Donovan May

Threshold of fragmentation for ultrasonic contrast agents.

Ultrasound contrast agents are small microbubbles that can be readily destroyed with sufficient acoustic pressure, typically, at a frequency in the low megaHertz range. Microvascular flow rate may be estimated by destroying the contrast agent in a vascular bed, and estimating the rate of flow of contrast agents back into the vascular bed. Characterization of contrast agent destruction provides important information for the design of this technique. In this paper, high-speed optical observation of an ultrasound contrast agent during acoustic insonation is performed. The resting diameter is shown to be a significant parameter in the prediction of microbubble destruction, with smaller diameters typically correlated with destruction. Pressure, center frequency, and transmission phase are each shown to have a significant effect on the fragmentation threshold. A linear prediction for the fragmentation threshold as a function of pressure, when normalized by the resting diameter, has a rate of change of 300 kPa/microm for the range of pressures from 310 to 1200 kPa, and a two-cycle excitation pulse with a center frequency of 2.25 MHz. A linear prediction for the fragmentation threshold as a function of frequency, when normalized by the resting diameter, has a rate of change of -1.2 MHz/microm for a transmission pressure of 800 kPa, and a two-cycle excitation pulse with a range of frequencies from 1 to 5 MHz.

Nondestructive subharmonic imaging

Ultrasound contrast agent microbubbles are intravascular agents that can be used to estimate blood perfusion. Blood perfusion may be estimated by destroying the bubbles in a vascular bed and observing the refresh of contrast agents back into the vascular bed. Contrast agents can be readily destroyed by traditional imaging techniques. The design of a nondestructive imaging technique is necessary for the accurate quantification of contrast agent refresh. In this work, subharmonic imaging is investigated as a method for nondestructive imaging with the contrast agent microbubble MP1950 (Mallinckrodt, Inc., St. Louis, MO). Optical observation during insonation, in conjunction with a modified Rayleigh-Plesset (R-P) analysis, provides insight into the mechanisms of and parameters required for subharmonic frequency generation. Subharmonic imaging with a transmission frequency that is the same as the resonant frequency of the bubble is shown to require a minimum pressure of insonation that is greater than the experimentally-observed bubble destruction threshold. Subharmonic imaging with a transmission frequency that is twice the resonant frequency of the bubble produces a subharmonic frequency response while minimizing bubble instability. Optimization is performed using optical experimental analysis and R-P analysis.

Optical observation of contrast agent destruction

Fragmentation of an ultrasound contrast agent on the time scale of microseconds provides opportunities for the advancement of microvascular detection, blood flow velocity estimation, and targeted drug delivery. Images captured by high-speed imaging systems show destruction of a microbubble during compression. Peak wall velocity of −700 m/s and peak acceleration of 1.2×1012 m/s2 is observed for insonation with a peak pressure of −1.1 MPa and a center frequency of 2.4 MHz. Theoretical calculations of wall velocity and acceleration using a modified Rayleigh–Plesset model predict a peak negative wall velocity of −680 m/s and peak acceleration of 2×1012 m/s2.

Dynamics and fragmentation of thick-shelled microbubbles

Localized delivery could decrease the systemic side effects of toxic chemotherapy drugs. The unique delivery agents we examine consist of microbubbles with an outer lipid coating, an oil layer, and a perfluorobutane gas core. These structures are 0.5-12 /spl mu/m in radius at rest. Oil layers of these acoustically active lipospheres (AALs) range from 0.3-1.5 /spl mu/m in thickness and thus the agents can carry a large payload compared to nano-scale drug delivery systems. We show that triacetin-based drug-delivery vehicles can be fragmented using ultrasound. Compared with a lipid-shelled contrast agent, the expansion of the drug-delivery vehicle within the first cycle is similar, and a subharmonic component is demonstrated at an equivalent radius, frequency, and driving pressure. For the experimental conditions explored here, the pulse length required for destruction of the drug-delivery vehicle is significantly greater, with at least five cycles required, compared with one cycle for the contrast agent. For the drug-delivery vehicle, the observed destruction mechanism varies with the initial radius, with microbubbles smaller than resonance size undergoing a symmetric collapse and producing a set of small, equal-sized fragments. Between resonance size and twice resonance size, surface waves become visible, and the oscillations become asymmetrical. For agents larger than twice the resonance radius, the destruction mechanism changes to a pinch-off, with one fragment containing a large fraction of the original volume.

DYNAMICS OF THERAPEUTIC ULTRASOUND CONTRAST AGENTS

Novel therapeutic contrast agents offer great potential for localized drug delivery. Localized delivery should significantly improve the efficacy of drug delivery and reduce any toxic exposure to the healthy tissue. This work describes a preliminary theoretical description of agents, such as those developed by the ImaRx Corporation, enclosed by a relatively thick fluid shell. A theoretical extension is made to a generalized Rayleigh-Plesset formulation that allows it to be solved for an encapsulating liquid shell of arbitrary thickness and density. The equation is used to investigate the role of shell thickness, density and viscosity on the radial dynamics and velocity of the inner and outer radii. Comparisons are made with experimental measurements of the maximum radial expansions for agents with triacetin shells. For a seven-cycle driving acoustic pulse with a center frequency of 1.5 MHz and peak amplitude of 1.6 MPa, the equation predicts maximum expansions from 5.5 to 1.3 times the initial radius for agents 1 to 10 microm, respectively, in initial radius with a 500-nm (28.0 cP) encapsulating shell. These predictions have reasonable agreement with the maximum radial expansions obtained from optical experimental data of fragmenting and intact agents. Approximate agreement between theory and experiment for a similar range of agent sizes is also demonstrated for a pulse with the same pressure amplitude at 2.5 MHz. At 2.5 MHz, smaller radial expansion amplitudes from 1.1 to 4.1 times the initial radius were found for agents 1 to 10 microm in initial radius, respectively. Discrepancies are attributed to shape instabilities and their associated fragmentation effects not incorporated in the equation. A significant difference in the inner and outer wall velocities is predicted for agents with a 500-nm triacetin shell. A 2.5 microm initial radius agent driven with a seven-cycle pulse at 2.5 MHz and 1.6 MPa achieves a maximum negative inner wall velocity of 364 m/s and outer wall velocity of 63 m/s. For parameters that correspond to large differences between the inner and outer wall velocities, fragmentation is typically observed experimentally.

Ultrasound contrast agents phagocytosed by neutrophils demonstrate acoustic activity

Ultrasound contrast agents are microbubbles composed of a thin lipid or albumin shell filled with air or a high molecular weight gas. These microbubbles are used for contrast-enhanced ultrasound (CEU) assessment of organ perfusion. In regions of inflammation, microbubbles are phagocytosed intact by activated neutrophils adherent to the venular wall. The authors hypothesized that microbubbles remain acoustically active following phagocytosis. Accordingly, they assessed the physical responses of both phagocytosed and free microbubbles by direct microscopic observation during delivery of repetitive single pulses of ultrasound at various acoustic pressures. Insonation results in oscillation in the bubbles volume. Microbubbles were optically recorded during insonation with a high-speed imaging system and diameter-time curves were analyzed to determine the effect of phagocytosis. Phagocytosed microbubbles retained their acoustic activity, although the intracellular environment increased viscoelastic damping experienced by microbubbles. With a pulse of high acoustic intensity (>1 MPa), phagocytosed microbubbles expanded up to 500% of their initial radii, which occasionally resulted in neutrophil rupture. Primary radiation force displaced phagocytosed microbubbles a distance of 100 microns with an acoustic pressure of -240 kPa and a pulse repetition frequency of 10 kHz, thus providing further evidence of acoustic activity. The authors conclude that phagocytosed microbubbles exhibit viscoelastic damping and yet are susceptible to acoustic destruction. They can generate non-linear echoes on the same order of magnitude as free microbubbles. These results indicate that CEU may be used to identify and assess regions of inflammation by detecting acoustic signals from microbubbles that are phagocytosed by activated neutrophils. In addition, the rapid expansion of a microbubble at high acoustic pressure may present a means to rupture a neutrophil or drug capsule at a specific site, resulting in delivery of a drug.

Acoustic fragmentation of therapeutic contrast agents designed for localized drug delivery

The current methods for delivery of lipid-soluble chemotherapeutic agents may be improved using acoustically active lipospheres (AALs). These microbubbles are designed to release a drug payload when disrupted with acoustic energy at the desired site. Delivery at localized areas would decrease the systemic side effects and increase uptake at the desired site. The drug delivery agents we are examining are provided by ImaRx Pharmaceuticals, and consist of an outer lipid shell with an inner oil layer of triacetin and a gas core. These AALs are 0.5-8 /spl mu/m in diameter when activated, with an oil shell from 0.3-1.0 /spl mu/m in thickness. The large volume of the oil shell allows these drug delivery vehicles to carry a large payload of hydrophobic drugs compared to nano-scale drug delivery systems. For these AALs, the observed destruction mechanism varies with the resting radius, with agents smaller than resonance size undergoing a symmetric collapse and producing a set of small equal-sized fragments. Between resonance size and twice resonance size, surface waves become visible, and the oscillations become asymmetrical. For agents greater than twice the resonance radius, the destruction mechanism changes to a pinch-off, with one fragment containing a large fraction of the original volume.

Ultrasound contrast agents used for localized drug delivery

The delivery of chemotherapy can be improved and localized using engineered drug delivery vehicles. Localized drug delivery would decrease the systemic side effects of toxic chemotherapy drugs. The unique delivery agents we are examining are provided by ImaRx Pharmaceuticals, and consist of an outer lipid shell with an inner oil layer and a gas core. The three microsphere types investigated contained soybean oil, corn oil, and triacetin, within the outer layer. These structures are 0.5-8 /spl mu/m in diameter at rest and thus can carry a large payload compared to nanoscale drug delivery systems. The oil shells of these delivery agents can be loaded with paclitaxel or other hydrophobic drugs. Oil layers of these acoustically active lipospheres range from 0.3-1.5 /spl mu/m in thickness. We show these drug delivery vehicles can be fragmented using ultrasound thus releasing their payload in a localized area.

Current Topics in Ultrasound Contrast Agent Application and Design

Ultrasound contrast agents are bubbles, 1-10 microns in radius, encapsulated by a lipid, protein, polymer or fluid shell. The agents have been used to distinguish the acoustic scattering signatures of blood from those of the surrounding tissue. This is possible due to the nonlinear response of the agent, which is similar to that of a free gas bubble. Upon sufficient forcing the agents will oscillate nonlinearly about their equilibrium radius, and for specific conditions, produce nonlinear resonance responses which are integer multiples of the primary resonance. Ultrasound tissue perfusion studies have been developed which are based on the destruction of contract agents coupled to the measurement of blood flow. Nevertheless, many outstanding issues remain in contrast agent design especially with respect to emerging applications. Even with the use of higher order harmonics there is a lack of an acoustic signature or destruction mechanism at frequencies above approximately 5.0 MHz with conventional agents. The design and use of a high frequency contrast agent is addressed by exploiting the multiple scattering response of agents modled as spherical elastic shells. Also considered is the nonlinear response of elastic-shelled agents. The considerations of shells modeled as linear and nonlinear elastic materials are discussed. The use of contrast agents for targeted drug delivery has recently received much attention. More specifically, the ImaRx Corporation (Tucson, Arizona) has developed thick fluid shelled agents, which release suspended taxol-based drugs from their shells upon destruction. Shape instabilities and surface waves correspond with the fragmentation and destruction of the agents. Finally, the interaction of multiple contrast agents has received little attention with respect to these emerging applications.

Dynamics and fragmentation of encapsulated gas bubbles

The development of ultrasound contrast agents has fostered an interest in the theoretical dynamics of encapsulated gas bubbles. Originally, micron‐sized agents were constructed for diagnostic imaging purposes with a thin shell (<15 nm) of lipid, protein, or polymeric materials. New therapeutic applications for localized drug delivery have led to an interest and development of micron‐size bubbles with a thick fluid shell (∼500 nm). Previously some experimental acoustic levitation studies have investigated the capillary wave development on millimeter‐size liquid shells. The nonlinear oscillations and fragmentation have not been extensively investigated from the theoretical standpoints. A generalized Rayleigh–Plesset equation has been developed that includes the effects of a thick viscous fluid shell. The equation is compared with other theoretical encapsulated bubble formulations and some recent experimental observations. For a 4.0 μ bubble, with a 500 nm shell of viscosity of 28 cP, insonified over 5 cycle...