From securing stealth to ensuring health

A material used to protect submarines from sonar detection is the latest technological breakthrough in ensuring the safe and effective dose of ultrasound in medicine. Practitioners and thousands of patients in physiotherapy departments worldwide will benefit from the latest technology, which will ensure a step forward in the reliability of delivered ultrasound treatment.

The material forms a key component in a novel desk-top ultrasound power meter developed by the UK’s national standards laboratory, The National Physical Laboratory (NPL) of Teddington, UK, in partnership with one of the leading manufacturers of ultrasound measurement equipment, Precision Acoustics Ltd (PA), Dorchester, UK.

An estimated 10,000 physiotherapy ultrasound units are currently in use in the UK alone. Key to ensuring that patients are receiving the most effective treatment for their soft tissue injuries is the assurance that the equipment delivers the correct level of ultrasound power. Physiotherapists will benefit from the latest measurement development to carry out their everyday treatments.

Maintaining accuracy of these devices is crucial to safety of patient treatment in both private and public sectors. Currently, the only way to guarantee the outputs of the physiotherapy devices is through costly measurement equipment retailing at around £1,500 to £2,500 and which are only suitable for use by trained hospital physicists

The new power meter provides the /answer in a matter of seconds. Compact and easy-to-use, the new device, which retails at under £400 (excl VAT), is smaller than an average shoe-box and will enable physiotherapists to conduct their own health-checks on ultrasound equipment each time they use it.

By simply placing the treatment head – the part of the physiotherapy equipment, which is applied to the patient’s body – in the water-filled well in the top of the device, the practitioner can check instantly that the equipment is delivering the right amount of power. The patient can be sure that they are receiving the best and most effective treatment.

“Until now, the only methods for checking equipment, have been too complex and costly for application at user level, either in the NHS or in private practice. The power meter was developed in response to demand for a cheaper and more user-friendly measuring system than those currently available. It is a good example of NPL partnering the instrumentation sector to deliver innovative measurement technologies” says Dr Bajram Zeqiri, Head of the Medical and Industrial Ultrasound Group at NPL.

The low cost and user-friendliness of the power meter makes it a very attractive alternative to the systems currently in use. Terri Gill, Managing Director of PAL. said,
“Once the device reaches the market, we anticipate a huge demand and with larger scale production to meet this demand, we expect the costs will eventually be even lower. Together with NPL , we are now looking into the feasibility of integrating the new system into future design for physiotherapy ultrasound equipment. Some manufacturers have already expressed interest.”

Scientists at NPL and PAL are very excited about the new technology as it looks to be flexible, offering the potential for application in a whole range of future measuring devices, from inexpensive, lower-accuracy devices such as the newly developed ultrasonic power meter, to more sophisticated meters which might replace traditional radiation force measuring devices. Dr Zeqiri adds, “The method looks to be fairly sensitive which means it might be developed to detect low ultrasonic powers, perhaps even down to a few mW or so.”

Key to the simplicity, efficiency and low cost of the power meter are the principles and materials used in its construction.

To address the problem of cost, NPL needed to find an alternative to conventional methods of measuring radiation force as a means of verifying the accuracy of ultrasound equipment. Conventional measuring devices (radiation forcebalances ) offer a highly accurate measurement of power output (to between ±7% and ±10% – well within the ±20% specified standard for physiotherapy ultrasound equipment), but are cumbersome, and costly to manufacture. More importantly, physiotherapists are not trained in the use of such equipment and have to rely on the availability of medical physicists to carry out the tests. Equipment is time-consuming to set up and, taking at least 15 minutes to carry out the check. This represents an ineffective use of practitioners’ time.

Working in partnership with PAL, NPL looked into the viability of using solid-state principles – rather than radiation force measurement – to achieve a measuring system. Using the known pyroelectric properties of polyvinylidene fluoride (pvdf) and the specially designed polyurethane rubber material, the team came up with a device which is not only considerably cheaper to produce, but which has the added advantage of being portable and extremely simple to use.

The power meter consists of a plastic cup-shaped vessel approximately 100mm in outside diameter and 40mm deep, the upper chamber of which is filled with tap water. A thin membrane of pvdf, only tens of microns thick, is stretched across the upper chamber of the vessel. This membrane is backed with a polyurethane rubber derived from a material originally developed for use as anti sonar-detection or ‘stealth’ coatings on submarines. At the frequencies of interest, this unique material acts as an acoustical absorber capable of absorbing more than 80% of the ultrasonic power within 1mm of the front surface of the absorber. Pic/diag here?. The acoustic energy produced by the treatment head is absorbed within the top surface of the rubber, resulting in a rapid increase of temperature of both the absorber surface and the adjacent pvdf film. Due to the pyroelectric response of the polymer film a voltage is simultaneously generated across the electrodes of the pvdf. The electronic circuitry which captures the largest voltage generated by the thin film, as a measure of the generated power, is neatly encased in the lower chamber of the vessel.