It might seem amazing that ultrasound, which is used to make pictures of a baby growing inside its mothers womb, can also burn out (or ablate) cancer cells inside the body but this is exactly what HIFU aims to do.

In the same way that generations of schoolboys have 'ablated' ants using only sunlight and a magnifying glass, doctors can now focus enough ultrasound onto a carefully selected region inside the body to destroy the targeted diseased tissue whilst leaving surrounding healthy tissue virtually untouched. It can even be focused through the skull onto brain tumours. This allows doctors to treat cancer and certain other conditions without the risks and side effects inherent in conventional surgery, chemotherapy or radiotherapy.

The killed cells are broken down and removed by natural processes and there is even evidence that the immune system is stimulated to attack any cells which survive the initial treatment. A wide range of conditions have been treated with HIFU but there is most experience with prostate cancer and with uterine fibroids: systems for treating these have been approved for general use in Europe and the USA.

NPL can play a vital role in this emerging procedure. Simply measuring ultrasound in these fields without destroying the measuring device is difficult enough, never mind making accurate measurements. Pressures of 500 atmospheres may occur at the focus and the intensity may be more than 10,000 Watts per square centimetre (that's 10 electric bar heaters crammed into 1 square centimetre!) and it is obviously essential that this energy is delivered at the right point. So the challenges for treatment planning and for targeting are formidable.