Characterization Of HIFU Acoustic Field Using An Approach:Measurement Combined With Theoretical Model

High intensity focused ultrasound (HIFU) is a non-invasive treatment technique for treatment of solid tumors. The ultrasound intensity at focus can reach as high as103W/cm2and HIFU exposure results in tumor ablation in seconds. In order to reduce the side effects and ensure the treatment safety, accurate prediction and measurement of the acoustic field distribution in the focal region is critical for ensuring a successful treatment. This paper introduces the measurement methods and theory models used for HIFU fields, and proposed a method for characterization of the HIFU field when shock wave generates.For the transducer with a wide aperture angle, a combined time-and frequency-domain method is applied to simulate the nonlinear field distribution. The spheroidal beam equation (SBE) is used to describe the field. The diffraction and absorption effects are calculated in frequency domain, while the nonlinearity effect is calculated in time domain. The results show that this method could calculate the shock wave effectively without oscillation at the shock front. In addition, the computation time by this method is about1/16of that by traditional frequency-domain method when the harmonic number is200.When the shock wave generates, conventional sensors for sound field characterization generally cannot endure the extreme conditions, such as strong pressure, broad spectrum. This paper proposes a combined experiment and simulation method to characterize the HIFU field from a focused tranduscer with a large apertuare angle. This approach consists of three step:(1) parameters of HIFU source is calibrated in the linear case;(2) relationship between the source pressure amplitude and the voltage excitation is determined by using the ratio of the second harmonic to the fundamental component of focal waveform;(3) HIFU field is calculated with SBE model. Measurement of the HIFU field by using the fiber optic probe hydrophone confirms the validity of this method. However, for very low acoustic excitations, the accuracy of this method is not guaranteed because of low second harmonic amplitude. When the shockwave appears, the measured waveforms differ from simulation results, but the spectrums agree well in the frequency range of50MHz.The proposed method shows potential prospect in the acoustic characterization of HIFU field generated from the focused transmitter with a large aperture. It will be helpful for the prediction of HIFU field distribution in vivo.