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· Tech Zoom: Aloka Debuts Ultrasound Systems for Cardiac and Vascular Markets

More Fit in the Future

Acoustic absorber improves ultrasound calibration

New technology developed by scientists at the National Physical Laboratory (NPL), Teddington, UK—the UK’s national measurement laboratory—reveals that calibration equipment for physiotherapy ultrasound machines quite often yields inaccurate results. In fact, equipment surveys suggest that up to 70% of physiotherapy machines currently in use could be outside specification, explains Mark Hodnett, senior research scientist at NPL.

“Calibration is vital to ensure safety and accuracy of device output,” Hodnett said. Three watts per square centimeter is the international limit for ultrasound intensity that can be produced by therapy machines, and all devices sold must comply with the limit to a tolerance of 30%. “Otherwise, how can a useful clinical treatment be given, if the machine’s output isn’t accurately known? Surveys have shown that up to 70% of devices in clinical use actually exceed their displayed output by up to 100%, and this is very likely due to poor, or no, calibration methods having been applied to them. Such devices have been reported to cause patient injury in the form of skin burns.”

NPL has developed an acoustic absorber that can be retrofit into current calibration equipment, increasing its accuracy.

To ameliorate the problem, NPL has developed an acoustic absorber that can be retrofit into current calibration equipment, increasing its accuracy. The absorber works by better accommodating spreading beams.

“Output power measurements are made using radiation force balances, which effectively ?weigh’ the ultrasound beam by way of a target placed in its path,” Hodnett noted. “If the target is large enough to intercept the whole beam, then the force experienced by the target is proportional to the power contained in the beam.”

The target then either reflects the beam or absorbs it. Hodnett says either approach works because both detect the momentum associated with the ultrasound beam. “If the beam spreads out as it propagates from the therapy applicator, which can be the case for small therapy applicators, the reflecting target approach starts to become inaccurate, because it relies on the ultrasound beam hitting it at a certain angle,” he explained. “However, the absorbing target configuration on the right of the image is more tolerant to ultrasound beams hitting it at a range of angles, and so gives an accurate result with small therapy applications.”

The absorber has been tested by medical physics staff at National Health Service Lothian, Scotland, where an ultrasound device showed power levels up to 100% higher than those indicated by the calibration equipment on its own. The unit would have delivered double its intended dose.

“Using absorbing targets to measure ultrasound is not a new idea,” Hodnett said. “The established theory of radiation force balance is based on their use. But the problem has been developing a material that does the job, and that’s what NPL has achieved in the past few years.”

The requirements for a suitable material were that it needed to be almost totally absorbing to ultrasound at MHz frequencies, but also nonreflecting. Bajram Zeqiri at NPL developed the material used in the acoustic absorber out of a substance he created for use on submarines to make them less detectable by underwater sonar. The absorbing material itself is sold under license by Precision Acoustics, Dorchester, UK.

“Ensuring that patients are treated with the correct level of ultrasonic power has always been a difficult problem,” said Steve Pye, MD, consultant medical physicist at NHS Lothian. “NPL delivered a double-whammy by making the calibration of physiotherapy equipment both simpler and more accurate.”

—Cat Vasko

Tech Zoom: Aloka Debuts Ultrasound Systems for Cardiac and Vascular Markets

Aloka Co Ltd, based in Wallingford, Conn, recently debuted two ultrasound systems with dedicated platforms for advanced cardiac and vascular imaging.

The ProSound Alpha 10—Aloka’s enhanced premium system—and the brand-new ProSound Alpha 7 represent the company’s efforts to expand its variety of ultrasound applications as well as the markets they serve.