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Accurate monitoring of high intensity focused ultrasound (HIFU) therapy is critical for widespread clinical use. Pulse-echo diagnostic ultrasound (DU) is known to exhibit temperature sensitivity through relative changes in time-of-flight between two sets of radio frequency (RF) backscatter measurements, one acquired before and one after therapy. These relative displacements, combined with knowledge of the exposure protocol, material properties, heat transfer, and measurement noise statistics, provide a natural framework for estimating the administered heating, and thereby therapy. The proposed method, termed displacement analysis, identifies the relative displacements using linearly independent displacement patterns, or modes, each induced by a particular time-varying heating applied during the exposure interval. These heating modes are themselves linearly independent. This relationship implies that a linear combination of displacement modes aligning the DU measurements is the response to an identical linear combination of heating modes, providing the heating estimate. Furthermore, the accuracy of coefficient estimates in this approximation is determined a priori, characterizing heating, thermal dose, and temperature estimates for any given protocol. Predicted performance is validated using simulations and experiments in alginate gel phantoms. Evidence for a spatially distributed interaction between temperature and time-of-flight changes is presented.

The importance of nonlinear acoustic wave propagation and ultrasound-induced cavitation in the acceleration of thermal lesion production by high intensity focused ultrasound was investigated experimentally and theoretically in a transparent protein-containing gel. A numerical model that accounted for nonlinear acoustic propagation was used to simulate experimental conditions. Various exposure regimes with equal total ultrasound energy but variable peak acoustic pressure were studied for single lesions and lesion stripes obtained by moving the transducer. Static overpressure was applied to suppress cavitation. Strong enhancement of lesion production was observed for high amplitude waves and was supported by modeling. Through overpressure experiments it was shown that both nonlinear propagation and cavitation mechanisms participate in accelerating lesion inception and growth. Using B-mode ultrasound, cavitation was observed at normal ambient pressure as weakly enhanced echogenicity in the focal region, but was not detected with overpressure. Formation of tadpole-shaped lesions, shifted toward the transducer, was always observed to be due to boiling. Boiling bubbles were visible in the gel and were evident as strongly echogenic regions in B-mode images. These experiments indicate that nonlinear propagation and cavitation accelerate heating, but no lesion displacement or distortion was observed in the absence of boiling.

Improved high-intensity focused ultrasound (HIFU) surgical applicators are required for use in a surgical environment. We report on the performance and characteristics of a new solid-cone HIFU applicator. Previous HIFU devices used a water-filled stand-off to couple the ultrasonic energy from the transducer to the treatment area. The new applicator uses a spherically-focused element and a solid aluminum cone to guide and couple the ultrasound to the tissue. Compared with the water-filled applicators, this new applicator is simpler to set up and manipulate, cannot leak, prevents the possibility of cavitation within the coupling device, and is much easier to sterilize and maintain during surgery. The design minimizes losses caused by shear wave conversion found in tapered solid acoustic velocity transformers operated at high frequencies. Computer simulations predicted good transfer of longitudinal waves. Impedance measurements, beam plots, Schlieren images, and force balance measurements verified strong focusing and suitable transfer of acoustic energy into water. At the focus, the -3 dB beam dimensions are 1.2 mm (axial)/spl times/0.3 mm (transverse). Radiation force balance measurements indicate a power transfer efficiency of 40%. In vitro and in vivo tissue experiments confirmed the applicator's ability to produce hemostasis.

A new method for computing wave scattering from rough surfaces, called the operator expansion (OE) method, has been proposed by D. M. Milder [J. Acoust. Soc. Am. 89(2), 529–541 (1991)]. In this paper, the OE method is examined in its application to acoustic scattering from one‐dimensional randomly rough surfaces with Gaussian and Pierson–Moskowitz roughness spectra satisfying the pressure release (Dirichlet) boundary condition. The operator expansion solution, which is expressed in a systematic series, is found to converge rapidly and monotonically for moderately rough surfaces, that is, for surfaces whose slope‐height roughness parameter khs, given by the product of acoustic wave number k, rms surface height h, and rms surface slope s, is less than about 0.25. Through comparison with a numerically exact integral equation solution, the OE method is found to be accurate over a wide range of incident and scattering angles. The method is currently used in a Monte Carlo computation of the scattering cross sec...

Bioheat transfer equation (BHTE) estimates of applied dose during HIFU therapy typically use prior knowledge or assume standard values for tissue properties that determine thermal diffusivity (K) and heat source (Q). We have developed a novel signal‐processing based technique to noninvasively estimate these parameters in situ based on analysis of raw backscattered RF data from two localized HIFU exposures, one at sub‐ablative intensities to determine K, and another at therapeutic intensities to obtain Q. Both exposures are performed prior to therapy. To estimate K, a short HIFU exposure is applied resulting in a temperature rise of less than 15° C. The Gaussian radius of the temperature induced apparent strain profile during cool down is estimated and fit to an approximate analytical heat diffusion expression to obtain K. Independent estimates of K derived using the transient hotwire technique validated the ultrasonic measurements. To estimate Q, an audio range hydrophone was acoustically coupled to the s...

The spatial distribution of tissue temperature is an essential indicator of thermal therapy progress, treatment safety and efficacy. Here, it is shown through in vitro experiments that temperature rise can be accurately measured during therapy delivery and post‐treatment cool down phases using RF backscatter data collected with a modified ultrasound scanner. RF data is acquired prior to, during, and after HIFU exposures, in tissue mimicking phantoms and excised animal tissue. Through two calibration experiments, initial estimates for key BHTE parameters (local thermal diffusivity, and magnitude of the HIFU heat source) and the temperature dependence of ultrasonic travel time are obtained prior to therapy. Tissue heterogeneity over a larger targeted region of interest is modeled as a change in the magnitude of the focal heat source. During therapy this magnitude is updated using an iterative optimization technique that minimizes the difference between predicted and measured travel time values. Temperature ...

In our research group, we desire measurements of the large pressure amplitudes generated by the shock waves used in shock wave lithotripsy (SWL) and the large acoustic intensities used in High Intensity Focused Ultrasound (HIFU). Conventional piezoelectric or PVDF hydrophones can not be used for such measurements as they are damaged either by cavitation, in SWL applications, or heat, in HIFU applications. In order to circumvent these difficulties, we have utilized optical fiber hydrophones in SWL that do not cavitate, and small glass probes and a scattering technique for measurements of large HIFU intensities. Descriptions of these techniques will be given as well as some typical data.

A dosimetry study of the high‐temperature and pressure regimes involved in high‐intensity focused ultrasound (HIFU) requires experiments on biological tissues because no synthetic tissue‐mimicking phantom is available. Unfortunately, the development of coagulative lesions cannot be observed in real‐time in opaque tissues. Furthermore, the natural heterogeneous structure of tissue complicates direct comparison with numerical models. In this study, a new optically transparent phantom is evaluated. It is principally composed of a polyacrylamide gel, and includes a thermally sensitive indicator protein that becomes optically diffusive when denatured. Various tests were undertaken to characterize the acoustical, thermal, and optical properties of this material for a range of protein concentrations. The attenuation coefficient can be usefully modified by adjusting the quantity of embedded proteins to permit some selection of acoustic regime. It is also possible to emphasize cavitation activity at lower BSA concentrations, or thermal effects at higher concentrations. This new phantom adequately matches tissue for most of the measured parameters and facilitates the study of the HIFU bioeffects. [Work supported by ONR Grant No. N00014‐96‐1‐0630 and NIH Grant No. HL64208‐02.]

Preliminary studies of the operator expansion method applied to scattering from rough surfaces satisfying the Dirichlet boundary condition [P. J. Kaczkowski and E. I. Thorsos, J. Acoust. Soc. Am. 90, 2258 (A) (1991)] have indicated that this relatively new method [D. M. Milder, J. Acoust. Soc. Am. 89, 529–541 (1991)] has a broad range of validity. For moderate rms surface slopes, the method is accurate over almost all scattering angles, and represents a vast improvement over the Kirchhoff approximation and small perturbation methods. Further study of the operator expansion method has led to new insights that establish a link between the formal validity of the method and the validity of the Rayleigh hypothesis. While the Rayleigh hypothesis appears to place a strict limit on the operator expansion, numerical examples will be presented that illustrate that the accuracy of the scattering cross section computed by the operator expansion method degrades only gradually as the rms slope is increased beyond that ...

A new method for computing wave scattering from rough surfaces has been proposed [D. M. Milder, J. Acoust. Soc. Am. 89, 529–541 (1991)]. The solution is obtained using a nonlocal operator that directly relates the surface values of the scattered field to the surface values of its normal derivative. This operator is expressed as a series of terms containing powers of an integral operator, which for a field incident on a flat surface provides the vertical derivative of the scattered field through Fourier transforms. The method has been applied to Dirichlet surfaces whose height varies randomly in only one direction (1‐D surfaces) with either Gaussian or power law (Pierson–Moskowitz) spectra. Through comparison with an exact numerical method, the tapered plane‐wave integral equation method [E. I. Thorsos, J. Acoust. Soc. Am. 83, 78–92 (1988)], the operator expansion (OE) solution is seen to have a remarkably broad region of validity, while its computational cost is orders of magnitude lower than that of the ...

A new method for imaging a moving fluid using acoustic tomography is evaluated by numerical simulation. A cross section of the medium is probed by high‐frequency acoustic waves from several different directions. It can be shown that the resulting measured travel time data contain sufficient information to reconstruct both the spatially varying sound speed and the transverse component of the fluid vorticity [K. B. Winters and D. Rouseff, Inverse Problems 6, L33 (1990)]. The results are exact to within the validity of the straight‐ray geometric acoustics approximation. To evaluate a discrete version of the reconstruction algorithm, a three‐dimensional stably stratified mixing layer is simulated. The flow exhibits characteristic features in both density (sound speed) and vorticity. The dynamics of the fluid flow can be described as the instability of a vortex sheet. The acoustic travel time is calculated by integrating through the simulated flow fields. The synthetic data are then inverted to yield reconstru...

Inversion for the one‐dimensional sound‐speed profile in the sofar waveguide is performed using damped least squares (Marquardt‐Levenberg algorithm). The study uses the split‐step parabolic equation method to solve for the synthetic acoustic pressure field in the ocean. Data for the inversion are also provided by the same model, and the robustness of this technique is investigated using different samples of the sound field: horizontal and vertical arrays of varying length and, single point data of varying bandwidth. The parameter resolution matrix and the singular value decomposition of the matrix of partial derivatives lend insight into the inversion and provide quantitative measures of the relative merit of different experimental configurations, and this will be discussed.