Temperature dependence of nickel ion release from nitinol medical devices.

Nitinol exhibits unique (thermo)mechanical properties that make it central to the design of many medical devices. However, nitinol nominally contains 50 atomic percent nickel, which if released in sufficient quantities, can lead to adverse health effects. While nickel release from nitinol devices is typically characterized using in vitro immersion tests, these evaluations require lengthy time periods.

A closer look at ultrasonic attenuation and heating in a tissue-mimicking material.

A well-characterized ultrasound tissue-mimicking material (TMM) can be important in determining the acoustic output and temperature rise from high intensity therapeutic ultrasound (HITU) devices and also in validating computer simulation models. A HITU TMM previously developed and characterized in our laboratory has been used in our acoustic and temperature measurements as well as modeled in our HITU simulation program. A discrepancy between thermal measurement and simulation, though, led us to further investigate the TMM properties.

Extending the Utility of the Parabolic Approximation in Medical Ultrasound Using Wide-Angle Diffraction Modeling.

Wide-angle parabolic models are commonly used in geophysics and underwater acoustics but have seen little application in medical ultrasound. Here, a wide-angle model for continuous-wave high-intensity ultrasound beams is derived, which approximates the diffraction process more accurately than the commonly used Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation without increasing implementation complexity or computing time. A method for preventing the high spatial frequencies often present in source boundary conditions from corrupting the solution is presented.

Experimental validation of a nonlinear derating technique based upon Gaussian-modal representation of focused ultrasound beams.

A technique useful for performing derating at acoustic powers where significant harmonic generation occurs is illustrated and validated with experimental measurements. The technique was previously presented using data from simulations. The method is based upon a Gaussian representation of the propagation modes, resulting in simple expressions for the modal quantities, but a Gaussian source is not required.

Thermal safety simulations of transient temperature rise during acoustic radiation force-based ultrasound elastography.

Ultrasound transient elastography is a new diagnostic imaging technique that uses acoustic radiation force to produce motion in solid tissue via a high-intensity, long-duration "push" beam. In our previous work, we developed analytical models for calculating transient temperature rise, both in soft tissue and at a bone/soft tissue interface, during a single acoustic radiation force impulse (ARFI) imaging frame. The present study expands on these temperature rise calculations, providing applicable range assessment and error analysis for a single ARFI frame.

Nonlinear derating of high-intensity focused ultrasound beams using Gaussian modal sums.

A method is introduced for using measurements made in water of the nonlinear acoustic pressure field produced by a high-intensity focused ultrasound transducer to compute the acoustic pressure and temperature rise in a tissue medium. The acoustic pressure harmonics generated by nonlinear propagation are represented as a sum of modes having a Gaussian functional dependence in the radial direction. While the method is derived in the context of Gaussian beams, final results are applicable to general transducer profiles.

A parametric study of error in the parabolic approximation of focused axisymmetric ultrasound beams.

The parabolic approximation results in a tractible model for studying ultrasound beams, but the limits of validity of the approximation are often presented only qualitatively. In this work the most common model for axisymmetric ultrasound beam propagation, the Kuznetsov-Zabolotskaya-Khokhlov equation, is directly compared with the more general Westervelt equation with regard to diffractive and absorptive effects in continuous wave beams. The parametric study compares the solutions of the two models as a function of source frequency and focusing geometry using peak focal pressure, the axial location at which that peak occurs, and the loss due to absorption as metrics.

Thresholds for nonlinear effects in high- intensity focused ultrasound propagation and tissue heating.

For a variety of reasons, including their simplicity and ability to capitalize upon superposition, linear acoustic propagation models are preferable to nonlinear ones in modeling the propagation of high-intensity focused ultrasound (HIFU) beams. However, under certain conditions, nonlinear models are necessary to accurately model the beam propagation and heating. In analyzing the performance of a HIFU system, it is advantageous to know before the analysis whether a linear model suffices.

Temperature modes for nonlinear Gaussian beams.

In assessing the influence of nonlinear acoustic propagation on thermal bioeffects, approximate methods for quickly estimating the temperature rise as operational parameters are varied can be very useful. This paper provides a formula for the transient temperature rise associated with nonlinear propagation of Gaussian beams. The pressure amplitudes for the Gaussian modes can be obtained rapidly using a method previously published for simulating nonlinear propagation of Gaussian beams.

Gaussian representation of high-intensity focused ultrasound beams.

A method for fast numerical simulation of high-intensity focused ultrasound beams is derived. The method is based on the frequency-domain representation of the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation, and assumes for each harmonic a Gaussian transverse pressure distribution at all distances from the transducer face. The beamwidths of the harmonics are constrained to vary inversely with the square root of the harmonic number, and as such this method may be viewed as an extension of a quasilinear approximation.