Rethinking ultrasonics

A new type of ultrasound detector that relies on optical sensors instead of piezoelectric transducers is being developed at University College London for use in the medical and industrial sectors.

The technology could be used for applications ranging from calibrating high-intensity focused ultrasound (HIFU) equipment, medical imaging or non-destructive testing in nuclear reactors.

Team leader Paul Beard, professor of biomedical photo-acoustics at UCL, said the detector’s optical sensing technology is based on a Fabry Perot polymer film interferometer.

The interferometer is fabricated through a vacuum process, which deposits two thin layers of gold with a thin polymer spacer material in the middle. The team, currently in the process of constructing a full prototype, proposes depositing this thin film interferometer on the tip of an optical-fibre cable.

The system would produce short pulses of low-energy laser light through the cable to stimulate the emission of ultrasonic acoustic waves from a target area. These waves create pressure that compresses or expands the polymer film inside the interferometer.

This stress changes the thickness of the polymer material and consequently changes the intensity of the laser light reflected back from the interferometer.

The reflected light will then be directed back up the fibre-optic cable to a photo diode that provides an electrical output proportional to the intensity of the light directed onto it.

The intensity of this light holds information on not only the pressure of the acoustic wave but also the heat that arises from the acoustic wave pressure.

Beard said the ability to measure pressure and temperature could significantly improve HIFU cancer therapy equipment, which uses a focused ultrasound beam to heat a tumour and destroy it.

He added: ‘There is a lot of interest in trying to optimise the process and understand the acoustic and thermal interactions that go on. There is a need for a sensor that can measure both the acoustic field and the temperature at the same time and also a sensor that is robust. Conventional sensors are usually damaged by the HIFU field.’

The first commercial product will likely be for HIFU equipment.