28 Feb 2023
Researchers at Caltech make progress in medical imaging by adapting astronomical optics.Caltech’s Andrew and Peggy Cherng Department of Medical Engineering have made what they call “a major step forward in medical imaging” by taking inspiration from the field of astronomy. The work is described in Nature Photonics.
In astronomy, the light that reaches telescopes is distorted by the earth’s atmosphere, resulting in blurry images of planets, satellites, and other cosmic objects. The earth’s atmosphere is a scattering medium; it scatters light, making images appear unfocused and cloudy.
Wavefront shaping is a method of generating focused light by reversing the optical distortion caused by the atmosphere. In this method, a reflective device, like a mirror, shapes light waves to counterbalance distortion.
Biological tissue is also a scattering medium. The movement of blood, the motion of breathing, and the constant pumping of the heart creates fast-changing distortion, or cloudiness, when taking microscopic images of blood vessels, nerves, and even cancer cells. Researchers in medical engineering have explored the use of wavefront shaping to cancel out distortions caused by biological tissue.
”When light goes through a scattered medium like a piece of tissue, it will simply scatter all over the place. That means we can't directly focus light deep in tissue,” said Lihong Wang, Bren Professor of Medical Engineering and Electrical Engineering, and corresponding author of the paper.
“Scattering has a cumulative effect. The more scattering photons go through, the more distortion we see. Through the use of wavefront shaping, we can mitigate the scattering effect and focus deeper into biological tissue.”
Satisfying three key metrics
Wang’s lab employs a photo-refractive crystal to act as a form of “magic mirror” that cancels out the distortion of light caused by tissue. However, using wavefront shaping to capture clearer images of biological tissue must satisfy three key metrics: speed; control of degrees of freedom; and mirror brightness. Previous methods have not been able to satisfy all three.
The first key metric is speed. Since biological tissue is alive and moving, the entire wavefront shaping process must be done within a millisecond. “Only when you have the same object in the same state at the same location during the time reversal process can we cancel the wavefront distortion,” said Wang, who is also the Andrew and Peggy Cherng Medical Engineering Leadership Chair.
The second key metric is the so-called “control degrees of freedom”. Rather than a conventional mirror you might use to get dressed in the morning, the "magic mirror" used in wavefront shaping is composed of many small panels of mirrors. The more panels, the more control researchers have to tune and shape light waves to cancel out distortion.
The third key metric, and the one most challenging to Wang and the team, is the brightness or reflectivity of the mirror—the so-called “energy gain”. With the magic mirror used in high-speed wavefront shaping with high control degrees of freedom, reflectivity is often too dim to be effective. The research team found a solution in how a laser is produced.
When light waves are passed through a material with properties that allow it to amplify light, the electrons in the gain medium release energy in the form of additional light. This process amplifies the light waves, forming the laser.
Similarly, a laser gain medium is used to amplify the scattered light waves getting to and reflecting off the magic mirror. “Metaphorically, this gain medium allows us to make the magic mirror shinier; it polishes the mirror,” said Wang. The magic mirror itself remains the same while the light moving to and from the mirror becomes amplified and brighter.
In astronomy, wavefront shaping can turn a fuzzy blob into a clearer image of a distant planet. Translated to medical engineering, this new process of medical wavefront shaping has the potential to sharply focus on tissue to detect cancer below the skin.
“We report this technique that simultaneously achieves high-speed, high-energy gain—that means high reflectivity—and high control degrees of freedom. That means all three metrics have been satisfied for the first time,” said Wang. “This is a major step forward.”
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