Ultrasound is often used to destroy cancerous cells. The sound waves emitted from an ultrasonic device raise the temperature of diseased cells, causing them to die. But there's a limit to how intense the sound wave can be — too much and they'll damage the surrounding healthy tissue.

Now scientists at the California Institute of Technology are experimenting with

sound "bullets" that could improve cancer treatments by better focussing the waves on precise areas. The technique could also improve sonar, which is used to create images of objects underwater.

At a recent gathering of the American Physical Society in San Diego, a team of researchers led by Chiara Daraio described their experiments with shaping sound

energy into focused bullets. 

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Ordinary ultrasound sends sound waves into a material. Those waves bounce back to the source, where a computer analyzes the signal and creates an image. But the images can be fuzzy, since the waves are traveling back and forth through a material and getting dispersed. Sonar

is similar. A submarine typically uses it to "ping" its surrounding. But again, the sound waves get dispersed through the water and may be scattered further by particles or small fish suspended in it. It's similar to how headlights scatter in the rain, making

it more difficult to see.


The Cal Tech team devised a technique that overcomes the challenges of dispersal and provides sharper images. Their innovation works in a way similar to the Newton's Cradle toy, in which a row of metal balls hang from strings. If you pull back on a ball at one end and allow it to swing and collide with the row of balls, the energy passes through all of the balls and gets transferred to the one on the opposite end, causing it to swing up.

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In the case of the Cal Tech's innovation, rows of tiny steel spheres are arranged rows and columns to form a cube shape. The sound waves travel through the balls, emerging as a pulse at the end of the array. The pulse of sound is concentrated and therefore has more power than the conventional ultrasound, which emits sound waves in a continous flow. More power results in a higher resolution image.

Daraio told Discovery News that because it's possible to

control the sound energy this way, it's possible to make a kind of ultrasound

machine that wouldn't suffer from the scattering and wave diffraction problems

that ordinary ultrasound does. The same is true of sonar. Because the pulse of

sound is short, rather than continuous, sending a high-power pulse through

tissue would be safer to do. When treating tumors this way, more energy could

be sent at one time, with tighter focus.

The next step is to refine the experiments and see if they

work for real imaging.

Credit: California Institute of Technology