Sound of accuracy

Submarine stealth technology has been adapted for safer physiotherapy ultrasound

Scientists at the National Physical Laboratory (NPL) have developed an acoustic absorber that they claim helps to ensure ultrasound machines used in physiotherapy are emitting an accurate level of power.

The polyurethane rubber absorber uses technology derived from that used to line submarines to make them invisible to radar. The flat shape of the device has been designed to ‘catch’ all the ultrasound beams that spread out from the machine’s applicator and give a more accurate reading of the power output compared with existing conical-shaped calibration devices.

‘If you imagine the ultrasound device is at the top pushing the sound out, the beams coming down, the equipment in widespread use for calibration is a conical target which intercepts the ultrasound beam,’ said Mark Hodnett, NPL research scientist.

‘If the sound is spraying out in a number of directions, like a poorly fitted shower head, and doesn’t hit the cone at the right angle, then you’re going to have problems measuring the power because the device is designed to assume that the sound is going to hit it at a certain angle of incidence. So if the sound comes out sideways, you’re going to measure less than it should be.

‘The new design is more tolerant to sound coming out in different directions. This means that if a machine has been inaccurately calibrated, a patient undergoing therapy could be saved from exposure to more power than is necessary, thus preventing further damage to skin tissue.

‘If a machine had been calibrated using an older-style conical target, the number on the dial might say three watts in terms of its acoustic power output. but if the transducer — the piece of equipment coupled to the patient — produces sound that goes out in a variety of directions, the output could be higher,’ said Hodnett.

He explained how the team was able to adapt the liner technology for calibration purposes.

‘It came out of developments here at NPL to develop acoustic absorbing materials to make submarines stealthy, so they would not show up on sonar, and changing the recipe so the material would absorb at higher frequencies. Sonar typically runs at kilohertz frequencies and lower, while ultrasound is in the megahertz ranges.’

Scientists at Lothian NHS, Edinburgh, tested the new absorber on an ultrasound machine and found it had an output of up to 100 per cent more than indicated on its dials.

Hodnett said: ‘In Lothian, it produced twice as much as what was on the dial because the manufacturer had set it up using a reflecting-style conical target balance, whereas the Lothian team used the absorbing targeting balance instead.

‘It’s an inaccuracy of display as much as anything else,’ he said . ‘So if it shows three watts on the machine, it was only after measuring the machine using the absorber that they discovered it was six instead.’

According to Hodnett, the absorber will therefore help verify that ultrasound machines will be manufactured to a pan-European standard.

‘There is an international requirement that the maximum intensity should not be higher than three watts/sq cm. There’s also a tolerance level on that value of + or -20 per cent, so a manufacturer must evaluate the uncertainties in their measurements and make sure that it is safe to be sold.

‘This means that if it says three watts on the dial, the output actually coming out must be 20 per cent within that value, between 2.4 and 3.6W,’ he said.

The NPL acoustic absorber, which can be retrofitted into current calibration equipment, is available from Precision Acoustics.