Acoustic characterization of single ultrasound contrast agent microbubbles

Coupling Two Ultra-high-Speed Cameras to Elucidate Ultrasound Contrast-Mediated Imaging and Therapy

Ultrasound contrast-mediated medical imaging and therapy both rely on the dynamics of micron- and nanometer-sized ultrasound cavitation nuclei, such as phospholipid-coated microbubbles and phase-change droplets. Ultrasound cavitation nuclei respond non-linearly to ultrasound on a nanosecond time scale that necessitates the use of ultra-high-speed imaging to fully visualize these dynamics in detail. In this study, we developed an ultra-high-speed optical imaging system that can record up to 20 million frames per second (Mfps) by coupling two small-sized, commercially available, 10-Mfps cameras. The timing and reliability of the interleaved cameras needed to achieve 20 Mfps was validated using two synchronized light-emitting diode strobe lights. Once verified, ultrasound-activated microbubble responses were recorded and analyzed. A unique characteristic of this coupled system is its ability to be reconfigured to provide orthogonal observations at 10 Mfps. Acoustic droplet vaporization was imaged from two orthogonal views, by which the 3-D dynamics of the phase transition could be visualized. This optical imaging system provides the temporal resolution and experimental flexibility needed to further elucidate the dynamics of ultrasound cavitation nuclei to potentiate the clinical translation of ultrasound-mediated imaging and therapy developments.

Ultrasound-Responsive Systems as Components for Smart Materials

Smart materials can respond to stimuli and adapt their responses based on external cues from their environments. Such behavior requires a way to transport energy efficiently and then convert it for use in applications such as actuation, sensing, or signaling. Ultrasound can carry energy safely and with low losses through complex and opaque media. It can be localized to small regions of space and couple to systems over a wide range of time scales. However, the same characteristics that allow ultrasound to propagate efficiently through materials make it difficult to convert acoustic energy into other useful forms. Recent work across diverse fields has begun to address this challenge, demonstrating ultrasonic effects that provide control over physical and chemical systems with surprisingly high specificity. Here, we review recent progress in ultrasound-matter interactions, focusing on effects that can be incorporated as components in smart materials. These techniques build on fundamental phenomena such as cavitation, microstreaming, scattering, and acoustic radiation forces to enable capabilities such as actuation, sensing, payload delivery, and the initiation of chemical or biological processes. The diversity of emerging techniques holds great promise for a wide range of smart capabilities supported by ultrasound and poses interesting questions for further investigations.

Vibration Characteristics and Persistence of Poloxamer- or Phospholipid-Coated Single Microbubbles under Ultrasound Irradiation

Microbubbles shelled with soft materials are expected to find applications as ultrasound-sensitive drug delivery systems, including through sonoporation. Microbubbles with specific vibrational characteristics and long intravascular persistence are required for clinical uses. To achieve this aim, the kinetics of the microbubble shell components at the gas/liquid interface while being subjected to ultrasound need to be better understood. This paper investigates the vibration characteristics and lifetime of single microbubbles coated with a poloxamer surfactant, Pluronic F-68, and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) under ultrasound irradiation. Air- and perfluorohexane (PFH)-filled microbubbles coated with Pluronic F-68 and DMPC at several concentrations (0 to 10^-2 mol L^-1) were produced. An optical measurement system using a laser Doppler vibrometer and microscope was used to observe the radial vibration mode of single microbubbles. The vibrational displacement amplitude and resonance radius of Pluronic- or DMPC-coated microbubbles were found to depend very little on the concentrations. The resonance radius was around 65 μm at 38.8 kHz under all the experimental conditions investigated. The lifetime of the microbubbles was investigated simultaneously by measuring their temporal change in volume, and it was increased with Pluronic concentration. Remarkably, the oscillation amplitude of the bubble has an effect on the bubble lifetime. In other words, larger oscillation under the resonance condition accelerates the diffusion of the inner gas.

A Review of Phospholipid Encapsulated Ultrasound Contrast Agent Microbubble Physics

Ultrasound contrast agent microbubbles have expanded the utility of biomedical ultrasound from anatomic imaging to the assessment of microvascular blood flow characteristics and ultrasound-assisted therapeutic applications. Central to their effectiveness in these applications is their resonant and non-linear oscillation behaviour. This article reviews the salient physics of an oscillating microbubble in an ultrasound field, with particular emphasis on phospholipid-coated agents. Both the theoretical underpinnings of bubble vibration and the experimental evidence of non-linear encapsulated bubble dynamics and scattering are discussed and placed within the context of current and emerging applications.

Laser-activated microparticles for multimodal imaging: ultrasound and photoacoustics

The increasing personalization of medical treatment demands refined imaging and increased monitoring capabilities, as well as an improved efficacy through targeted drug delivery. Such a transition in health care can be facilitated by the use of multimodal contrast agents. In this paper, we present a novel type of multimodal contrast agents, that enhances contrast both in ultrasound and in photoacoustic imaging, while at the same time being capable of triggered drug delivery. Upon pulsed laser irradiation, polymeric microparticles-containing a dye and an oil core-can create a cavitation bubble that subsequently emits a strong acoustic wave. We investigated different formulations of these particles, by changing the oil content, dye concentration and probing conditions using a combination of pulsed laser excitation and an ultrasound chirp. We demonstrated that capsules with a core containing a low boiling point oil give the highest photoacoustic and acoustic response. The laser activation threshold for this system is high in the visible range, but within the near infrared medical limits. The same system also produces a stable bubble. US scattering by these stable bubbles results in medically relevant frequencies, making the particles of interest for biomedical and pre-clinical imaging. Finally, the system has potential to carry a functional drug-load, and a route to these applications is discussed.

The effect of resonance on transient microbubble acoustic response: Experimental observations and numerical simulations

A large number of acoustic signals from single lipid-shelled Definity^® (Lantheus Medical Imaging, N. Billerica, MA) microbubbles have been measured using a calibrated microacoustic system, and a unique transient characteristic of resonance has been identified in the onset of scatter. Comparison of the numerically obtained response of microbubbles with acoustic measurements provides good agreement for a soft shell that is characterized by small area dilatation modulus and strain softening behavior, and identifies time to maximum radial excursion and scatter as a robust marker of resonance during transient response. As the sound amplitude increases a two-population pattern emerges in the time delay vs the fundamental acoustic scatter plots, consisting of an initial part pertaining to microbubbles with less than resonant rest radii, which corresponds to the weaker second harmonic resonance, and the dominant resonant envelope pertaining to microbubbles with resonant and greater than resonant rest radii, which corresponds to the primary and subharmonic resonances. Consequently, a wider resonant spectrum is observed. It is a result of the strain softening nature of soft lipid shells, based on which the microbubble sizes corresponding to the above resonances decrease as the sound amplitude increases. This bares an impact on the selection of an optimal microbubble size pertaining to subharmonic imaging.

Focal areas of increased lipid concentration on the coating of microbubbles during short tone-burst ultrasound insonification

Acoustic behavior of lipid-coated microbubbles has been widely studied, which has led to several numerical microbubble dynamics models that incorporate lipid coating behavior, such as buckling and rupture. In this study we investigated the relationship between microbubble acoustic and lipid coating behavior on a nanosecond scale by using fluorescently labeled lipids. It is hypothesized that a local increased concentration of lipids, appearing as a focal area of increased fluorescence intensity (hot spot) in the fluorescence image, is related to buckling and folding of the lipid layer thereby highly influencing the microbubble acoustic behavior. To test this hypothesis, the lipid microbubble coating was fluorescently labeled. The vibration of the microbubble (n = 177; 2.3–10.3 μm in diameter) upon insonification at an ultrasound frequency of 0.5 or 1 MHz at 25 or 50 kPa acoustic pressure was recorded with the UPMC Cam, an ultra-high-speed fluorescence camera, operated at ~4–5 million frames per second. During short tone-burst excitation, hot spots on the microbubble coating occurred at relative vibration amplitudes > 0.3 irrespective of frequency and acoustic pressure. Around resonance, the majority of the microbubbles formed hot spots. When the microbubble also deflated acoustically, hot spot formation was likely irreversible. Although compression-only behavior (defined as substantially more microbubble compression than expansion) and subharmonic responses were observed in those microbubbles that formed hot spots, both phenomena were also found in microbubbles that did not form hot spots during insonification. In conclusion, this study reveals hot spot formation of the lipid monolayer in the microbubble’s compression phase. However, our experimental results show that there is no direct relationship between hot spot formation of the lipid coating and microbubble acoustic behaviors such as compression-only and the generation of a subharmonic response. Hence, our hypothesis that hot spots are related to acoustic buckling could not be verified.

Biophysical insight into mechanisms of sonoporation

This study presents a unique approach to understanding the biophysical mechanisms of ultrasound-triggered cell membrane disruption (i.e., sonoporation). We report direct correlations between ultrasound-stimulated encapsulated microbubble oscillation physics and the resulting cellular membrane permeability by simultaneous microscopy of these two processes over their intrinsic physical timescales (microseconds for microbubble dynamics and seconds to minutes for local macromolecule uptake and cell membrane reorganization). We show that there exists a microbubble oscillation-induced shear-stress threshold, on the order of kilopascals, beyond which endothelial cellular membrane permeability increases. The shear-stress threshold exhibits an inverse square-root relation to the number of oscillation cycles and an approximately linear dependence on ultrasound frequency from 0.5 to 2 MHz. Further, via real-time 3D confocal microscopy measurements, our data provide evidence that a sonoporation event directly results in the immediate generation of membrane pores through both apical and basal cell membrane layers that reseal along their lateral area (resealing time of ∼<2 min). Finally, we demonstrate the potential for sonoporation to indirectly initiate prolonged, intercellular gaps between adjacent, confluent cells (∼>30-60 min). This real-time microscopic approach has provided insight into both the physical, cavitation-based mechanisms of sonoporation and the biophysical, cell-membrane-based mechanisms by which microbubble acoustic behaviors cause acute and sustained enhancement of cellular and vascular permeability.

Analysis of the Uncertainty in Microbubble Characterization

There is increasing interest in the use of microbubble contrast agents for quantitative imaging applications such as perfusion and blood pressure measurement. The response of a microbubble to ultrasound excitation is, however, extremely sensitive to its size, the properties of its coating and the characteristics of the sound field and surrounding environment. Hence the results of microbubble characterization experiments can be significantly affected by experimental uncertainties, and this can limit their utility in predictive modelling. The aim of this study was to attempt to quantify these uncertainties and their influence upon measured microbubble characteristics. Estimates for the parameters characterizing the microbubble coating were obtained by fitting model data to numerical simulations of microbubble dynamics. The effect of uncertainty in different experimental parameters was gauged by modifying the relevant input values to the fitting process. The results indicate that even the minimum expected uncertainty in, for example, measurements of microbubble radius using conventional optical microscopy, leads to variations in the estimated coating parameters of ∼20%. This should be taken into account in designing microbubble characterization experiments and in the use of data obtained from them.

Influence of Guided Waves in Tibia on Non-linear Scattering of Contrast Agents

The aim of this study was to elucidate the linear and non-linear responses of ultrasound contrast agent (UCA) to frequency-dispersive guided waves from the tibia cortex, particularly two individual modes, S0 (1.23 MHz) and A1 (2.06 MHz). The UCA responses to guided waves were illustrated through the Marmottant model derived from measured guided waves, and then verified by continuous infusion experiments in a vessel-tibia flow phantom. These UCA responses were further evaluated by the enhanced ratio of peak values and the resolutions of UCA backscattered echoes. Because of the individual modes S0 and A1 in the tibia, the peak values of the UCA backscattered echoes were enhanced by 83.57 ± 7.35% (p < 0.05) and 80.77 ± 6.60% (p < 0.01) in the UCA subharmonic frequency and subharmonic imaging, respectively. However, corresponding resolutions were 0.78 ± 0.07 (p < 0.05) and 0.72 ± 0.12 (p < 0.01) times those without guided wave disturbances, respectively. Even though the resolution was partly degenerated, the subharmonic detection sensitivity of UCA was improved by the guided waves. Thus, UCA responses to the double-frequency guided waves should be further explored to benefit the detection of capillary perfusion in tissue layers near the bone cortex, particularly for perfusion imaging in the free flaps and skeletal muscles.

Individual lipid encapsulated microbubble radial oscillations: Effects of fluid viscosity

Ultrasound-stimulated microbubble dynamics have been shown to be dependent on intrinsic bubble properties, including size and shell characteristics. The effect of the surrounding environment on microbubble response, however, has been less investigated. In particular, microbubble optimization studies are generally conducted in water/saline, characterized by a 1 cP viscosity, for application in the vasculature (i.e., 4 cP). In this study, ultra-high speed microscopy was employed to investigate fluid viscosity effects on phospholipid encapsulated microbubble oscillations at 1 MHz, using a single, eight-cycle pulse at peak negative pressures of 100 and 250 kPa. Microbubble oscillations were shown to be affected by fluid viscosity in a size- and pressure-dependent manner. In general, the oscillation amplitudes exhibited by microbubbles between 3 and 6 μm in 1 cP fluid were larger than in 4 cP fluid, reaching a maximum of 1.7-fold at 100 kPa for microbubbles 3.8 μm in diameter and 1.35-fold at 250 kPa for microbubbles 4.8 μm in diameter. Simulation results were in broad agreement at 250 kPa, however generally underestimated the effect of fluid viscosity at 100 kPa. This is the first experimental demonstration documenting the effects of surrounding fluid viscosity on microbubble oscillations, resulting in behavior not entirely predicted by current microbubble models.

Interpreting attenuation at different excitation amplitudes to estimate strain-dependent interfacial rheological properties of lipid-coated monodisperse microbubbles

Broadband attenuation of ultrasound measured at different excitation pressures being different raises a serious theoretical concern, because the underlying assumption of linear and independent propagation of different frequency components nominally requires attenuation to be independent of excitation. Here, this issue is investigated by examining ultrasound attenuation through a monodisperse lipid-coated microbubble suspension measured at four different acoustic excitation amplitudes. The attenuation data are used to determine interfacial rheological properties (surface tension, surface dilatational elasticity, and surface dilatational viscosity) of the encapsulation according to three different models. Although different models result in similar rheological properties, attenuation measured at different excitation levels (4-110 kPa) leads to different values for them; the dilatation elasticity (0.56 to 0.18 N/m) and viscosity (2.4 × 10(-8) to 1.52 × 10(-8) Ns/m) both decrease with increasing pressure. Numerically simulating the scattered response, nonlinear energy transfer between frequencies are shown to be negligible, thereby demonstrating the linearity in propagation and validating the attenuation analysis. There is a second concern to the characterization arising from shell properties being dependent on excitation amplitude, which is not a proper constitutive variable. It is resolved by arriving at a strain-dependent rheology for the encapsulation. The limitations of the underlying analysis are discussed.

On the relationship between microbubble fragmentation, deflation and broadband superharmonic signal production

Acoustic angiography imaging of microbubble contrast agents uses the superharmonic energy produced from excited microbubbles and enables high-contrast, high-resolution imaging. However, the exact mechanism by which broadband harmonic energy is produced is not fully understood. To elucidate the role of microbubble shell fragmentation in superharmonic signal production, simultaneous optical and acoustic measurements were performed on individual microbubbles at transmit frequencies from 1.75 to 3.75 MHz and pressures near the shell fragmentation threshold for microbubbles of varying diameter. High-amplitude, broadband superharmonic signals were produced with shell fragmentation, whereas weaker signals (approximately 25% of peak amplitude) were observed in the presence of shrinking bubbles. Furthermore, when populations of stationary microbubbles were imaged with a dual-frequency ultrasound imaging system, a sharper decline in image intensity with respect to frame number was observed for 1-μm bubbles than for 4-μm bubbles. Finally, in a study of two rodents, increasing frame rate from 4 to 7 Hz resulted in decreases in mean steady-state image intensity of 27% at 1000 kPa and 29% at 1300 kPa. Although the existence of superharmonic signals when bubbles shrink has the potential to prolong the imaging efficacy of microbubbles, parameters such as frame rate and peak pressure must be balanced with expected re-perfusion rate to maintain adequate contrast during in vivo imaging.

Acoustic characterization of monodisperse lipid-coated microbubbles: relationship between size and shell viscoelastic properties

The acoustic attenuation spectrum of lipid-coated microbubble suspensions was measured in order to characterize the linear acoustic behavior of ultrasound contrast agents. For that purpose, microbubbles samples were generated with a very narrow size distribution by using microfluidics techniques. A performance as good as optical characterization techniques of single microbubbles was achieved using this method. Compared to polydispersions (i.e., contrast agents used clinically), monodisperse contrast agents have a narrower attenuation spectrum, which presents a maximum peak at a frequency value corresponding to the average single bubble resonance frequency. The low polydispersity index of the samples made the estimation of the lipid viscoelastic properties more accurate since, as previously reported, the shell linear parameters may change with the equilibrium bubble radius. The results showed the great advantage of dealing with monodisperse populations rather than polydisperse populations for the acoustic characterization of ultrasound contrast agents.

Encapsulated microbubbles and echogenic liposomes for contrast ultrasound imaging and targeted drug delivery

Micron- to nanometer-sized ultrasound agents, like encapsulated microbubbles and echogenic liposomes, are being developed for diagnostic imaging and ultrasound mediated drug/gene delivery. This review provides an overview of the current state of the art of the mathematical models of the acoustic behavior of ultrasound contrast microbubbles. We also present a review of the in vitro experimental characterization of the acoustic properties of microbubble based contrast agents undertaken in our laboratory. The hierarchical two-pronged approach of modeling contrast agents we developed is demonstrated for a lipid coated (Sonazoid™) and a polymer shelled (poly D-L-lactic acid) contrast microbubbles. The acoustic and drug release properties of the newly developed echogenic liposomes are discussed for their use as simultaneous imaging and drug/gene delivery agents. Although echogenicity is conclusively demonstrated in experiments, its physical mechanisms remain uncertain. Addressing questions raised here will accelerate further development and eventual clinical approval of these novel technologies.

Ultrasound assisted particle and cell manipulation on-chip

Ultrasonic fields are able to exert forces on cells and other micron-scale particles, including microbubbles. The technology is compatible with existing lab-on-chip techniques and is complementary to many alternative manipulation approaches due to its ability to handle many cells simultaneously over extended length scales. This paper provides an overview of the physical principles underlying ultrasonic manipulation, discusses the biological effects relevant to its use with cells, and describes emerging applications that are of interest in the field of drug development and delivery on-chip.

Nonlinear resonance behavior and linear shell estimates for Definity™ and MicroMarker™ assessed with acoustic microbubble spectroscopy

There is a growing interest in microbubble based ultrasound contrast imaging applications in the 5-15 MHz range. In this study, individual microbubbles were insonified at low pressures (≤ 25 kPa) using an "acoustic spectroscopy" approach which entailed transmitting a sequence of tone bursts with center frequencies ranging from 4 to 13.5 MHz. The fundamental (transmit) frequency radial excursion amplitude was calculated from the scattered signals to produce a resonance curve for each bubble. For diameters between 2.5 to 4 μm, 69% of Target-Ready MicroMarker™ (Bracco, Geneva; Visualsonics, Canada) exhibited asymmetric resonance, characterized by a skewing of the resonance curve and indicative of nonlinear behavior. For Definity™ (Lantheus Medical Imaging, N. Billerica, MA), these responses were observed for 8% of diameters between 1.7 to 3.1 μm. For the subset of bubbles exhibiting linear, symmetric resonance curves, resonant frequencies, shell elasticity, and viscosity values were estimated. Between 10 to 12 MHz, for example, Target-Ready MicroMarker between 2.7 to 3.3 μm in diameter was resonant, where Definity was resonant between 1.7 to 2.6 μm. From 4 to 13.5 MHz, Target-Ready MicroMarker is characterized by a stiffer shell (3 < χ(0) < 5) N/m than Definity (0.5 < χ(0) < 2.5) N/m, and distinct strain-softening and shear-thinning rheological behavior. For Definity, no clear strain or shear-rate dependence of the shell properties is evident.

In vivo characterization of ultrasound contrast agents: microbubble spectroscopy in a chicken embryo

The dynamics of coated microbubbles was studied in an in vivo model. Biotinylated lipid-coated microbubbles were prepared in-house and were injected into a chick embryo chorioallantoic membrane (CAM) model on the fifth day of incubation. The microbubbles, ranging between 1.0 and 3.5 μm in diameter, were insonified in the frequency range of 4-7 MHz. Two amplitudes of acoustic pressure were applied: 300 kPa and 400 kPa. The fundamental and subharmonic responses were recorded optically with an ultra-fast camera (Brandaris 128) at 20 million frames per second. A subharmonic response was observed for 44% of the studied bubbles. From the data the frequency of the maximum fundamental and subharmonic response was derived for each individual bubble and resulted in the resonance curves of the microbubbles. All the bubbles showed shell (strain) hardening behavior for a higher acoustic pressure. We conclude that the subharmonic oscillations observed in this study belonged to the transmit at resonance (TR) regime.

Investigating the subharmonic response of individual phospholipid encapsulated microbubbles at high frequencies: a comparative study of five agents

There are a range of contrast ultrasound applications above 10 MHz, a frequency regime in which nonlinear microbubble behavior is poorly understood. Lipid-encapsulated microbubbles have considerable potential for use at higher frequencies because they have been shown to exhibit pronounced nonlinear activity at frequencies up to 40 MHz. The objective of this work was to investigate the influence of agent formulation on the subharmonic response of lipid-encapsulated microbubbles at high frequencies with a view to providing information relevant to improving contrast agent design and imaging performance. An optical-acoustical setup was used to measure the subharmonic emissions from small (d < 3 μm) individual lipid-encapsulated microbubbles as a function of transmit pressure, size and composition. In this study, five agent formulations (Definity™, MicroMarker™ and three in-house agents manipulated to exhibit different levels of shell microstructure heterogeneity) were insonified at 25 MHz over a peak negative pressure (P(n)) range of 0.02-1.2 MPa. All agents exhibited distinctly different subharmonic behavior, both in terms of amplitude and active sizes. MicroMarker™ exhibited the strongest, broadest and most consistent subharmonic response, 22% greater in power than that of Definity™ and as much as 50% greater than the in-house formulations. No clear relation between in-house agents' shell microstructure and nonlinear response was found, other than the variability in the nonlinear response itself. An analysis of the response of MicroMarker™ bubbles suggests that these bubbles exhibit "expansion-dominated" oscillations, in contrast to "compression-only" oscillations observed for similar bubbles at lower frequencies (f < 11 MHz).

Estimating concentration of ultrasound contrast agents with backscatter coefficients: experimental and theoretical aspects

Ultrasound contrast agents (UCAs) have been explored as a means to enhance therapeutic techniques. Because the effectiveness of these techniques relies on the UCA concentration at a target site, it would be beneficial to estimate UCA concentration noninvasively. In this study, a noninvasive method for estimating UCA concentration was developed in vitro. Backscatter coefficients (BSCs) estimated from measurements of Definity(®) UCAs were fitted to a theoretical scattering model in the 15-25 MHz range using a Levenberg-Marquardt regression technique. The model was defined by the UCA size distribution and concentration, and therefore concentration estimates were extracted directly from the fit. Calculation of the BSC was accomplished using planar reference measurements from the back wall of a Plexiglas(®) chamber and an average of 500 snapshots of ultrasonic backscatter from UCAs flowing through the chamber. In order to verify the ultrasonically derived UCA concentration estimates, a sample of the UCAs was extracted from the flow path and the concentration was estimated with a hemacytometer. UCA concentrations of 1, 2, and 5 times the dose recommended by the manufacturer were used in experiments. All BSC-based estimates were within one standard deviation of hemacytometer based estimates for peak rarefactional pressures of 100-400 kPa.

Spontaneous echocardiographic contrast associated with portosystemic shunt due to persistent patent ductus venosus

We describe a case of an infant with a single ventricle physiology, who presented with spontaneous microbubbles originating from her inferior vena cava. Imaging revealed a persistent patent ductus venosus, leading to a portosystemic shunt, streaming the microbubbles into the heart. We discuss the possible mechanisms for this rare phenomenon in a child.

The inertial terms in equations of motion for bubbles in tubular vessels or between plates

Equations resembling the Rayleigh-Plesset and Keller-Miksis equations are frequently used to model bubble dynamics in confined spaces, using the standard inertial term RR+3R([middle dot]) (2)/2, where R is the bubble radius. This practice has been widely assumed to be defensible if the bubble is much smaller than the radius of the confining vessel. This paper questions this assumption, and provides a simple rigid wall model for worst-case quantification of the effect on the inertial term of the specific confinement geometry. The relevance to a range of scenarios (including bubbles confined in microfluidic devices; or contained in test chambers for insonification or imaging; or in blood vessels) is discussed.

Combined optical and acoustical detection of single microbubble dynamics

A detailed understanding of the response of single microbubbles subjected to ultrasound is fundamental to a full understanding of the contrast-enhancing abilities of microbubbles in medical ultrasound imaging, in targeted molecular imaging with ultrasound, and in ultrasound-mediated drug delivery with microbubbles. Here, single microbubbles are isolated and their ultrasound-induced radial dynamics recorded with an ultra-high-speed camera at up to 25 million frames per second. The sound emission is recorded simultaneously with a calibrated single element transducer. It is shown that the sound emission can be predicted directly from the optically recorded radial dynamics, and vice versa, that the nanometer-scale radial dynamics can be predicted from the acoustic response recorded in the far field.

Regimes of bubble volume oscillations in a pipe

The effect of an acoustically driven bubble on the acoustics of a liquid-filled pipe is theoretically analyzed and the dimensionless groups of the problem are identified. The different regimes of bubble volume oscillations are predicted theoretically with these dimensionless groups. Three main regimes can be identified: (1) For small bubbles and weak driving, the effect of the bubble oscillations on the acoustic field can be neglected. (2) For larger bubbles and still small driving, the bubble affects the acoustic field, but due to the small driving, a linear theory is sufficient. (3) For large bubbles and large driving, the two-way coupling between the bubble and the flow dynamics requires the solution of the full nonlinear problem. The developed theory is then applied to an air bubble in a channel of an inkjet printhead. A numerical model is developed to test the predictions of the theoretical analysis. The Rayleigh-Plesset equation is extended to include the influence of the bubble volume oscillations on the acoustic field and vice versa. This modified Rayleigh-Plesset equation is coupled to a channel acoustics calculation and a Navier-Stokes solver for the flow in the nozzle. The numerical simulations indeed confirm the predictions of the theoretical analysis.

A new transducer receive transfer function calibration method: application to microbubble backscattering cross-section measurements at high frequency

When comparing acoustic scattering experiments with theory, the relationship between the pressure generated by a scatterer at the surface of a transducer and the induced voltage must be known. Methods have been previously proposed to measure the receive transfer function that rely on several assumptions. A new, experimental method for measuring the acoustic response of a spherically-focused transducer, using a hydrophone at twice the focal distance, is proposed that requires a minimum number of assumptions and calculations. The receive transfer function of a spherically-focused, high-frequency transducer was calculated, and found to be within 10% of the receive transfer function calculated assuming reciprocity. Further, using the receive transfer function, the effective backscattering cross-section of bound microbubbles interrogated at 30 MHz was measured to be, on average, 65% of the geometric backscattering cross-section, with significant size-independent variability. These results give insight into selecting the optimal microbubble size distribution for linear microbubble imaging at high frequencies.

Parametric array technique for microbubble excitation

This study investigates the use of an acoustic parametric array as a means for microbubble excitation. The excitation wave is generated during propagation in a nonlinear medium of two high-frequency carrier waves, whereby the frequency of the excitation wave is the difference frequency of the carrier waves. Carrier waves of around 10 and 25 MHz are used to generate low-frequency waves between 0.5 and 3.5 MHz at amplitudes in the range of 25 to 80 kPa in water. We demonstrate with high-speed camera observations that it is possible to induce microbubble oscillations with the low frequency signal arising from the nonlinear propagation process. As an application, we determined the resonance frequency of Definity contrast agent microbubbles with radius ranging from 1.5 to 5 μm by sweeping the difference frequency in the range from 0.5 to 3.5 MHz.

Vibration of a single microcapsule with a hard plastic shell in an acoustic standing wave field

Observation techniques for measuring the small vibration of a single microcapsule of tens of nanometers in an acoustic standing wave field are discussed. First, simultaneous optical observation of a microbubble vibration by two methods is investigated, using a high-speed video camera, which permits two-dimensional observation of the bubble vibration, and a laser Doppler vibrometer (LDV), which can observe small bubble vibration amplitudes at high frequency. Bubbles of tens of micrometers size were trapped at the antinode of an acoustic standing wave generated in an observational cell. Bubble vibration at 27 kHz could be observed and the experimental results for the two methods showed good agreement. The radial vibration of microcapsules with a hard plastic shell was observed using the LDV and the measurement of the capsule vibration with radial oscillation amplitude of tens of nanometers was successful. The acoustic radiation force acting on microcapsules in the acoustic standing wave was measured from the trapped position of the standing wave and the radial oscillation amplitude of the capsules was estimated from the theoretical equation of the acoustic radiation force, giving results in good agreement with the LDV measurements. The radial oscillation amplitude of a capsule was found to be proportional to the amplitude of the driving sound pressure. A larger expansion ratio was observed for capsules closer to the resonance condition under the same driving sound pressure and frequency.

Reflector-based phase calibration of ultrasound transducers

Recently, the measurement of phase transfer functions (PTFs) of piezoelectric transducers has received more attention. These PTFs are useful for e.g. coding and interference based imaging methods, and ultrasound contrast microbubble research. Several optical and acoustic methods to measure a transducer's PTF have been reported in literature. The optical methods require a setup to which not all ultrasound laboratories have access to. The acoustic methods require accurate distance and acoustic wave speed measurements. A small error in these leads to a large error in phase, e.g. an accuracy of 0.1% on an axial distance of 10cm leads to an uncertainty in the PTF measurement of ±97° at 4MHz. In this paper we present an acoustic pulse-echo method to measure the PTF of a transducer, which is based on linear wave propagation and only requires an estimate of the wave travel distance and the acoustic wave speed. In our method the transducer is excited by a monofrequency sine burst with a rectangular envelope. The transducer initially vibrates at resonance (transient regime) prior to the forcing frequency response (steady state regime). The PTF value of the system is the difference between the phases deduced from the transient and the steady state regimes. Good agreement, to within 7°, was obtained between KLM simulations and measurements on two transducers in a 1-8MHz frequency range. The reproducibility of the method was ±10°, with a systematic error of 2° at 1MHz increasing to 16° at 8MHz. This work demonstrates that the PTF of a transducer can be measured in a simple laboratory setting.

Subharmonic behavior of phospholipid-coated ultrasound contrast agent microbubbles

Coated microbubbles, unlike tissue are able to scatter sound subharmonically. Therefore, the subharmonic behavior of coated microbubbles can be used to enhance the contrast in ultrasound contrast imaging. Theoretically, a threshold amplitude of the driving pressure can be calculated above which subharmonic oscillations of microbubbles are initiated. Interestingly, earlier experimental studies on coated microbubbles demonstrated that the threshold for these bubbles is much lower than predicted by the traditional linear viscoelastic shell models. This paper presents an experimental study on the subharmonic behavior of differently sized individual phospholipid coated microbubbles. The radial subharmonic response of the microbubbles was recorded with the Brandaris ultra high-speed camera as a function of both the amplitude and the frequency of the driving pulse. Threshold pressures for subharmonic generation as low as 5 kPa were found near a driving frequency equal to twice the resonance frequency of the bubble. An explanation for this low threshold pressure is provided by the shell buckling model proposed by Marmottant et al. [J. Acoust. Soc. Am. 118, 3499-3505 (2005)]. It is shown that the change in the elasticity of the bubble shell as a function of bubble radius as proposed in this model, enhances the subharmonic behavior of the microbubbles.

Acoustic sizing of an ultrasound contrast agent

Because the properties of ultrasound contrast agent populations after administration to patients are largely unknown, methods able to study them noninvasively are required. In this study, we acoustically performed a size distribution measurement of the ultrasound contrast agent Definity(®). Single lipid-shelled microbubbles were insonified at 25 MHz, which is considerably higher than their resonance frequency, so that their acoustic responses depended on their geometrical cross sections only. We calculated the size of each microbubble from their measured backscattered pressures. The acoustic size measurements were compared with optical reference size measurements to test their accuracy. Our acoustic sizing method was applied to 88 individual Definity(®) bubbles to derive a size distribution of this agent. The size distribution obtained acoustically showed a mean diameter (2.5 μm) and a standard deviation (0.9 μm) in agreement within 8% with the optical reference measurement. At 25 MHz, this method can be applied to bubble sizes larger than 1.2 μm in diameter. It was observed that similar sized bubbles can give different responses (up to a factor 1.5), probably because of shell differences. These limitations should be taken into account when implementing the method in vivo. This acoustic sizing method has potential for estimating the size distribution of an ultrasound contrast agent noninvasively.

Advances in molecular imaging with ultrasound

Ultrasound imaging has long demonstrated utility in the study and measurement of anatomic features and noninvasive observation of blood flow. Within the last decade, advances in molecular biology and contrast agents have allowed researchers to use ultrasound to detect changes in the expression of molecular markers on the vascular endothelium and other intravascular targets. This new technology, referred to as ultrasonic molecular imaging, is still in its infancy. However, in preclinical studies, ultrasonic molecular imaging has shown promise in assessing angiogenesis, inflammation, and thrombus. In this review, we discuss recent advances in microbubble-type contrast agent development, ultrasound technology, and signal processing strategies that have the potential to substantially improve the capabilities and utility of ultrasonic molecular imaging.

High-resolution, high-contrast ultrasound imaging using a prototype dual-frequency transducer: in vitro and in vivo studies

With recent advances in animal models of disease, there has been great interest in capabilities for highresolution contrast-enhanced ultrasound imaging. Microbubble contrast agents are unique in that they scatter broadband ultrasound energy because of their nonlinear behavior. For optimal response, it is desirable to excite the microbubbles near their resonant frequency. To date, this has been challenging with high-frequency imaging systems because most contrast agents are resonant at frequencies in the order of several megahertz. Our team has developed a unique dual-frequency confocal transducer which enables low-frequency excitation of bubbles near their resonance with one element, and detection of their emitted high-frequency content with the second element. Using this imaging approach, we have attained an average 12.3 dB improvement in contrast-to-tissue ratios over fundamental mode imaging, with spatial resolution near that of the high-frequency element. Because this detection method does not rely on signal decorrelation, it is not susceptible to corruption by tissue motion. This probe demonstrates contrast imaging capability with significant tissue suppression, enabling high-resolution contrast-enhanced images of microvascular blood flow. Additionally, this probe can readily produce radiation force on flowing contrast agents, which may be beneficial for targeted imaging or therapy.

Microbubble characterization through acoustically induced deflation

Ultrasound contrast agents (UCA) populations are typically polydisperse and contain microbubbles with radii over a given range. Although the behavior of microbubbles of certain sizes might be masked by the behavior of others, the acoustic characterization of UCA is typically made on full populations. In this paper, we have combined acoustic and optical methods to investigate the response of isolated lipid-shelled microbubbles to low-pressure (49 and 62 kPa peak negative pressure) ultrasound tone bursts. These bursts induced slow deflation of the microbubbles. The experimental setup included a microscope connected to a fast camera acquiring one frame per pulse transmitted by a single-element transducer. The behavior of each bubble was measured at multiple frequencies, by cyclically changing the transmission frequency over the range of 2 to 4 MHz during subsequent pulse repetition intervals. The bubble echoes were captured by a second transducer and coherently recorded. More than 50 individual microbubbles were observed. Microbubbles with radii larger than 3 mum did not experience any size reduction. Smaller bubbles slowly deflated, generally until the radius reached a value around 1.4 microm, independent of the initial microbubble size. The detected pressure amplitude backscattered at 2.5 cm distance was very low, decreasing from about 5 Pa down to 1 Pa at 2 MHz as the bubbles deflated. The resonant radius was evaluated from the echo amplitude normalized with respect to the geometrical cross section. At 2-MHz excitation, deflating microbubbles showed highest normalized echo when the radius was 2.2 microm while at higher excitation frequencies, the resonant radius was lower. The relative phase shift of the echo during the deflation process was also measured. It was found to exceed pi/2 in all cases. A heuristic procedure based on the analysis of multiple bubbles of a same population was used to estimate the undamped natural frequency. It was found that a microbubble of 1.7 microm has an undamped natural frequency of 2 MHz. The difference between this size and the resonant radius is discussed as indicative of significant damping.

Radial modulation of single microbubbles

Radial modulation imaging is a new promising technique to improve contrast-enhanced ultrasound images. The method is based on dual-frequency insonation of contrast agent microbubbles. A low-frequency (LF) pulse is used to modulate the responses of the microbubbles to a high-frequency (HF) imaging pulse. Inverting the LF pulse induces amplitude and phase differences in the HF response of contrast agent microbubbles, which can be detected using Doppler techniques. Although the technique has been successfully implemented, no consensus persists on parameter choice and resulting effects. In a separate study, "compression-only" behavior of coated microbubbles was observed. Compression-only behavior could be beneficial for radial modulation imaging. This was investigated using high-speed camera recordings and simulations. We recorded the vibrations of 78 single microbubbles in a dual-frequency ultrasound field. The results showed that the LF pulse induced significant compression-only behavior, which for microbubble sizes below and at HF resonance resulted in high radial amplitude modulation. It, however, also appeared that, for radial modulation imaging, microbubble size is more important than resonance and compression-only effects.

Ultrasonic characterization of ultrasound contrast agents

The main constituent of an ultrasound contrast agent (UCA) is gas-filled microbubbles. An average UCA contains billions per ml. These microbubbles are excellent ultrasound scatterers due to their high compressibility. In an ultrasound field they act as resonant systems, resulting in harmonic energy in the backscattered ultrasound signal, such as energy at the subharmonic, ultraharmonic and higher harmonic frequencies. This harmonic energy is exploited for contrast enhanced imaging to discriminate the contrast agent from surrounding tissue. The amount of harmonic energy that the contrast agent bubbles generate depends on the bubble characteristics in combination with the ultrasound field applied. This paper summarizes different strategies to characterize the UCAs. These strategies can be divided into acoustic and optical methods, which focus on the linear or nonlinear responses of the contrast agent bubbles. In addition, the characteristics of individual bubbles can be determined or the bubbles can be examined when they are part of a population. Recently, especially optical methods have proven their value to study individual bubbles. This paper concludes by showing some examples of optically observed typical behavior of contrast bubbles in ultrasound fields.