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Tuesday, January 8, 2013

Surgical ultrasound

Ultrasound has been used both for imaging and for disrupting tissues (like fragmenting kidney stones). However, it delivers too broad a punch (generally over 2 mm wide and 10 mm deep) to be useful for delicate repair work. You wouldn’t want to target a small blood vessel with such an inaccurate tool. High-frequency ultrasound would be more precise but among other problems, it can generate excessive heat. Luckily, researchers from the University of Michigan, led by Hyoung Won Baac, may have come up with a solution. They designed a laser-generated focused ultrasound (LGFU) that can target an area only 75 microns across using pressure rather than heat.

First, what is ultrasound? Like ‘ultraviolet’, ultrasound is composed of waves (sound instead of light) with a frequency above that which humans can hear (about 20 khz). That’s it. Just inaudible sound waves. Also as with the light spectrum, different ultrasound frequencies have different properties. Most medical ultrasounds have frequencies of a few Mhz (100 times higher than a person can hear). Baac’s device can produce ultrasound with frequencies above 15 Mhz.

To create their high-frequency LGFU, the scientists used a carbon nanotube lens to convert the light energy of a laser to sound. The lens also amplified those sound waves to create a pressure front that was able to target and break up a kidney stone only a few microns across without generating excessive heat. Even more impressively, the researchers were able to use the LGFU to remove an individual cell from amongst its neighbors.

The scientists predict that doctors will one day perform pain-free surgeries with ultrasound scalpels so precise that they evade all nerve cells. But before then, the device has to be tested on animals or humans, which hasn’t happened yet.

Baac HW, Ok JG, Maxwell A, Lee KT, Chen YC, Hart AJ, Xu Z, Yoon E, & Guo LJ (2012). Carbon-nanotube optoacoustic lens for focused ultrasound generation and high-precision targeted therapy. Scientific reports, 2 PMID: 23251775.