Assessment of Endogenous and Therapeutic Arteriogenesis by Contrast Ultrasound Molecular Imaging of Integrin Expression

Multi-modality imaging for assessment of the microcirculation in peripheral artery disease: Bench to clinical practice

Peripheral artery disease (PAD) is a highly prevalent disorder with a high risk of mortality and amputation despite the introduction of novel medical and procedural treatments. Microvascular disease (MVD) is common among patients with PAD, and despite the established role as a predictor of amputations and mortality, MVD is not routinely assessed as part of current standard practice. Recent pre-clinical and clinical perfusion and molecular imaging studies have confirmed the important role of MVD in the pathogenesis and outcomes of PAD. The recent advancements in the imaging of the peripheral microcirculation could lead to a better understanding of the pathophysiology of PAD, and result in improved risk stratification, and our evaluation of response to therapies. In this review, we will discuss the current understanding of the anatomy and physiology of peripheral microcirculation, and the role of imaging for assessment of perfusion in PAD, and the latest advancements in molecular imaging. By highlighting the latest advancements in multi-modality imaging of the peripheral microcirculation, we aim to underscore the most promising imaging approaches and highlight potential research opportunities, with the goal of translating these approaches for improved and personalized management of PAD in the future.

Perfusion imaging techniques in lower extremity peripheral arterial disease

Lower limb peripheral arterial disease (PAD) characterizes the impairment of blood flow to extremities caused by arterial stenoses or occlusions. Evaluation of PAD is based on clinical examination, calculation of ankle-brachial index and imaging studies such as ultrasound, CT, MRI and digital subtraction angiography. These modalities provide significant information about location, extension and severity of macrovasular lesions in lower extremity arterial system. However, they can be also used to evaluate limb perfusion, using appropriate techniques and protocols. This information may be valuable for assessment of the severity of ischemia and detection of hypoperfused areas. Moreover, they can be used for planning of revascularization strategy in patients with severe PAD and evaluation of therapeutic outcome. These techniques may also determine prognosis and amputation risk in patients with PAD. This review gives a basic overview of the perfusion techniques for lower limbs provided by imaging modalities such as ultrasound, CT, MRI, digital subtraction angiography and scintigraphy and their clinical applications for evaluation of PAD and revascularization outcome.

Contrast-Enhanced Ultrasound Reveals Partial Perfusion Recovery After Hindlimb Ischemia as Opposed to Full Recovery by Laser Doppler Perfusion Imaging

Mouse models are critical in developing new therapeutic approaches to treat peripheral arterial disease (PAD). Despite decades of research and numerous clinical trials, the efficacy of available therapies is limited. This may suggest shortcomings in our current animal models and/or methods of assessment. We evaluated perfusion measurement methods in a mouse model of PAD by comparing laser Doppler perfusion imaging (LDPI, the most common technique), contrast-enhanced ultrasound (CEUS, an emerging technique) and fluorescent microspheres (conventional standard). Mice undergoing a femoral artery ligation were assessed by LDPI and CEUS at baseline and 1, 4, 7, 14, 28, 60, 90 and 150 d post-surgery to evaluate perfusion recovery in the ischemic hindlimb. Fourteen days after surgery, additional mice were measured with fluorescent microspheres, LDPI, and CEUS. LDPI and CEUS resulted in broadly similar trends of perfusion recovery until 7 d post-surgery. However, by day 14, LDPI indicated full recovery of perfusion, whereas CEUS indicated ∼50% recovery, which failed to improve even after 5 mo. In agreement with the CEUS results, fluorescent microspheres at day 14 post-surgery confirmed that perfusion recovery was incomplete. Histopathology and photoacoustic microscopy provided further evidence of sustained vascular abnormalities.

Preparation, characterization, and antibacterial activity of tigecycline-loaded, ultrasound-activated microbubbles

Central nervous system infectious disease caused by the multidrug-resistant Acinetobacter baumannii (AB) seriously threatens human life in clinic. Tigecycline has good sensitivity in killing AB, but due to its wide tissue distribution and blood-brain barrier, concentration in cerebrospinal fluid is low, therefore, the clinical effect is limited. Herein, we designed micro-bubbled tigecycline, aimed to enhance its anti-MDRAB effects under ultrasound. The lipid microbubbles with different ratios of lipids to drugs (a ratio of 10:1, 20:1, and 40:1) were prepared by the mechanical shaking method. The morphology, zeta potential and particle size of microbubbles were tested to screen out the much better formulation. Encapsulation efficiency and drug loading amount were determined by ultracentrifugation combined with high-performance liquid chromatography. Then the in vitro antibacterial activity against AB was conducted using the selected ultrasound-activated microbubble. Results showed the selected microbubbles with high encapsulation efficiency and good stability. The mechanical shaking method is feasible for preparation of drug-loaded and ultrasound-activated lipid microbubbles. Using 0.2 mg/mL microbubbles, combined with 1 MHz, 2.5 W/cm² and 1 min of ultrasound exhibited a potent anit-AB in vitro. This study indicates that tigecycline treatment in form of ultrasound-activated microbubble is a promising strategy against AB infections.

Modeling acute and chronic vascular responses to a major arterial occlusion

OBJECTIVE

To incorporate chronic vascular adaptations into a mathematical model of the rat hindlimb to simulate flow restoration following total occlusion of the femoral artery.

METHODS

A vascular wall mechanics model is used to simulate acute and chronic vascular adaptations in the collateral arteries and collateral-dependent arterioles of the rat hindlimb. On an acute timeframe, the vascular tone of collateral arteries and distal arterioles is determined by responses to pressure, shear stress, and metabolic demand. On a chronic timeframe, sustained dilation of arteries and arterioles induces outward vessel remodeling represented by increased passive vessel diameter (arteriogenesis), and low venous oxygen saturation levels induce the growth of new capillaries represented by increased capillary number (angiogenesis).

RESULTS

The model predicts that flow compensation to an occlusion is enhanced primarily by arteriogenesis of the collateral arteries on a chronic time frame. Blood flow autoregulation is predicted to be disrupted and to occur for higher pressure values following femoral arterial occlusion.

CONCLUSIONS

Structural adaptation of the vasculature allows for increased blood flow to the collateral-dependent region after occlusion. Although flow is still below pre-occlusion levels, model predictions indicate that interventions which enhance collateral arteriogenesis would have the greatest potential for restoring flow.

Recent Advances in Metal-Based Magnetic Composites as High-Efficiency Candidates for Ultrasound-Assisted Effects in Cancer Therapy

Metal-based magnetic materials have been used in different fields due to their particular physical or chemical properties. The original magnetic properties can be influenced by the composition of constituent metals. As utilized in different application fields, such as imaging monitoring, thermal treatment, and combined integration in cancer therapies, fabricated metal-based magnetic materials can be doped with target metal elements in research. Furthermore, there is one possible new trend in human activities and basic cancer treatment. As has appeared in characterizations such as magnetic resonance, catalytic performance, thermal efficiency, etc., structural information about the real morphology, size distribution, and composition play important roles in its further applications. In cancer studies, metal-based magnetic materials are considered one appropriate material because of their ability to penetrate biological tissues, interact with cellular components, and induce noxious effects. The disruptions of cytoskeletons, membranes, and the generation of reactive oxygen species (ROS) further influence the efficiency of metal-based magnetic materials in related applications. While combining with cancer cells, these magnetic materials are not only applied in imaging monitoring focus areas but also could give the exact area information in the cure process while integrating ultrasound treatment. Here, we provide an overview of metal-based magnetic materials of various types and then their real applications in the magnetic resonance imaging (MRI) field and cancer cell treatments. We will demonstrate advancements in using ultrasound fields co-worked with MRI or ROS approaches. Besides iron oxides, there is a super-family of heterogeneous magnetic materials used as magnetic agents, imaging materials, catalytic candidates in cell signaling and tissue imaging, and the expression of cancer cells and their high sensitivity to chemical, thermal, and mechanical stimuli. On the other hand, the interactions between magnetic candidates and cancer tissues may be used in drug delivery systems. The materials’ surface structure characteristics are introduced as drug loading substrates as much as possible. We emphasize that further research is required to fully characterize the mechanisms of underlying ultrasounds induced together, and their appropriate relevance for materials toxicology and biomedical applications.

Therapeutic Angiogenesis for Ovarian Transplantation through Ultrasound-Targeted Microbubble Destruction

Timely angiogenesis and effective microcirculation perfusion are essential for the survival and functional recovery of transplanted ovaries. Ultrasound-targeted microbubble destruction (UTMD) can lead to angiogenesis and increase flow perfusion by causing transient inflammation. The purpose of this study was to evaluate the effects of UTMD on transplanted ovarian revascularization and survival. In vitro, for the criteria of cell viability and tube formation capability, the optimal exposure parameters were determined to be a microbubble concentration of 1 × 10⁸/mL, mechanical index of 1 and exposure time of 30 s. After ovarian transplantation, 40 female Sprague Dawley rats were divided into four groups: transplantation alone, ultrasound alone, microbubbles alone and ultrasound and microbubbles (UTMD). At 7 d after transplantation, ovarian perfusion was assessed using qualitative and quantitative methods. The effect of angiogenesis was assessed by contrast-enhanced ultrasound, laser Doppler perfusion imaging and histologic analysis. The results, in which ovarian perfusion was highest in the UTMD group, suggest that UTMD can effectively improve ovarian perfusion. Compared with the other three groups, the number of follicles, microvascular density and rate of Ki-67-positive cells increased significantly in the UTMD group, while apoptosis decreased significantly (p < 0.05). The study indicates that UTMD promoted ovarian re-vascularization after ovarian transplantation and maintained follicular reserve.

Enhanced Ultrasound Contrast of Renal-Clearable Luminescent Gold Nanoparticles

Renal-clearable nanoparticles are typically fast eliminated through the free glomerular filtration, which show weak interaction with the renal compartments and negligible ultrasound signals, raising challenges in direct imaging of kidney diseases. Here, we report the ultrasmall renal-clearable luminescent gold nanoparticles (AuNPs) with both pH-induced charge reversal and aggregation properties, and discover that enhanced ultrasound contrast could be facilely acquired through the increased tubular reabsorption and in situ aggregation of AuNPs in renal tubule cells in injured kidneys. The tuning elimination pathway of the renal-clearable luminescent AuNPs is further demonstrated to provide a synergistical fluorescence and ultrasound imaging strategy for diagnosing early kidney injury with precise anatomical information.

Molecular Ultrasound Imaging

In the last decade, molecular ultrasound imaging has been rapidly progressing. It has proven promising to diagnose angiogenesis, inflammation, and thrombosis, and many intravascular targets, such as VEGFR2, integrins, and selectins, have been successfully visualized in vivo. Furthermore, pre-clinical studies demonstrated that molecular ultrasound increased sensitivity and specificity in disease detection, classification, and therapy response monitoring compared to current clinically applied ultrasound technologies. Several techniques were developed to detect target-bound microbubbles comprising sensitive particle acoustic quantification (SPAQ), destruction-replenishment analysis, and dwelling time assessment. Moreover, some groups tried to assess microbubble binding by a change in their echogenicity after target binding. These techniques can be complemented by radiation force ultrasound improving target binding by pushing microbubbles to vessel walls. Two targeted microbubble formulations are already in clinical trials for tumor detection and liver lesion characterization, and further clinical scale targeted microbubbles are prepared for clinical translation. The recent enormous progress in the field of molecular ultrasound imaging is summarized in this review article by introducing the most relevant detection technologies, concepts for targeted nano- and micro-bubbles, as well as their applications to characterize various diseases. Finally, progress in clinical translation is highlighted, and roadblocks are discussed that currently slow the clinical translation.

Ultrasound Imaging Based on Molecular Targeting for Quantitative Evaluation of Hepatic Ischemia-Reperfusion Injury

The aim of the present study was to quantitatively diagnose and monitor the therapy response of hepatic ischemia-reperfusion injury (IRI) with the use of targeted ultrasound (US) imaging. Targeted microbubbles (MBs) were fabricated, and the binding of intracellular adhesion molecule 1 (ICAM-1) antibodies to MBs was observed. To establish a quantitative method based on targeted US imaging, contrast-enhanced US was applied for IRI rats. After andrographolide treatment, the IRI rats were subjected to the quantitative targeted US imaging for a therapeutic effect. Effective binding of ICAM-1 antibodies to MBs was observed. According to the quantitative targeted US imaging, the ICAM-1 normalized intensity difference (NID) in the IRI rats (38.74 ± 15.08%) was significantly higher than that in the control rats (10.08 ± 2.52%, p = 0.048). Further, different degrees of IRI (mild IRI, moderate to severe IRI) were distinguished by the use of the NID (37.14 ± 2.14%, 22.34 ± 1.08%, p = 0.002). Analysis of mRNA expression demonstrated the accuracy of analyzing the NID by using quantitative targeted US imaging (R²  = 0.7434, p < 0.001). Andrographolide treatment resulted in an obviously weakened NID of ICAM-1 (17.7 ± 4.8% vs 34.2 ± 6.6%, p < 0.001). The study showed the potential of the quantitative targeted US imaging method for the diagnosis and therapeutic monitoring of IRI.

Identification of neovascularization by contrast-enhanced ultrasound to detect unstable carotid stenosis

Background

Plaque neovascularization accompanies local inflammation and critically contributes to plaque instability. Correct identification of intraplaque neovascularization by contrast–enhanced ultrasound (CEUS) may provide an additional risk marker in carotid stenosis. This pilot study investigates the correlation between histological evaluation of carotid plaque specimens and pre-surgery CEUS to identify neovascularization.

Methods

17 patients with high-grade internal carotid artery (ICA) stenosis were studied. CEUS was performed in all patients shortly before carotid endarterectomy. Neovascularization, infiltration of T cells and macrophages along with intraplaque hemorrhage were studied in excised plaques by immunohistochemistry. Ultrasound-based four-level and two-level classification systems for neovascularization were used. CEUS findings were compared with histological findings.

Results

Scores on the CEUS-based four-level and two-level classifications were robustly correlated with the density of intraplaque vessels (r = 0.635, p = 0.006 and r = 0.578, p = 0.015, respectively). Histological evaluation of regions with strong and prolonged intraplaque enhancement typically showed strong intraplaque neovascularization in conjunction with acute intraplaque hemorrhage. Moreover, higher grades of intraplaque neovascularization as determined by ultrasound were associated with a higher percentage of macrophage-rich areas.

Conclusion

CEUS is a technique well suited to gauge the degree of neovascularization of carotid plaques. Future research will have to define the reliability and validity of CEUS in everyday clinical practice. Further, our study suggests that CEUS may also be useful to pick up features of vulnerable plaques such as acute intraplaque hemorrhages.

Ultrasound Molecular Imaging of Angiogenesis Using Vascular Endothelial Growth Factor-Conjugated Microbubbles

Imaging of angiogenesis receptors could provide a sensitive and clinically useful method for detecting neovascularization such as occurs in malignant tumors, and responses to antiangiogenic therapies for such tumors. We tested the hypothesis that microbubbles (MB) tagged with human VEGF₁₂₁ (MB_VEGF) bind to the kinase insert domain receptor (KDR) in vitro and angiogenic endothelium in vivo, and that this specific binding can be imaged on a clinical ultrasound system. In this work, targeted adhesion of MB_VEGF was evaluated in vitro using a parallel plate flow system containing adsorbed recombinant human KDR. There was more adhesion of MB_VEGF to KDR-coated plates when the amount of VEGF₁₂₁ on each MB or KDR density on the plate was increased. MB_VEGF adhesion to KDR-coated plates decreased with increasing wall shear rate. On intravital microscopic imaging of bFGF-stimulated rat cremaster muscle, there was greater microvascular adhesion of MB_VEGF compared to that of isotype IgG-conjugated control MB (MB_CTL). To determine if MB_VEGF could be used to ultrasonically image angiogenesis, ultrasound imaging was performed in mice bearing squamous cell carcinoma after intravenous injection of MB_VEGF. Ultrasound videointensity enhancement in tumor was significantly higher for MB_VEGF (17.3 ± 9.7 dB) compared to MB_CTL (3.8 ± 4.4 dB, n = 6, p < 0.05). This work demonstrates the feasibility of targeted ultrasound imaging of an angiogenic marker using MB_VEGF. This approach offers a noninvasive bedside method for detecting tumor angiogenesis and could be extended to other applications such as molecular monitoring of therapeutic angiogenesis or antiangiogenic therapies in cardiovascular disease or cancer.

Molecular Acoustic Angiography: A New Technique for High-resolution Superharmonic Ultrasound Molecular Imaging

Ultrasound molecular imaging utilizes targeted microbubbles to bind to vascular targets such as integrins, selectins and other extracellular binding domains. After binding, these microbubbles are typically imaged using low pressures and multi-pulse imaging sequences. In this article, we present an alternative approach for molecular imaging using ultrasound that relies on superharmonic signals produced by microbubble contrast agents. Bound bubbles were insonified near resonance using a low frequency (4 MHz) element and superharmonic echoes were received at high frequencies (25-30 MHz). Although this approach was observed to produce declining image intensity during repeated imaging in both in vitro and in vivo experiments because of bubble destruction, the feasibility of superharmonic molecular imaging was demonstrated for transmit pressures, which are sufficiently high to induce shell disruption in bound microbubbles. This approach was validated using microbubbles targeted to the αvβ3 integrin in a rat fibrosarcoma model (n = 5) and combined with superharmonic images of free microbubbles to produce high-contrast, high-resolution 3-D volumes of both microvascular anatomy and molecular targeting. Image intensity over repeated scans and the effect of microbubble diameter were also assessed in vivo, indicating that larger microbubbles yield increased persistence in image intensity. Using ultrasound-based acoustic angiography images rather than conventional B-mode ultrasound to provide the underlying anatomic information facilitates anatomic localization of molecular markers. Quantitative analysis of relationships between microvasculature and targeting information indicated that most targeting occurred within 50 μm of a resolvable vessel (>100 μm diameter). The combined information provided by these scans may present new opportunities for analyzing relationships between microvascular anatomy and vascular targets, subject only to limitations of the current mechanically scanned system and microbubble persistence to repeated imaging at moderate mechanical indices.

Nanoscale theranostics for physical stimulus-responsive cancer therapies

Physical stimulus-responsive therapies often employing multifunctional theranostic agents responsive to external physical stimuli such as light, magnetic field, ultra-sound, radiofrequency, X-ray, etc., have been widely explored as novel cancer therapy strategies, showing encouraging results in many pre-clinical animal experiments. Unlike conventional cancer chemotherapy which often accompanies with severe toxic side effects, physical stimulus-responsive agents usually are non-toxic by themselves and would destruct cancer cells only under specific external stimuli, and thus could offer greatly reduced toxicity and enhanced treatment specificity. In addition, physical stimulus-responsive therapies can also be combined with other traditional therapeutics to achieve synergistic anti-tumor effects via a variety of mechanisms. In this review, we will summarize the latest progress in the development of physical stimulus-responsive therapies, and discuss the important roles of nanoscale theranostic agents involved in those non-conventional therapeutic strategies.

The Optimized Fabrication of Nanobubbles as Ultrasound Contrast Agents for Tumor Imaging

Nanobubbles, which have the potential for ultrasonic targeted imaging and treatment in tumors, have been a research focus in recent years. With the current methods, however, the prepared uniformly sized nanobubbles either undergo post-formulation manipulation, such as centrifugation, after the mixture of microbubbles and nanobubbles, or require the addition of amphiphilic surfactants. These processes influence the nanobubble stability, possibly create material waste, and complicate the preparation process. In the present work, we directly prepared uniformly sized nanobubbles by modulating the thickness of a phospholipid film without the purification processes or the addition of amphiphilic surfactants. The fabricated nanobubbles from the optimal phospholipid film thickness exhibited optimal physical characteristics, such as uniform bubble size, good stability, and low toxicity. We also evaluated the enhanced imaging ability of the nanobubbles both in vitro and in vivo. The in vivo enhancement intensity in the tumor was stronger than that of SonoVue after injection (UCA; 2 min: 162.47 ± 8.94 dB vs. 132.11 ± 5.16 dB, P < 0.01; 5 min: 128.38.47 ± 5.06 dB vs. 68.24 ± 2.07 dB, P < 0.01). Thus, the optimal phospholipid film thickness can lead to nanobubbles that are effective for tumor imaging.

Quantitative ultrasound molecular imaging

Ultrasound molecular imaging using targeting microbubbles is predominantly a semi-quantitative tool, thus limiting its potential diagnostic power and clinical applications. In the work described here, we developed a novel method for acoustic quantification of molecular expression. E-Selectin expression in the mouse heart was induced by lipopolysaccharide. Real-time ultrasound imaging of E-selectin expression in the heart was performed using E-selectin-targeting microbubbles and a clinical ultrasound scanner in contrast pulse sequencing mode at 14 MHz, with a mechanical index of 0.22-0.26. The level of E-selectin expression was quantified using a novel time-signal intensity curve analytical method based on bubble elimination, which consisted of curve-fitting the bi-exponential equation [Formula: see text] to the elimination phase of the myocardial time-signal intensity curve. Ar and Af represent the maximum signal intensities of the retained and freely circulating bubbles in the myocardium, respectively; λr and λf represent the elimination rate constants of the retained and freely circulating bubbles in the myocardium, respectively. Ar correlated strongly with the level of E-selectin expression (|r|>0.8), determined using reverse transcriptase real-time quantitative polymerase chain reaction, and the duration of post-lipopolysaccharide treatment-both linearly related to cell surface E-selectin protein (actual bubble target) concentration in the expression range imaged. Compared with a conventional acoustic quantification method (which used retained bubble signal intensity at 20 min post-bubble injection), this new approach exhibited greater dynamic range and sensitivity and was able to simultaneously quantify other useful characteristics (e.g., the microbubble half-life). In conclusion, quantitative determination of the level of molecular expression is feasible acoustically using a time-signal intensity curve analytical method based on bubble elimination.

Imaging of small animal peripheral artery disease models: recent advancements and translational potential

Peripheral artery disease (PAD) is a broad disorder encompassing multiple forms of arterial disease outside of the heart. As such, PAD development is a multifactorial process with a variety of manifestations. For example, aneurysms are pathological expansions of an artery that can lead to rupture, while ischemic atherosclerosis reduces blood flow, increasing the risk of claudication, poor wound healing, limb amputation, and stroke. Current PAD treatment is often ineffective or associated with serious risks, largely because these disorders are commonly undiagnosed or misdiagnosed. Active areas of research are focused on detecting and characterizing deleterious arterial changes at early stages using non-invasive imaging strategies, such as ultrasound, as well as emerging technologies like photoacoustic imaging. Earlier disease detection and characterization could improve interventional strategies, leading to better prognosis in PAD patients. While rodents are being used to investigate PAD pathophysiology, imaging of these animal models has been underutilized. This review focuses on structural and molecular information and disease progression revealed by recent imaging efforts of aortic, cerebral, and peripheral vascular disease models in mice, rats, and rabbits. Effective translation to humans involves better understanding of underlying PAD pathophysiology to develop novel therapeutics and apply non-invasive imaging techniques in the clinic.

The future of collateral artery research

In the event of obstructive coronary artery disease, collateral arteries have been deemed an alternative blood source to preserve myocardial tissue perfusion and function. Monocytes play an important role in modulating this process, by local secretion of growth factors and extracellular matrix degrading enzymes. Extensive efforts have focused on developing compounds for augmenting the growth of collateral vessels (arteriogenesis). Nonetheless, clinical trials investigating the therapeutic potential of these compounds resulted in disappointing outcomes. Previous studies focused on developing compounds that stimulated collateral vessel growth by enhancing monocyte survival and activity. The limited success of these compounds in clinical studies, led to a paradigm shift in arteriogenesis research. Recent studies have shown genetic heterogeneity between CAD patients with sufficient and insufficient collateral vessels. The genetic predispositions in patients with poorly developed collateral vessels include overexpression of arteriogenesis inhibiting signaling pathways. New directions of arteriogenesis research focus on attempting to block such inhibitory pathways to ultimately promote arteriogenesis. Methods to detect collateral vessel growth are also critical in realizing the therapeutic potential of newly developed compounds. Traditional invasive measurements of intracoronary derived collateral flow index remain the gold standard in quantifying functional capacity of collateral vessels. However, advancements made in hybrid diagnostic imaging modalities will also prove to be advantageous in detecting the effects of pro-arteriogenic compounds.

Real-time contrast ultrasound muscle perfusion imaging with intermediate-power imaging coupled with acoustically durable microbubbles

BACKGROUND

There is growing interest in limb contrast-enhanced ultrasound (CEU) perfusion imaging for the evaluation of peripheral artery disease. Because of low resting microvascular blood flow in skeletal muscle, signal enhancement during limb CEU is prohibitively low for real-time imaging. The aim of this study was to test the hypothesis that this obstacle can be overcome by intermediate- rather than low-power CEU when performed with an acoustically resilient microbubble agent.

METHODS

Viscoelastic properties of Definity and Sonazoid were assessed by measuring bulk modulus during incremental increases in ambient pressure to 200 mm Hg. Comparison of in vivo microbubble destruction and signal enhancement at a mechanical index (MI) of 0.1 to 0.4 was performed by sequential reduction in pulsing interval from 10 to 0.05 sec during limb CEU at 7 MHz in mice and 1.8 MHz in dogs. Destruction was also assessed by broadband signal generation during passive cavitation detection. Real-time CEU perfusion imaging with destruction-replenishment was then performed at 1.8 MHz in dogs using an MI of 0.1, 0.2, or 0.3.

RESULTS

Sonazoid had a higher bulk modulus than Definity (66 ± 12 vs 29 ± 2 kPa, P = .02) and exhibited less inertial cavitation (destruction) at MIs ≥ 0.2. On in vivo CEU, maximal signal intensity increased incrementally with MI for both agents and was equivalent between agents except at an MI of 0.1 (60% and 85% lower for Sonazoid at 7 and 1.8 MHz, respectively, P < .05). However, on progressive shortening of the pulsing interval, Definity was nearly completely destroyed at MIs ≥ 0.2 at 1.8 and 7 MHz, whereas Sonazoid was destroyed only at 1.8 MHz at MIs ≥ 0.3. As a result, real-time CEU perfusion imaging demonstrated approximately fourfold greater enhancement for Sonazoid at an MI of 0.3 to 0.4.

CONCLUSIONS

Robust signal enhancement during real-time CEU perfusion imaging of the limb is possible when using intermediate-power imaging coupled with a durable microbubble contrast agent.

Ultrasound molecular imaging of transient acute myocardial ischemia with a clinically translatable P- and E-selectin targeted contrast agent: correlation with the expression of selectins

OBJECTIVE

The diagnosis of acute coronary syndrome remains challenging especially in patients without clear symptoms or electrocardiographic and/or biomarker features. A hallmark of ischemia/reperfusion is activation of endothelial cells leading to altered expression of molecular markers, including selectins. In this context, we aimed to validate the value of ultrasound molecular imaging for detecting transient myocardial ischemia by using a clinically translatable dual P- and E-selectin-targeted ultrasound contrast agent (UCA) and microbubble (MB(selectin)).

MATERIAL AND METHODS

Transient (20 minutes) myocardial ischemia of rat heart was produced by ligation of the left anterior descending coronary artery ligation followed by 2-, 5-, or 24-hour reperfusion. Imaging of the transient ischemic event was achieved by the use of MB(selectin). Performance of this clinically translatable targeted UCA was compared with that of antibody-targeted streptavidin MBs. Finally, immunohistochemistry staining of rat myocardial ischemic tissue was performed to assess expression of selectins accessible to targeted UCA.

RESULTS

In rats subjected to myocardial ischemia (20 minutes) followed by reperfusion (2 hours), injection of MB(selectin) produced high late phase (ie, 10-minute postinjection) ultrasound molecular imaging enhancement in the myocardium, which colocalized with the ischemic area. Late phase enhancement persisted 5 and 24 hours after reperfusion. Similarly, the use of MBP and MBE, comprising antibodies specific for P- and E-selectin, respectively, showed high late-phase enhancement within the ischemic area compared with remote myocardial tissue. Two and 5 hours after ischemia has resolved, a persistent expression of these 2 selectins was detected. After 24 hours of reperfusion, only MBE produced late phase enhancement within the ischemic myocardium. Immunohistochemical findings revealed that both P- and E-selectin were expressed and accessible on the surface of the activated endothelium 2 and 5 hours after the acute ischemic event, whereas only E-selectin remained accessible after 24 hours.

CONCLUSIONS

Ultrasound molecular imaging of transient myocardial ischemia using dual selectin-targeted UCA is able to monitor the time course of expression of selectins after resolution of the ischemic event, paving the way for a large clinical diagnostic window.

Ischemic memory imaging in nonhuman primates with echocardiographic molecular imaging of selectin expression

BACKGROUND

Selectins are adhesion molecules that are expressed by the vascular endothelium upon activation and may be an imaging target for detecting myocardial ischemia long after resolution. The aim of this study was to test the hypothesis that molecular imaging of selectins with myocardial contrast echocardiographic (MCE) molecular imaging could be used to detect recent brief ischemia in closed-chest nonhuman primates.

METHODS

Myocardial ischemia was produced in anesthetized adult rhesus macaques (n = 6) by percutaneous balloon catheter occlusion of the left anterior descending or circumflex coronary artery for 5 to 10 min. Three separate macaques served as nonischemic controls. MCE perfusion imaging was performed during coronary occlusion to measure risk area and at 100 to 110 min to exclude infarction. MCE molecular imaging was performed at 30 and 90 min after reperfusion using a lipid microbubble bearing dimeric recombinant human P-selectin glycoprotein ligand-1 (MB-YSPSL). Collection of blood for safety data, electrocardiography, and echocardiography were performed at baseline and before and 10 min after each MB-YSPSL injection.

RESULTS

Vital signs, oxygen saturation, electrocardiographic results, ventricular systolic function, pulmonary vascular resistance, and serum safety markers were unchanged by intravenous injection of MB-YSPSL. On echocardiography, left ventricular dysfunction in the risk area had resolved by 30 min, and there was no evidence of infarction on MCE perfusion imaging. On selectin-targeted MCE molecular imaging, signal enhancement was greater (P < .05) in the risk area than remote territory at 30 min (25 ± 11 vs 11 ± 4 IU) and 90 min (13 ± 3 vs 3 ± 2 IU) after ischemia. There was no enhancement (<1 IU) in control nonischemic subjects.

CONCLUSIONS

In primates, MCE molecular imaging of selectins using MB-YSPSL, a recombinant ligand appropriate for humans, is both safe and effective for imaging recent myocardial ischemia. This technique may be useful for detecting recent ischemia in patients with chest pain even in the absence of necrosis.

Cardiovascular molecular imaging with contrast ultrasound: principles and applications

Methods for imaging the molecular or cellular profile of tissue are being developed for all forms of non-invasive cardiovascular imaging. It is thought that these technologies will potentially improve patient outcomes by allowing diagnosis of disease at an early-stage, monitoring disease progression, providing important information on patient risk, and for tailoring therapy to the molecular basis of disease. Molecular imaging is also already assuming an important role in science by providing a better understanding of the molecular basis of cardiovascular pathology, for assessing response to new therapies, and for rapidly optimizing new or established therapies. Ultrasound-based molecular imaging is one of these new approaches. Contrast-enhanced ultrasound molecular imaging relies on the detection of novel site-targeted microbubbles (MB) or other acoustically active particles which are administered by intravenous injection, circulate throughout the vascular compartment, and are then retained and imaged within regions of disease by ligand-directed binding. The technique is thought to be advantageous in practical terms of cost, time, and ease of use. The aim of this review is to discuss the molecular participants of cardiovascular disease that have been targeted for ultrasound imaging, general features of site-targeted MB, imaging protocols, and potential roles of ultrasound molecular imaging in cardiovascular research and clinical medicine.

Targeted contrast-enhanced ultrasound imaging of angiogenesis in an orthotopic mouse tumor model of renal carcinoma

Previous studies have reported that microbubbles bearing targeting ligands to molecular markers of angiogenesis can be successfully detected by ultrasound imaging in various animal models of solid cancer. In the present study, we sought to investigate the activity of microbubbles targeted to vascular endothelial growth factor receptor 2 (VEGFR2) in an orthotopic model of renal cell carcinoma (RCC). Microbubbles conjugated to an anti-VEGFR2 antibody (MBV) were compared with microbubbles conjugated to an isotype control antibody (MBC) or naked microbubbles (MBN). An orthotopic mouse model of human RCC was established by surgically implanting an established tumor within the renal capsule in mice. Tumor growth and blood flow were verified by B-mode and color Doppler ultrasound imaging. VEGFR2 expression within the tumor and renal parenchyma was detected by immunohistochemistry. The duration of contrast enhancement of MBV was much longer than those of MBN and MBC when assessed over 10 min. The baseline-subtracted contrast intensity within the tumor was higher for MBV than for MBC and MBN (p < 0.01). Additionally, the contrast intensity for MBV was significantly higher in the tumor region than in normal parenchyma (p < 0.01). Microbubbles targeting VEGFR2 exhibit suitable properties for imaging angiogenesis in orthotopic models of renal cell carcinoma, with potential applications in life science research and clinical medicine.

New progress in angiogenesis therapy of cardiovascular disease by ultrasound targeted microbubble destruction

Angiogenesis plays a vital part in the pathogenesis and treatment of cardiovascular disease and has become one of the hotspots that are being discussed in the past decades. At present, the promising angiogenesis therapies are gene therapy and stem cell therapy. Besides, a series of studies have shown that the ultrasound targeted microbubble destruction (UTMD) was a novel gene delivery system, due to its advantages of noninvasiveness, low immunogenicity and toxicity, repeatability and temporal and spatial target specificity; UTMD has also been used for angiogenesis therapy of cardiovascular disease. In this review, we mainly discuss the combination of UTMD and gene therapy or stem cell therapy which is applied in angiogenesis therapy in recent researches, and outline the future challenges and good prospects of these approaches.

Feasibility of lactadherin-bearing clinically available microbubbles as ultrasound contrast agent for angiogenesis

OBJECTIVES

Phagocytosis of apoptotic cells is carried out through bridging of phosphatidylserine (PS)-expressing apoptotic cells and integrin αvβ3-expressing phagocytes with lactadherin. The objective of this study was to examine whether microbubbles targeted to integrin αvβ3 could be produced by conjugating a PS-containing clinically available ultrasound contrast agent with lactadherin.

MATERIALS AND METHODS

PS-containing perfluorobutane-filled microbubbles were incubated with R-phycoerythrin (PE)-labeled lactadherin, and the presence of PE-positive bubbles was examined by FACS analysis. Secondly, the attachment of lactadherin to integrin αvβ3-expressing cells (human umbilical vein endothelial cells (HUVEC)) was also examined by FACS analysis. Finally, the adhesion of PS-containing bubbles to HUVEC was examined using a parallel plate flow chamber. The number of adherent bubbles with or without the intermediation of lactadherin was compared.

RESULTS

The more lactadherin was added to the bubble suspension, the more PE-positive bubbles were detected. The size of bubbles was not increased even after conjugation with lactadherin (2.90 ± 0.04 vs. 2.81 ± 0.02 μm). Binding between lactadherin and HUVEC was also confirmed by FACS analysis. The parallel plate flow chamber study revealed that the number of PS-containing bubbles adherent to HUVEC was increased about five times by the intermediation of lactadherin (12.1 ± 6.0 to 58.7 ± 33.1 bubbles).

CONCLUSION

Because integrin αvβ3 is well-known to play a key role in angiogenesis, the complex of PS-containing bubbles and lactadherin has feasibility as a clinically translatable targeted ultrasound contrast agent for angiogenesis.

(64)Cu-labeled LyP-1-dendrimer for PET-CT imaging of atherosclerotic plaque

The ability to detect and quantify macrophage accumulation can provide important diagnostic and prognostic information for atherosclerotic plaque. We have previously shown that LyP-1, a cyclic 9-amino acid peptide, binds to p32 proteins on activated macrophages, facilitating the visualization of atherosclerotic plaque with PET. Yet, the in vivo plaque accumulation of monomeric [(18)F]FBA-LyP-1 was low (0.31 ± 0.05%ID/g). To increase the avidity of LyP-1 constructs to p32, we synthesized a dendritic form of LyP-1 on solid phase using lysine as the core structural element. Imaging probes (FAM or 6-BAT) were conjugated to a lysine or cysteine on the dendrimer for optical and PET studies. The N-terminus of the dendrimer was further modified with an aminooxy group in order to conjugate LyP-1 and ARAL peptides bearing a ketone. Oxime ligation of peptides to both dendrimers resulted in (LyP-1)4- and (ARAL)4-dendrimers with optical (FAM) and PET probes (6-BAT). For PET-CT studies, (LyP-1)4- and (ARAL)4-dendrimer-6-BAT were labeled with (64)Cu (t1/2 = 12.7 h) and intravenously injected into the atherosclerotic (ApoE(-/-)) mice. After two hours of circulation, PET-CT coregistered images demonstrated greater uptake of the (LyP-1)4-dendrimer-(64)Cu than the (ARAL)4-dendrimer-(64)Cu in the aortic root and descending aorta. Ex vivo images and the biodistribution acquired at three hours after injection also demonstrated a significantly higher uptake of the (LyP-1)4-dendrimer-(64)Cu (1.1 ± 0.26%ID/g) than the (ARAL)4-dendrimer-(64)Cu (0.22 ± 0.05%ID/g) in the aorta. Similarly, subcutaneous injection of the LyP-1-dendrimeric carriers resulted in preferential accumulation in plaque-containing regions over 24 h. In the same model system, ex vivo fluorescence images within aortic plaque depict an increased accumulation and penetration of the (LyP-1)4-dendrimer-FAM as compared to the (ARAL)4-dendrimer-FAM. Taken together, the results suggest that the (LyP-1)4-dendrimer can be applied for in vivo PET imaging of plaque and that LyP-1 could be further exploited for the delivery of therapeutics with multivalent carriers or nanoparticles.

Improving ultrasound gene transfection efficiency by controlling ultrasound excitation of microbubbles

Ultrasound application in the presence of microbubbles has shown great potential for non-viral gene transfection via transient disruption of cell membrane (sonoporation). However, improvement of its efficiency has largely relied on empirical approaches without consistent and translatable results. The goal of this study is to develop a rational strategy based on new results obtained using novel experimental techniques and analysis to improve sonoporation gene transfection. In this study, we conducted experiments using targeted microbubbles that were attached to cell membrane to facilitate sonoporation. We quantified the dynamic activities of microbubbles exposed to pulsed ultrasound and the resulting sonoporation outcome, and identified distinct regimes of characteristic microbubble behaviors: stable cavitation, coalescence and translation, and inertial cavitation. We found that inertial cavitation generated the highest rate of membrane poration. By establishing direct correlation of ultrasound-induced bubble activities with intracellular uptake and pore size, we designed a ramped pulse exposure scheme for optimizing microbubble excitation to improve sonoporation gene transfection. We implemented a novel sonoporation gene transfection system using an aqueous two phase system (ATPS) for efficient use of reagents and high throughput operation. Using plasmids coding for the green fluorescence protein (GFP), we achieved a sonoporation transfection efficiency in rate aortic smooth muscle cells (RASMCs) of 6.9%±2.2% (n=9), comparable with lipofection (7.5%±0.8%, n=9). Our results reveal characteristic microbubble behaviors responsible for sonoporation and demonstrated a rational strategy to improve sonoporation gene transfection.

Contrast ultrasound and targeted microbubbles: diagnostic and therapeutic applications in progressive diabetic nephropathy

Diabetic nephropathy remains one of the most common causes for end-stage renal disease worldwide. Although therapies aimed at optimizing glycemic control and systemic blood pressure have benefit, the reduction in progressive nephropathy remains modest at best. Thus, research continues to focus on newer therapies to address the unmet needs for additional renal protective strategies. The ability to noninvasively image the molecular and cellular processes that underlie diabetic nephropathy would be useful in risk stratifying patients with diabetes, and more importantly would aid in the evaluation of novel therapies to prevent and treat nephropathy. In addition, the development of ultrasound technologies that allow targeted gene delivery using high-power ultrasound and DNA-bearing microbubbles may have applicability for gene therapy to prevent diabetic nephropathy. This review highlights contrast-enhanced ultrasound imaging techniques for the evaluation of renal pathologies, including perfusion and molecular imaging techniques, and ultrasound-mediated gene delivery for therapeutic applications in diabetic nephropathy, that have potential for translation to clinical practice.

Microbubbles as ultrasound contrast agents for molecular imaging: preparation and application

OBJECTIVE

The purpose of this review is to describe trends in microbubble application in molecular imaging.

CONCLUSION

Microbubbles are used for contrast ultrasound imaging as blood-pool agents in cardiology and radiology. Their promise as targeted agents for molecular imaging is now being recognized. Microbubbles can be functionalized with ligand molecules that bind to molecular markers of disease. Potential clinical applications of molecular imaging with microbubble-based ultrasound contrast agents are in the monitoring of the biomarker status of vascular endothelium, visualizing tumor vasculature, and imaging inflammation and ischemia-reperfusion injury zones and thrombi.

Molecular imaging of vasa vasorum neovascularization via DEspR-targeted contrast-enhanced ultrasound micro-imaging in transgenic atherosclerosis rat model

PURPOSE

Given that carotid vasa vasorum neovascularization is associated with increased risk for stroke and cardiac events, the present in vivo study was designed to investigate molecular imaging of carotid artery vasa vasorum neovascularization via target-specific contrast-enhanced ultrasound (CEU) micro-imaging.

PROCEDURES

Molecular imaging was performed in male transgenic rats with carotid artery disease and non-transgenic controls using dual endothelin1/VEGFsp receptor (DEspR)-targeted microbubbles (MB(D)) and the Vevo770 micro-imaging system and CEU imaging software.

RESULTS

DEspR-targeted CEU-positive imaging exhibited significantly higher contrast intensity signal (CIS)-levels and pre-/post-destruction CIS-differences in seven of 13 transgenic rats, in contrast to significantly lower CIS-levels and differences in control isotype-targeted microbubble (MB(C))-CEU imaging (n = 8) and in MB(D) CEU-imaging of five non-transgenic control rats (P < 0.0001). Ex vivo immunofluorescence analysis demonstrated binding of MB(D) to DEspR-positive endothelial cells; and association of DEspR-targeted increased contrast intensity signals with DEspR expression in vasa vasorum neovessel and intimal lesions. In vitro analysis demonstrated dose-dependent binding of MB(D) to DEspR-positive human endothelial cells with increasing %cells bound and number of MB(D) per cell, in contrast to MB(C) or non-labeled microbubbles (P < 0.0001).

CONCLUSION

In vivo DEspR-targeted molecular imaging detected increased DEspR-expression in carotid artery lesions and in expanded vasa vasorum neovessels in transgenic rats with carotid artery disease. Future studies are needed to determine predictive value for stroke or heart disease in this transgenic atherosclerosis rat model and translational applications.

Effect of surface architecture on in vivo ultrasound contrast persistence of targeted size-selected microbubbles

Ultrasound molecular imaging is a powerful diagnostic modality using microbubbles coated with targeting ligands specific for endothelial biomarkers. The circulation persistence of ligand-bearing contrast agents is a key determinant in their contrast enhancement and targeting capability. Prior studies have shown that targeted microbubbles with ligands attached to the shell using the conventional exposed-ligand architecture (ELA) could trigger undesired ligand-induced complement activation and decreased circulation time. Microbubbles with the buried-ligand architecture (BLA), however, were found to inhibit complement activation and prolong circulation time. In the present study, we extended the stealth BLA microbubble design to size-selected (4 to 5-μm diameter) microbubbles targeted with cyclic RGD peptide using the postlabeling technique. Microbubble circulation persistence was measured in the healthy mouse kidney using a Visualsonics Vevo 770 scanner operating at 40 MHz in fundamental mode. The circulation persistence for targeted BLA microbubbles was significantly longer compared with their ELA counterparts and similar to no-ligand controls. Use of the BLA instead of the ELA increased the circulation half-life approximately two-fold. Analysis of the time-intensity and time-fluctuation curves with a two-compartment pharmacokinetic model showed a minimal degree of nonspecific vascular adhesion for any group. These results demonstrate the importance of surface architecture in the design of targeted microbubbles for ultrasound molecular imaging.

Retrograde perfusion and filling of mouse coronary vasculature as preparation for micro computed tomography imaging

Visualization of the vasculature is becoming increasingly important for understanding many different disease states. While several techniques exist for imaging vasculature, few are able to visualize the vascular network as a whole while extending to a resolution that includes the smaller vessels. Additionally, many vascular casting techniques destroy the surrounding tissue, preventing further analysis of the sample. One method which circumvents these issues is micro-Computed Tomography (μCT). μCT imaging can scan at resolutions <10 microns, is capable of producing 3D reconstructions of the vascular network, and leaves the tissue intact for subsequent analysis (e.g., histology and morphometry). However, imaging vessels by ex vivo μCT methods requires that the vessels be filled with a radiopaque compound. As such, the accurate representation of vasculature produced by μCT imaging is contingent upon reliable and complete filling of the vessels. In this protocol, we describe a technique for filling mouse coronary vessels in preparation for μCT imaging. Two predominate techniques exist for filling the coronary vasculature: in vivo via cannulation and retrograde perfusion of the aorta (or a branch off the aortic arch), or ex vivo via a Langendorff perfusion system. Here we describe an in vivo aortic cannulation method which has been specifically designed to ensure filling of all vessels. We use a low viscosity radiopaque compound called Microfil which can perfuse through the smallest vessels to fill all the capillaries, as well as both the arterial and venous sides of the vascular network. Vessels are perfused with buffer using a pressurized perfusion system, and then filled with Microfil. To ensure that Microfil fills the small higher resistance vessels, we ligate the large branches emanating from the aorta, which diverts the Microfil into the coronaries. Once filling is complete, to prevent the elastic nature of cardiac tissue from squeezing Microfil out of some vessels, we ligate accessible major vascular exit points immediately after filling. Therefore, our technique is optimized for complete filling and maximum retention of the filling agent, enabling visualization of the complete coronary vascular network--arteries, capillaries, and veins alike.

Imaging: guiding the clinical translation of cardiac stem cell therapy

Stem cells have been touted as the holy grail of medical therapy, with promises to regenerate cardiac tissue, but it appears the jury is still out on this novel therapy. Using advanced imaging technology, scientists have discovered that these cells do not survive nor engraft long-term. In addition, only marginal benefit has been observed in large-animal studies and human trials. However, all is not lost. Further application of advanced imaging technology will help scientists unravel the mysteries of stem cell therapy and address the clinical hurdles facing its routine implementation. In this review, we will discuss how advanced imaging technology will help investigators better define the optimal delivery method, improve survival and engraftment, and evaluate efficacy and safety. Insights gained from this review may direct the development of future preclinical investigations and clinical trials.

Quantification and monitoring of inflammation in murine inflammatory bowel disease with targeted contrast-enhanced US

PURPOSE

To evaluate ultrasonography (US) by using contrast agent microbubbles (MBs) targeted to P-selectin (MB(P-selectin)) to quantify P-selectin expression levels in inflamed tissue and to monitor response to therapy in a murine model of chemically induced inflammatory bowel disease (IBD).

MATERIALS AND METHODS

All procedures in which laboratory animals were used were approved by the institutional administrative panel on laboratory animal care. Binding affinity and specificity of MB(P-selectin) were tested in cell culture experiments under flow shear stress conditions and compared with control MBs (MB(Control)). In vivo binding specificity of MB(P-selectin) to P-selectin was tested in mice with trinitrobenzenesulfonic acid-induced colitis (n = 22) and control mice (n = 10). Monitoring of anti-tumor necrosis factor α antibody therapy was performed over 5 days in an additional 30 mice with colitis by using P-selectin-targeted US imaging, by measuring bowel wall thickness and perfusion, and by using a clinical disease activity index score. In vivo targeted contrast material-enhanced US signal was quantitatively correlated with ex vivo expression levels of P-selectin as assessed by quantitative immunofluorescence.

RESULTS

Attachment of MB(P-selectin) to endothelial cells was significantly (P = .0001) higher than attachment of MB(Control) and significantly (ρ = 0.83, P = .04) correlated with expression levels of P-selectin on endothelial cells. In vivo US signal in mice with colitis was significantly higher (P = .0001) with MB(P-selectin) than with MB(Control). In treated mice, in vivo US signal decreased significantly (P = .0001) compared with that in nontreated mice and correlated well with ex vivo P-selectin expression levels (ρ = 0.69; P = .04). Colonic wall thickness (P ≥ .06), bowel wall perfusion (P ≥ .85), and clinical disease activity scoring (P ≥ .06) were not significantly different between treated and nontreated mice at any time.

CONCLUSION

Targeted contrast-enhanced US imaging enables noninvasive in vivo quantification and monitoring of P-selectin expression in inflammation in murine IBD.

Contrast-enhanced ultrasound in oncology

In patients with known malignant disease, 51% of liver lesions less than 1.5 cm turn out to be benign. Whether the probability of malignancy is high or low, further investigations are often necessary to definitely exclude malignancy. Contrast-enhanced ultrasonography has a prominent role in lesion characterization with a diagnostic accuracy comparable with computed tomography and magnetic resonance imaging. Anti-angiogenic treatment is common in most oncological institutions and the response evaluation is a new challenge with a research focus on the change in tumour vasculature and perfusion. In planning biopsies, CEUS can identify necrotic and viable areas of tumours and improve the diagnostic accuracy.

Future applications of contrast ultrasound

Contrast agents are currently used during echocardiography for enhancement of structure and function, as well as for perfusion imaging. The next frontiers in contrast ultrasonography are targeted imaging, and using microbubbles for therapeutic purposes. Targeted imaging is the detection of specific components of cardiovascular disease in vivo, with microbubbles which may non-specifically attach to diseased endothelial cells, or with microbubbles which have been specifically designed to detect a pathologic process. Therapeutic applications of contrast ultrasonography include the use of microbubbles to enhance delivery of agents (like drugs, genes, growth factors, etc.) to the endothelium or perivascular cells. This review will discuss differences in contrast agents used for current applications versus targeted imaging, technical considerations required to achieve site-specific imaging, and potential applications of this technology. The potential for contrast ultrasonography to enhance drug and gene delivery to tissue will also be discussed.