Ultrasound has long been used to see inside the body, but focused high-frequency sound is now targeting cancer:
If Zhen Xu hadn’t annoyed her lab mates, she might never have discovered a groundbreaking treatment for liver cancer.
As a PhD student in biomedical engineering at the University of Michigan in the US during the early 2000s, Xu was trying to find a way for doctors to destroy and remove diseased tissue without the need for invasive surgery. She’d landed on the idea of using high-frequency sound waves – ultrasound – to mechanically break up tissue and was testing her theory on pig hearts.
Ultrasound isn’t supposed to be audible to human ears, but Xu was using such a powerful amplifier in her experiments that other researchers she shared the laboratory with began to complain about noise. “Nothing had worked anyway,” she says. So she decided to humour her colleagues by increasing the rate of ultrasound pulses, which would bring the sound level outside the range of human hearing.
To her shock, increasing the number of pulses per second — which also meant each pulse reduced in length to a microsecond — was not only less disruptive to those around her, but also more effective on living tissue than the approach she’d tried previously. As she watched, a hole appeared in the pig heart tissue within a minute of ultrasound application.
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For treatment of liver cancer, histotripsy devices channel ultrasound waves into a focal zone of about two by four millimetres — “basically, the tip of your colouring pen”, Xu says. Then, a robotic arm guides the transducer over the tumour to target the correct area.
The ultrasound is delivered in quick bursts. These pulses create tiny “microbubbles” that expand and then collapse in microseconds, breaking apart the tumour tissue as they do. The patient’s immune system is then able to clean up the remains.