Moth eyes inspire improved x-ray imaging

Ashley N. Paddock, ashley.paddock@photonics.com

A new class of nanoscale materials modeled after a moth’s eye could improve the light-capturing efficiency of x-ray machines and similar medical imaging devices.

As with butterflies, moths have large compound eyes composed of many thousand ommatidia – structures comprising a primitive cornea and lens, connected by photoreceptor cells. Unlike those of butterflies, however, moth eyes are extraordinarily antireflective, bouncing back only a small portion of the light that strikes them. This adaptation makes the insects less visible to predators during their nocturnal flights. Because of this, engineers have looked to the moth eye to help design more efficient coatings for solar panels and antireflective surfaces for military devices.

City University of New York professor Yasha Yi and colleagues at Tongji University in Shanghai have taken this feature a step further: They have used the moth eye as a model for developing nanoscale materials that someday could reduce the x-ray radiation dosages received by patients, while improving the resolution of the resulting images.

The scientists focused their experiment on “scintillation” materials – compounds that, when struck by incoming particles, absorb the energy in the form of light. Such scintillators are used in radiographic imaging devices to convert the x-rays exiting the body into visible light signals picked up by a detector to form an image.

A higher x-ray dosage improves output but is not healthy for patients. As an alternative, Yi’s team found that improving the scintillator’s efficiency at converting x-rays to light improved the output. Their new nanomaterial does just that.

The material consists of a 500-nm-thick thin film composed of a cerium-doped lutetium oxyorthosilicate crystal.

“We need a thin film to fabricate the light-extraction structure,” Yi told BioPhotonics. The layer was needed so as not to perturb the scintillation material’s light emission layer.

The crystals are encrusted with tiny pyramid-shaped silicon nitride protuberances. Each protuberance, or corneal nipple, is modeled after the structure in a moth’s eye and is designed to extract more light from the film.

A scanning electron microscope image of a leaf miner moth’s eye. Moths’ large compound eyes are extraordinarily antireflective, bouncing back only a small portion of light that strikes them; now, researchers have used the moth eye as a model for new nanoscale materials for improved x-ray imaging. Courtesy of Dartmouth College.

Within a 100 x 100-µm square, about the same density as the actual moth eye, the scientists can fit between 100,000 and 200,000 protuberances. They made the side walls of the device rougher, improving its ability to scatter light and enhancing the scintillator’s efficiency.

“The light-extraction efficiency enhancement is very sensitive to the dimension of the protuberances,” Yi said. More research is needed to see whether there are benefits to adding more, he said.

During lab experiments, Yi discovered that adding the thin film to the scintillator of an x-ray mammographic unit increased the intensity of the emitted light by as much as 175 percent compared with the output of a traditional scintillator.

The work appeared in Optics Letters (http://dx.doi.org/10.1364/ OL.37.002808) and represents a proof-of-concept evaluation of the use of the moth eye-based nanostructures in medical imaging materials. It also could be applied to various light-emitting devices, Yi said.

He estimates that it will take at least another three to five years to evaluate and perfect the film, and five years before it will reach a clinical setting.

The team plans to continue investigations to understand and improve the light-enhancement mechanism.