by Wayne Gillam, University of Washington - Department of Electrical & Computer Engineering

An illustration of the endoscope lens system designed by UW ECE and Physics Professor Arka Majumdar and his research team. A coin-shaped metalens at the front end of the endoscope directs three cones of red, green, and blue light to a rainbow of focal spots that probe the three-dimensional target (spoked wheel at right). The lens system then directs reflected light through the endoscope (middle) via an optical fiber bundle. A color camera creates the image at the other end of the endoscope (left) with a built-in color filter, shown here as pixelated color cubes. The camera (left) outputs three color images (RGB), corresponding to the three different depths described by the lens system. Credit: Aamod Shanker

The human body contains a vast, complex, and interconnected web of organic tunnels and passageways that weave their way through the cardiovascular, respiratory, and digestive systems. For physicians, reaching into this maze of arteries, bronchial tubes, and gastrointestinal chambers to view and treat diseased or damaged tissue can be—to put it mildly—challenging.

Many of these conduits are long and winding but small in diameter, and they can narrow down to microscopic dimensions. Medical devices built to navigate and optically view these areas must be flexible, maneuverable, and carry a light source.

One such device is an endoscope, which is a long, thin, flexible tube with a light—and sometimes a camera—attached. It has long been a tool of choice for doctors and surgeons alike for detecting, viewing, and treating a wide range of diseases and conditions, such as blood clots in the heart, airway blockages, and the early stages of colon cancer.

However, this device has its limitations. The size of most endoscopes used today is too big and bulky to access many of the body's smaller spaces, such as arteries in the brain or bronchioles in the lungs. One of the limiting factors is the camera lens and light source needed at the front end, which the physician uses to move the scope toward detecting, viewing, and treating diseased or damaged tissue.

But now, a research team led by University of Washington Department of Electrical & Computer Engineering and Physics Professor Arka Majumdar has designed a new kind of lens system for the tip of an endoscope, which could enable physicians to view and treat areas deep inside the body.

The research team reported their findings in a paper published this month in Light: Science & Applications. To build this device, the team engineered a metalens—a flat, lightweight optical component that uses microscopic nanostructures to manipulate light. Metalenses are used in a wide variety of technologies where space is limited; for example, in smartphone cameras.

"A lot of people developing endoscopy are working on the software, the computational side," Majumdar said. "Our team chose to develop a metalens for the optical hardware in an endoscope, and as our work progressed, we realized that many improvements were possible."

The tiny, flat metalens the team designed could enable the diameter of the smallest endoscopes in use today to be shrunk by more than 50%, which in turn would empower the user to look deeper into hard-to-reach spaces inside the body. The advance could allow physicians to access areas never seen before by optical imaging, such as blood clots deep in the brain and diseased arteries anywhere in the body, including the heart. This is very good news for patients, especially for treatment of common cardiovascular diseases, such as heart attack and stroke, which—when combined—are the number one cause of premature death in the world.

An endoscope equipped with a lens system like the one the research team has developed could provide physicians with visual feedback in real time. This translates to higher efficiency, fewer medical errors, and a higher success rate for each medical procedure. The system also provides higher resolution and better contrast than an X-ray, without the unwanted side effects of radiation.

"We are trying to extend the eyes of the surgeon or the physician deeper into the body," said Eric Seibel, a UW research professor in mechanical engineering, who co-authored the paper and has been engineering endoscopes for decades. "This is an area that I have been working in for 25 years, and this new technology has the potential to leapfrog over my past work. I'm very excited about this device."

Leveraging chromatic aberration in a metalens

This new metalens system uses quantitative phase imaging—a microscopy technique that measures the phase of light as it passes through a transparent or semi-transparent sample—and depth sensing to render a three dimensional, full-color video in real time with very little computing needed. It is also tiny, with an aperture width of 0.5 millimeters, which is about the width of five human hairs laid side to side. Today, there is a good amount of research being done on metalenses that use quantitative phase imaging and depth sensing, but not with an endoscopic application in mind. That is part of what makes this device unique.

Another unique aspect of this optical hardware is that it uses chromatic aberration for depth sensing and building the three-dimensional image. Chromatic aberration is present in almost all lenses and is usually considered to be a nuisance. It is a failure of a lens to focus all colors to the same point, and it can cause colored fringes to appear in images produced by an uncorrected lens. But the research team found a way to turn this flaw into a feature.

"Using a tiny, flat metalens, we create a chromatic splitting along focus, causing each color to converge at a different depth. In the reverse direction, this longitudinal rainbow effect allows for mapping depth into the color channels of a camera," said Aamod Shanker, who was the paper's lead author and is now a research fellow at the Iberian National Laboratory, Portugal. Shanker took part in this work while he was a postdoctoral researcher in Majumdar's lab and in the lab of Steve Brunton, a UW professor in mechanical engineering, who also co-authored the paper.

This research took place primarily in Majumdar's lab and at the Washington Nanofabrication Facility over a two-year period. Other UW ECE-affiliated co-authors of the paper were postdoctoral researchers Johannes Fröch and Saswata Mukherjee as well as UW ECE doctoral student Maksym Zhelyeznyakov.

A vision for the future

Now that the team has produced a proof of concept, next steps include building a prototype to test in a physical model of a human organ. Majumdar and Seibel estimate that this will be about a two-year process. The type of optics the team has developed, meta optics, can be fabricated and packaged at a large scale, which provides a unique opportunity to bring this lens system into the medical marketplace. They also plan to guide the technology through clinical studies over the next decade and beyond.

"This chromatic aberration providing information about depth is good, but this is still experimental," Majumdar said. "To get what a surgeon needs, we need to improve the image quality, and that's what we are pushing toward."

Seibel emphasized the human impact this technology could have in the future.

"This is an application of meta optics that has the potential to make a practical impact in everybody's lives, with a significant amount of development work," he said. "It may take 20 more years to make that impact. But it's a technology that has great potential, and everyone should start paying attention to it."

More information: Aamod Shanker et al, Quantitative phase imaging endoscopy with a metalens, Light: Science & Applications (2024). DOI: 10.1038/s41377-024-01587-y

Journal information: Light: Science & Applications 

Provided by University of Washington - Department of Electrical & Computer Engineering