This review offers a critical analysis of the state of the art of medical microbubbles and their application in therapeutic delivery and monitoring. When driven by an ultrasonic pulse, these small gas bubbles oscillate with a wall velocity on the order of tens to hundreds of meters per second and can be deflected to a vessel wall or fragmented into particles on the order of nanometers. While single-session molecular imaging of multiple targets is difficult with affinity-based strategies employed in some other imaging modalities, microbubble fragmentation facilitates such studies. Similarly, a focused ultrasound beam can be used to disrupt delivery vehicles and blood vessel walls, offering the opportunity to locally deliver a drug or gene. Clinical translation of these vehicles will require that current challenges be overcome, where these challenges include rapid clearance and low payload. The technology, early successes with drug and gene delivery, and potential clinical applications are reviewed.

Ultrasound microbubble contrast agents: fundamentals and application to gene and drug delivery.

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

A Molecular Imaging Primer: Modalities, Imaging Agents, and Applications

Microbubbles in ultrasound-triggered drug and gene delivery

Single-bubble sonoluminescence occurs when an acoustically trapped and periodically driven gas bubble collapses so strongly that the energy focusing at collapse leads to light emission. Detailed experiments have demonstrated the unique properties of this system: the spectrum of the emitted light tends to peak in the ultraviolet and depends strongly on the type of gas dissolved in the liquid; small amounts of trace noble gases or other impurities can dramatically change the amount of light emission, which is also affected by small changes in other operating parameters (mainly forcing pressure, dissolved gas concentration, and liquid temperature). This article reviews experimental and theoretical efforts to understand this phenomenon. The currently available information favors a description of sonoluminescence caused by adiabatic heating of the bubble at collapse, leading to partial ionization of the gas inside the bubble and to thermal emission such as bremsstrahlung. After a brief historical review, the authors survey the major areas of research: Section II describes the classical theory of bubble dynamics, as developed by Rayleigh, Plesset, Prosperetti, and others, while Sec. III describes research on the gas dynamics inside the bubble. Shock waves inside the bubble do not seem to play a prominent role in the process. Section IV discusses the hydrodynamic and chemical stability of the bubble. Stable single-bubble sonoluminescence requires that the bubble be shape stable and diffusively stable, and, together with an energy focusing condition, this fixes the parameter space where light emission occurs. Section V describes experiments and models addressing the origin of the light emission. The final section presents an overview of what is known, and outlines some directions for future research.

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Single bubble sonoluminescence

Background—Preclinical atherosclerosis is associated with increased endothelial cell (EC) expression of leukocyte adhesion molecules (LAMs), which mediate monocyte adhesion during atherogenesis. Identification of cell-surface LAMs may uniquely allow assessment of endothelial function, but there are no in vivo methods for detecting LAMs. We tested a new microbubble designed to bind to and allow specific ultrasound detection of intercellular adhesion molecule-1 (ICAM-1). Methods and Results—A perfluorobutane gas–filled lipid-derived microsphere with monoclonal antibody to ICAM-1 covalently bound to the bubble shell was synthesized. Bubbles with either nonspecific IgG or no protein on the shell were synthesized as controls. Coverslips of cultured human coronary artery ECs were placed in a parallel-plate perfusion chamber and exposed to 1 of the 3 microbubble species, followed by perfusion with culture medium. Experiments were performed with either normal or interleukin-1β–activated ECs overexpressing ICAM-1,...

Microbubbles Targeted to Intercellular Adhesion Molecule-1 Bind to Activated Coronary Artery Endothelial Cells

The goal of targeted imaging is to produce an enhanced view of physiological processes or pathological tissue components. Contrast agents may improve the specificity of imaging modalities through selective targeting, and this may be particularly significant when using ultrasound (US) to image inflammatory processes or thrombi. One means of selective targeting involves the attachment of contrast agents to the desired site with the use of a specific binding mechanism. Because molecular binding mechanisms are effective over distances on the order of nanometers, targeting effectiveness would be greatly increased if the agent is initially concentrated in a particular region, and if the velocity of the agent is decreased as it passes the potential binding site. Ultrasonic transmission produces a primary radiation force that can manipulate microbubbles with each acoustic pulse. Observations demonstrate that primary radiation force can displace US contrast agents from the center of the streamline to the wall of a 200-μm cellulose vessel in vitro. Here, the effects of radiation force on contrast agents in vivo are presented for the first time. Experimental results demonstrate that radiation force can displace a contrast agent to the wall of a 50-μm blood vessel in the mouse cremaster muscle, can significantly reduce the velocity of flowing contrast agents, and can produce a reversible aggregation. Acoustic radiation force presents a means to localize and concentrate contrast agents near a vessel wall, which may assist the delivery of targeted agents.

Acoustic radiation force in vivo: A mechanism to assist targeting of microbubbles

Optimal use of encapsulated microbubbles for ultrasound contrast agents and drug delivery requires an understanding of the complex set of phenomena that affect the contrast agent echo and persistence. With the use of a video microscopy system coupled to either an ultrasound flow phantom or a chamber for insonifying stationary bubbles, we show that ultrasound has significant effects on encapsulated microbubbles. In vitro studies show that a train of ultrasound pulses can alter the structure of an albumin-shelled bubble, initiate various mechanisms of bubble destruction or produce aggregation that changes the echo spectrum. In this analysis, changes observed optically are compared with those observed acoustically for both albumin and lipid-shelled agents. We show that, when insonified with a narrowband pulse at an acoustic pressure of several hundred kPa, a phospholipid-shelled bubble can undergo net radius fluctuations of at least 15%; and an albumin-shelled bubble initially demonstrates constrained expansion and contraction. If the albumin shell contains air, the shell may not initially experience surface tension; therefore, the echo changes more significantly with repeated pulsing. A set of observations of contrast agent destruction is presented, which includes the slow diffusion of gas through the shell and formation of a shell defect followed by rapid diffusion of gas into the surrounding liquid. These observations demonstrate that the low-solubility gas used in these agents can persist for several hundred milliseconds in solution. With the transmission of a high-pulse repetition rate and a low pressure, the echoes from, contrast agents can be affected by secondary radiation force. Secondary radiation force is an attractive force for these experimental conditions, creating aggregates with distinct echo characteristics and extended persistence. The scattered echo from an aggregate is several times stronger and more narrowband than echoes from individual bubbles.

Optical and acoustical observations of the effects of ultrasound on contrast agents

Background—Albumin microbubbles that are used for contrast echocardiography persist within the myocardial microcirculation after ischemia/reperfusion (I-R). The mechanism responsible for this pheno...

Microbubble Persistence in the Microcirculation During Ischemia/Reperfusion and Inflammation Is Caused by Integrin- and Complement-Mediated Adherence to Activated Leukocytes

Rationale and Objectives:During echo examinations with microbubble contrast, individual “dots” of ultrasound reflection can be visualized. To address the question whether these signals represent individual microbubbles, very dilute suspensions of ultrasound contrast agents or individual microbubbles

Gary H. Brandenburger

Papers

Microbubbles Targeted to Intercellular Adhesion Molecule-1 Bind to Activated Coronary Artery Endothelial Cells

Acoustic radiation force in vivo: A mechanism to assist targeting of microbubbles

Optical and acoustical observations of the effects of ultrasound on contrast agents

Microbubble Persistence in the Microcirculation During Ischemia/Reperfusion and Inflammation Is Caused by Integrin- and Complement-Mediated Adherence to Activated Leukocytes

Detection of individual microbubbles of ultrasound contrast agents: imaging of free-floating and targeted bubbles.