Precision Ultrasound

Targeted ultrasound is widely used for a variety of medical applications, but the relatively large focal area (typically on the order of a centimeter or more) makes it difficult to focus the beam precisely. Recently, researchers found a way to fine-tune the accuracy by nearly 100 times taking the target range down to an area as small as 75 by 400 microns (thousandths of a millimeter).

Instead of starting with sound, the new method uses a 6-millimeter optoacoustic lens coated with carbon nanotubes to convert laser light into high-frequency sound waves. As the nanotubes absorb the light, they warm up, transferring heat to a second layer – a rubbery synthetic material called polydimethylsiloxane – which expands as it absorbs heat and amplifies the signal through rapid thermal expansion. The resultant sound waves are at a frequency 10,000 times higher than humans can hear.

The highly-focused sound waves are capable of blasting a 150-micron hole in a kidney stone. With this kind of precision, ultrasound could someday be used to clear cholesterol deposits from clogged vessels or target specific clumps of cancer cells.