Abstract
Multiple ultrasound elastography techniques rely on acoustic radiation force (ARF) in monitoring high-intensity focused ultrasound (HIFU) therapy. However, ARF is dependent on tissue attenuation and sound speed, both of which are also known to change with temperature making the therapy monitoring more challenging. Furthermore, the viscoelastic properties of tissue are also temperature dependent, which affects the displacements induced by ARF. The aim of this study is to quantify the temperature dependent changes in the acoustic and viscoelastic properties of liver and investigate their effect on ARF induced displacements by using both experimental methods and simulations. Furthermore, the temperature dependent viscoelastic properties of liver are experimentally measured over a frequency range of 0.1–200 Hz at temperatures reaching 80 °C, and both conventional and fractional Zener models are used to fit the data.The fractional Zener model was found to fit better with the experimental viscoelasticity data with respect to the conventional model with up to two orders of magnitude lower sum of squared errors (SSE). The characteristics of experimental displacement data were also seen in the simulations due to the changes in attenuation coefficient and lesion development. At low temperatures before thermal ablation, attenuation was found to affect the displacement amplitude. At higher temperature, the decrease in displacement amplitude occurs approximately at 60–70 °C due to the combined effect of viscoelasticity changes and lesion growth overpowering the effect of attenuation. The results suggest that it is necessary to monitor displacement continuously during HIFU therapy in order to ascertain when ablation occurs.
Highlights
One of the main barriers for the wide spread use of high-intensity focused ultrasound (HIFU) has been the lack of robust, reliable and cost-efficient monitoring methods
For both conventional and fractional Zener models the magnitude of the average modulus were fit by using a nonlinear least squares method
It is likely that at high frequencies the dynamic mechanical analysis (DMA) system cannot distinguish between ambient system noise and the response due to the tissue
Summary
One of the main barriers for the wide spread use of high-intensity focused ultrasound (HIFU) has been the lack of robust, reliable and cost-efficient monitoring methods. The increase in stiffness has been shown to be reversible at temperatures below 60 ◦C (Wu et al, 2001), and irreversible once temperatures exceed 60 ◦C due to collagen denaturation (Wu et al, 2001; Lepetit et al, 2000). This suggests that measurements of stiffness can be used to distinguish thermally ablated tissue (Varghese et al, 2002, 2003)
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