Researchers from North Carolina State University have developed a technique that allows ultrasound to penetrate bone or metal, using customized structures that offset the distortion usually caused by so-called ‘aberrating layers’.  The study is published in the journal Physical Review X.

The team designed complementary metamaterials that will make it easier for medical professionals to use ultrasound for diagnostic or therapeutic applications, such as monitoring blood flow in the brain or to treat brain tumours.  This has been difficult in the past because the skull distorts the ultrasound’s acoustic field.  These metamaterials could also be used in industrial settings.  For example, it would allow ultrasound to detect cracks in airplane wings under the wing’s metal skin.

Ultrasound imaging works by emitting high frequency acoustic waves. When those waves bounce off an object, they return to the ultrasound equipment, which translates the waves into an image.  But some materials, such as bone or metal, have physical characteristics that block or distort ultrasound’s acoustic waves. These materials are called aberrating layers.

The researchers addressed this problem by designing customized metamaterial structures that take into account the acoustic properties of the aberrating layer and offsetting them. The metamaterial structure uses a series of membranes and small tubes to achieve the desired acoustic characteristics.

In simulations, only 28 percent of ultrasound wave energy makes it past an aberrating layer of bone when the metamaterial structure is not in place. But with the metamaterial structure, the simulation shows that 88 percent of ultrasound wave energy passes through the aberrating layer.

The technique can be used for ultrasound imaging, as well as therapeutically, such as using ultrasound to ablate brain tumours.  The team state that in effect, it’s as if the aberrating layer isn’t even there.

The researchers have tested the technique using computer simulations and are in the process of developing and testing a physical prototype.

Source:  NC State University