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- Abstract
- I. INTRODUCTION
- II. ANALYTIC APPROACHES
- III. NUMERICAL MODEL
- IV. RESULTS
- V. CONCLUSION
- References

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Acoust Phys. Author manuscript; available in PMC 2010 July 21.

Published in final edited form as:

Acoust Phys. 2009 July 21; 55(4-5): 463–476.

doi: 10.1134/S1063771009040034PMCID: PMC2776740

NIHMSID: NIHMS117317

O.V. Bessonova and V.A. Khokhlova

Department of Acoustics, Physics Faculty, Moscow State University, Leninskie gory, Moscow, 119992, Russia

Center for Industrial and Medical Ultrasound, Applied Physics Laboratory, University of Washington, Seattle, Washington, 981056, US

O.V. Bessonova: ur.usm.syhp.663sca@aglo; V.A. Khokhlova: ur.usm.syhp.663sca@arev; M.R. Bailey: ude.notgnihsaw.lpa@yeliab

See other articles in PMC that cite the published article.

In this work, the influence of nonlinear and diffraction effects on amplification factors of focused ultrasound systems is investigated. The limiting values of acoustic field parameters obtained by focusing of high power ultrasound are studied. The Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation was used for the numerical modeling. Solutions for the nonlinear acoustic field were obtained at output levels corresponding to both pre- and post- shock formation conditions in the focal area of the beam in a weakly dissipative medium. Numerical solutions were compared with experimental data as well as with known analytic predictions.

The study of problems that involve the focusing of intense ultrasonic beams is an important area of nonlinear acoustics [1]. Recently, there has been increasing interest in these problems mainly due to the development of new medical devices for nonlinear diagnostic ultrasound imaging, for noninvasive destruction of tumors (high-temperature hyperthermia or acoustic surgery), for cessation of internal bleeding (acoustic hemostasis), and for kidney stone comminution [2, 3]. All of these applications rely on the focusing of acoustic waves in a nonlinear medium. In ultrasound surgery systems which are already used in clinical practice, the intensity levels in the focal area can reach 10000 - 30000 W/cm^{2} [4]. At these intensities, the shock formation distance for an initially harmonic plane wave with a frequency of 1.5 MHz - typical for medical applications - is only 3 - 5 mm. In most ultrasound surgery devices, this distance is less than the length of the focal area of the beam, therefore it is necessary to account for nonlinear effects when characterizing the acoustic fields of such systems [3].

With an increase of pressure amplitude at the source, the combined effects of nonlinearity and diffraction result in changes of the focusing gains of the acoustic parameters of the beam [5]. Moreover, these changes are different for each parameter of the field. With a further increase of the source output, nonlinear saturation phenomenon occurs and the parameters of the field at the focus no longer depend on the source pressure amplitude. Knowledge of these saturation values in acoustic focusing systems is an interesting problem for fundamental studies of nonlinear waves as well as for practical applications.

Approximate analytic models for estimation of focal pressures and saturation levels in nonlinear beams were proposed more than 50 years ago [6, 7]. These results are still used to estimate the limiting pressures obtained due to focusing. It was shown that analytic predictions in general agree with experimental data; however, they cannot provide quantitatively correct values for various parameters of the acoustic field [8]. The paraxial approach can be used to analytically calculate the change of focusing gains in pre-shock formation regimes [9]. More accurate and detailed investigation of nonlinear focused fields became possible using numerical modeling [5, 10-12]. The change of focusing gains and saturation of acoustic fields at the focus due to nonlinear effects were investigated for initially Gaussian beams [5]. However, the Gaussian source is an idealized model; real transducers have a finite size and therefore have more complicated spatial field distributions. In the paper [11] the nonlinear effects in the field of a weakly focused piston source with parameters typical for medical diagnostic sensors were investigated in more detail. The problem of determining the change in focusing gains due to nonlinear-diffractive effects in strongly focused beams, used in ultrasound surgery, has not yet been solved.

In this work, this problem is investigated numerically using the Khokhlov-Zabolotskaya-Kuznetzov equation. Simulations are performed over a wide range of parameters typical for medical ultrasound transducers in a fluid of low absorption for beams of initially harmonic waves with uniform pressure amplitude at the source. The pressure waveforms, spatial distributions of peak pressures, intensity, and heat deposition (mainly due to absorption of the wave energy at the shocks) are calculated. The maximum focusing gains of the systems working in nonlinear regimes, and also the limiting values of the acoustic parameters in the focal area of the beam are obtained. Numerical solutions are compared with experimental data as well as with known analytic estimates. The results obtained in this work can be used for characterizing the fields of high power focused ultrasound sources in water and in low-absorptive tissue phantoms, for determining the focal values of acoustic parameters of nonlinear fields, and for choosing the optimal operating parameters of medical ultrasound transducers.

In this section, a short overview of the most common analytic approaches used for calculation of saturation levels at the focus of spherical transducers excited by initially harmonic waves are presented. The analytic solutions will be compared with the numerical results obtained in this paper. The first approach was proposed by Naugolnykh and Romanenko [6]. They considered a converging spherical wave, which propagates from the surface of a spherical cap with a radius *F* towards the focal region defined by a sphere of radius *r _{f}*. The propagation of this wave is described by the one-dimensional generalized simple wave equation [13]:

$$\frac{\partial p}{\partial x}+\frac{p}{x}-\frac{\epsilon}{{\rho}_{0}{c}_{0}^{3}}p\frac{\partial p}{\partial \tau}=0.$$

(1)

Here *p* is the acoustic pressure, *x* is the propagation coordinate, τ = *t − x/c*_{0} is the retarded time, *ε* is the coefficient of nonlinearity, *ρ*_{0} is the ambient density, and *c*_{0} is the sound speed. The distance *r _{f}* is defined so that the pressure amplitude of the one-dimensional linear spherically converging wave at

$$2\mathit{ik}\frac{\partial A}{\partial x}+{\Delta}_{\perp}A=0,$$

(2)

with boundary condition

$$A(x=0)=\{\begin{array}{l}{A}_{0}{e}^{i({\omega}_{0}\tau +{\mathit{kr}}^{2}/2F)},r<{a}_{0}\\ 0,r>{a}_{0}\end{array}.$$

(3)

Here *A*_{0} is the pressure amplitude at the source, *k* is the wave number, *ω*_{0} = 2*π f*_{0} is the angular frequency of the wave, *r* is the coordinate across the beam axis, and *a*_{0} is the source radius (Fig. 1). Using the exact solution of Eq. (2) on the beam axis for a focused piston source, Eq. (3):

$$A(x)=\frac{2{A}_{0}}{1-x/F}\mathrm{sin}\left(G\frac{1-x/F}{2x/F}\right),$$

(4)

we obtain that the pressure amplitude at the geometrical focus of the beam *A*(*x* = *F*) is equal to *A*_{0}*G*, where
$G=k{a}_{0}^{2}/2F$ is the focusing gain for the pressure. Substitution of *A*(*x* = *F*), Eq. (4), into the linear solution for spherically converging wave yields the value of *r _{f}* =

$$\begin{array}{c}{p}_{\mathit{sat}}({r}_{f})=\frac{{\rho}_{0}{c}_{0}^{3}}{2\epsilon {f}_{0}{r}_{f}}\frac{1}{\mathrm{ln}(F/{r}_{f})}=\frac{{\rho}_{0}{c}_{0}^{3}}{2\epsilon {f}_{0}F}\frac{G}{\mathrm{ln}\phantom{\rule{0.2em}{0ex}}G}=\frac{\pi {\rho}_{0}{c}_{0}^{2}}{2\epsilon}{\left(\frac{{a}_{0}}{F}\right)}^{2}\frac{1}{\mathrm{ln}\phantom{\rule{0.2em}{0ex}}G}\\ {I}_{\mathit{sat}}({r}_{f})=\frac{{\rho}_{0}{c}_{0}^{5}{G}^{2}}{12{(\mathrm{ln}\phantom{\rule{0.2em}{0ex}}G)}^{2}{f}^{2}{F}^{2}{\epsilon}^{2}}=\frac{{\pi}^{2}{\rho}_{0}{c}_{0}^{3}}{12\epsilon}{\left(\frac{{a}_{0}}{F}\right)}^{4}\frac{1}{{(\mathrm{ln}\phantom{\rule{0.2em}{0ex}}G)}^{2}}.\end{array}$$

(5)

Distributions of the dimensionless amplitude of harmonic wave along the axis *z* = *x*/*F* of the piston transducer for 1D spherically convergent wave (solid line) and linear focused beam (dashed line) with the focusing gain *G* = 10. The waveforms of the same **...**

From the solution given by Eq. (5), it can be seen that the level of saturation pressure *p _{sat}* depends on the source geometry (convergence angle of the wave from the source to the focus sin

Various other analytical models for estimating limiting values of acoustic fields at the focus have been developed as well. Ostrovskii and Sutin employed an approximate approach for step-by-step calculation of the acoustic field of a focused acoustic beam [3]. In this approach, first, the nonlinear focusing of the beam is considered while ignoring diffraction effects. Then, at some distance from the focus, nonlinear propagation is neglected and the linear diffraction problem is solved. Finally, near the focus, nonlinear effects again dominate over diffraction effects and the wave transforms into a sequence of pulses with nearly planar shock fronts. The saturation pressures obtained using this method agrees within an order of magnitude with the values obtained using Eq. (5).

The model of one-dimensional nonlinear propagation of a non-diffractive beam in a focused tube with a Gaussian cross-section was also considered [14]. The saturation pressure at the focus for the sawtooth wave given by this model

$${p}_{\mathit{sat}}=\frac{{\rho}_{0}{c}_{0}^{3}}{2\epsilon {f}_{0}F}\frac{G}{\mathrm{ln}\phantom{\rule{0.2em}{0ex}}\left(2G\right)}$$

is very similar to Eq. (5) and practically coincides with it for high linear focusing gains *G*.

Some analytical results have been obtained for estimation of focusing gains in nonlinear beams. It was shown that the pressure amplitude at the focus of a nonlinear beam can increase fourfold and intensity can increase twofold as compared to the linear beam [7]. Nonlinear increase of focusing gain for peak positive pressure was also calculated using the paraxial approximation, but only for quasi-linear propagation, far from the shock solutions [9].

In the present work, a numerical approach will be used and the results for saturation levels will be compared to the analytical estimation, Eq. (5).

The propagation of high intensity focused acoustic waves will be described by the KZK equation [1]:

$$\frac{\partial}{\partial \tau}\left[\frac{\partial p}{\partial x}-\frac{\epsilon p}{{\rho}_{0}{c}_{0}^{3}}\frac{\partial p}{\partial \tau}-\frac{b}{2{\rho}_{0}{c}_{0}^{3}}\frac{{\partial}^{2}p}{\partial {\tau}^{2}}\right]=\frac{{c}_{0}}{2}{\Delta}_{\perp}p$$

(6)

where Δ_{┴} is the transverse Laplacian, *Δ*_{┴} = 1/*r* /*r*(*r*/*r*) for axially symmetric beam, *b* = *ξ* + 4/3·*η* is the dissipative coefficient, which is assumed to be small.

The boundary condition for a circular focused transducer with uniform amplitude distribution in the parabolic approximation is written as

$$p(x=0,r,\tau )=\{\begin{array}{l}{p}_{0}\mathrm{sin}\left[{\omega}_{0}\left(\tau +{r}^{2}/2{c}_{0}F\right)\right],r\le {a}_{0}\\ 0,r>{a}_{0}\end{array}.$$

(7)

Equations (6) and (7) can be rewritten in dimensionless variables:

$$\frac{\partial}{\partial \theta}\left[\frac{\partial P}{\partial z}-NP\frac{\partial P}{\partial \theta}-A\frac{{\partial}^{2}P}{\partial {\theta}^{2}}\right]=\frac{1}{4G}{\mathrm{\Delta}}_{\perp}P,$$

(8)

$$P(z=0,R,\theta )=\{\begin{array}{ll}\mathrm{sin}\left(\theta +G{R}^{2}\right),& R\le 1\\ 0,& R>1\end{array}.$$

(9)

Here *P* = *p*/*p*_{0} is the acoustic pressure normalized to the initial amplitude *p*_{0} at the source; *θ* = *ω*_{0}*τ* is the dimensionless time; *z* = *x/F* is the dimensionless propagation coordinate normalized to the focal distance, and *R* = *r*/*a*_{0} is the dimensionless transverse coordinate normalized to the source radius.

Equation (8) contains three dimensionless parameters: *N* = *F*/*x _{s}* is the parameter of nonlinearity,

Equation (8) with boundary condition (9) is solved numerically at each step along the coordinate *z* using an operator splitting procedure. A combined time- and frequency- domain approach is used to model diffraction, nonlinearity, and absorption:

$$\frac{\partial P}{\partial z}=\frac{1}{4G}{\mathrm{\Delta}}_{\perp}{\int}_{\theta}^{}\mathit{Pd}{\theta}^{\prime}+\mathit{NP}\frac{\partial P}{\partial \theta}+A\frac{{\partial}^{2}P}{\partial {\theta}^{2}}={L}_{\u0434\u0438\u0444\u0440}+{L}_{\u043d\u0435\u043b\u0438\u043d}+{L}_{\u043f\u043e\u0433\u043b}.$$

(10)

Both the temporal waveform and its spectral representation are necessary to solve different operators *L* in Eq. (10). Both representations are related by the Fourier transform:

$$P(z,\theta ,R)=\sum _{k=-\infty}^{\infty}{C}_{n}(z,R){e}^{-\mathit{in}\theta},$$

(11)

where *C _{n}* is the complex amplitude of the

At each integration step along the beam axis from layer *z* to layer *z*+*hz*, the operator splitting procedure consists of three substeps. At the first substep, diffraction effects are calculated using independent parabolic equations for each of *N*_{max} harmonics of the wave: *C _{n}*/

The algorithm described above was used to obtain the non-dimensional waveforms *P*(*z, θ, R*); the peak positive *P*_{+} and peak negative *P*_{−} pressures, and also the time-averaged intensity of the wave:

$$\stackrel{\sim}{I}\left(z,R\right)=\langle 2{P}^{2}\left(z,\theta ,R\right)\rangle =\frac{1}{\pi}\underset{0}{\overset{2\pi}{\int}}{P}^{2}\left(z,\theta ,R\right)d\theta =4{\sum _{n=1}^{\infty}\mid {C}_{n}\left(z,R\right)\mid}^{2}=\sum _{n=1}^{\infty}{\stackrel{\sim}{I}}_{n}\left(z,R\right),$$

(12)

where *Ĩ _{n}*(

The heating rate is calculated at each step of the grid along *z* as the intensity difference

$$H\left(z,R\right)=-\frac{\stackrel{\sim}{I}\left(z+\mathit{hz},R\right)-\stackrel{\sim}{I}\left(z,R\right)}{\mathit{hz}}$$

(13)

before and after calculation of nonlinear and dissipative operators. The energy absorbed at the shock fronts due to the numerical viscosity of the Godunov scheme is also taken into account in Eq. (13).

Equation (8) was numerically solved using a wide range of values for parameters *G* and *N*. The linear focusing gain of the beam *G* was varied from 10, which corresponds to the case of weakly focused diagnostic transducers, to high values *G* = 40 ÷ 60, which are typical for transducers used for noninvasive surgery. The nonlinear parameter *N*, which is determined by the source amplitude, was varied over the range 0 ≤ *N* ≤ 6. The following values of the parameters of the numerical scheme were used: the number of the calculated number of harmonics in the spectrum was *N _{max}* = 256; the number of the time grid points over the wave period was 512; the integration distance along the beam axis was 0 ≤

One of the most important characteristics of focusing systems is the focusing gain, or amplification factor, i.e. the ratio between the value of some acoustic field parameter at the focus *x* = *F* and the corresponding quantity at the source. For the case of a focused linear harmonic wave, Eq. (4), the focusing gain for the pressure amplitude
$G={A}_{F}/{A}_{0}=k{a}_{0}^{2}/2F$ uniquely determines the amplification of all acoustic parameters of the field at the focus. The peak positive and peak negative pressures in the profile increase *G* times while the mean intensity of the wave and heat deposition increase *G*^{2} times.

The relationship between acoustic parameters in nonlinear beams is much more complicated. Focal waveform becomes asymmetric due to the combined effects of nonlinearity and diffraction; and the peak positive pressure noticeably exceeds the peak negative pressure. To determine the wave intensity and heat deposition, it is necessary to know not only the pressure amplitude but also the temporal waveform or wave spectrum, Eq. (12). With an increase of the initial wave amplitude, i.e. with an increase of the parameter *N* in Eq. (8), the focusing gains will change in different ways for different acoustic parameters and for different values of linear focusing gain *G*.

Figure 2 shows the correction indices *K* = *G _{nonlin}*/

Dependences of correction factors to the focusing gains in nonlinear beam on nonlinear parameter *N* for the peak positive *p*_{+}^{F} (a) and peak negative *p*_{−}^{F} (b) pressures, and intensity *I*^{F} (c). Correction factors are defined as *K*_{P+} = *p*_{+}^{F}/*p*_{0}*G*, *K*_{P−} **...**

The analysis of simulation results indicates that the maxima of the curves in Fig. 2 correspond to such values of *N* (proportional to the source amplitude) when the shocks form close to the focus. With increase of linear focusing gain *G*, the shocks form at lower initial wave amplitude and thus the maximum of enhancement occurs at smaller values of *N*. With further increase of the source amplitude, the shock front forms in the prefocal region, which leads to additional losses of wave energy on the way to the focus, and to a decrease of the correction indices *K _{P+}* and

Using the results shown in Fig. 2, the focal values of peak pressures and intensity can be obtained for any piston transducer at any level of its excitation. Thus, they can be used as calibration curves for nonlinear corrections to acoustic quantities at the focus of ultrasound transducers operating at high intensity levels. These results are of practical importance and can be used for regulating the fields of high power focused ultrasound sources, for determination of the focal values of acoustic parameters of nonlinear fields, and for choosing optimal operating levels.

For higher source output levels, shock fronts form closer to the source, an effective absorption of energy occurs at the shock fronts, and saturation of the acoustic field at the focus ensues. The calculated saturation curves for peak pressures (a, b) and intensities (c) are shown in Figure 3. On the right side of each plot, horizontal lines depict the levels of saturation corresponding to Eq. (5). For convenience, graphs are presented in dimensionless quantities. The ordinate axes on the top two figures correspond to the values proportionate to peak pressures at the focus
${\mathit{NK}}_{{P}_{+}}={p}_{+}\left(F\right)\cdot \epsilon {\omega}_{0}F/{c}_{0}^{3}{\rho}_{0}G$ and
${\mathit{NK}}_{{P}_{-}}={p}_{-}\left(F\right)\cdot \epsilon {\omega}_{0}F/{c}_{0}^{3}{\rho}_{0}G$, on the bottom figure – intensity of the wave
${N}^{2}{K}_{\stackrel{\sim}{I}}=I\left(F\right)\cdot 2{\left(\epsilon {\omega}_{0}F\right)}^{2}/{c}_{0}^{5}{\rho}_{0}{G}^{2}$. As can be seen in Fig.3, the saturation levels for peak negative pressures are about twice lower and for peak positive pressure about twice higher than analytic predictions given by Eq. (5). At the same time, for intensity and half-sum of peak pressures, the results obtained from the simple model of one-dimensional spherically converging wave, Eq. (5), and calculated data are very close. Also, it is important to note that saturation at the focus is reached at lower values of *N* for transducers with higher linear focusing gains *G*.

Saturation curves at the focus for dimensionless peak pressures (*NK*_{P±} ~ *p*_{±}^{F}, a, b) and intensity *Ĩ* (*N*^{2}*K*_{Ĩ}~ *I*^{F}, c). The value of parameter *N* is proportional to the source pressure output *p*_{0}, the curves are presented for various **...**

Calculations have shown that for weakly focused transducers (*G* <10), the maximum of the field can occur in the lobe preceding the main focal lobe, even though saturation at the focus has not yet occurred. Such situation is shown in the Figure 4, where the distribution of dimensionless intensity of the wave along the beam axis is presented for *G* = 10 and *N* = 4. For strongly focused transducers (*G* = 20, 40, 60), the maximum of the field always occurs spatially within the focal lobe up to the saturation levels.

Distribution of the dimensionless intensity *Ĩ* along the beam axis under conditions of well developed shocks (*G* = 10, *N* = 4).

Figure 5 illustrates how the acoustic field of a nonlinear spherically converging one-dimensional wave, Eq. (1), differs from the field of a real transducer, which has strongly oscillatory structure in the nearfield. Figure 5 presents the dimensionless peak pressures *P _{+}* and

Distributions of the dimensionless peak pressures *P*_{+} and *P*_{−} along beam axis (*G* = 10) for various values of nonlinear parameter *N* = 0.25 (a), 0.33 (b), and 1.17 (c). Solid lines correspond to the peak pressure in one-dimensional spherically convergent **...**

In Figure 5, it can be seen that the positions of the spatial maxima for *P _{+}* and

In this work, the focusing gains of nonlinear beams are calculated at the geometrical focus. However, as can be seen from Fig. 5, the maximum values of various acoustic parameters of the field in space are different than corresponding values at the geometrical focus. For example, for values of the nonlinear parameter *N* where the maximum focusing gain of the peak positive pressure is achieved (Fig. 2), the maximum pressure *P _{+}* in space differs from the pressure at the geometrical focus for

The difference of saturation levels for peak positive pressure, calculated at the geometrical focus (Fig. 3), and at the point of spatial maximum of the field, changes none monotonically and corresponds to a difference of 12 % (*G* = 10), 0.38 % (*G* = 20), 6 % (*G* = 40) and 20 % (*G* = 60). Therefore, it is necessary to take into account these differences when estimating focusing gains and limiting values of acoustic field parameters for high power ultrasound transducers.

As nonlinear effects increase, not only the focusing gains and locations of spatial maxima of different acoustic parameters of the beam change, but also the spatial structure becomes different [19]. Figure 6 shows the spatial distributions of the positive *P _{+}* and negative

Spatial distributions in (*z, R*) coordinates of the peak positive *P*_{+} and negative *P*_{−} pressures, intensity *Ĩ*, and heat deposition *H* for linear (*N* = 0, a-b) and nonlinear (*N* = 0.25, c-e) beams (*G* = 40).

Since various parameters of the acoustic wave are responsible for different effects of ultrasound on tissue, it is necessary to take into account the mentioned changes that can occur in the spatial localization of acoustic parameters in nonlinear fields when planning the therapeutic impact of high power ultrasound on biological tissue. The negative phase of the waveform determines cavitation impact, while the absorption of the wave at the shocks leads to faster heat deposition. It is expected, therefore, that in high power focused fields, cavitation phenomena will be more pronounced in a wider area and closer to the transducer as compared to thermal effects, and that very high heating rates are possible in the focal area [12]. It is also clear that in the nonlinear regime of focusing, the wave intensity cannot be used to estimate heat deposition.

To illustrate the practical use of the calibration curves depicted in Fig. 2, a specific example will be considered. The acoustic field generated in water by a transducer with frequency *f*_{0} = 2 MHz, radius *a*_{0} = 22.5 mm and focal length *F* = 44.4 mm is used as an example. These parameters correspond to *G* = 48. If the acoustic power is 120 W, then the pressure amplitude at the source is *p*_{0} = 0.4 MPa and the initial intensity is *I*_{0} = 5 W/cm^{2}, which corresponds to a value for the nonlinear parameter of *N* = 0.25 [10]. Based on the curves for nonlinear correction of focusing gains (Fig. 2), the values of acoustic parameters of the beam at the focus for the quantities *N* = 0.25 and *G* = 48 can be estimated. As can be seen from the figure, at this output level, the values of the correction indices for focusing gains of peak positive pressure and intensity are close to their maximum values. Let us choose the values of correction indices between the curves for *G* = 40 and *G* = 60: *K _{P+}* = 3.27,

Comparison of measured data (solid lines) with the results of numerical modeling (dashed lines) for the pressure waveform at the focus: (a) - the measured signal; (b) - two periods in the wave profile between vertical lines on the graph of the measured **...**

The breaking point of piezoceramics and the presence of cavitation activity make it technically difficult to achieve saturation levels experimentally. Using the results shown in Fig. 3, we can estimate the limiting values of the field of the transducer considered above: the limiting peak positive pressure is 117 MPa, peak negative pressure is 39 MPa, and intensity is 96 kW/cm^{2}. In the experiment, the fields at the focus were measured up to the values of *p _{+}^{F}* = 80 MPa,

For a source with higher frequency *f*_{0} = 5.5 MHz, radius *a*_{0} = 9.5 mm and *F* = 19 mm (*G* = 55) the limiting peak positive pressure at the focus is 113 MPa, limiting peak negative pressure is 34.7 MPa, intensity is 85 kW/cm^{2}. In the experiment the pressure was measured up to *p*_{+} = 34.5 MPa and *p*_{−} = 15.5 MPa, which is also far from saturation [8].

In this work, the nonlinear-diffractive effects which occur in high power sound beams in a weakly dissipative medium are investigated numerically. The quantitative data for nonlinear corrections of focusing gains and saturation of the field at the focus are obtained. Various characteristics of nonlinearly distorted waveforms are calculated over a wide range of parameters for piston transducers. It is shown that as the pressure amplitude at the source increases, the focusing gains of the field for peak positive pressure and intensity change none monotonically; at first, they noticeably increase (up to 3.5 times for *p*_{+} and 1.4 times for *I*) and then they decrease. The maxima on these curves correspond to the initial amplitude when the shock front is formed in the wave profile near the focus. The effect of enhancement of field concentration is more pronounced for sources with higher linear focusing gains *G*. For peak negative pressures, the focusing gain decreases monotonically as the source pressure amplitude increases and is only 60% of its linear value when the focusing gain for peak positive pressure is at a maximum.

It was established that the present analytical estimations (5) for saturation levels at the focus underestimate the values for peak positive pressure and overestimate the values for peak negative pressure (by about 2 times). At the same time, these estimations are sufficiently close to numerically calculated intensities and to half-sum of peak pressure values. The main differences in the spatial distribution of different acoustic parameters in nonlinear acoustic fields were presented: peak positive pressure, intensity and heat deposition are strongly localized and, on the contrary, the area of peak negative pressure is extended and shifted towards the source.

The results of modeling are in good agreement with the experimental data and can be used for calibration of real therapeutic ultrasound transducers and for optimization of clinical protocols.

The authors wish to thank M.V. Averiyanov for help in the optimization of numerical algorithm. The work was partially supported by RFBR 06-02-16860, INTAS 05-1000008-7841, ISTC 3691, NSBRI SMS00402, and NIH R01EB007643 grants.

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