate, just 150 microns thick, to the tip of a coherent fiber bundle, a type of optical fiber that is commonly used in endoscopy appli­cations. The coherent fiber bundle used in the experiment was about 350 microns wide and consisted of 10,000 cores.

When the central fiber core is illu­minated, it emits a beam that is reflected back into the fiber bundle and serves as a virtual guide star for measuring how the light is being transmitted, known as the optical transfer function. The optical transfer function provides crucial data the system uses to calibrate itself on the fly. A key component of the new setup is a spatial light modulator, which is used to mani­pulate the direction of the light and enable remote focusing. The spatial light modulator compensates the optical transfer function and images onto the fiber bundle. The back-reflected light from the fiber bundle is captured on the camera and super­posed with a reference wave to measure the light’s phase.

The position of the virtual guide star determines the instrument’s focus, with a minimal focus diameter of approxi­mately one micron. The researchers used an adaptive lens and a 2D galvo­meter mirror to shift the focus and enable scanning at different depths. The team tested their device by using it to image a 3D specimen under a 140-micron thick cover slip. Scanning the image plane in 13 steps over 400 microns with an image rate of 4 cycles per second, the device success­fully imaged particles at the top and bottom of the 3D specimen. However, its focus deterio­rated as the galvo­meter mirror’s angle increased. The researchers suggest future work could address this limitation. In addition, using a galvometer scanner with a higher frame rate could allow faster image acqui­sition.

“The novel approach enables both real-time calibration and imaging with minimal invasive­ness, important for in-situ 3D imaging, lab-on-a-chip-based mechanical cell mani­pulation, deep tissue in vivo opto­genetics, and key-hole technical inspec­tions,” said Czarske. (Source: OSA)

Reference: J. W. Czarske et al.: Fast 3D Imaging with Lensless Holographic Endoscopy Employing Coherent Fiber Bundles, Session FM3F, Frontiers in Optics / Laser Science 2019, Washington D. C.

Link: Measurement and Sensor System, Technical University Dresden, Dresden, Germany

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