Molecular evaluation of thrombosis using X-ray phase contrast imaging with microbubbles targeted to P-selectin in mice

In-situ synchrotron X-ray imaging of ultrasound (US)-generated bubbles: Influence of US frequency on microbubble cavitation for membrane fouling remediation

Gaining an in-depth understanding of the characteristics and dynamics of ultrasound (US)--generated bubbles is crucial to effectively remediate membrane fouling. The goal of present study is to conduct in-situ visualization of US-generated microbubbles in water to examine the influence of US frequency on the dynamics of microbubbles. This study utilized synchrotron in-line phase contrast imaging (In-line PCI) available at the biomedical imaging and therapy (BMIT) beamlines at the Canadian Light Source (CLS) to enhance the contrast of liquid/air interfaces at different US frequencies of 20, 28 and 40 KHz at 60 Watts. A high-speed camera was used to capture 2,000 frames per second of the bubble cavitation generated in water under the ultrasound influence. Key parameters at the polychromatic beamlines were optimized to maximize the phase contrast of gas/liquid of the microbubbles with a minimum size of 5.5 µm. ImageJ software was used to analyze the bubble characteristics and their behavior under the US exposure including the microbubble number, size, and fraction of the total area occupied by the bubbles at each US frequency. Furthermore, the bubble characteristics over the US exposure time and at different distances from the transducer were studied. The qualitative and quantitative data analyses showed that the microbubble number or size did not change over time; however, it was observed that most bubbles were created at the middle of the frames and close to the US field. The number of bubbles created under the US exposure increased with the frequency from 20 kHz to 40 kHz (about 4.6 times). However, larger bubbles were generated at 20 kHz such that the average bubble radius at 20 kHz was about 6.8 times of that at 40 kHz. Microbubble movement/traveling through water was monitored, and it was observed that the bubble velocity increased as the frequency was increased from 20 kHz to 40 kHz. The small bubbles moved faster, and the majority of them traveled upward towards the US transducer location. The growth pattern (a correlation between the mean growth ratio and the exposure time) of bubbles at 20 kHz and 60 W was obtained by tracking the oscillation of 22 representative microbubbles over the 700 ms of imaging. The mean growth ratio model was also obtained.

Developing a Microbubble-Based Contrast Agent for Synchrotron In-Line Phase Contrast Imaging

OBJECTIVE

X-ray phase contrast imaging generates contrast from refraction of X-rays, enhancing soft tissue contrast compared to conventional absorption-based imaging. Our goal is to develop a contrast agent for X-ray in-line phase contrast imaging (PCI) based on ultrasound microbubbles (MBs), by assessing size, shell material, and concentration.

METHODS

Polydisperse perfluorobutane-core lipid-shelled MBs were synthesized and size separated into five groups between 1 and 10 μm. We generated two size populations of polyvinyl-alcohol (PVA)-MBs, 2-3 μm and 3-4 μm, whose shells were either coated or integrated with iron oxide nanoparticles (SPIONs). Microbubbles were then embedded in agar at three concentrations: 5 × 10⁷, 5 × 10⁶ and 5 × 10⁵ MBs/ml. In-line phase contrast imaging was performed at the Canadian Light Source with filtered white beam micro-computed tomography. Phase contrast intensity was measured by both counting detectable MBs, and comparing mean pixel values (MPV) in minimum and maximum intensity projections of the overall samples.

RESULTS

Individual lipid-MBs 6-10 μm, lipid-MBs 4-6 μm and PVA-MBs coated with SPIONs were detectable at each concentration. At the highest concentration, lipid-MBs 6-10 μm and 4-6 μm showed an overall increase in positive contrast, whereas at a moderate concentration, only lipid-MBs 6-10 μm displayed an increase. Negative contrast was also observed from two largest lipid-MBs at high concentration.

CONCLUSION

These data indicate that lipid-MBs larger than 4 μm are candidates for PCI, and 5 × 10⁶ MBs/ml may be the lowest concentration suitable for generating visible phase contrast in vivo.

SIGNIFICANCE

Identifying a suitable MB for PCI may facilitate future clinical translation.

Development of a Translatable Ultrasound Molecular Imaging Agent for Inflammation

This study details the development, characterization and non-clinical efficacy of an ultrasound molecular imaging agent intended for molecular imaging of P-selectin in humans. A targeting ligand based on a recently discovered human selectin ligand was manufactured as fusion protein, and activity for human and mouse P- and E-selectin was evaluated by functional immunoassay. The targeting ligand was covalently conjugated to a lipophilic anchor inserted into a phospholipid microbubble shell. Three lots of the targeted microbubble drug product, TS-07-009, were produced, and assays for size distribution, zeta potential and morphology were established. The suitability of TS-07-009 as a molecular imaging agent was evaluated in vitro in a flow-based adhesion assay and in vivo using a canine model of transient myocardial ischemia. Selectivity for P-selectin over E-selectin was observed in both the human and murine systems. Contrast agent adhesion increased with P-selectin concentration in a dynamic adhesion assay. Significant contrast enhancement was observed on ultrasound imaging with TS-07-009 in post-ischemic canine myocardium at 30 or 90 min of re-perfusion. Negligible enhancement was observed in resting (no prior ischemia) hearts or with a control microbubble 90 min after ischemia. The microbubble contrast agent described here exhibits physiochemical properties and in vivo behavior suitable for development as a clinical imaging agent.

Polydopamine-modified dual-ligand nanoparticles as highly effective and targeted magnetic resonance/photoacoustic dual-modality thrombus imaging agents

Background

Platelet activation and subsequent aggregation are the initial stages of thrombosis. A molecular probe that specifically targets activated platelets and remains retained under high shear stress in vivo can enhance the imaging effect to achieve early and accurate diagnosis.

Methods and materials

In this study, we constructed nanoparticles (NPs) using polydopamine to carry two peptides that simultaneously bind integrin αIIbβ3 and P-selectin on activated platelets to enhance the targeting of NPs to thrombus.

Results

The targeting specificity and binding stability of the NPs on red and white thrombi were demonstrated in vitro using a simulated circulatory device and the targeting effect of the NPs on mixed thrombus was studied by  magnetic resonance (MR)/photoacoustic (PA) dual-modality imaging in vivo. NPs that were surface modified with both peptides have higher selectivity and retention to red and white thrombi in vitro than NPs with a single or no peptide, and the targeting effect was closely related to the number and distribution of activated platelets as well as the structure and type of thrombus. The NPs also have MR/PA dual-modality imaging functionality, significantly enhancing the imaging of mixed thrombus in vivo.

Conclusion

These dual-targeted NPs have improved targeting specificity and binding stability to different thrombi under high shear stress and are beneficial for the early diagnosis of thrombosis.

Microbubbles containing gadolinium as contrast agents for both phase contrast and magnetic resonance imaging

Portal vein imaging is an important method for investigating portal venous disorders. However, the diagnostic requirements are not usually satisfied when using single imaging techniques. Diagnostic accuracy can be improved by combining different imaging techniques. Contrast agents that can be used for combined imaging modalities are needed. In this study, the feasibility of using microbubbles containing gadolinium (MCG) as contrast agents for both phase contrast imaging (PCI) and magnetic resonance imaging (MRI) are investigated. MCG were made by encapsulating sulfur hexafluoride (SF₆) gas with gadolinium and lyophilized powder. Absorption contrast imaging (ACI) and PCI of MCG were performed and compared in vitro. MCG were injected into the main portal trunk of living rats. PCI and MRI were performed at 2 min and 10 min after MCG injection, respectively. PCI exploited the differences in the refractive index and visibly showed the MCG, which were not detectable by ACI. PCI could facilitate clear revelation of the MCG-infused portal veins. The diameter of the portal veins could be determined by the largest MCG in the same portal vein. The minimum diameter of clearly detected portal veins was about 300 µm by MRI. These results indicate that MCG could enhance both PCI and MRI for imaging portal veins. The detection sensitivity of PCI and MRI could compensate for each other when using MCG contrast agents for animals.