Medical ultrasound explodes nanodroplets

A technique which may one day be used for tumor therapy and imaging could be greatly improved thanks to new insights obtained by scientists from the University of Twente, the Erasmus MC and the TU Delft.

The imaging method the team has been developing hings on nanodroplets of a special liquid called perfluorocarbon that can be injected in a human body. These droplets can move out of the vascular system and enter the space in between the tumour cells. The idea is to activate these droplets with an intense pulse of ultrasound. This sound causes the droplets to vaporise, forming tiny bubbles of gas which can be viewed using ultrasound imaging equipment.

Researchers from the three universities solved a long standing mystery of how superheated nanodroplets vaporise when hit by a pulse of ultrasound. Using images captured by the world’s fastest camera, the Brandaris 128, they were able to see that the ultrasound was being focused in a single spot within the droplet. This was peculiar, because the wavelength of the ultrasound emitted is many times larger than the droplet, causing negligible focusing.

The explanation can be found in a unique phenomenon that occurs in the propagation of ultrasound. Sound is a wave movement of high and low pressure that moves at the speed of sound. However, in the body, a high pressure propagates faster than a low pressure, distorting the wave and creating a shock wave. “In fact, a whole series of higher harmonics develops from the original sound”, says one of the authors, acoustical imaging expert Dr. Martin Verweij (Applied Sciences). “The wavelength of these higher harmonics is much smaller, around the size of the droplets, and these waves can focus inside the droplet.”


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