Optical observations of acoustical radiation force effects on individual air bubbles

Previous studies dealing with contrast agent microbubbles have demonstrated that ultrasound (US) can significantly influence the movement of microbubbles. In this paper, we investigated the influence of the acoustic radiation force on individual air bubbles using high-speed photography. We emphasize the effects of the US parameters (pulse length, acoustic pressure) on different bubble\ud patterns and their consequences on the translational motion of the bubbles. A stream of uniform air bubbles with diameter ranging from 35 um to 79 um was generated and insonified with a single US pulse emitted at a frequency of 130 kHz. The bubble sizes have been chosen to be above, below, and at resonance. The peak acoustic pressures used in these experiments ranged from 40 kPa to 120 kPa. The axial displacements of the bubbles produced by the action of the US pulse were optically recorded using a high-speed camera at 1 kHz frame rate. The experimental results were compared to a simplified force balance theoretical model, including the action of the primary radiation force and the fluid drag force. Although the model is quite simple and does not take into account phenomena like bubble shape oscillations and added mass, the experimental findings agree with the predictions. The measured axial displacement increases quasilinearly with the burst length and the transmitted acoustic pressure. The axial displacement varies with the size and the density of the air bubbles, reaching a maximum at the resonance size of 48 um. The predicted displacement values differ by 15% from the measured data, except for resonant bubbles for which the displacement was overestimated by about 40%. This study demonstrates that even a single US pulse produces radiation forces that are strong enough to affect the bubble position

Air bubble in an ultrasound field:theoretical and optical results

The radial motion of a gas bubble has been widely investigated in various studies using different theoretical models. The aim of this study is to compare, qualitatively and quantitatively, the results obtained by optical recording with those of a theoretical model. Bubble oscillations were optically recorded using an ultrafast digital camera, Brandaris. The radius-time, R(t), curves are directly computed from 128 video frames. The resting diameters of the air bubbles were 26-100 /spl mu/m. The ultrasound field was defined as an 8 cycle pulse at a frequency of 130 kHz generating an acoustic pressure of 10-150 kPa. The time and the frequency response of the bubble radial motion were compared to the Keller model. From the results, it is concluded that the Keller model can be used to accurately predict the fundamental and harmonic behavior of gas bubbles

A new multifrequency transducer for microemboli detection and classification

The classification of circulating microemboli as gaseous or particulate matter is essential to establish the relevance of detected embolic signals. Transcranial Doppler (TCD) technology has not yet fully succeeded in characterizing the composition of microemboli unambiguously. Recently, the authors proposed a new approach to detect, characterize and size gaseous emboli. The method is based on the nonlinear properties of gaseous bubbles. The application of this approach requires a dedicated transducer with the ability to transmit the adequate frequencies and simultaneously receive the high frequency scattered nonlinear components. The paper presents a multifrequency emboli transducer composed of two independent transmitting elements and a separate receiving part. The transmitting part can cover a frequency band between 100 kHz and 600 kHz. The reception of the signal is performed by a 110 /spl mu/m PVDF layer sensitive over a frequency band ranging from 50 kHz to 2 MHz. Experimental results show that a specific range of gaseous embolus size was detected by each transmitting element. Using the 130 kHz outer element in transmission, microemboli between 35 /spl mu/m and 105 /spl mu/m can be discriminated through their second harmonic or subharmonic emissions while gaseous microemboli between 10 /spl mu/m and 40 /spl mu/m were accurately classified using the 360 kHz inner element. The in vitro results demonstrate that nonlinear properties of microemboli combined with the new transducer offer a real opportunity to characterize and size microemboli

Microbubble shape oscillations excited through ultrasonic parametric driving\ud

An air bubble driven by ultrasound can become shape-unstable through a parametric instability. We report time-resolved optical observations of shape oscillations (mode n=2 to 6) of micron-sized single air bubbles. The observed mode number n was found to be linearly related to the ambient radius of the bubble. Above the critical driving pressure threshold for shape oscillations, which is minimal at the resonance of the volumetric radial mode, the observed mode number n is independent of the forcing pressure amplitude. The microbubble shape oscillations were also analyzed numerically by introducing a small nonspherical linear perturbation to a Rayleigh-Plesset-type equation, capturing the experimental observations in detail.\ud \u