Ultrasound method could lead to non-invasive surgical knife

University of Michigan engineering researchers who developed the new therapeutic ultrasound approach said in a statement that it could lead to an invisible knife for non-invasive surgery.

Ultrasound technology enables far more than glimpses into the womb as doctors routinely use focused sound waves to destroy kidney stones and prostate tumours.

The tools work primarily by focusing sound waves tightly enough to generate heat, said Jay Guo, a professor of electrical engineering and computer science, mechanical engineering and macromolecular science and engineering. Guo is a co-author of a paper on the new technique published in the current issue of Nature’s journal Scientific Reports.

The beams that today’s technology produces can be unwieldy, said Hyoung Won Baac, a research fellow at Harvard Medical School who worked on this project as a doctoral student in Guo’s lab.

‘A major drawback of current strongly focused ultrasound technology is a bulky focal spot, which is on the order of several millimetres,’ Baac said. ‘A few centimetres is typical. Therefore, it can be difficult to treat tissue objects in a high-precision manner, for targeting delicate vasculature, thin tissue layer and cellular texture. We can enhance the focal accuracy 100-fold.’

The team was able to concentrate high-amplitude sound waves to 75 x 400 micrometers and their beam can blast and cut with pressure, rather than heat. Guo speculated that it might be able to operate painlessly because its beam is so finely focused it could avoid nerve fibres. The device has not yet been tested on animals or humans, however.

‘We believe this could be used as an invisible knife for non-invasive surgery,’ Guo said. ‘Nothing pokes into your body, just the ultrasound beam. And it is so tightly focused, you can disrupt individual cells.’

To achieve this super-fine beam, Guo’s team took an optoacoustic approach that converts light from a pulsed laser to high-amplitude sound waves through a specially designed lens.

The general technique has been around for many years but for today’s medical applications the process doesn’t normally generate a sound signal strong enough to be useful.

The Michigan University researchers’ system is claimed to be unique because it performs three functions: it converts the light to sound; it focuses it to a tiny spot; and it amplifies the sound waves.

To achieve the amplification, the researchers coated their lens with a layer of carbon nanotubes and a layer of a rubbery material called polydimethylsiloxane. The carbon nanotube layer absorbs the light and generates heat from it. Then the rubbery layer, which expands when exposed to heat, boosts the signal by the rapid thermal expansion.

The resulting sound waves are 10,000 times higher frequency than humans can hear. They work in tissues by creating shock waves and microbubbles that exert pressure towards the target, which Guo envisions could be tiny cancerous tumours, artery-clogging plaques or single cells to deliver drugs. The technique might also have applications in cosmetic surgery.

In experiments, the researchers demonstrated micro-ultrasonic surgery, accurately detaching a single ovarian cancer cell and blasting a hole measuring less than 150 micrometers in an artificial kidney stone in less than a minute.