A modeling approach to predict acoustic nonlinear field generated by a transmitter with an aluminum lens.

PURPOSE: In this work, the authors propose a modeling approach to compute the nonlinear acoustic field generated by a flat piston transmitter with an attached aluminum lens. METHODS: In this approach, the geometrical parameters (radius and focal length) of a virtual source are initially determined by Snell's refraction law and then adjusted based on the Rayleigh integral result in the linear case. Then, this virtual source is used with the nonlinear spheroidal beam equation (SBE) model to predict the nonlinear acoustic field in the focal region. RESULTS: To examine the validity of this approach, the calculated nonlinear result is compared with those from the Westervelt and (Khokhlov-Zabolotskaya-Kuznetsov) KZK equations for a focal intensity of 7 kW/cm(2). Results indicate that this approach could accurately describe the nonlinear acoustic field in the focal region with less computation time. CONCLUSIONS: The proposed modeling approach is shown to accurately describe the nonlinear acoustic field in the focal region. Compared with the Westervelt equation, the computation time of this approach is significantly reduced. It might also be applicable for the widely used concave focused transmitter with a large aperture angle.

Comparative study of lesions created by high-intensity focused ultrasound using sequential discrete and continuous scanning strategies.

Lesion formation and temperature distribution induced by high-intensity focused ultrasound (HIFU) were investigated both numerically and experimentally via two energy-delivering strategies, i.e., sequential discrete and continuous scanning modes. Simulations were presented based on the combination of Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation and bioheat equation. Measurements were performed on tissue-mimicking phantoms sonicated by a 1.12-MHz single-element focused transducer working at an acoustic power of 75 W. Both the simulated and experimental results show that, in the sequential discrete mode, obvious saw-tooth-like contours could be observed for the peak temperature distribution and the lesion boundaries, with the increasing interval space between two adjacent exposure points. In the continuous scanning mode, more uniform peak temperature distributions and lesion boundaries would be produced, and the peak temperature values would decrease significantly with the increasing scanning speed. In addition, compared to the sequential discrete mode, the continuous scanning mode could achieve higher treatment efficiency (lesion area generated per second) with a lower peak temperature. The present studies suggest that the peak temperature and tissue lesion resulting from the HIFU exposure could be controlled by adjusting the transducer scanning speed, which is important for improving the HIFU treatment efficiency.

Controllable in vivo hyperthermia effect induced by pulsed high intensity focused ultrasound with low duty cycles.

High intensity focused ultrasound (HIFU)-induced hyperthermia is a promising tool for cancer therapy. Three-dimensional nonlinear acoustic-bioheat transfer-blood flow-coupling model simulations and in vivo thermocouple measurements were performed to study hyperthermia effects in rabbit auricular vein exposed to pulsed HIFU (pHIFU) at varied duty cycles (DCs). pHIFU-induced temperature elevations are shown to increase with increasing DC. A critical DC of 6.9% is estimated for temperature at distal vessel wall exceeding 44 °C, although different tissue depths and inclusions could affect the DC threshold. The results demonstrate clinic potentials of achieving controllable hyperthermia by adjusting pHIFU DCs, while minimizing perivascular thermal injury.

A feasibility study of temperature rise measurement in a tissue phantom as an alternative way for characterization of the therapeutic high intensity focused ultrasonic field.

The feasibility that temperature field measurements in vitro as an alternative way to characterize the high intensity focused ultrasound (HIFU) field used in therapeutic applications has been explored in a phantom study. Thermocouples (copper-constantan, diameter 0.125 mm) are embedded in a phantom filled with tissue mimicking material that simulates the thermal and acoustic properties of soft-tissue. The temperature rises as a function of ultrasound exposure time near the focus of a HIFU transducer (1.1 MHz, active radius a=32 mm, geometric focal length=62 mm) of various acoustic powers up to 30 W are measured and compared with predicted values using a simple nonlinear Gaussian model. The experimental results can be explained well by the model if no acoustic cavitation takes place. When the acoustic power become higher (>5 W) and the local temperature elevation >15 degrees C and the local temperature is >40 degrees C at the focal point, cavitation vapor bubbles appear. The presence of the cavitation bubbles may increase the temperature rise rate initially. The bubble aggregates may form along the beam axis under sonication and then eventually makes the temperature elevation reach a saturated value. When acoustic cavitation occurs, the bubble-assisted enhancement of the initial temperature rise (exposure time t<2s) can still be predicted by the theory.