[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Active response of a microbubble is characterized by its resonance behavior where the microbubble might oscillate after the excitation waveform has been turned off. We investigate in this paper an excitation approach based on this resonance phenomenon using chirps. The technique, called chirp reversal, consists in transmitting a first excitation signal, the up-sweep chirp (UPF) of increasing frequency with time, and a second excitation signal, the down-sweep (DNF) that is a replica of the first signal, but time reversed with a sweep of decreasing frequency with time. Simulations using a modified Rayleigh-Plesset equation were carried out to determine bubble response to chirp reversal. In addition, optical observations and acoustical measurements were carried out to corroborate the theoretical findings. Results of simulations show differences between bubbles' oscillations in response to up-sweep and down-sweep chirps mainly for transmitted center frequencies above the bubble's resonance frequency. Bubbles that are at resonance or far away from resonance engender identical responses. From the optical data, the larger bubbles showed different dynamics when up-sweep or down-sweep chirps were transmitted. Smaller bubbles (< 2 microm diameter) appear to be less sensitive to frequency sweep at 1.7 MHz center frequency. However, driven at a higher center frequency, smaller bubbles tend to be more sensitive. These results were confirmed through the acoustical measurements. We concluded that simulations and experimental data show that significant differences might be observed between bubbles' responses to UPF and DNF chirps. We demonstrate in this study that, for an optimal use of chirp reversal, the transmit frequency should be higher than the resonance frequency of the contrast microbubbles.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 02/2009; 56(6):1199-206. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The occurrence of nonspherical oscillations (or surface modes) of coated microbubbles, used as ultrasound contrast agents in medical imaging, is investigated using ultra-high-speed optical imaging. Optical tweezers designed to micromanipulate single bubbles in 3-D are used to trap the bubbles far from any boundary, enabling a controlled study of the nonspherical oscillations of free-floating bubbles. Nonspherical oscillations appear as a parametric instability and display subharmonic behavior: they oscillate at half the forcing frequency, which was fixed at 1.7 MHz in this study. Surface modes are shown to preferentially develop for a bubble radius near the resonance of radial oscillations. In the studied range of acoustic pressures, the growth of surface modes saturates at a level far below bubble breakage. With the definition of a single, dimensionless deformation parameter, the amplitude of nonspherical deformation is quantified as a function of the bubble radius (between 1.5 and 5 microm) and of the acoustic pressure (up to 200 kPa).
Ultrasound in Medicine & Biology 06/2008; 34(9):1465-73. · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate an excitation approach for contrast agents based on chirps. This technique, named chirp reversal, consists in transmitting an up sweep frequency chirp (UPF) followed by a down sweep frequency chirp (DNF). Simulations using a modified Rayleigh-Plesset equation were carried out. Chirps with center frequencies from 1.4 MHz to 2 MHz, pressures from 50 kPa to 200 kPa and frequency bandwidths from 30% to 65% were considered. High speed optical observations and acoustical measurements were performed using individual contrast bubbles of radii from 1 mum to 5 mum and a diluted solution of contrast agent respectively. Simulations showed differences between bubbles' oscillations following UPF and DNF chirps in terms of amplitude and duration. Maximal differences occurred for bubbles that were around 80% and 140% of the resonance size. Bubbles at resonance or far away from resonance provided identical responses to UPF and DNF chirps. Larger bandwidths and higher acoustic pressures accentuate further the difference between the UPF and DNF responses. These findings were confirmed through optical data and acoustical measurements. The results reveal the potential of chirp reversal for contrast agent detection.
The Journal of the Acoustical Society of America 06/2008; 123(5):3110. · 1.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new optical characterization of the behavior of single ultrasound contrast bubbles is presented. The method consists of insonifying individual bubbles several times successively sweeping the applied frequency, and to record movies of the bubble response up to 25 million frames/s with an ultrahigh speed camera operated in a segmented mode. The method, termed microbubble spectroscopy, enables to reconstruct a resonance curve in a single run. The data is analyzed through a linearized model for coated bubbles. The results confirm the significant influence of the shell on the bubble dynamics: shell elasticity increases the resonance frequency by about 50%, and shell viscosity is responsible for about 70% of the total damping. The obtained value for shell elasticity is in quantative agreement with previously reported values. The shell viscosity increases significantly with the radius, revealing a new nonlinear behavior of the phospholipid coating.
The Journal of the Acoustical Society of America 02/2007; 121(1):648-56. · 1.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Chirp reversal consists of transmitting first a chirp with upsweep frequencies (UPF) then a second chirp with downsweep frequencies (DNF). We have shown that contrast bubbles react differently to these chirps allowing the use of chirp reversal for ultrasound contrast imaging. The aim of the current study is to explore how chirp reversal can be optimized in terms of transmit frequency, bubble size, acoustic pressure and frequency bandwidth. Simulations were carried out to determine ultrasound parameters that provide largest differences in bubble's response to UPF and DNF chirps. Scattered pressures were calculated using a modified Rayleigh-Plesset equation. To evaluate the performance of chirp reversal for bubble detection, the echo from an UPF chirp was correlated to the time reversed echo from a DNF chirp for each scanning parameter. Simulations and optical measurements allow for an optimization of chirp reversal and demonstrate a potential application for contrast imaging when appropriate scanning parameters are selected.
Proceedings of the IEEE Ultrasonics Symposium 01/2007;
[Show abstract][Hide abstract] ABSTRACT: Here we present optical tweezers as a micromanipulation tool for the study of ultrasound contrast agent (UCA) microbubbles. Optical trapping and the resulting manipulation of individual and multiple microbubbles enables the study of their dynamics with controlled boundary conditions. The bubble response to ultrasound is recorded optically using the ultra high-speed Brandaris 128 camera. In our experiments, the amplitude of bubble oscillations was found to be strongly influenced by the vicinity of the sample chamber wall. Experiments with two trapped bubbles also showed a considerable influence of the neighboring bubble on the amplitude of oscillations of the other one
[Show abstract][Hide abstract] ABSTRACT: We study non-spherical oscillations (surface modes) of the coated microbubbles used in ultrasound contrast agents. We show that an acoustic pressure of 100 kPa, at a frequency of 1.7 MHz, is sufficient to excite surface modes for radii between 2 and 4 mum. Surface modes develop as a parametric instability. We derive their evolution equation accounting for the effect of the shell viscoelasticity. We identify a critical radius (1 to 3 mum, depending on the mode number) below which no surface mode can occur
[Show abstract][Hide abstract] ABSTRACT: We present a new contrast imaging approach based on chirps named chirp reversal contrast imaging. The technique consists of transmitting a first excitation signal being a chirp of increasing frequency with time (the so-called upsweep) and a second excitation signal, the downsweep, being a replica of the first signal, but time reversed with a sweep of decreasing frequency with time. Simulations and optical observations were carried out to explore the potential of the chirp reversal approach in detecting microbubbles. Simulations using a Rayleigh-Plesset equation were performed considering various microbubbles excited with chirps at 1.7 MHz center frequency and 50% bandwidth. Optical observations with the Brandaris camera were carried out using BR14 bubbles of radii from 1 mum to 5 mum. Chirps with center frequencies of 1.7 MHz and 50% bandwidth were transmitted with peak negative pressures ranging from 70 kPa to 200 kPa. Simulations showed that for larger bubbles (>2 mum), significant differences occur between upsweep chirp response and down sweep response at 1.7 MHz transmit frequency. Optical observations confirmed these results. From the optical radius-time curves, the larger bubbles showed different dynamics when upsweep or downsweep frequencies were used in transmission. Upsweep excitation chirps produce highly damped responses with large amplitude excursions whereas the response to downsweep chirps showed a pronounced resonance behavior with smaller amplitudes. Smaller bubbles (
Proceedings of the IEEE Ultrasonics Symposium 01/2006;
[Show abstract][Hide abstract] ABSTRACT: We investigate the nonspherical oscillations, or surface modes, of bubbles of radius between 10 and 60 microns within an ultrasonic field of frequency of 130 kHz. We show experimentally that a threshold in acoustic pressure is required to trigger the surface modes, that they appear only after a few cycles of ultrasons, and that the observed mode number (2 to 6) is linearly related to the resting radius of the bubble and does not depend significantly on the acoustic pressure. We relate the observations to a parametric instability: The amplitude of nonspherical oscillations is modulated by the radial dynamics. Using a simple, linear radial dynamics, we reproduce the dependence of the observed mode number with the radius. A more accurate, nonlinear radial dynamics model determined from a modified Rayleigh-Plesset equation yields excellent agreement, both for the threshold in acoustic pressure and for the mode number, in the whole parameter space. The implications of these results for the coated microbubbles widely used as ultrasound contrast agents in medical acoustics are discussed.
[Show abstract][Hide abstract] ABSTRACT: We present a model applicable to ultrasound contrast agent bubbles that takes into account the physical properties of a lipid monolayer coating on a gas microbubble. Three parameters describe the properties of the shell: a buckling radius, the compressibility of the shell, and a break-up shell tension. The model presents an original non-linear behavior at large amplitude oscillations, termed compression-only, induced by the buckling of the lipid monolayer. This prediction is validated by experimental recordings with the high-speed camera Brandaris 128, operated at several millions of frames per second. The effect of aging, or the resultant of repeated acoustic pressure pulses on bubbles, is predicted by the model. It corrects a flaw in the shell elasticity term previously used in the dynamical equation for coated bubbles. The break-up is modeled by a critical shell tension above which gas is directly exposed to water.
The Journal of the Acoustical Society of America 01/2005; 118(6):3499-3505. · 1.65 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The resonance frequencies of individual SonoVue™ contrast agent bubbles were measured optically by recording the radius-time curves of a single microbubble at 24 different frequencies. For these experiments, the Brandaris 128 fast framing camera was operated in a special segmented mode. The resonance frequencies found for SonoVue™ microbubbles are in good agreement with the modified Herring model for coated bubbles indicating that the shell only slightly affects the resonance frequency of this class of contrast bubbles.
[Show abstract][Hide abstract] ABSTRACT: We have investigated surface vibrations generated by ultrasound excitation of individual unencapsulated micron-sized bubbles. In addition, we present surface modes (n=2 and 3) observed for phospholipid-coated ultrasound contrast agents excited through excitation of radial modes at frequencies between 1 and 4 MHz. Even higher modes of vibration (up to mode 5) are observed for coated microbubbles at insonation frequencies of 10 and 19 MHz. The potential relevance of surface modes for medical ultrasound is discussed, including the possible implications for current theoretical models of ultrasound contrast agents.