Developing calibration and measurement techniques
In response to the changing requirements of medical ultrasound equipment, we develop novel calibration and measurement techniques to underpin ultrasound safety. Acoustic output measurements of medical ultrasound devices are essential for manufacturers wanting to demonstrate compliance with standards and market their devices worldwide. Clinical demands and technology developments are rapidly enhancing clinical ultrasound applications, posing measurement challenges which shape NPL's research programme.
Ultrasound imaging for applications such as skin cancer detection, ophthalmology and pre-clinical imaging is using increasingly higher centre frequencies, 70 MHz and beyond, and broad bandwidth transducers. This drives our research into new techniques for providing primary standard calibrations of ultrasound hydrophones. Currently, primary calibrations are performed using optical interferometry, employing conventional piezoelectric transducers. Although, by employing nonlinear propagation, acoustic signals of up to 60 MHz can be generated and used to calibrate hydrophones, uncertainties are degraded due to the difficulties in producing sufficient pressure amplitudes.
We are researching the use of photoacoustic methods to generate broadband, high amplitude, ultrasound signals. A high energy pulsed laser incident on a substrate coated with a strong optical absorber generates broadband ultrasound signals of high acoustic pressure amplitude with frequency content beyond 110 MHz. This new approach is now being trialled as a source for the next generation of NPL's primary standard calibration facility for hydrophones, to continue disseminating the acoustic pascal to the user community worldwide.
To meet clinical diagnostic needs, ultrasound scanners are becoming more complex in their beam patterns, with rapid frame-rate imaging and non-repetitive pulse regimes. This presents challenges for quantifying acoustic output, particularly at the end-user level where national guidelines recommend regular checks of on-screen thermal and mechanical indices, related to patient safety. Our research is developing thermal-based methods for measuring the acoustic output power from diagnostic ultrasound scanners. These methods exploit the pyroelectric response of thin membrane materials, backed with a high–performance acoustical absorber. Early tests have shown that this provides a high-sensitivity measurement capability suitable for use in hospital environments.
Ultrasound materials measurement capabilities are being developed further to improve the understanding of the interactions between high frequency sound and tissue-like materials used with phantoms. Measurement of properties such as speed of sound and attenuation are well established, and capability is being extended to develop standard protocols for measuring the non-linearity parameter of soft tissue, an important parameter governing heat deposition in a medium.
Ongoing research at NPL is also developing unique capabilities for characterising acoustic cavitation, generated by devices such as ultrasonic cleaners, cell disruptors, materials processors and tissue aspirators. This will inform a new generation of standards, specifying how to measure these challenging fields, allowing exploitation and scale-up of advanced manufacturing processes.