Metamaterials allow ultrasound to penetrate bone and metal

Metamaterials negate the properties of the so-called aberrating layers, allowing ultrasound to penetrate metai and bone (Image: Yun Jing)

Score another point for metamaterials. Researchers at North Carolina State University have designed complementary metamaterials that will aid medical professionals and engineers in diagnosing problems under the skin. These metamaterials are structured to account for so-called "aberrating layers" that block or distort the acoustic waves used in ultrasounds, making it possible to now conduct ultrasounds of a person's head or an airplane's wing – among other things.

Ultrasound imaging involves emitting high-frequency acoustic waves that can be translated into an image of an object by measuring how long they take to return to the equipment. It's great for examining a pregnant woman's womb or scanning the ocean for solid objects (the latter being sonar), but certain materials lying between the scanner and the target area – such as bone and metal – block or distort a large portion of the signal. These are known as aberrating layers.

What the researchers designed is akin to an acoustic cloak. The complementary metamaterials are composed of a series of membranes and small tubes that jut out in different directions and have different thicknesses. This offsets the acoustic properties of the aberrating layers, effectively canceling them out.

The technique was tested using computer simulations. With the metamaterial structure included, 88 percent of ultrasound wave energy survived the trip through an aberrating layer of bone, while just 28 percent made it through without the complementary metamaterials. "In effect, it's as if the aberrating layer isn't even there," observed senior author Dr. Yun Jing.

The researchers are now developing and testing a physical prototype that they believe will lead to applications in both medical and industrial settings. "For example," Jing said, "it would allow you to use ultrasound to detect cracks in airplane wings under the wing's metal 'skin.'"

It could also find various non-invasive diagnostic and therapeutic uses, such as monitoring blood flow in the brain or treating brain tumors (with blasts of focused energy that burn them). "This has been difficult in the past," explained lead author Tarry Chen Shen, "because the skull distorts the ultrasound's acoustic field."

A paper describing the research was published in the journal Physical Review X.

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