Contrast enhancement by lipid-based MRI contrast agents in mouse atherosclerotic plaques; a longitudinal study

Self-gated, dynamic contrast-enhanced magnetic resonance imaging with compressed-sensing reconstruction for evaluating endothelial permeability in the aortic root of atherosclerotic mice

High-risk atherosclerotic plaques are characterized by active inflammation and abundant leaky microvessels. We present a self-gated, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) acquisition with compressed sensing reconstruction and apply it to assess longitudinal changes in endothelial permeability in the aortic root of Apoe-/- atherosclerotic mice during natural disease progression. Twenty-four, 8-week-old, female Apoe-/- mice were divided into four groups (n = 6 each) and imaged with self-gated DCE-MRI at 4, 8, 12, and 16 weeks after high-fat diet initiation, and then euthanized for CD68 immunohistochemistry for macrophages. Eight additional mice were kept on a high-fat diet and imaged longitudinally at the same time points. Aortic-root pseudo-concentration curves were analyzed using a validated piecewise linear model. Contrast agent wash-in and washout slopes (b1 and b2 ) were measured as surrogates of aortic root endothelial permeability and compared with macrophage density by immunohistochemistry. b2 , indicating contrast agent washout, was significantly higher in mice kept on an high-fat diet for longer periods of time (p = 0.03). Group comparison revealed significant differences between mice on a high-fat diet for 4 versus 16 weeks (p = 0.03). Macrophage density also significantly increased with diet duration (p = 0.009). Spearman correlation between b2 from DCE-MRI and macrophage density indicated a weak relationship between the two parameters (r = 0.28, p = 0.20). Validated piecewise linear modeling of the DCE-MRI data showed that the aortic root contrast agent washout rate is significantly different during disease progression. Further development of this technique from a single-slice to a 3D acquisition may enable better investigation of the relationship between in vivo imaging of endothelial permeability and atherosclerotic plaques' genetic, molecular, and cellular makeup in this important model of disease.

One-Pot, One-Step Synthesis of Drug-Loaded Magnetic Multimicelle Aggregates

Lipid-based formulations provide a nanotechnology platform that is widely used in a variety of biomedical applications because it has several advantageous properties including biocompatibility, reduced toxicity, relative ease of surface modifications, and the possibility for efficient loading of drugs, biologics, and nanoparticles. A combination of lipid-based formulations with magnetic nanoparticles such as iron oxide was shown to be highly advantageous in a growing number of applications including magnet-mediated drug delivery and image-guided therapy. Currently, lipid-based formulations are prepared by multistep protocols. Simplification of the current multistep procedures can lead to a number of important technological advantages including significantly decreased processing time, higher reaction yield, better product reproducibility, and improved quality. Here, we introduce a one-pot, single-step synthesis of drug-loaded magnetic multimicelle aggregates (MaMAs), which is based on controlled flow infusion of an iron oxide nanoparticle/lipid mixture into an aqueous drug solution under ultrasonication. Furthermore, we prepared molecular-targeted MaMAs by directional antibody conjugation through an Fc moiety using Cu-free click chemistry. Fluorescence imaging and quantification confirmed that antibody-conjugated MaMAs showed high cell-specific targeting that was enhanced by magnetic delivery.

Contrast agents for cardiovascular magnetic resonance imaging: an overview

Contrast agents for Cardiovascular Magnetic Resonance (CMR) play a major role in research and clinical cardiology.

Molecular imaging of inflammation in atherosclerosis

Acute rupture of vulnerable plaques frequently leads to myocardial infarction and stroke. Within the last decades, several cellular and molecular players have been identified that promote atherosclerotic lesion formation, maturation and plaque rupture. It is now widely recognized that inflammation of the vessel wall and distinct leukocyte subsets are involved throughout all phases of atherosclerotic lesion development. The mechanisms that render a stable plaque unstable and prone to rupture, however, remain unknown and the identification of the vulnerable plaque remains a major challenge in cardiovascular medicine. Imaging technologies used in the clinic offer minimal information about the underlying biology and potential risk for rupture. New imaging technologies are therefore being developed, and in the preclinical setting have enabled new and dynamic insights into the vessel wall for a better understanding of this complex disease. Molecular imaging has the potential to track biological processes, such as the activity of cellular and molecular biomarkers in vivo and over time. Similarly, novel imaging technologies specifically detect effects of therapies that aim to stabilize vulnerable plaques and silence vascular inflammation. Here we will review the potential of established and new molecular imaging technologies in the setting of atherosclerosis, and discuss the cumbersome steps required for translating molecular imaging approaches into the clinic.

Paramagnetic liposomes for molecular MRI and MRI-guided drug delivery

Liposomes are a versatile class of nanoparticles with tunable properties, and multiple liposomal drug formulations have been clinically approved for cancer treatment. In recent years, an extensive library of gadolinium (Gd)-containing liposomal MRI contrast agents has been developed for molecular and cellular imaging of disease-specific markers and for image-guided drug delivery. This review discusses the advances in the development and novel applications of paramagnetic liposomes in molecular and cellular imaging, and in image-guided drug delivery. A high targeting specificity has been achieved in vitro using ligand-conjugated paramagnetic liposomes. On targeting of internalizing cell receptors, the effective longitudinal relaxivity r1 of paramagnetic liposomes is modulated by compartmentalization effects. This provides unique opportunities to monitor the biological fate of liposomes. In vivo contrast-enhanced MRI studies with nontargeted liposomes have shown the extravasation of liposomes in diseases associated with endothelial dysfunction, such as tumors and myocardial infarction. The in vivo use of targeted paramagnetic liposomes has facilitated the specific imaging of pathophysiological processes, such as angiogenesis and inflammation. Paramagnetic liposomes loaded with drugs have been utilized for therapeutic interventions. MR image-guided drug delivery using such liposomes allows the visualization and quantification of local drug delivery.

Self-gated CINE MRI for combined contrast-enhanced imaging and wall-stiffness measurements of murine aortic atherosclerotic lesions

Background

High-resolution contrast-enhanced imaging of the murine atherosclerotic vessel wall is difficult due to unpredictable flow artifacts, motion of the thin artery wall and problems with flow suppression in the presence of a circulating contrast agent.

Methods and Results

We applied a 2D-FLASH retrospective-gated CINE MRI method at 9.4T to characterize atherosclerotic plaques and vessel wall distensibility in the aortic arch of aged ApoE^−/− mice after injection of a contrast agent. The method enabled detection of contrast enhancement in atherosclerotic plaques in the aortic arch after I.V. injection of micelles and iron oxides resulting in reproducible plaque enhancement. Both contrast agents were taken up in the plaque, which was confirmed by histology. Additionally, the retrospective-gated CINE method provided images of the aortic wall throughout the cardiac cycle, from which the vessel wall distensibility could be calculated. Reduction in plaque size by statin treatment resulted in lower contrast enhancement and reduced wall stiffness.

Conclusions

The retrospective-gated CINE MRI provides a robust and simple way to detect and quantify contrast enhancement in atherosclerotic plaques in the aortic wall of ApoE^−/− mice. From the same scan, plaque-related changes in stiffness of the aortic wall can be determined. In this mouse model, a correlation between vessel wall stiffness and atherosclerotic lesions was found.