Acoustic Characterization and Enhanced Ultrasound Imaging of Long-Circulating Lipid-Coated Microbubbles

A vortex-based hydrodynamic cavitation manufacturing platform to generate albumin microbubbles for delivery of chemotherapies to cancerous tumours

A novel approach was developed to create stable protein-based microbubbles using a vortex-driven hydrodynamic cavitation device. Such microbubbles, tiny gas-filled spheres, combined with ultrasound, can enhance drug uptake leading to inhibition of cancerous cell growth, boosting the effectiveness of anti-cancer drug molecules. The optimal conditions for the fabrication of stable bovine serum albumin (BSA) microbubbles were found to be a 15 wt% bovine serum albumin (BSA) solution at 60 °C with a pH of 6 and an ionic strength of 1.0 M. This resulted in stable BSA microbubbles with an approximate diameter of 7 μm. Curcumin-encapsulated BSA microbubbles (CBMs, 63 ± 1 μM curcumin per 101⁰ microbubbles) were created using these optimised fabrication parameters as a model system for delivering chemotherapeutic agents. The maximum percentage of curcumin release from the CBMs into phosphate buffered saline with sonication (85 %) was significantly greater than without sonication (24 %). These microbubbles were then tested to assess their effectiveness in delivering curcumin to triple-negative breast cancer cells (MDAMB-231) using a cell-to-MB ratio of 1:100, an ultrasound intensity of 0.5 W/cm2, and an ultrasound exposure time of 10 s to maximise uptake. Kinetic studies demonstrated a significant enhancement in the uptake of curcumin by MDAMB-231 cells when encapsulated into the microbubbles with ultrasound application. A substantial reduction in cellular proliferation was observed in both 2D cell culture and 3D tumour spheroid models when MDAMB-231 cells were exposed to microbubbles loaded with curcumin and ultrasound was applied. The vortex-based hydrodynamic cavitation device successfully generated curcumin loaded microbubbles with a long shelf life (120 days at 4 °C), mild preparation conditions, and enhanced uptake into cancerous tumour spheroid models. This data demonstrates the potential of this device for the commercial manufacture of drug loaded microbubble-based delivery systems.

The Clinical Utility of Liver-Specific Ultrasound Contrast Agents During Hepatocellular Carcinoma Imaging

Hepatocellular carcinoma (HCC) is the most common form of hepatic malignancy, with high mortality rates recorded globally. Early detection through clinical biomarkers, medical imaging and histological assessment followed by rapid intervention are integral for positive patient outcomes. Although contrast-enhanced computed tomography scans and magnetic resonance imaging are recognised as the reference standard for the diagnosis and staging of HCC in international guidelines, ultrasound (US) examination is recommended as a screening tool for patients at risk. Contrast-enhanced US (CEUS) elevates the standard of an US examination using US contrast agents (UCAs), capable of diagnosing focal liver lesions with high efficacy. Most UCAs are purely intravascular, offering clinicians a dynamic representation of a lesions' arterial phase vascular kinetics, which is seldom seen in such detail during computed tomography or magnetic resonance imaging assessments. Despite its benefits, there is incongruity between international societies on the role of CEUS in the HCC clinical pathway. The transient nature of pure blood-pool agents is suggested to be insufficient to justify CEUS as a primary modality due to the inability to consistently perform whole liver imaging, alongside disputes regarding its capabilities to differentiate HCC from intrahepatic cholangiocarcinoma. A sinusoidal phase UCA affords clinicians the opportunity to perform whole liver imaging through Kupffer cell uptake in addition to visualising lesion vascular kinetics in the arterial and portal venous phases. Therefore, the purpose of this review was to examine the role of CEUS in the HCC clinical pathway in its current practice and observe how a Kupffer cell sinusoidal phase UCA may supplement contemporary diagnostic techniques through a multi-modality, multi-agent approach.

One-Step Preparation of Magnetic Lipid Bubbles: Magnetothermal Effect Induces the Simultaneous Formation of Gas Nuclei and Self-Assembly of Phospholipids

In recent years, enveloped micro-nanobubbles have garnered significant attention in research due to their commendable stability, biocompatibility, and other notable properties. Currently, the preparation methods of enveloped micro-nanobubbles have limitations such as complicated preparation process, large bubble size, wide distribution range, low yield, etc. There exists an urgent demand to devise a simple and efficient method for the preparation of enveloped micro-nanobubbles, ensuring both high concentration and a uniform particle size distribution. Magnetic lipid bubbles (MLBs) are a multifunctional type of enveloped micro-nanobubble combining magnetic nanoparticles with lipid-coated bubbles. In this study, MLBs are prepared simply and efficiently by a magneto internal heat bubble generation process based on the interfacial self-assembly of iron oxide nanoparticles induced by the thermogenic effect in an alternating magnetic field. The mean hydrodynamic diameter of the MLBs obtained was 384.9 ± 8.5 nm, with a polydispersity index (PDI) of 0.248 ± 0.021, a zeta potential of -30.5 ± 1.0 mV, and a concentration of (7.92 ± 0.46) × 109 bubbles/mL. Electron microscopy results show that the MLBs have a regular spherical stable core-shell structure. The superparamagnetic iron oxide nanoparticles (SPIONs) and phospholipid layers adsorbed around the spherical gas nuclei of the MLBs, leading the particles to demonstrate commendable superparamagnetic and magnetic properties. In addition, the effects of process parameters on the morphology of MLBs, including phospholipid concentration, phospholipid proportiona, current intensity, magnetothermal time, and SPION concentration, were investigated and discussed to achieve controlled preparation of MLBs. In vitro imaging results reveal that the higher the concentration of MLBs loaded with iron oxide nanoparticles, the better the in vitro ultrasound (US) imaging and magnetic resonance imaging (MRI) results. This study proves that the magneto internal heat bubble generation process is a simple and efficient technique for preparing MLBs with high concentration, regular structure, and commendable properties. These findings lay a robust foundation for the mass production and application of enveloped micro-nanobubbles, particularly in biomedical fields and other related domains.

Role of Ultrasonic Microbubbles in Treating Rheumatoid Arthritis: Enhancing the Efficacy of Tocilizumab via Contrast-Enhanced Ultrasound-Monitored, Ultrasound-Targeted Microbubble Destruction

OBJECTIVE

Our aim was to explore the feasibility of using ultrasound-targeted microbubble destruction (UTMD) to deliver tocilizumab and enhance its efficacy in treating rheumatoid arthritis (RA).

METHODS

Rats with adjuvant-induced arthritis were randomly assigned to one of five treatment groups: group 1, tocilizumab + microbubbles (MBs) + UTMD; group 2, tocilizumab + MBs; group 3, tocilizumab + saline; group 4, MBs + UTMD; group 5, no treatment. We employed a commercially available ultrasound (US) machine capable of performing contrast-enhanced ultrasound (CEUS) and UTMD simultaneously using a single probe. CEUS was performed to monitor the entry and collapse of MBs. After treatment, the rats' left hindlimb paws were harvested for immunohistochemical staining of interleukin-6 (IL-6) and tumor necrosis factor α (TNF-α).

RESULTS

After injection of the mixture of drugs and MBs with UTMD, significant enhancement was seen in the inflamed hindlimb paw regions, which subsided immediately on exposure to low-frequency US beams and re-appeared in the intervals between beam exposures. IL-6 expression was significantly lower in groups 1, 2 and 3 than in groups 4 and 5 (p < 0.01). Group 1 had the lowest level of IL-6 expression (p [G1 vs. G2] < 0.01, p [G1 vs. G3] < 0.01). The levels of TNF-α expression in groups 1, 2, and 3 were significantly lower than those in groups 4 and 5, but no difference was observed in these levels between groups 1-3.

CONCLUSION

UTMD shows promise in enhancing the treatment efficacy of anti-IL-6 drugs for RA treatment.

Microbubble Delivery Platform for Ultrasound-Mediated Therapy in Brain Cancers

The blood-brain barrier (BBB) is one of the most selective endothelial barriers that protect the brain and maintains homeostasis in neural microenvironments. This barrier restricts the passage of molecules into the brain, except for gaseous or extremely small hydrophobic molecules. Thus, the BBB hinders the delivery of drugs with large molecular weights for the treatment of brain cancers. Various methods have been used to deliver drugs to the brain by circumventing the BBB; however, they have limitations such as drug diversity and low delivery efficiency. To overcome this challenge, microbubbles (MBs)-based drug delivery systems have garnered a lot of interest in recent years. MBs are widely used as contrast agents and are recently being researched as a vehicle for delivering drugs, proteins, and gene complexes. The MBs are 1-10 μm in size and consist of a gas core and an organic shell, which cause physical changes, such as bubble expansion, contraction, vibration, and collapse, in response to ultrasound. The physical changes in the MBs and the resulting energy lead to biological changes in the BBB and cause the drug to penetrate it, thus enhancing the therapeutic effect. Particularly, this review describes a state-of-the-art strategy for fabricating MB-based delivery platforms and their use with ultrasound in brain cancer therapy.

Perfluorocarbon nanodroplet size, acoustic vaporization, and inertial cavitation affected by lipid shell composition in vitro

Perfluorocarbon nanodroplets (PFCnDs) are ultrasound contrast agents that phase-transition from liquid nanodroplets to gas microbubbles when activated by laser irradiation or insonated with an ultrasound pulse. The dynamics of PFCnDs can vary drastically depending on the nanodroplet composition, including the lipid shell properties. In this paper, we investigate the effect of varying the ratio of PEGylated to non-PEGylated phospholipids in the outer shell of PFCnDs on the acoustic nanodroplet vaporization (liquid to gas phase transition) and inertial cavitation (rapid collapse of the vaporized nanodroplets) dynamics in vitro when insonated with focused ultrasound. Nanodroplets with a high concentration of PEGylated lipids had larger diameters and exhibited greater variance in size distribution compared to nanodroplets with lower proportions of PEGylated lipids in the lipid shell. PFCnDs with a lipid shell composed of 50:50 PEGylated to non-PEGylated lipids yielded the highest B-mode image intensity and duration, as well as the greatest pressure difference between acoustic droplet vaporization onset and inertial cavitation onset. We demonstrate that slight changes in lipid shell composition of PFCnDs can significantly impact droplet phase transitioning and inertial cavitation dynamics. These findings can help guide researchers to fabricate PFCnDs with optimized compositions for their specific applications.

Ultrasound and nanomaterial: an efficient pair to fight cancer

Ultrasounds are often used in cancer treatment protocols, e.g. to collect tumor tissues in the right location using ultrasound-guided biopsy, to image the region of the tumor using more affordable and easier to use apparatus than MRI and CT, or to ablate tumor tissues using HIFU. The efficacy of these methods can be further improved by combining them with various nano-systems, thus enabling: (i) a better resolution of ultrasound imaging, allowing for example the visualization of angiogenic blood vessels, (ii) the specific tumor targeting of anti-tumor chemotherapeutic drugs or gases attached to or encapsulated in nano-systems and released in a controlled manner in the tumor under ultrasound application, (iii) tumor treatment at tumor site using more moderate heating temperatures than with HIFU. Furthermore, some nano-systems display adjustable sizes, i.e. nanobubbles can grow into micro-bubbles. Such dual size is advantageous since it enables gathering within the same unit the targeting properties of nano bubbles via EPR effect and the enhanced ultrasound contrasting properties of micro bubbles. Interestingly, the way in which nano-systems act against a tumor could in principle also be adjusted by accurately selecting the nano-system among a large choice and by tuning the values of the ultrasound parameters, which can lead, due to their mechanical nature, to specific effects such as cavitation that are usually not observed with purely electromagnetic waves and can potentially help destroying the tumor. This review highlights the clinical potential of these combined treatments that can improve the benefit/risk ratio of current cancer treatments.

Targeted Delivery of Salusin-α Into Rabbit Carotid Arterial Endothelium Using SonoVue

OBJECTIVES

A new method based on the adhesion of SonoVue to plasmids was assessed to achieve targeted gene delivery into the vascular endothelium.

METHODS

pEGFP-Salusin-α and pcDNA3.1-Salusin-α plasmids were transfected into the arterial endothelium of different rabbit groups. Western blotting was performed to analyze the expression of EGFP and salusin-α in the common carotid arteries of rabbits from different groups, and ELISA was performed to detect plasma salusin-α levels in rabbits from each group; simultaneously, blood parameters of different groups of rabbits were measured.

RESULTS

Green fluorescence was observed in the right common carotid artery of rabbits transfected with pEGFP-Salusin-α, but not in the endothelial cells of not-transfected control rabbits. The expression of salusin-α in the transfected animals was higher than that in the control not-transfected animals (P < .05). In rabbits transfected with pcDNA3.1-Salusin-α plasmid, salusin-α expression was higher than in the not-transfected control animals (P < .05). However, there was no significant difference in plasma salusin-α levels between transfected animals and controls (P > .05). Blood parameters were also measured in both groups.

CONCLUSIONS

Our data confirm the establishment of a new method using SonoVue for targeted gene delivery into the arterial endothelium. Our study outcomes propose a new method of intervention in atherosclerosis and a new tool for targeted gene delivery.

Contrast-enhanced ultrasound imaging using long-circulating cationic magnetic microbubbles in vitro and in vivo validations

Traditional encapsulated microbubbles are recently used as delivery carriers for drugs and genes, but they have low efficiency. If the local microbubble concentration could be increased, this might be able to improve the therapeutic efficacy of diseases. In this study, we developed novel cationic magnetic microbubbles (MB_M), which could simultaneously realize targeted aggregation under a magnetic field as well as ultrasonographic real-time visualization. Their physicochemical properties, biocompatibility, ultrasonography, magnetic response characteristics, and biodistribution were systematically evaluated. Here, the MB_M were 2.55±0.14µm in size with a positive zeta potential, and had a good biocompatibility. They were able to enhance ultrasonographic contrast both in vitro and in vivo. MB_M could be attracted by an external magnet for directional movement and aggregation in vitro. We confirmed that MB_M also had a great magnetic response in vivo, by means of fluorescence imaging and contrast-enhanced ultrasound imaging. Following intravenous injection into tumor-bearing mice, MB_M showed excellent stability in the internal circulation, and could accumulate in the tumor vasculature through magnetic targeting. With the excellent combination of magnetic response and acoustic properties, cationic magnetic microbubbles (MB_M) have promising potential for use as a new kind of drug/gene carrier for theranostics in the future.

Tailor-Made Single-Core PLGA Microbubbles as Acoustic Cavitation Enhancers for Therapeutic Applications

Microbubbles (MBs), being gas bubbles encapsulated inside a solid shell, have been investigated extensively in the field of therapeutic ultrasound as acoustic cavitation enhancers. Hard-shell MBs have an advantage over soft-shell MBs due to their improved stability. Poly(lactic-co-glycolic acid) (PLGA) is one of the most attractive polymers for hard-shell MB synthesis; however, very little is known regarding the effect of synthesis parameters on the acoustic cavitation activity of PLGA MBs and the tunability of this activity. In this study, by manipulating the synthesis parameters, we were able to control the characteristics of the MBs, such as their internal structure, gas core, size distribution, and shell thickness, which significantly affect the total acoustic cavitation activity that they exhibit (i.e., their cavitation dose). We showed that single-core MBs filled with C₃F₈ gas can produce cavitation effects for extended periods under continuous circulation. These MBs exhibited high stability, and their cavitation activity was not affected by prior circulation in the system. Preliminary in vivo results demonstrated that intravenously injected MBs did not cause adverse effects and produced cavitation activity that increased the permeability of the pig blood-brain barrier. Although more tests should be performed to evaluate the MB long-term safety and activity in vivo, these encouraging results suggest that our PLGA MBs have potential for future therapeutic applications as cavitation enhancers.

Effectiveness of Oil-Layered Albumin Microbubbles Produced Using Microfluidic T-Junctions in Series for In Vitro Inhibition of Tumor Cells

This work focuses on the synthesis of oil-layered microbubbles using two microfluidic T-junctions in series and evaluation of the effectiveness of these microbubbles loaded with doxorubicin and curcumin for cell invasion arrest from 3D spheroid models of triple negative breast cancer (TNBC), MDA-MB-231 cell line. Albumin microbubbles coated in the drug-laden oil layer were synthesized using a new method of connecting two microfluidic T-mixers in series. Double-layered microbubbles thus produced consist of an innermost core of nitrogen gas encapsulated in an aqueous layer of bovine serum albumin (BSA) which in turn, is coated with an outer layer of silicone oil. In order to identify the process conditions leading to the formation of double-layered microbubbles, a regime map was constructed based on capillary numbers for aqueous and oil phases. The microbubble formation regime transitions from double-layered to single layer microbubbles and then to formation of single oil droplets upon gradual change in flow rates of aqueous and oil phases. In vitro dissolution studies of double-layered microbubbles in an air-saturated environment indicated that a complete dissolution of such bubbles produces an oil droplet devoid of a gas bubble. Incorporation of doxorubicin and curcumin was found to produce a synergistic effect, which resulted in higher cell deaths in 2D monolayers of TNBC cells and inhibition of cell proliferation from 3D spheroid models of TNBC cells compared to the control.

Dual-Mode Contrast Agents with RGD-Modified Polymer for Tumour-Targeted US/NIRF Imaging

BACKGROUND

Cancer diagnosis and treatment during the early stages of disease remain extremely challenging clinical tasks. The development of effective multimode contrast agents could greatly facilitate the early detection of cancer.

MATERIALS AND METHODS

We prepared dual-mode contrast agents using a biotin/avidin bioamplification system. Through in vivo and in vitro experiments, we verified the imaging performance of this contrast agents in both fluorescence and ultrasound and its targeting specificity for MDA-MB-231 cells.

RESULTS

The RGD peptide-labelled microbubbles showed excellent targeting of αvβ3 integrin expressed by MDA-MB-231 cells in vitro and in vivo. The signal intensity and time duration of ultrasound imaging using these particles were superior to those obtained with a typical ultrasound contrast agent in the clinic. The tumour areas also demonstrated high Cy5.5 accumulation by fluorescence imaging.

CONCLUSION

The results show that this targeted dual-mode imaging system yields outstanding US/NIRF imaging results, possibly allowing the early clinical diagnosis of cancer.

Ultrasound-responsive lipid microbubbles for drug delivery: A review of preparation techniques to optimise formulation size, stability and drug loading

Lipid-shelled microbubbles have received extensive interest to enhance ultrasound-responsive drug delivery outcomes due to their high biocompatibility. While therapeutic effectiveness of microbubbles is well established, there remain limitations in sample homogeneity, stability profile and drug loading properties which restrict these formulations from seeing widespread use in the clinical setting. In this review, we evaluate and discuss the most encouraging leads in lipid microbubble design and optimisation. We examine current applications in drug delivery for the systems and subsequently detail shell compositions and preparation strategies that improve monodispersity while retaining ultrasound responsiveness. We review how excipients and storage techniques help maximise stability and introduce different characterisation and drug loading techniques and evaluate their impact on formulation performance. The review concludes with current quality control measures in place to ensure lipid microbubbles can be reproducibly used in drug delivery.

Photo- and Sono-Dynamic Therapy: A Review of Mechanisms and Considerations for Pharmacological Agents Used in Therapy Incorporating Light and Sound

As irreplaceable energy sources of minimally invasive treatment, light and sound have, separately, laid solid foundations in their clinic applications. Constrained by the relatively shallow penetration depth of light, photodynamic therapy (PDT) typically involves involves superficial targets such as shallow seated skin conditions, head and neck cancers, eye disorders, early-stage cancer of esophagus, etc. For ultrasound-driven sonodynamic therapy (SDT), however, to various organs is facilitated by the superior... transmission and focusing ability of ultrasound in biological tissues, enabling multiple therapeutic applications including treating glioma, breast cancer, hematologic tumor and opening blood-brain-barrier (BBB). Considering the emergence of theranostics and precision therapy, these two classic energy sources and corresponding sensitizers are worth reevaluating. In this review, three typical therapies using light and sound as a trigger, PDT, SDT, and combined PDT and SDT are introduced. The therapeutic dynamics and current designs of pharmacological sensitizers involved in these therapies are presented. By introducing both the history of the field and the most up-to-date design strategies, this review provides a systemic summary on the development of PDT and SDT and fosters inspiration for researchers working on 'multi-modal' therapies involving light and sound.

Scoping Review of Targeted Ultrasound Contrast Agents in the Detection of Angiogenesis

A systematic search was conducted to categorize targeted ultrasound contrast agents (UCAs) used in cancer-related angiogenesis detection. We identified 15 unique contrast agents from 2008 to March 2018. Most primary research articles studied UCAs targeted to vascular endothelial growth factor receptor or αv β3 -integrin. Breast cancer and colon cancer are the most common neoplastic processes in which these agents were studied. BR55 (Bracco Research SA, Geneva, Switzerland), a vascular endothelial growth factor receptor-targeting UCA, is the first targeted UCA that has completed phase 0 trials. Our review identifies a gap in the literature regarding the application of targeted UCAs in cancer models beyond breast and colon cancers and identifies other promising UCAs.

Phase Change Ultrasound Contrast Agents with a Photopolymerized Diacetylene Shell

Phase change contrast agents for ultrasound (US) imaging consist of nanodroplets (NDs) with a perfluorocarbon (PFC) liquid core stabilized with a lipid or a polymer shell. Liquid ↔ gas transition, occurring in the core, can be triggered by US to produce acoustically active microbubbles (MBs) in a process named acoustic droplet vaporization (ADV). MB shells containing polymerized diacetylene moiety were considered as a good trade off between the lipid MBs, showing optimal attenuation, and the polymeric ones, displaying enhanced stability. This work reports on novel perfluoropentane and perfluorobutane NDs stabilized with a monolayer of an amphiphilic fatty acid, i.e. 10,12-pentacosadiynoic acid (PCDA), cured with ultraviolet (UV) irradiation. The photopolymerization of the diacetylene groups, evidenced by the appearance of a blue color due to the conjugation of ene-yne sequences, exhibits a chromatic transition from the nonfluorescent blue color to a fluorescent red color when the NDs are heated or the pH of the suspension is basic. An estimate of the molecular weights reached by the polymerized PCDA in the shell, poly(PCDA), has been obtained using gel permeation chromatography and MALDI-TOF mass spectrometry. The poly(PCDA)/PFC NDs show good biocompatibility with fibroblast cells. ADV efficiency and acoustic properties before and after the transition were tested using a 1 MHz probe, revealing a resonance frequency between 1 and 2 MHz similar to other lipidic MBs. The surface of PCDA shelled NDs can be easily modified without influencing the stability and the acoustic performances of droplets. As a proof of concept we report on the conjugation of cyclic RGD and PEG chains of the particles to support targeting ability toward endothelial cells.

Effect of Bubble Concentration on the in Vitro and in Vivo Performance of Highly Stable Lipid Shell-Stabilized Micro- and Nanoscale Ultrasound Contrast Agents

Ultrasound (US) is a widely used diagnostic imaging tool because it is inexpensive, safe, portable, and broadly accessible. Ultrasound contrast agents (UCAs) are employed to enhance backscatter echo and improve imaging contrast. The most frequently utilized UCAs are echogenic bubbles made with a phospholipid or protein-stabilized hydrophobic gas core. While clinically utilized, applications of UCAs are often limited by rapid signal decay (<5 min) in vivo under typical ultrasound imaging protocols. Here, we report on a formulation of lipid shell-stabilized perfluoropropane (C₃F₈) microbubbles and nanobubbles with a significantly prolonged in vivo stability. Microbubbles (875 ± 280 nm) of the target size were prepared by utilizing a multiple-step centrifugation cycle, while nanobubbles (299 ± 189 nm) were isolated from the activated vial using a single centrifugation step. To provide in-depth acoustic characterization of the new construct we evaluated the effect of size and concentration on their in vitro and in vivo performance. In vitro and in vivo characterization were carried out for a range of bubble concentrations normalized by total gas volume quantified via headspace gas chromatography/mass spectrometry (GC/MS). In vitro characterization revealed that nanobubbles at different concentrations are more consistently stable over time with the highest and lowest dilutions (50-fold decrease) only differing in US signal after 8 min exposure by 10.34%, while for microbubbles the difference was 86.46%. As expected, due to the difference in hydrodynamic diameter and scattering cross section difference, nanobubbles showed lower overall initial signal intensity. In vivo experiments showed that both microbubbles and nanobubbles with similar initial peak signal intensity are comparably stable over time with 66.8% and 60.6% remaining signal after 30 min, respectively. This study demonstrates that bubble concentration has significant effects on the persistence of both microbubbles and nanobubbles in vitro and in vivo, but the effects are more pronounced in larger bubbles. These effects should be taken into account when selecting the appropriate bubble parameters for future imaging applications.

Ultrasound Technology for Molecular Imaging: From Contrast Agents to Multimodal Imaging

Ultrasound (US) takes advantage of ultrasound contrast agents (UCAs) to further increase the sensitivity and specificity of monitoring at the cellular level, which has had a considerable effect on the modern molecular imaging field. Gas-filled microbubbles (MBs) as UCAs in the bloodstream generate resonant volumetric oscillations in response to rapid variations in acoustic pressure, which are related to both the acoustic parameters of applied ultrasound and the physicochemical properties of the contrast agents. Nanoscale UCAs have been developed and have attracted much attention due to their utility in detecting extravascular lesions. Ultrasound molecular assessment is achieved by binding disease-specific ligands to the surface of UCAs, which have been designed to target tissue biomarkers in the area of interest, such as blood vessels, inflammation, or thrombosis. Additionally, the development of multimodal imaging technology is conducive for integration of the advantages of various imaging techniques to acquire additional diagnostic information. In this review paper, the present status and the critical issues for developing ultrasound contrast agents and multimodal imaging applications are described. Conventional MB UCAs are first introduced, including their research material, diagnostic applications, and intrinsic limitations. Then, recent progress in developing targeted UCAs and phase-inversion contrast agents for diagnostic purposes is discussed. Finally, we review the present status and the critical issues for developing ultrasound-based multimodal imaging applications and summarize the existing challenges and future prospects.