The Generation and Control of Ultrasonic Waves in Nonlinear Media

The objective of this thesis is to utilise modern open-design ultrasound research platforms to develop new and advance several existing techniques that incorporate nonlinear phenomena. Acoustically, nonlinearity refers to changes in speed of sound, attenuation or elasticity that vary with frequency, temperature or pressure. These effects cannot be linearised by the wave equation and require fluid dynamics and elasticity equations to be fully understood. While this is a hindrance and source of error in many areas of ultrasound such as high-intensity focused ultrasound (HIFU) and medical imaging, nonlinearities do have uses in non-destructive guided wave (GW) testing. These effects are influenced greatly by the transducer surface pressure, and so precise control of the excitation is necessary to achieve the desired nonlinear effect, if any, in the medium. In this thesis, aided by the use of two new research platforms, several new ultrasound techniques were developed. It was shown the frequency content in the electrical waveform is pertinent and so distortion must be minimised. This requirement conflicts with several hardware limitations, however. Accordingly, a genetic algorithm was applied to find novel switched waveform designs. It was found to achieve a 2% granularity in amplitude control with harmonic reduction, where existing waveform designs could not produce any. This fine amplitude control is a requirement for array applications. Following this, a technique to control the direction of GWs without knowledge of the waveguide was devised. Recordings of a propagating GW, induced by the first element of an array transducer, were re-transmitted in a recursive fashion. The effect was that the transducer's transmissions constructively interfered with the transverse wave, causing most of the guided wave energy to travel in the direction of the transducer's spatial influence. Experimental results show a 34 dB enhancement in one direction compared with the other. GWs were then applied to bone for two purposes: for assessment of osteoporosis and for measurement of skull properties to assist transcranial therapy. It was shown that existing methods for obtaining dispersion curves are ineffectual due to limitations in the available sampling area. A signal processing scheme was devised to temporally align transverse dispersive waves so that beamforming style techniques could be applied to prove or disprove the existence of certain modes. The technique in combination with multiplication was applied to numerical, ex vivo and in vivo experiments. It was found to improve the contrast of the higher order modes. The technique could improve the reliability of osteoporosis diagnosis with ultrasound, but may also prove useful for acquiring dispersion images in NDT. Numerically the technique was shown to improve the S3 and A3 mode intensity by 6 dB and 13 dB respectively compared with an existing Fourier method. In skull, a relationship was found between the curved therapeutic array geometry and the delay profile necessary to form GWs in skull. Several numerical models were tested and it was shown that the thickness could be obtained from the group velocity. The estimated maximum error using this technique was 0.2 mm. Since the data is co-registered with the therapeutic elements, this method could be used to improve the accuracy of thermal treatments in the brain. Finally, the application of switched excitation for HIFU was considered. To improve on cost, efficiency and size, alternative excitation methods have the potential to replace the linear amplifier circuitry currently used in HIFU. In this final study, harmonic reduction pulse width modulation (HRPWM) was proposed as an algorithmic solution to the design of switched waveforms. Its appropriateness for HIFU was assessed by design of a high power 5 level unfiltered amplifier and subsequent thermal-only lesioning of ex vivo chicken breast. HRPWM produced symmetric, thermal-only lesions that were the same size as their linear amplifier equivalents (p > 0.05). These results demonstrate that HRPWM can minimise HIFU drive circuity size without the need for filters to remove harmonics or adjustable power supplies to achieve array apodisation. Overall it has been shown in this thesis that precise control of the nonlinear wave phenomena can be afforded when using open-platform ultrasound research hardware. The methods described within may reduce the cost and increase the efficacy of future commercial systems.