Tiny endoscope images objects smaller than a single cell

Researchers from Technische Universität Dresden have developed a 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 µm across — about the width of a few human hairs twisted together.

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 fibres, eliminating the need for bulky components and allowing for significantly thinner endoscopes. However, these technologies suffer from limitations such as an inability to tolerate temperature fluctuations or bending and twisting of the fibre.

A major hurdle to making these technologies practical is that they require complicated calibration processes, in many cases while the fibre is collecting images. To address this, the researchers added a thin glass plate, just 150 µm thick, to the tip of a coherent fibre bundle — a type of optical fibre that is commonly used in endoscopy applications. The coherent fibre bundle used in the experiment was about 350 µm wide and consisted of 10,000 cores.

When the central fibre core is illuminated, it emits a beam that is reflected back into the fibre 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.

“The lensless fibre 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 fibre,” said lead author Juergen W Czarske, Director and C4-Professor at TU Dresden.

A key component of the set-up 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 fibre bundle. The back-reflected light from the fibre 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 meanwhile determines the instrument’s focus, with a minimal focus diameter of approximately 1 µm. 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 µm-thick cover slip. Scanning the image plane in 13 steps over 400 µm 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.

As a minimally invasive tool for imaging features inside living tissues, the extremely thin endoscope could enable a variety of research and medical applications. It is likely to be especially useful for optogenetics — research approaches that use light to stimulate cellular activity — and could also prove useful for monitoring cells and tissues during medical procedures as well as for technical inspections.

“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 keyhole technical inspections,” Czarske said.

Image caption: The self-calibrating endoscope produces 3D images of objects smaller than a single cell. Image credit: J Czarske, TU Dresden, Germany.

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