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A scoping study of the potential of photoacoustics to support primary standard calibration of medical ultrasonic hydrophones up to 100 MHz.

Rajagopal, S; Zeqiri, B; Cox, B* (2015) A scoping study of the potential of photoacoustics to support primary standard calibration of medical ultrasonic hydrophones up to 100 MHz. NPL Report. AC 14

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Miniature medical hydrophone devices based on the piezo-electric effect, and hydrophones founded on fibre-optic Fabry-Perot interferometric principles are used to characterise the spatial and temporal properties of ultrasound fields. The quantity delivered by the hydrophone is an electrical waveform which can be transformed into the true acoustic waveform using a frequency dependent calibration factor with units of volts per pascal. Primary-level hydrophone calibration is carried out using an optical displacement interferometer. There is a pressing need to calibrate these hydrophones as high as 100 MHz due to an increasing proliferation of high frequency medical ultrasound scanners. However, calibration of these devices on the interferometer is extremely difficult at elevated frequencies due to the small, tens of picometre displacements generated by conventional mainstream ultrasound transducers. Environmental disturbances such as vibrations from seismic activity, movement of people in and around the laboratory and equipment cooling fans impose fundamental limitations on the smallest discernible displacements which can be measured, regardless of the type detection scheme used to build these extremely sensitive measurement instruments. Therefore, a viable alternative to support calibrations would be to boost the high frequency displacements, ideally to a few hundred picometres. Photoacoustics has a potential to do this. The photoacoustic effect refers to the phenomenon when a material illuminated with light absorbs photon energy, converting it into propagating acoustic waves within the material via one of several mechanisms. In principle, photoacoustics could be used to generate acoustic frequencies up to hundreds of gigahertz. Recently, photoacoustic sources made of nanocomposite materials have been able to generate planar ultrasound peak-positive pressure pulses as high as 12 MPa in the medical imaging frequency range. In order to understand the state of the art in photoacoustics research, a detailed literature review was conducted to establish its potential to support high frequency hydrophone calibration using the NPL primary standard displacement interferometer. In parallel with the review, a photoacoustic measurement facility was setup, with early measurements being conducted. The literature review findings and early experimental results have shown photoacoustics to be a promising new area of science for NPL offering the potential to tackle the challenge of high frequency primary standard calibration of hydrophones.

Item Type: Report/Guide (NPL Report)
NPL Report No.: AC 14
Subjects: Acoustics
Acoustics > Ultrasound
Last Modified: 02 Feb 2018 13:13
URI: http://eprintspublications.npl.co.uk/id/eprint/6734

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