Influence of boundary conditions on a one-dimensional ultrasound backscattering model of blood.

A simulation model of one-dimensional (1-D) ultrasound (US) propagation in blood was used to study the relation between the backscattering coefficient and hematocrit. In this model, an ultrasonic plane wave was propagated in plasma normal to randomly placed slabs of constant thickness whose acoustical properties are the same as red blood cells, and the corresponding intensity reflection coefficient was calculated. The simulation results were compared to the 1-D Percus-Yevick (P-Y) theory as presented in the literature. Previous investigators have reported a close agreement over a limited range of simulation parameters between their results and the P-Y theory. However, a more careful investigation using a wider range of parameters has revealed major discrepancies. It is shown that these arise from an inappropriate choice of boundary conditions. By averaging the material properties beyond the boundaries of the simulation, as suggested by earlier theoretical work, the results are now in excellent agreement with the P-Y theory over a wide range of simulation parameters.

Ultrasound-induced tissue ablation: studies on isolated, perfused porcine liver.

Minimally invasive methods for the treatment of cancers, such as high-intensity focused ultrasound (HIFU) and high-energy shock waves (SW), have been proposed recently. Their feasibility for treatment of human cancer needs to be confirmed. A simplified model of isolated perfused pig liver that is close to the human liver in vivo has been proposed. The objective was to study the feasibility of deep focused tissue ablation with HIFU and SW in large organs approaching the size of the human liver. The model was demonstrated to be physiologically valid during the first 2 h of anoxic perfusion with a composite saline solution; arterial and portal pressure, enzymes, urea levels and bile secretion remained stable. It can simulate the major effects of perfusion and physical phenomena that occur in vivo during treatment. Histological analysis revealed no major changes. Previous results obtained in vivo in animal models at a depth of 2-3 cm were successfully reproduced and deeper lesion arrays at 4, 6, 8 and 9 cm from the surface were produced using the same principles. The depth of 9 cm from the liver surface is consistent with an extracorporeal treatment of most of the liver segments in man. Other applications of the model are proposed, particularly for the study of the role of interferences such as ribs and intestinal gas, blood perfusion and respiratory movements.

A piezocomposite shock wave generator with electronic focusing capability: application for producing cavitation-induced lesions in rabbit liver.

In this work, a piezocomposite shock wave generator with electronic focusing capability is presented. The system is composed of a bidimensional array and its electronic hardware. The array is composed of 274 independent piezocomposite transducers arranged in a spherical shell of 280 mm in diameter and focused at 190 mm from its surface. The electronic hardware includes 274 x 6.6 kV distinct impulse generators. For the purpose of performing the electronic steering of shock waves, the delay time of each channel can be adjusted from 100 ns to 100 microseconds in steps of 100 ns. In order to enhance the effect of cavitation at the focus for the purpose of tissue destruction, the pressure-time waveform starts with a half cycle of negative pressure with a peak amplitude of about -150 x 10(5) Pa, followed by a very steep shock front with a positive peak pressure > 1000 x 10(5) Pa and a rise time of about 10 ns. Using this generator, the cavitation-induced lesions in rabbit liver were studied. To obtain a predefined lesion volume, two methods of scanning were used: mechanical and electronic. Comparison of the lesions obtained by these two methods shows that they have identical macroscopic and histological characteristics, which justify the feasibility of electronic beam steering of shock waves in tissue destruction applications.

Sources of error in maximum velocity estimation using linear phased-array Doppler systems with steady flow.

Using linear-array Doppler ultrasound (US) transducers, the measured maximum velocity may be in error and lead to incorrect clinical diagnosis. This study investigates the existence and cause of maximum velocity estimation errors for steady flow of a blood-mimicking fluid in a tissue-mimicking phantom. A specially designed system was used that enabled fine control of flow rate, transducer positioning and transducer angle relative to the flow phantom. Doppler machine settings (transducer aperture size, focal depth, beam-steering, gain) were varied to investigate a wide range of clinical applications. To estimate the maximum velocity, a new signal-to-noise ratio (SNR) independent method was developed to calculate the maximum frequency from an ensemble averaged Doppler power spectrum. This enabled the impact of each factor on the total Doppler error to be determined. When using the new maximum frequency estimator, it was found that the effect of transducer focal depth, intratransducer, intramachine, intermachine (that was tested) and beam-steering did not significantly contribute to maximum velocity estimation errors. Instead, it was the dependence of the maximum velocity on the Doppler angle that made, by far, the greatest contribution to the estimation error. Because our maximum frequency estimator took into account the effect of intrinsic spectral broadening, the degree of overestimation error was not as great as that previously published. Thus, the effects of Doppler angle and intrinsic spectral broadening are the chief sources of Doppler US error and should be the focus of future efforts to improve the accuracy.

Comparison between the effects of cavitation induced by two different pressure-time shock waveform pulses.

Acoustic cavitation generates very large localized pressures and temperatures, and thus provides a mechanism whereby physical and biological effects are produced in a high-intensity acoustic field. In this work, we studied the influence of the temporal form of a pressure pulse waveform on the destructive effects of transient cavitation. Two different shock pressure-time waveforms with nearly the same acoustic energy content were used. The first pressure waveform starts with a tensile wave followed by a compressive one, and the second pressure waveform starts with a compressive wave followed by a tensile one. These two pressure waveforms are called direct and inverse-mode pulses respectively. Based on the measurements presented in this work, we can state that, between the two types of shock pressure pulses studied, the direct-mode pulse amplifies systematically tile cavitation effect. This conclusion was achieved from a series of several quantitative and qualitative experiments: cavitation bubble collapse time, disintegration efficacy of plaster balls (a kidney stone-mimicking material), macroscopic study of lesions in agar gel and in vitro isolated rabbit liver tissue destruction. Considering these results and those obtained by other research groups, we can express that the temporal form of a shock pressure pulse has a major role on the cavitation effects.

New X-wave solutions of free-space scalar wave equation and their finite size realization.

Based on the method proposed by Donnelly and Ziolkowski [1], [2], a new general solution has been obtained for the isotropic/homogeneous scalar wave equation in cylindrical coordinates. It is shown that well-known limited diffraction beams such as Durnin's Bessel beams [4], Lu and Greenleaf's X-wave [15], localized waves of Donnelly and Ziolkowski [1], [2], and limited-diffraction, band-limited waves of Li and Bharath [19], [20] can be obtained from this generic solution as particular cases. In addition, we have obtained new X-wave solutions and have calculated the field characteristics for one of them using a finite aperture realization. It is shown that with a proper choice of the free parameter values, well-behaved X-waves with narrow beamwidths and large depths of field can be achieved. For similar source spectra, the results are compared with Lu and Greenleaf's zeroth-order X-wave, and it is shown that the depth of field and beamwidth are very comparable.

Modeling of pulsed finite-amplitude focused sound beams in time domain.

A time-domain numerical model is presented for simulating the finite-amplitude focused acoustic pulse propagation in a dissipative and nonlinear medium with a symmetrical source geometry. In this method, the main effects responsible in finite-amplitude wave propagation, i.e., diffraction, nonlinearity, and absorption, are taken into account. These effects are treated independently using the method of fractional steps with a second-order operator-splitting algorithm. In this method, the acoustic beam propagates, plane-by-plane, from the surface of a highly focused radiator up to its focus. The results of calculations in an ideal (linear and nondissipative) medium show the validity of the model for simulating the effect of diffraction in highly focused pulse propagation. For real media, very good agreement was obtained in the shape of the theoretical and experimental pressure-time waveforms. A discrepancy in the amplitudes was observed with a maximum of around 20%, which can be explained by existing sources of error in our measurements and on the assumptions inherent in our theoretical model. The model has certain advantages over other time-domain methods previously reported in that it: (1) allows for arbitrary absorption and dispersion, and (2) makes use of full diffraction formulation. The latter point is particularly important for studying intense sources with high focusing gains.