Bubble dynamics involved in ultrasonic imaging

Toward a complete understanding of quasi-static bubble growth and departure

The distortion by gravity of a quasi-static bubble attached on an upward facing surface in a quiescent liquid is investigated. The contact angle evolution during the growth of such a bubble is studied, and the consequences on the motion of the contact line between the solid and the interface are discussed. From the initial case of a bubble attached to the rim of a nucleation site, the contact line can move inside the cavity for a highly wetting fluid, causing premature departure. For a higher wettability, the contact can either remain attached to the rim of the cavity or spread out of the cavity, depending on the cavity size and geometry. For the latter case, the bubble growth is investigated taking into account a contact angle hysteresis. Finally, a comprehensive map detailing various geometrical characteristics of bubbles is presented, ranging over several orders of magnitude of Bond numbers and normalized volumes. The map aims at being used as a tool for investigating bubble growth in a similar situation.

Experimental acoustic characterization of an endoskeletal antibubble contrast agent: First results

Purpose

An antibubble is an encapsulated gas bubble with an incompressible inclusion inside the gas phase. Current‐generation ultrasound contrast agents are bubble‐based: they contain encapsulated gas bubbles with no inclusions. The objective of this work is to determine the linear and nonlinear responses of an antibubble contrast agent in comparison to two bubble‐based ultrasound contrast agents, that is, reference bubbles and SonoVueTM.

Methods

Side scatter and attenuation of the three contrast agents were measured, using single‐element ultrasound transducers, operating at 1.0, 2.25, and 3.5 MHz. The scatter measurements were performed at acoustic pressures of 200 and 300 kPa for 1.0 MHz, 300 kPa, and 450 kPa for 2.25 MHz, and 370 and 560 kPa for 3.5 MHz. Attenuation measurements were conducted at pressures of 13, 55, and 50 kPa for 1.0, 2.25, and 3.5 MHz, respectively. In addition, a dynamic contrast‐enhanced ultrasound measurement was performed, imaging the contrast agent flow through a vascular phantom with a commercial diagnostic linear array probe.

Results

Antibubbles generated equivalent or stronger harmonic signal, compared to bubble‐based ultrasound contrast agents. The second harmonic side‐scatter amplitude of the antibubble agent was up to 3 dB greater than that of reference bubble agent and up to 4 dB greater than that of SonoVueTM at the estimated concentration of 8×104 bubbles/mL. For ultrasound with a center transmit frequency of 1.0 MHz, the attenuation coefficient of the antibubble agent was 8.7 dB/cm, whereas the attenuation coefficient of the reference agent was 7.7 and 0.3 dB/cm for SonoVueTM. At 2.25 MHz, the attenuation coefficients were 9.7, 3.0, and 0.6 dB/cm, respectively. For 3.5 MHz, they were 4.4, 1.8, and 1.0 dB/cm, respectively. A dynamic contrast‐enhanced ultrasound recording showed the nonlinear signal of the antibubble agent to be 31% greater than for reference bubbles and 23% lower than SonoVueTM at a high concentration of 2×106 bubbles/mL.

Conclusion

Endoskeletal antibubbles generate comparable or greater higher harmonics than reference bubbles and SonoVueTM. As a result, antibubbles with liquid therapeutic agents inside the gas phase have high potential to become a traceable therapeutic agent.

Formulation and Characterization of Chemically Cross-linked Microbubble Clusters

The purpose of this study is to introduce a new concept of chemically cross-linked microbubble clusters (CCMCs), which are individual microbubble ultrasound contrast agents (UCAs) physically tethered together. We demonstrate a facile means of their production, characterize their size and stability, and describe how they can potentially be used in biomedical applications. By tethering UCAs together into CCMCs, we propose that novel methods of ultrasound mediated imaging and therapy can be developed through unique interbubble interactions in an ultrasound field. One of the major challenges in generating CCMCs is controlling aggregate sizes and maintaining stability against Ostwald ripening and coalescence. In this study, we demonstrate that chemically cross-linked microbubble clusters can produce small (<10 μm) quasi-stable complexes that slowly fuse into bubbles with individual gas cores. Furthermore, we demonstrate that this process can be driven with low-intensity ultrasound pulses, enabling a rapid fusion of clusters which could potentially be used to develop novel ultrasound contrast imaging and drug delivery strategies in future studies. The development of novel microbubble clusters presents a simple yet robust process for generating novel UCAs with a design that could allow for more versatility in contrast-enhanced ultrasound (CEUS), molecular imaging, and drug delivery applications. Additionally, microbubble clustering is a unique way to control size, shell, and gas compositions that can be used to study bubble ripening and coalescence in a highly controlled environment or study the behavior of mixed-microbubble populations.

Streaming flow from ultrasound contrast agents by acoustic waves in a blood vessel model

To elucidate the effects of streaming flow on ultrasound contrast agent (UCA)-assisted drug delivery, streaming velocity fields from sonicated UCA microbubbles were measured using particle image velocimetry (PIV) in a blood vessel model. At the beginning of ultrasound sonication, the UCA bubbles formed clusters and translated in the direction of the ultrasound field. Bubble cluster formation and translation were faster with 2.25MHz sonication, a frequency close to the resonance frequency of the UCA. Translation of bubble clusters induced streaming jet flow that impinged on the vessel wall, forming symmetric vortices. The maximum streaming velocity was about 60mm/s at 2.25MHz and decreased to 15mm/s at 1.0MHz for the same acoustic pressure amplitude. The effect of the ultrasound frequency on wall shear stress was more noticeable. Maximum wall shear stress decreased from 0.84 to 0.1Pa as the ultrasound frequency decreased from 2.25 to 1.0MHz. The maximum spatial gradient of the wall shear stress also decreased from 1.0 to 0.1Pa/mm. This study showed that streaming flow was induced by bubble cluster formation and translation and was stronger upon sonication by an acoustic wave with a frequency near the UCA resonance frequency. Therefore, the secondary radiant force, which is much stronger at the resonance frequency, should play an important role in UCA-assisted drug delivery.

Inertial cavitation threshold of nested microbubbles

Cavitation of ultrasound contrast agents (UCAs) promotes both beneficial and detrimental bioeffects in vivo (Radhakrishnan et al., 2013) [1]. The ability to determine the inertial cavitation threshold of UCA microbubbles has potential application in contrast imaging, development of therapeutic agents, and evaluation of localized effects on the body (Ammi et al., 2006) [2]. This study evaluates a novel UCA and its inertial cavitation behavior as determined by a home built cavitation detection system. Two 2.25 MHz transducers are placed at a 90° angle to one another where one transducer is driven by a high voltage pulser and the other transducer receives the signal from the oscillating microbubble. The sample chamber is placed in the overlap of the focal region of the two transducers where the microbubbles are exposed to a pulser signal consisting of 600 pulse trains per experiment at a pulse repetition frequency of 5 Hz where each train has four pulses of four cycles. The formulation being analyzed is comprised of an SF6 microbubble coated by a DSPC PEG-3000 monolayer nested within a poly-lactic acid (PLA) spherical shell. The effect of varying shell diameters and microbubble concentration on cavitation threshold profile for peak negative pressures ranging from 50 kPa to 2 MPa are presented and discussed in this paper. The nesting shell decreases inertial cavitation events from 97.96% for an un-nested microbubble to 19.09% for the same microbubbles nested within a 2.53 μm shell. As shell diameter decreases, the percentage of inertially cavitating microbubbles also decreases. For nesting formulations with average outer capsule diameters of 20.52, 14.95, 9.95, 5.55, 2.53, and 1.95 μm, the percentage of sample destroyed at 1 MPa was 51.02, 38.94, 33.25, 25.27, 19.09, and 5.37% respectively.

State-of-the-art materials for ultrasound-triggered drug delivery

Ultrasound is a unique and exciting theranostic modality that can be used to track drug carriers, trigger drug release and improve drug deposition with high spatial precision. In this review, we briefly describe the mechanisms of interaction between drug carriers and ultrasound waves, including cavitation, streaming and hyperthermia, and how those interactions can promote drug release and tissue uptake. We then discuss the rational design of some state-of-the-art materials for ultrasound-triggered drug delivery and review recent progress for each drug carrier, focusing on the delivery of chemotherapeutic agents such as doxorubicin. These materials include nanocarrier formulations, such as liposomes and micelles, designed specifically for ultrasound-triggered drug release, as well as microbubbles, microbubble-nanocarrier hybrids, microbubble-seeded hydrogels and phase-change 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.

Size distribution of microbubbles as a function of shell composition

The effect of modifying the shell composition of a population of microbubbles on their size demonstrated through experiment. Specifically, these variations include altering both the mole fraction and molecular weight of functionalized polymer, polyethylene glycol (PEG) in the microbubble phospholipid monolayer shell (1-15 mol% PEG, and 1000-5000 g/mole, respectively). The size distribution is measured with an unbiased image segmentation program written in MATLAB which identifies and sizes bubbles from micrographs. For a population of microbubbles with a shell composition of 5 mol% PEG2000, the mean diameter is 1.42 μm with a variance of 0.244 μm. For the remainder of the shell compositions studied herein, we find that the size distributions do not show a statistically significant correlation to either PEG molecular weight or mole fraction. All the measured distributions are nearly Gaussian in shape and have a monomodal peak.

Determination of the interfacial rheological properties of a poly(DL-lactic acid)-encapsulated contrast agent using in vitro attenuation and scattering

The stabilizing encapsulation of a microbubble-based ultrasound contrast agent (UCA) critically affects its acoustic properties. Polymers, which behave differently from materials commonly used (i.e., lipids or proteins) for monolayer encapsulation, have the potential for better stability and improved control of encapsulation properties. Air-filled microbubbles coated with poly(DL-lactic acid) (PLA) are characterized here using in vitro acoustic experiments and several models of encapsulation. The interfacial rheological properties of the encapsulation are determined according to each model using attenuation of ultrasound through a suspension of microbubbles. Then the model predictions are compared with scattered non-linear (sub- and second harmonic) responses. For this microbubble population (average diameter, 1.9 μm), the peak in attenuation measurement indicates a weighted-average resonance frequency of 2.5-3 MHz, which, in contrast to other encapsulated microbubbles, is lower than the resonance frequency of a free bubble of similar size (diameter, 1.9 μm). This apparently contradictory result stems from the extremely low surface dilational elasticity (around 0.01-0.07 N/m) and the reduced surface tension of the poly(DL-lactic acid) encapsulation, as well as the polydispersity of the bubble population. All models considered here are shown to behave similarly even in the non-linear regime because of the low surface dilational elasticity value. Pressure-dependent scattering measurements at two different excitation frequencies (2.25 and 3 MHz) revealed strongly non-linear behavior with 25-30 dB and 5-20 dB enhancements in fundamental and second-harmonic responses, respectively, for a contrast agent concentration of 1.33 μg/mL in the suspension. Sub-harmonic responses are registered above a relatively low generation threshold of 100-150 kPa, with up to 20 dB enhancement beyond that pressure. Numerical predictions from all models show good agreement with the experimentally measured fundamental response, but not with the experimental second-harmonic response. The characteristic features of sub-harmonic responses and the steady response beyond the threshold are matched well by model predictions. However, prediction of the threshold value depends on estimated properties and size distribution. The variation in size distribution from sample to sample leads to variation in estimates of encapsulation properties: the lowest estimated value for surface dilational viscosity better predicts the sub-harmonic threshold.

Influence of shell composition on the resonance frequency of microbubble contrast agents

The effect of variations in microbubble shell composition on microbubble resonance frequency is revealed through experiment. These variations are achieved by altering the mole fraction and molecular weight of functionalized polyethylene glycol (PEG) in the microbubble phospholipid monolayer shell and measuring the microbubble resonance frequency. The resonance frequency is measured via a chirp pulse and identified as the frequency at which the pressure amplitude loss of the ultrasound wave is the greatest as a result of passing through a population of microbubbles. For the shell compositions used herein, we find that PEG molecular weight has little to no influence on resonance frequency at an overall PEG mole fraction (0.01) corresponding to a mushroom regime and influences the resonance frequency markedly at overall PEG mole fractions (0.050-0.100) corresponding to a brush regime. Specifically, the measured resonance frequency was found to be 8.4, 4.9, 3.3 and 1.4 MHz at PEG molecular weights of 1000, 2000, 3000 and 5000 g/mol, respectively, at an overall PEG mole fraction of 0.075. At an overall PEG mole fraction of just 0.01, on the other hand, resonance frequency exhibited no systematic variation, with values ranging from 5.7 to 4.9 MHz. Experimental results were analyzed using the Sarkar bubble dynamics model. With the dilatational viscosity held constant (10(-8) N·s/m) and the elastic modulus used as a fitting parameter, model fits to the pressure amplitude loss data resulted in elastic modulus values of 2.2, 2.4, 1.6 and 1.8 N/m for PEG molecular weights of 1000, 2000, 3000 and 5000 g/mol, respectively, at an overall PEG mole fraction of 0.010 and 4.2, 1.4, 0.5 and 0.0 N/m, respectively, at an overall PEG mole fraction of 0.075. These results are consistent with theory, which predicts that the elastic modulus is constant in the mushroom regime and decreases with PEG molecular weight to the inverse 3/5 power in the brush regime. Additionally, these results are consistent with inertial cavitation studies, which revealed that increasing PEG molecular weight has little to no effect on inethe rtial cavitation threshold in the mushroom regime, but that increasing PEG molecular weight decreases inertial cavitation markedly in the brush regime. We conclude that the design and synthesis of microbubbles with a prescribed resonance frequency is attainable by tuning PEG composition and molecular weight.

Bursting bubbles and bilayers

This paper discusses various interactions between ultrasound, phospholipid monolayer-coated gas bubbles, phospholipid bilayer vesicles, and cells. The paper begins with a review of microbubble physics models, developed to describe microbubble dynamic behavior in the presence of ultrasound, and follows this with a discussion of how such models can be used to predict inertial cavitation profiles. Predicted sensitivities of inertial cavitation to changes in the values of membrane properties, including surface tension, surface dilatational viscosity, and area expansion modulus, indicate that area expansion modulus exerts the greatest relative influence on inertial cavitation. Accordingly, the theoretical dependence of area expansion modulus on chemical composition - in particular, poly (ethylene glyclol) (PEG) - is reviewed, and predictions of inertial cavitation for different PEG molecular weights and compositions are compared with experiment. Noteworthy is the predicted dependence, or lack thereof, of inertial cavitation on PEG molecular weight and mole fraction. Specifically, inertial cavitation is predicted to be independent of PEG molecular weight and mole fraction in the so-called mushroom regime. In the “brush” regime, however, inertial cavitation is predicted to increase with PEG mole fraction but to decrease (to the inverse 3/5 power) with PEG molecular weight. While excellent agreement between experiment and theory can be achieved, it is shown that the calculated inertial cavitation profiles depend strongly on the criterion used to predict inertial cavitation. This is followed by a discussion of nesting microbubbles inside the aqueous core of microcapsules and how this significantly increases the inertial cavitation threshold. Nesting thus offers a means for avoiding unwanted inertial cavitation and cell death during imaging and other applications such as sonoporation. A review of putative sonoporation mechanisms is then presented, including those involving microbubbles to deliver cargo into a cell, and those - not necessarily involving microubbles - to release cargo from a phospholipid vesicle (or reverse sonoporation). It is shown that the rate of (reverse) sonoporation from liposomes correlates with phospholipid bilayer phase behavior, liquid-disordered phases giving appreciably faster release than liquid-ordered phases. Moreover, liquid-disordered phases exhibit evidence of two release mechanisms, which are described well mathematically by enhanced diffusion (possibly via dilation of membrane phospholipids) and irreversible membrane disruption, whereas liquid-ordered phases are described by a single mechanism, which has yet to be positively identified. The ability to tune release kinetics with bilayer composition makes reverse sonoporation of phospholipid vesicles a promising methodology for controlled drug delivery. Moreover, nesting of microbubbles inside vesicles constitutes a truly “theranostic” vehicle, one that can be used for both long-lasting, safe imaging and for controlled drug delivery.

Targeted microbubbles in the experimental and clinical setting

BACKGROUND

Microbubbles have improved ultrasonography imaging techniques over the past 2 decades. Their safety, versatility, and easiness of use have rendered them equal or even superior in some instances to other imaging modalities such as computed tomography and magnetic resonance imaging. Herein, we conducted a literature review to present their types, general behavior in tissues, and current and potential use in clinical practice.

METHODS

A literature search was conducted for all preclinical and clinical studies involving microbubbles and ultrasonography.

RESULTS

Different types of microbubbles are available. These generally improve the enhancement of tissues during ultrasonography imaging. They also can be attached to ligands for the target of several conditions such as inflammation, angiogenesis, thrombosis, apoptosis, and might have the potential of carrying toxic drugs to diseased sites, thereby limiting the systemic adverse effects.

CONCLUSIONS

The use of microbubbles is evolving rapidly and can have a significant impact on the management of various conditions. The potential for their use as targeting agents and gene and drug delivery vehicles looks promising.

Real-time and spatial quantification using contrast-enhanced ultrasonography of spinal cord perfusion during experimental spinal cord injury

STUDY DESIGN

Experimental study in male Wistar rats.

OBJECTIVE

To quantify temporal and spatial changes simultaneously in spinal cord blood flow and hemorrhage during the first hour after spinal cord injury (SCI), using contrast-enhanced ultrasonography (CEU).

SUMMARY OF BACKGROUND DATA

Post-traumatic ischemia and hemorrhage worsen the primary lesions induced by SCI. Previous studies did not simultaneously assess temporal and spatial changes in spinal cord blood flow.

METHODS

SCI was induced at Th10 in 12 animals, which were compared with 11 sham-operated controls. Spinal cord blood flow was measured in 7 adjacent regions of interest and in the sum of these 7 regions. Blood flow was quantified using CEU with intravenous microbubble injection. Spinal cord hemorrhage was measured on conventional B-mode sonogram slices.

RESULTS

CEU allowed us to measure the temporal and spatial changes in spinal cord blood flow in both groups. In the SCI group, spinal cord blood flow was significantly decreased in the global region of interest (P = 0.0016), at the impact site (epicenter), and in the 4 regions surrounding the epicenter, compared with the sham group. The blood flow decrease was maximum at the epicenter. No statistically significant differences between the sham groups were found for the most rostral and caudal regions of interest. Hemorrhage size increased significantly with time (P < 0.0001), from 30.3 mm(2) (±2) after 5 minutes to 39.6 mm(2) (±2.3) after 60 minutes.

CONCLUSION

CEU seems reliable for quantifying temporal and spatial changes in spinal cord blood flow. After SCI, bleeding occurs in the spinal cord parenchyma and increases significantly throughout the first hour.

Production of uniformly sized serum albumin and dextrose microbubbles

Uniformly-sized preparations with average microbubble (MB) diameters from 1 to 7 μm were produced reliably by sonicating decafluorobutane-saturated solutions of serum albumin and dextrose. Detailed protocols for producing and size-separating the MBs are presented, along with the effects that changing each production parameter (serum albumin concentration, sonication power, sonication time, etc.) had on MB size distribution and acoustic stability. These protocols can be used to produce MBs for experimental applications or serve as templates for developing new protocols that yield MBs with physical and acoustic properties better suited to specific applications. Size stability and ultrasonic performance quality control tests were developed to assure that successive MB preparations perform identically and to distinguish the physical and acoustic properties of identically sized MBs produced with different serum albumin-dextrose formulations and sonication parameters. MBs can be stored at 5 °C for protracted periods (2 weeks to one year depending on formulation).

Laser-nucleated acoustic cavitation in focused ultrasound

Acoustic cavitation can occur in therapeutic applications of high-amplitude focused ultrasound. Studying acoustic cavitation has been challenging, because the onset of nucleation is unpredictable. We hypothesized that acoustic cavitation can be forced to occur at a specific location using a laser to nucleate a microcavity in a pre-established ultrasound field. In this paper we describe a scientific instrument that is dedicated to this outcome, combining a focused ultrasound transducer with a pulsed laser. We present high-speed photographic observations of laser-induced cavitation and laser-nucleated acoustic cavitation, at frame rates of 0.5×10(6) frames per second, from laser pulses of energy above and below the optical breakdown threshold, respectively. Acoustic recordings demonstrated inertial cavitation can be controllably introduced to the ultrasound focus. This technique will contribute to the understanding of cavitation evolution in focused ultrasound including for potential therapeutic applications.

Contrast-enhanced and targeted ultrasound

Ultrasonic imaging is becoming the most popular medical imaging modality, owing to the low price per examination and its safety. However, blood is a poor scatterer of ultrasound waves at clinical diagnostic transmit frequencies. For perfusion imaging, markers have been designed to enhance the contrast in B-mode imaging. These so-called ultrasound contrast agents consist of microscopically small gas bubbles encapsulated in biodegradable shells. In this review, the physical principles of ultrasound contrast agent microbubble behavior and their adjustment for drug delivery including sonoporation are described. Furthermore, an outline of clinical imaging applications of contrast-enhanced ultrasound is given. It is a challenging task to quantify and predict which bubble phenomenon occurs under which acoustic condition, and how these phenomena may be utilized in ultrasonic imaging. Aided by high-speed photography, our improved understanding of encapsulated microbubble behavior will lead to more sophisticated detection and delivery techniques. More sophisticated methods use quantitative approaches to measure the amount and the time course of bolus or reperfusion curves, and have shown great promise in revealing effective tumor responses to anti-angiogenic drugs in humans before tumor shrinkage occurs. These are beginning to be accepted into clinical practice. In the long term, targeted microbubbles for molecular imaging and eventually for directed anti-tumor therapy are expected to be tested.

Microfoam formation in a capillary

The ultrasound-induced formation of bubble clusters may be of interest as a therapeutic means. If the clusters behave as one entity, i.e., one mega-bubble, its ultrasonic manipulation towards a boundary is straightforward and quick. If the clusters can be forced to accumulate to a microfoam, entire vessels might be blocked on purpose using an ultrasound contrast agent and a sound source. In this paper, we analyse how ultrasound contrast agent clusters are formed in a capillary and what happens to the clusters if sonication is continued, using continuous driving frequencies in the range 1-10 MHz. Furthermore, we show high-speed camera footage of microbubble clustering phenomena. We observed the following stages of microfoam formation within a dense population of microbubbles before ultrasound arrival. After the sonication started, contrast microbubbles collided, forming small clusters, owing to secondary radiation forces. These clusters coalesced within the space of a quarter of the ultrasonic wavelength, owing to primary radiation forces. The resulting microfoams translated in the direction of the ultrasound field, hitting the capillary wall, also owing to primary radiation forces. We have demonstrated that as soon as the bubble clusters are formed and as long as they are in the sound field, they behave as one entity. At our acoustic settings, it takes seconds to force the bubble clusters to positions approximately a quarter wavelength apart. It also just takes seconds to drive the clusters towards the capillary wall. Subjecting an ultrasound contrast agent of given concentration to a continuous low-amplitude signal makes it cluster to a microfoam of known position and known size, allowing for sonic manipulation.

Microbubble Compositions, Properties and Biomedical Applications

Over the last decade, there has been significant progress towards the development of microbubbles as theranostics for a wide variety of biomedical applications. The unique ability of microbubbles to respond to ultrasound makes them useful agents for contrast ultrasound imaging, molecular imaging, and targeted drug and gene delivery. The general composition of a microbubble is a gas core stabilized by a shell comprised of proteins, lipids or polymers. Each type of microbubble has its own unique advantages and can be tailored for specialized functions. In this review, different microbubbles compositions and physiochemical properties are discussed in the context of current progress towards developing novel constructs for biomedical applications, with specific emphasis on molecular imaging and targeted drug/gene delivery.

Anniversary paper: evolution of ultrasound physics and the role of medical physicists and the AAPM and its journal in that evolution

Ultrasound has been the greatest imaging modality worldwide for many years by equipment purchase value and by number of machines and examinations. It is becoming increasingly the front end imaging modality; serving often as an extension of the physician's fingers. We believe that at the other extreme, high-end systems will continue to compete with all other imaging modalities in imaging departments to be the method of choice for various applications, particularly where safety and cost are paramount. Therapeutic ultrasound, in addition to the physiotherapy practiced for many decades, is just coming into its own as a major tool in the long progression to less invasive interventional treatment. The physics of medical ultrasound has evolved over many fronts throughout its history. For this reason, a topical review, rather than a primarily chronological one is presented. A brief review of medical ultrasound imaging and therapy is presented, with an emphasis on the contributions of medical physicists, the American Association of Physicists in Medicine (AAPM) and its publications, particularly its journal Medical Physics. The AAPM and Medical Physics have contributed substantially to training of physicists and engineers, medical practitioners, technologists, and the public.

Sonazoid-enhanced ultrasonography for evaluation of the enhancement patterns of focal liver tumors in the late phase by intermittent imaging with a high mechanical index

OBJECTIVE

The purpose of this study was to evaluate the enhancement patterns of focal liver tumors in the late phase of Sonazoid-enhanced ultrasonography by intermittent imaging with a high mechanical index (MI).

METHODS

A total of 142 patients with 208 lesions, including 109 hepatocellular carcinomas (HCCs), 61 metastases, 30 hemangiomas, and 8 focal nodular hyperplasias (FNHs), were enrolled in this prospective study. Contrast-enhanced ultrasonography with intermittent scanning at 2 frames per second (MI, 0.7-1.2) was conducted in the late phase (>5 minutes after bolus intravenous injection of the perflubutane-based contrast agent Sonazoid; Daiichi Sankyo, Tokyo, Japan). Two blinded readers classified the enhancement patterns of the lesions. The sensitivity, specificity, and positive predictive value (PPV) of the dominant enhancement patterns and inter-reader agreement were assessed.

RESULTS

A combination of diffuse enhancement with intratumoral vessels and intratumoral vessels alone yielded sensitivity of 85% (average of both readers), specificity of 88%, and a PPV of 88% for HCC. For metastasis, a combination of peripheral ringlike enhancement with peritumoral vessels and peripheral ringlike enhancement with intratumoral vessels yielded sensitivity of 79%, specificity of 95%, and a PPV of 85%. For hemangiomas, a combination of peripheral nodular enhancement with peritumoral vessels and peripheral nodular enhancement without peritumoral vessels yielded sensitivity of 75%, specificity of 99%, and a PPV of 92%. Diffuse enhancement with spoked wheel arteries yielded sensitivity of 82%, specificity of 100%, and a PPV of 87% for FNHs. Good inter-reader agreement was achieved.

CONCLUSIONS

Sonazoid-enhanced ultrasonography using intermittent imaging with a high MI can potentially be used for evaluating the enhancement patterns of focal liver tumors in the late phase.

Ultrasonic imaging of molecular targets

Today nuclear medicine is the only modality that is clinically successful in molecular imaging. However, other modalities compete with its excellent sensitivity in imaging molecular targets. In the last 10 years ultrasound imaging has shown the potential to provide sufficiently high sensitivity for the molecular imaging of vascular targets. These advances are based on the joint development of microbubble contrast media and the methods to image them with high sensitivity. Ultrasound-contrast-enhanced imaging strategies make use of the specific physical properties of microbubbles such as resonance, nonlinear oscillation, and collapse. The size of microbubbles limits their use to the vascular space. Thus, the main applications of ultrasound for molecular imaging are inflammation, thrombi, and angiogenesis, for which successful contrast enhancement has been achieved in animal models. Main molecular targets used to date include selectins, alpha(v)beta(3) or alpha(5)beta(1) integrins, glycoprotein (GP) IIb/IIIa, intracellular adhesion molecule ICAM-1, and vascular endothelial growth factor receptor VEGFR2. Results from animal studies indicate that ultrasound could play a major role in vascular molecular imaging for diagnosis and treatment monitoring. Additional effects of insonified microbubbles (e.g., opening of the blood-brain barrier or increased transfection efficiency in gene therapy) are attributed to the transient opening of cell membranes known as "sonoporation" and demonstrate further potential for integrated diagnosis and therapy.

Dynamic adsorption properties of n-alkyl glucopyranosides determine their ability to inhibit cytolysis mediated by acoustic cavitation

Suspensions of human leukemia (HL-60) cells readily undergo cytolysis when exposed to ultrasound above the acoustic cavitation threshold. However, n-alkyl glucopyranosides (hexyl, heptyl, and octyl) completely inhibit ultrasound-induced (1057 kHz) cytolysis (Sostaric, et al. Free Radical Biol. Med. 2005, 39, 1539-1548). The efficacy of protection from ultrasound-induced cytolysis was determined by the n-alkyl chain length of the glucopyranosides, indicating that protection efficacy depended on adsorption of n-alkyl glucopyranosides to the gas/solution interface of cavitation bubbles and/or the lipid membrane of cells. The current study tests the hypothesis that "sonoprotection" (i.e., protection of cells from ultrasound-induced cytolysis) in vitro depends on the adsorption of glucopyranosides at the gas/solution interface of cavitation bubbles. To test this hypothesis, the effect of ultrasound frequency (from 42 kHz to 1 MHz) on the ability of a homologous series of n-alkyl glucopyranosides to protect cells from ultrasound-induced cytolysis was investigated. It is expected that ultrasound frequency will affect sonoprotection ability since the nature of the cavitation bubble field will change. This will affect the relative importance of the possible mechanisms for ultrasound-induced cytolysis. Additionally, ultrasound frequency will affect the lifetime and rate of change of the surface area of cavitation bubbles, hence the dynamically controlled adsorption of glucopyranosides to their surface. The data support the hypothesis that sonoprotection efficiency depends on the ability of glucopyranosides to adsorb at the gas/solution interface of cavitation bubbles.

Oil-filled polymer microcapsules for ultrasound-mediated delivery of lipophilic drugs

The use of ultrasound contrast agents as local drug delivery systems continues to grow. Current limitations are the amount of drug that can be incorporated as well as the efficiency of drug release upon insonification. This study focuses on the synthesis and characterisation of novel polymeric microcapsules for ultrasound-triggered delivery of lipophilic drugs. Microcapsules with a shell of fluorinated end-capped poly(L-lactic acid) were made through pre-mix membrane emulsification and contained, apart from a gaseous phase, different amounts of hexadecane oil as a drug-carrier reservoir. Mean number weighted diameters were between 1.22 microm and 1.31 microm. High-speed imaging at approximately 10 million fames per second showed that for low acoustic pressures (1 MHz, 0.24 MPa) microcapsules compressed but remained intact. At higher diagnostic pressures of 0.51 MPa, microcapsules cracked, thereby releasing the encapsulated gas and model lipophilic drug. Using conventional ultrasound B-mode imaging at a frequency of 2.5 MHz, a marked enhancement of scatter intensity over a tissue-mimicking phantom was observed for all differently loaded microcapsules. The partially oil-filled microcapsules with high drug loads and well-defined acoustic activation thresholds have great potential for ultrasound-triggered local delivery of lipophilic drugs under ultrasound image-guidance.

Three-dimensional reconstruction of fine vascularity in ultrasound breast imaging using contrast-enhanced spatial compounding: in vitro analyses

RATIONALE AND OBJECTIVES

Ultrasound image quality can be improved by imaging an object (here: the female breast) from different viewing angles in one image plane. With this technique, which is commonly referred to as spatial compounding, a more isotropic resolution is achieved while speckle noise and further artifacts are reduced. We present results obtained from a combination of spatial compounding with contrast-enhanced ultrasound imaging in three dimensions to reduce contrast specific artifacts (depth dependency, shadowing, speckle) and reconstruct vascular structures.

MATERIALS AND METHODS

We used a conventional ultrasound scanner and a custom made mechanical system to rotate an ultrasound curved array probe around an object (360 degrees, 36 transducer positions). For 10 parallel image planes, ultrasound compound images were generated of a flow-mimicking phantom consecutively supplied with water and contrast agent. These compound images were combined to form a volume dataset and postprocessed to obtain a sonographic subtraction angiography.

RESULTS

Image quality was significantly improved by spatial compounding for the native (ie, without contrast agent), and, in particular, for the contrast-enhanced case. After subtracting the native images from the contrast-enhanced ones, only structures supplied with contrast agent remain. This technique yields much better results for compound images than for conventional ultrasound images because speckle noise and an anisotropic resolution affect the latter.

CONCLUSIONS

With the presented approach contrast specific artifacts can be eliminated efficiently, and a subtraction angiography can be computed. A speckle reduced three-dimensional reconstruction of submillimeter vessel structures was achieved for the first time. In the future, this technique can be applied in vivo to image the vascularity of cancer in the female breast.

Full angle spatial compounding for improved replenishment analyses in contrast perfusion imaging: in vitro studies

For contrast enhanced perfusion imaging semi-quantitative methods (such as the bolus-, replenishment- or depletion-method) are commonly used to analyze the dynamic changes in concentration of contrast agent induced by insonification. In order to apply these methods and to decrease artifacts from tissue nonlinearity, perfusion imaging is conducted using decreased transmit power. However, echo signals from deeper structures are often too weak to be successfully analyzed. Furthermore, shadowing artifacts may occur as a result of high concentration of contrast agent in the beam path. Thus, those semi-quantitative methods often fail or yield ambiguous diagnoses. Imaging an object (e.g., the female breast) from multiple viewing angles (spatial compounding) may overcome these issues. In addition, spatial compounding achieves an isotropic resolution and reduces speckle and further common artifacts. In this paper we present results obtained from a combination of spatial compounding with contrast enhanced perfusion imaging. Applying the replenishment method, we extracted perfusion-related parameters and compared the conventional parametric images with the compound parametric images. We found that the compounded parametric images outperform the conventional images due to reduced noise and suppression of artifacts.

Ultrasonographic contrast-agent imaging of sub-millimeter vessel structures with spatial compounding:in vitroanalyses / Kontrastmittelgestützte Ultraschallabbildung von Sub-Millimeter-Gefäßstrukturen mittels Spatial Compounding:In-vitroAnalysen

In clinical diagnostics, ultrasonographic contrast-agent imaging gives access to medical parameters such as perfusion and vascularization. In addition to the artifacts that are typical for ultrasonic imaging, e.g., speckle noise and depth-dependent sensitivity and resolution, contrast-agent imaging shows more pronounced depth dependence and may suffer from shadowing artifacts that arise from high attenuation of the ultrasound waves by the contrast agent at high concentrations. By imaging an object from different viewing angles in one 2D image plane and summing the images obtained (spatial compounding), image quality can be increased and artifacts can be suppressed. In the present study, we combined both techniques to overcome the limitations of contrast-agent imaging. We used a commercially available ultrasound scanner and a custom-made high-precision mechanical system to rotate the ultrasound transducer fully around the object under investigation. Using this set-up, ultrasound data were acquired in reflection mode to generate a 360 degrees compound scan of a flow-mimicking phantom supplied with contrast agent.

Ultrasonic bubbles in medicine: influence of the shell

Ultrasound contrast agents consist of microscopically small bubbles encapsulated by an elastic shell. These microbubbles oscillate upon ultrasound insonification, and demonstrate highly nonlinear behavior, ameliorating their detectability. (Potential) medical applications involving the ultrasonic disruption of contrast agent microbubble shells include release-burst imaging, localized drug delivery, and noninvasive blood pressure measurement. To develop and enhance these techniques, predicting the cracking behavior of ultrasound-insonified encapsulated microbubbles has been of importance. In this paper, we explore microbubble behavior in an ultrasound field, with special attention to the influence of the bubble shell. A bubble in a sound field can be considered a forced damped harmonic oscillator. For encapsulated microbubbles, the presence of a shell has to be taken into account. In models, an extra damping parameter and a shell stiffness parameter have been included, assuming that Hooke's Law holds for the bubble shell. At high acoustic amplitudes, disruptive phenomena have been observed, such as microbubble fragmentation and ultrasonic cracking. We analyzed the occurrence of ultrasound contrast agent fragmentation, by simulating the oscillating behavior of encapsulated microbubbles with various sizes in a harmonic acoustic field. Fragmentation occurs exclusively during the collapse phase and occurs if the kinetic energy of the collapsing microbubble is greater than the instantaneous bubble surface energy, provided that surface instabilities have grown big enough to allow for break-up. From our simulations it follows that the Blake critical radius is not a good approximation for a fragmentation threshold. We demonstrated how the phase angle differences between a damped radially oscillating bubble and an incident sound field depend on shell parameters.