Marlies Overvelde

Changes in microbubble dynamics near a boundary revealed by combined optical micromanipulation and high-speed imaging

The authors report optical observations of the change in the dynamics of one and the same ultrasound contrast agent microbubble due to the influence of interfaces and neighboring bubbles. The bubble is excited by a 2.25MHz ultrasound burst and its oscillations are recorded with an ultrahigh-speed camera at 15 million frames per second. The position of an individual bubble relative to a rigid wall or second bubble is precisely controlled using optical tweezers based on Laguerre-Gaussian laser beams [P. Prentice et al., Opt. Express 12, 593 (2004); V. Garbin et al., Jpn. J. Appl. Phys. 44, 5773 (2005)]. This allows for repeated experiments on the very same bubble and for a quantitative comparison of the effect of boundaries on bubble behavior.

Dynamics of coated microbubbles adherent to a wall.

Molecular imaging with ultrasound is a promising noninvasive technique for disease-specific imaging, enabling for instance, the diagnosis of thrombus and inflammation. Selective imaging is performed by using ultrasound contrast agent microbubbles functionalized with ligands, which bind specifically to the target molecules. Here, we investigate in a model system, the influence of adherence at a wall on the dynamics of the microbubbles, in particular, on the frequency of maximum response, by recording the radial response of individual microbubbles as a function of the applied acoustic pressure and frequency. The frequency of maximum response of adherent microbubbles was found to be over 50% lower than for bubbles in the unbounded fluid and over 30% lower than that of a nonadherent bubble in contact with the wall. The change is caused by adhesion of the bubbles to the wall as no influence was found due solely to the presence of the targeting ligands on the bubble dynamics. The shift in the frequency of maximum response may prove to be important for molecular imaging with ultrasound as this application would benefit from an acoustic imaging method to distinguish adherent microbubbles from freely circulating microbubbles.

History force on coated microbubbles propelled by ultrasound

In this paper the unsteady translation of coated microbubbles propelled by acoustic radiation force is studied experimentally. A system of two pulsating microbubbles of the type used as contrast agent in ultrasound medical imaging is considered, which attract each other as a result of the secondary Bjerknes force. Optical tweezers are used to isolate the bubble pair from neighboring boundaries so that it can be regarded as if in an unbounded fluid and the hydrodynamic forces acting on the system can be identified unambiguously. The radial and translational dynamics, excited by a 2.25 MHz ultrasound wave, is recorded with an ultrahigh speed camera at 15×106 frames/s. The time-resolved measurements reveal a quasisteady component of the translational velocity, at an average translational Reynolds number 〈Ret〉 ≈ 0.5, and an oscillatory component at the same frequency as the radial pulsations, as predicted by existing models. Since the coating enforces a no-slip boundary condition, an increased viscous dissipation is expected due to the oscillatory component, similar to the case of an oscillating rigid sphere that was first described by Stokes [“On the effect of the internal friction of fluids on the motion of pendulums,” Trans. Cambridge Philos. Soc. 9, 8 (1851) ]. A history force term is therefore included in the force balance, in the form originally proposed by Basset and extended to the case of time-dependent radius by Takemura and Magnaudet [“The history force on a rapidly shrinking bubble rising at finite Reynolds number,” Phys. Fluids 16, 3247 (2004) ]. The instantaneous values of the hydrodynamic forces extracted from the experimental data confirm that the history force accounts for the largest part of the viscous force. The trajectories of the bubbles predicted by numerically solving the equations of motion are in very good agreement with the experiment.

Unbinding of targeted ultrasound contrast agent microbubbles by secondary acoustic forces

Targeted molecular imaging with ultrasound contrast agent microbubbles is achieved by incorporating targeting ligands on the bubble coating and allows for specific imaging of tissues affected by diseases. Improved understanding of the interplay between the acoustic forces acting on the bubbles during insonation with ultrasound and other forces (e.g. shear due to blood flow, binding of targeting ligands to receptors on cell membranes) can help improve the efficacy of this technique. This work focuses on the effects of the secondary acoustic radiation force, which causes bubbles to attract each other and may affect the adhesion of targeted bubbles. First, we examine the translational dynamics of ultrasound contrast agent microbubbles in contact with (but not adherent to) a semi-rigid membrane due to the secondary acoustic radiation force. An equation of motion that effectively accounts for the proximity of the membrane is developed, and the predictions of the model are compared with experimental data extracted from optical recordings at 15 million frames per second. A time-averaged model is also proposed and validated. In the second part of the paper, initial results on the translation due to the secondary acoustic radiation force of targeted, adherent bubbles are presented. Adherent bubbles are also found to move due to secondary acoustic radiation force, and a restoring force is observed that brings them back to their initial positions. For increasing magnitude of the secondary acoustic radiation force, a threshold is reached above which the adhesion of targeted microbubbles is disrupted. This points to the fact that secondary acoustic radiation forces can cause adherent bubbles to detach and alter the spatial distribution of targeted contrast agents bound to tissues during activation with ultrasound. While the details of the rupture of intermolecular bonds remain elusive, this work motivates the use of the secondary acoustic radiation force to measure the strength of adhesion of targeted microbubbles.

1F-2 Optical Trapping of Ultrasound Contrast Agent Microbubbles: Study of the Bubble-Wall and Bubble-Bubble Interaction in Ultrasound

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

Shell buckling enhances subharmonic behavior of phospholipid coated ultrasound contrast agent microbubbles.

Subharmonic behavior of coated microbubbles can greatly enhance the contrast in ultrasound imaging. The threshold driving pressure above which subharmonic oscillations are initiated can be calculated from a linearized Rayleigh‐Plesset‐type equation. Earlier experimental studies on a suspension of phospholipid‐coated microbubbles showed a lower threshold than predicted from traditional elastic shell models. Here we present an experimental study of the subharmonic behavior of individual BR‐14 microbubbles (Bracco Research) with initial radii between 1.6 and 4.8 μm. The subharmonic behavior was studied as a function of the amplitude and the frequency of the driving pressure pulse. The radial response of the microbubbles was recorded with the Brandaris ultrahigh‐speed camera, while the resulting acoustic response was measured with a calibrated transducer. It is shown that the threshold pressure is minimum near a driving frequency equal to half the resonance frequency of the bubble, as expected. We found a thr...

Ultrasound Contrast Agent Microbubble Dynamics

Ultrasound contrast agents are traditionally used in ultrasound-assisted organ perfusion imaging. Recently the use of coated microbubbles has been proposed for molecular imaging applications where the bubbles are covered with a layer of targeting ligands to bind specifically to their target cells. In this chapter we describe contrast agent microbubble behavior starting from the details of free bubble dynamics leading to a set of equations describing the dynamics of coated microbubbles. Experimentally, the dynamics of ultrasound contrast agent microbubbles is temporally resolved using the ultra-high speed camera Brandaris 128. The influence of a neighboring wall is investigated by combining the Brandaris camera with optical tweezers. It was observed that the presence of the wall can alter the bubble response. A detailed description of the bubble-wall interaction may therefore lead to improved molecular imaging strategies.

Influence of finite wall impedance on contrast agent bubble behavior near a membrane.

Experiments investigating the radial dynamics of ultrasound contrast agent (UCA) microbubbles in the vicinity of an optically transparent membrane show that bubble oscillation amplitude and frequency of peak response decrease as bubbles move closer to the membrane (Overvelde et al. Proc. 19th Intl. Congr. Acoust., 2007). However, treating the membrane as a rigid wall predicts that the peak oscillation amplitude should increase at small standoff distances, contrary to experimental observations. Here we present a model describing UCA bubble dynamics near a locally reacting wall having finite acoustic impedance. If finite wall impedance is included in the model, the predicted bubble behavior is in good agreement with observations. The hybrid time‐frequency domain model is based on a linear frequency domain solution [Ingard, J. Acoust. Soc. Am. 23, 329 (1951)] which has been adapted to account for weakly nonlinear bubble oscillations and UCA shell dynamics. Wall impedance parameters are derived from independe...

Shell buckling increases the nonlinear dynamics of ultrasound contrast agents at low acoustic pressures.

The key feature of ultrasound contrast agents in distinguishing blood pool and tissue echoes is based on the nonlinear behavior of the bubbles. Here we investigate the nonlinear properties of the shell which lead to an increased nonlinear bubble response, especially at low acoustic pressures. The microbubbles were studied in free space away from the wall using the Brandaris camera coupled to an optical tweezers setup. The microbubble spectroscopy method [Van der Meer et al., JASA, 121, 648 (2007)] was employed to characterize BR‐14 microbubbles (Bracco, Geneva). For increasing applied pressures the bubble resonance curves become asymmetrical and the frequency of maximum response decreases, up to 50% at a pressure of 25 kPa. It was found that the skewing of the nonlinear resonance curve is the origin of the so‐called thresholding behavior below resonance. Traditional bubble models account for a purely elastic shell predict linear behavior, whereas the shell buckling model by Marmottant et al. [JASA, 118, 3...

Optical micromanipulation and force spectroscopy of ultrasound contrast microbubbles for targeted molecular imaging

The conventional applications of optical tweezers (micromanipulation, force sensor) can be extended to low-index particles, to understand the dynamics of ultrasound contrast microbubbles for targeted molecular imaging.