Nonlinear change of on-axis pressure and intensity maxima positions and its relation with the linear focal shift effect.

A comprehensive experimental, analytical and numerical study of the true focal region drift relative to the geometrical focus (focal shift effect) in acoustic focused beams and its nonlinear evolution is presented. For this aim, the concept of Fresnel number, proportional to the linear gain, is introduced as a convenient parameter for characterizing focused sources. It is shown that the magnitude of the shift is strongly dependent on the Fresnel number of the source, being larger for weakly focused systems where a large initial shift occurs. Analytical expressions for axial pressure distributions in linear regime are presented for the general case of truncated Gaussian beams. The main new contribution of this work is the examination of the connection between the linear and nonlinear stages of the focal shift effect, and its use for the estimation of the more complicated nonlinear stage. Experiments were carried out using a continuous-wave ultrasonic beam in water, radiated by a focused source with nominal frequency f=1 MHz, aperture radius a=1.5 cm and geometrical focal distance R=11.7 cm, corresponding to a Fresnel number N(F)=1.28. The maximum measured shifts for peak pressure and intensity were 4.4 and 1.1cm, respectively. The evolution of the different maxima with the source amplitude, and the disparity in their axial positions, is interpreted in terms of the dynamics of the nonlinear distortion process. Analytical results for the particular case of a sound beam with initial Gaussian distribution are also presented, demonstrating that the motion of peak pressure and peak intensity may occur in opposite directions.

Frequency-dependent effects of low-intensity ultrasound on activity of isolated heart.

The experiments with low-intensity ultrasound stimulation of isolated rat heart revealed frequency-dependent effects of ultrasound in a frequency range of 45-298 kHz on cardiac activity.

Effect of low-frequency low-intensity ultrasound on contractile function of isolated heart.

Stimulation of the isolated heart with low-intensity low-frequency ultrasound produced a positive inotropic effect (at certain ultrasound intensity). This effect manifested in increased systolic and pulse intraventricular pressures and persisted for tens of minutes after the end of sonication.