Ultrasound phased array simulations for hyperthermia

Hyperthermia uses elevated temperature to achieve therapeutic benefit in cancer treatment. Ultrasound is one of the most popular modalities in hyperthermia. The numerical simulations for the pressure, power, and temperature fields generated by ultrasound phased arrays are explored in this thesis.;Angular spectrum approach rapidly and accurately simulates the pressure fields for large ultrasound phased arrays. Angular spectrum simulations with the spatial propagator produce accurate results in the central portion of the computational grid while significant errors are produced near the edge due to the finite extent of the window applied to the spatial propagator. Angular spectrum simulations with the spectral propagator are more erroneous due to the undersampling in the spatial frequency domain. However, the error is greatly reduced by angular restriction in non-attenuating media. The spatial and spectral propagators achieve similar accuracy in attenuating media or for apodized sources. Angular spectrum simulations with pressure sources are more accurate than those with normal velocity sources. The pressure source plane should be larger than the array aperture and at least one wavelength away from the array. While the above criteria are established for homogeneous and linear simulations, the angular spectrum approach can also simulate nonlinear harmonic propagations in layered tissue media. Results show that the absorption of these nonlinear harmonics is important to high temperature therapy, but is negligible to mild temperature hyperthermia. The pressure fields simulated with the angular spectrum approach are used as inputs to the bio-heat transfer equation to model temperatures.;The power and temperature distributions are simulated in tumor-tissue models, and the goal is to achieve relatively uniform temperature between 41--43°C in tumor and minimize the temperature rise in normal tissue. The power fields are produced either with single-focus spot scans optimized by the thermal inverse method, or with multiple-focus scans optimized by the waveform diversity method. Results show that the single-focus spot scan with the thermal inverse optimization achieves good results for small tumors but produces excessive intervening tissue heating for large tumors. The multiple-focus scan with the waveform diversity optimization achieves superior results in heating large deep-seated tumors without inducing excessive intervening tissue heating.