Photoacoustic ultrasound source for hydrophone calibrations

High frequency applications of ultrasound are becoming increasingly common in areas such as ophthalmology, dermatology, paediatric and peripheral vascular imaging, where frequencies in the range 40–70MHz are used. In ablative therapies and lithotripsy, low frequency (<10 MHz) and high amplitude (tens of MPa) ultrasound waves are used to ablate tumours and break kidney stones.

The nonlinear propagation of such waves in water can generate harmonic frequency components up to 100 MHz and beyond.

Therefore, a need has arisen for end users such as medical device manufacturers and academic researchers to have access to calibration data as high as 100MHz.

Ultrasound fields generated via pulsed laser excitation of highly photo-absorptive nanocomposites (made of carbon nanotubes dispersed in polymeric material) can generate high-pressure (up to 10 MPa) ultrasound pulses with frequency content up to 100 MHz.

The advantages of such a nanocomposite source are that a single ultrasound pulse spans the measurement bandwidth of most hydrophones and, unlike piezoelectric transducers, high-pressure levels can be achieved without focusing, which ensures errors due to spatial-averaging are small.

Additionally, the high signal-to-noise lowers the uncertainty in the measurement. This will facilitate the accurate characterisation required for patient safety, performance validation and the development of new high frequency ultrasound technologies.