Numerical simulation of the transient temperature field from an annular focused ultrasonic transducer

Numerical and Experimental Evaluation of High-Intensity Focused Ultrasound-Induced Lesions in Liver Tissue Ex Vivo

OBJECTIVES

Recent advances in the field of acoustics and piezoelectric and ultrasound transducers have led to new approaches to the diagnosis and treatment of certain diseases. One method of treatment with ultrasonic waves is high-intensity focused ultrasound (HIFU) treatment, which is a thermal therapeutic method used to treat malignant tumors. Although a variety of treatment-planning strategies using ultrasonic waves have been investigated, little clinical success has been achieved. Computational modeling is a powerful tool for predicting device performance.

METHODS

The heating induced by a concave transducer with operating powers of 85 and 135 W was studied, and the experimental results presented in this article verify its applicability. Numerical simulations of the nonlinear acoustic field were performed by using the Westervelt and Khokhlov-Zabolotskaya-Kuznetsov equations. Heat transfer was measured for the 2 operational powers, and the results were compared with ex vivo experimental results. In addition, thermal dose contours for both the simulation and experimental results were calculated to investigate the ablated area.

RESULTS

Good agreement was found between the experimental and numerical results. The results show that the average temperature deviations calculated at the focal point were 12.8% and 4.3% for transducer powers of 85 and 135 W, respectively.

CONCLUSIONS

This study provides guidance to HIFU practitioners in determining lesion size and identifying nonlinear effects that should be considered in HIFU procedures.

Potential femoral head osteonecrosis model induced by high-intensity focused ultrasound

Osteonecrosis of the femoral head is a common disease that can result in complex hip replacement. To evaluate potential treatments, a model that consistently creates osteonecrosis is needed. We studied and demonstrated the possibility of developing an osteonecrosis model using high-intensity focused ultrasound (HIFU) on canine femora in vitro. To achieve these goals, the temperature in the medullary cavity of the femoral head was measured. A phenomenological model was developed to fit the measured temperature variations with the HIFU parameters for similar HIFU experiments on femoral heads. The average temperature discrepancy between model and measured values was less than 0.83°C. Histology confirmed that the temperature in the medullary cavity can be elevated to a level at which an acute thermal injury is created. HIFU has the potential to be used in a non-invasive model of osteonecrosis.

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.

A comparison of the thermal-dose equation and the intensity-time product, Itm, for predicting tissue damage thresholds

Thermal dose is the most generally accepted concept for estimating temperature-related tissue damage thresholds in high-intensity focused ultrasound (HIFU) procedures. However, another approach based on the intensity-time product I t(m) =D has been used, where D is a tissue-dependent damage threshold, I is the spatial-peak, temporal-average intensity and t is time. In this study, these two approaches were compared analytically by substituting a well-known soft-tissue solution for temperature vs. time into the thermal dose equation. From power law fits of I vs. t, m was found to fall between about 0.3 and 0.8. In terms of the intensity required for cell death for a given exposure time, the standard deviation of the error between the full thermal-dose formulation and the I t(m) =D prediction based upon the power-law fit was less than 5% for focal beam diameters up to 3 mm. Thus, for the practical range of HIFU parameters examined, the intensity-time product relationship is equivalent to the thermal dose formulation.