Ultrasound finds new roles beyond medical imaging

By SIMON HADLINGTON


For most of us, the word "ultrasound" conjures up a grainy scene of grey and white swirls which, with guidance from a skilled radiographer, we can more or less accept as being an image of a baby developing in the womb.

For decades, ultrasound has been used as an important technique for imaging within the body, particularly to check the health of the growing foetus in pregnant women.

It is safe, cheap and reliable. But now ultrasound is beginning to find new roles in medicine, from "acoustic surgery" - killing cancerous tumours without recourse to a scalpel - to potentially helping to deliver drugs, and even genes, into cells.

For medical imaging the ultrasound has low power. It can penetrate the skin but does not have sufficient energy to do damage. But for decades medical scientists have pondered how to harness ultrasound energy to destroy unhealthy tissue, such as cancerous tumours, within the body.

Increasing the energy of the sound wave, by lowering its frequency, would not work. As soon as the sound wave encountered the surface of the skin it would heat it up dangerously.

"But, if you took two high-frequency beams, each on their own being harmless, and directed them into the body from two different points, where they intersect you would get twice the energy," says Professor Tim Mason, the director of the Sonochemistry Centre at Coventry University.

"So if you had several beams meeting at a point, you could obtain sufficient energies at the focus to heat up tissue and kill it at that specific point."

This is called high-intensity focused ultrasound (Hifu).

Professor Gail ter Haar, head of ultrasound therapy at the Institute of Cancer Research at the Royal Marsden Hospital in London, has been researching Hifu for more than 20 years and built the first clinical prototype, which has been tested on 74 patients with various types of cancer.

One of the main difficulties had been in monitoring what was happening. It has been necessary to alternate Hifu treatment with ultrasound imaging to monitor the therapy, making the process time consuming, but still successful in killing tissue.

Another approach being investigated by the team is in sealing off the blood vessels that feed tumours. A short burst of Hifu can cauterise small blood vessels, and it is possible that this could be a way of "starving" cancers of their blood supply.

Meanwhile, a new Chinese system has been developed that could bring the technique closer to routine use. The machine, the first Hifu instrument of its type, is being tested on patients at the Churchill Hospital in Oxford.

The Chinese system has overcome the monitoring problem by having an imaging ultrasound transducer alongside the Hifu transducer, so that the treatment can be observed as it is being done.

Ultrasound is also finding other new applications in medicine. At the University of Wales College of Medicine, Dr Nazar Amso and his research team are working with colleagues at Cardiff University to investigate the effect of ultrasound waves on the porosity of cell membranes.

Many medical interventions require large, unwieldy molecules to enter cells across the cell membrane. One possible way of increasing the "leakiness" of cells is to expose them to ultrasonic waves.

The researchers are now planning experiments with cell cultures to see if similar effects can be produced in a more complex cellular system. If ultrasound can be shown to increase the porosity of cells in the body safely and reversibly, it could have important applications in many areas.

Ultrasound facts

* Ultrasound is beyond the range of human hearing and is generally defined in physics as sound with a frequency of more than 20,000 Hertz.

* It is propagated as a wave, and travels by the mechanical vibration of molecules through a medium.

* Ultrasound waves are generated by devices called transducers, which contain "piezoelectric" materials. These vibrate at given frequencies when an electric field is applied to them.


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