The feasibility of ultrasound elastography in monitoring high-intensity focused ultrasound therapy

• Main Author: Suomi, Visa
• Format: Dissertation
• Language: English
• Published: University of Oxford 2016

High-intensity focused ultrasound (HIFU) therapy is a non-invasive therapy method for the treatment of solid tumours. One of the main barriers to wider clinical adoption of HIFU is the lack of reliable and cost-efficient therapy monitoring. One promising technique is acoustic radiation force (ARF) ultrasound elastography, which relies on the temperature dependence of tissue properties to affect displacement amplitudes. An extensive literature review of tissue acoustic, thermal and mechanical properties identified an absence of data on the temperature dependence of the viscoelastic properties. Experiments were carried out in ex vivo liver to determine these properties over the range of temperatures and frequencies relevant to ARF. ARF induced displacements were quantified experimentally in a tissue-mimicking phantom. The comparison of sine and square modulated ARF excitation showed that square modulation results in higher peak amplitudes and smaller second harmonic components in the frequency domain. A simulation model was developed, which accurately reproduced the observed displacements with different modulation frequencies and power levels. The effect of temperature on ARF displacements was measured experimentally during HIFU therapy in ex vivo liver. Simulations captured the same phenomena as observed in the experiments. ARF displacements during HIFU therapy were shown to initially depend on attenuation before ablation, but once the temperature exceeded the ablation threshold, viscoelastic properties dominated. A second barrier for HIFU is the need to reliably deliver acoustic energy to the target. Nonlinear acoustic simulations were conducted using patient derived computed tomography (CT) data to evaluate the HIFU therapy in the kidney. The simulations revealed that attenuation and refraction contributed roughly equally to intensity loss. Focal splitting associated with refraction was found to be the main effect reducing the heating efficacy, and a method to employ phase correction at the source was proposed to mitigate the effect.