Magnetic resonance imaging of atherosclerotic plaque with ultrasmall superparamagnetic particles of iron oxide in hyperlipidemic rabbits

Atherosclerotic plaque composition: analysis with multicolor CT and targeted gold nanoparticles

PURPOSE

To investigate the potential of spectral computed tomography (CT) (popularly referred to as multicolor CT), used in combination with a gold high-density lipoprotein nanoparticle contrast agent (Au-HDL), for characterization of macrophage burden, calcification, and stenosis of atherosclerotic plaques.

MATERIALS AND METHODS

The local animal care committee approved all animal experiments. A preclinical spectral CT system in which incident x-rays are divided into six different energy bins was used for multicolor imaging. Au-HDL, an iodine-based contrast agent, and calcium phosphate were imaged in a variety of phantoms. Apolipoprotein E knockout (apo E-KO) mice were used as the model for atherosclerosis. Gold nanoparticles targeted to atherosclerosis (Au-HDL) were intravenously injected at a dose of 500 mg per kilogram of body weight. Iodine-based contrast material was injected 24 hours later, after which the mice were imaged. Wild-type mice were used as controls. Macrophage targeting by Au-HDL was further evaluated by using transmission electron microscopy and confocal microscopy of aorta sections.

RESULTS

Multicolor CT enabled differentiation of Au-HDL, iodine-based contrast material, and calcium phosphate in the phantoms. Accumulations of Au-HDL were detected in the aortas of the apo E-KO mice, while the iodine-based contrast agent and the calcium-rich tissue could also be detected and thus facilitated visualization of the vasculature and bones (skeleton), respectively, during a single scanning examination. Microscopy revealed Au-HDL to be primarily localized in the macrophages on the aorta sections; hence, the multicolor CT images provided information about the macrophage burden.

CONCLUSION

Spectral CT used with carefully chosen contrast agents may yield valuable information about atherosclerotic plaque composition.

Biomimetic MRI contrast agent for imaging of inflammation in atherosclerotic plaque of ApoE-/- mice: a pilot study

OBJECTIVE

Atherosclerosis involves an inflammatory process characterized by cellular and molecular responses. A slow-clearance blood-pool paramagnetic agent (CMD-A2-Gd-DOTA: P717) chemically modified to create a functionalized product (F-P717) for targeting inflammation in vessel walls was evaluated in vivo in mice.

METHODS AND RESULTS

Carboxylate and sulfate groups were grafted onto the macromolecular paramagnetic Gd-DOTA-dextran backbone. Products were also fluorescently labeled with rhodamine isothiocyanate. Pre- and postcontrast MRI was performed on a 2-Tesla magnet in ApoE-/- and control C57BL/6 mice after P717 or F-P717 injection at a dose of 60 micromol Gd/kg. Axial T1-weighted images of the abdominal aorta were obtained using a 2D multislice spin-echo sequence. F-P717 significantly enhanced the magnetic resonance imaging (MRI) signal in the abdominal aortic wall of ApoE-/- mice (>50% signal-to-noise ratio increase between 10 and 30 minutes), but not of control mice. P717 produced only moderate (<20%) MRI signal enhancement within the same time frame. The MRI data were correlated to histopathology. Immunofluorescence in ApoE-/- mice colocalized F-P717 but not P717 with the inflammatory area revealed by P-selectin labeling.

CONCLUSION

This study demonstrates the efficacy of F-P717 as a new molecular imaging agent for noninvasive in vivo MRI location of inflammatory vascular tree lesions in ApoE-/- mice.

Imaging atherosclerosis and vulnerable plaque

Identifying patients at high risk for an acute cardiovascular event such as myocardial infarction or stroke and assessing the total atherosclerotic burden are clinically important. Currently available imaging modalities can delineate vascular wall anatomy and, with novel probes, target biologic processes important in plaque evolution and plaque stability. Expansion of the vessel wall involving remodeling of the extracellular matrix can be imaged, as can angiogenesis of the vasa vasorum, plaque inflammation, and fibrin deposits on early nonocclusive vascular thrombosis. Several imaging platforms are available for targeted vascular imaging to acquire information on both anatomy and pathobiology in the same imaging session using either hybrid technology (nuclear combined with CT) or MRI combined with novel probes targeting processes identified by molecular biology to be of importance. This article will discuss the current state of the art of these modalities and challenges to clinical translation.

[State of the art: high resolution MR imaging of carotid atherosclerotic plaque]

A third of cerebrovascular accidents are a complication from carotid artery plaque. In addition to the degree of stenosis, plaque composition and morphology are key elements in determining the probability of complication from the atherosclerotic plaque. High resolution MRI can characterize plaque composition and morphology and therefore help identify unstable plaque. The purpose of this review is to summarize recent concepts on unstable plaque and underlying inflammation. The signal characteristics of the different components of plaque on high resolution MRI then be reviewed. Finally, current morphological and functional criteria for unstable plaque will be discussed.

Cardiovascular MRI in small animals

Imaging studies of cardiovascular disease in small rodents have become a prerequisite in preclinical cardiovascular research. Transgenic and gene-knockout models of cardiovascular diseases enables the investigation of the influence of single genes or groups of genes on disease pathogenesis. In addition, experimental and genetically altered models provide valuable in vivo platforms to investigate the efficacy of novel drugs and contrast agents. Owing to the excellent soft tissue contrast, high spatial and temporal resolution, as well as the tomographic nature of MRI, anatomy and function can be assessed with unique accuracy and reproducibility. Furthermore, using novel targeted MRI contrast agents, molecular changes associated with cardiovascular disease can be investigated in the same imaging session. This review focuses on recent advances in hardware, imaging sequences and probe design.

Early identification of aortic valve sclerosis using iron oxide enhanced MRI

PURPOSE

To test the ability of MION-47 enhanced MRI to identify tissue macrophage infiltration in a rabbit model of aortic valve sclerosis (AVS).

MATERIALS AND METHODS

The aortic valves of control and cholesterol-fed New Zealand White rabbits were imaged in vivo pre- and 48 h post-intravenous administration of MION-47 using a 1.5 Tesla (T) MR clinical scanner and a CINE fSPGR sequence. MION-47 aortic valve cusps were imaged ex vivo on a 3.0T whole-body MR system with a custom gradient insert coil and a three-dimensional (3D) FIESTA sequence and compared with aortic valve cusps from control and cholesterol-fed contrast-free rabbits. Histopathological analysis was performed to determine the site of iron oxide uptake.

RESULTS

MION-47 enhanced the visibility of both control and cholesterol-fed rabbit valves in in vivo images. Ex vivo image analysis confirmed the presence of significant signal voids in contrast-administered aortic valves. Signal voids were not observed in contrast-free valve cusps. In MION-47 administered rabbits, histopathological analysis revealed iron staining not only in fibrosal macrophages of cholesterol-fed valves but also in myofibroblasts from control and cholesterol-fed valves.

CONCLUSION

Although iron oxide labeling of macrophage infiltration in AVS has the potential to detect the disease process early, a macrophage-specific iron compound rather than passive targeting may be required.

Nanomedicine strategies for molecular targets with MRI and optical imaging

The science of 'theranostics' plays a crucial role in personalized medicine, which represents the future of patient management. Over the last decade an increasing research effort has focused on the development of nanoparticle-based molecular-imaging and drug-delivery approaches, emerging as a multidisciplinary field that shows promise in understanding the components, processes, dynamics and therapies of a disease at a molecular level. The potential of nanometer-sized agents for early detection, diagnosis and personalized treatment of diseases is extraordinary. They have found applications in almost all clinically relevant biomedical imaging modality. In this review, a number of these approaches will be presented with a particular emphasis on MRI and optical imaging-based techniques. We have discussed both established molecular-imaging approaches and recently developed innovative strategies, highlighting the seminal studies and a number of successful examples of theranostic nanomedicine, especially in the areas of cardiovascular and cancer therapy.

MR plaque imaging of the carotid artery

Atherosclerotic carotid plaque represents a major cause of cerebral ischemia. The detection of vulnerable plaque is important for preventing future cardiovascular events. The key factors in advanced plaque that are most likely to lead to patient complications are the condition of the fibrous cap, the size of the necrotic core and hemorrhage, and the extent of inflammatory activity within the plaque. Magnetic resonance (MR) imaging has excellent soft tissue contrast and can allow for a more accurate and objective estimation of carotid wall morphology and plaque composition. Recent advances in MR imaging techniques have permitted serial monitoring of atherosclerotic disease evolution and the identification of intraplaque risk factors for accelerated progression. The purpose of this review article is to review the current state of techniques of carotid wall MR imaging and the characterization of plaque components and surface morphology with MR imaging, and to describe the clinical practice of carotid wall MR imaging for the determination of treatment plan.

Recent developments and new perspectives on imaging of atherosclerotic plaque: role of anatomical, cellular and molecular MRI part III

Atherosclerotic plaque disruption accounts for the major part of cardiovascular mortality and the risk of disruption appears to depend on plaque composition. Carotid plaques in patients, scheduled for endarterectomy, have been successfully characterised with MRI. MRI has the advantage of combining information about morphology and function. Unfortunately, the tortuosity and size of the coronary arteries, and the respiratory and cardiac motion hinder the in vivo characterisation of human coronary plaque. In addition to plaque composition several molecular markers of the different processes involved in atherosclerosis, such as integrins, matrix metalloproteinases and fibrin seem to correlate with risk of plaque rupture and clinical outcome. These molecular markers can be targeted with antibodies coupled to carriers, which are loaded with gadolinium for detection (molecular MRI). Several cellular/molecular MRI studies in animal models and some in human patients have been conducted with varying levels of success. The advent of clinical high field magnets, the development of contrast agent carriers with high relaxivity and the development of relatively new MR contrast techniques appear to be promising in the field of plaque imaging. Future MRI studies will have to focus on the molecular target of the atherosclerotic process, which has the highest prognostic value with regard to acute coronary syndromes and on the most suitable contrast agent to visualize that target.

Imaging of the unstable plaque: how far have we got?

Rupture of unstable plaques may lead to myocardial infarction or stroke and is the leading cause of morbidity and mortality in western countries. Thus, there is a clear need for identifying these vulnerable plaques before the rupture occurs. Atherosclerotic plaques are a challenging imaging target as they are small and move rapidly, especially in the coronary tree. Many of the currently available imaging tools for clinical use still provide minimal information about the biological characteristics of plaques, because they are limited with respect to spatial and temporal resolution. Moreover, many of these imaging tools are invasive. The new generation of imaging modalities such as magnetic resonance imaging, nuclear imaging such as positron emission tomography and single photon emission computed tomography, computed tomography, fluorescence imaging, intravascular ultrasound, and optical coherence tomography offer opportunities to overcome some of these limitations. This review discusses the potential of these techniques for imaging the unstable plaque.

Assessment of macrophage infiltration in a murine model of abdominal aortic aneurysm

PURPOSE

To evaluate the use of an ultrasmall superparamagnetic iron oxide (USPIO) contrast agent as a marker for the detection of macrophage in a preclinical abdominal aortic aneurysm animal (AAA) model.

MATERIALS AND METHODS

Osmotic pumps were implanted subcutaneously in apoE(-/-) mice for continuous infusion of Angiotensin II (Ang-II). Weekly bright-blood gradient echo scans were performed on the suprarenal abdominal aorta to evaluate aneurysm development. Once an AAA was detected, animals were administered 1000 mumol/kg of the USPIO contrast agent ferumoxtran-10 (Combidex) followed by in vivo scanning 24 h post-USPIO administration. After in vivo imaging, aortas were harvested for ex vivo imaging, histology, iron quantification, and gene expression analysis.

RESULTS

Reduced signal intensity was evident in the post-USPIO transverse images of the abdominal aorta. The areas of reduced signal were primarily along the aneurysm shoulder and outer perianeurysm areas and corresponded to regions of macrophage infiltration and colocalized USPIO determination by means of histological staining. The absolute iron content measured significantly correlated to the area of signal reduction in the ex vivo images (r = 0.9; P < 0.01). In the AAA tissue, the macrophage-driven cytokine gene expression was up-regulated along with a matrix metalloproteinase known to mediate extracellular matrix breakdown in this disease model.

CONCLUSION

These results demonstrate the feasibility of using an USPIO contrast agent as a surrogate for detecting the acute inflammatory process involved in the development of abdominal aneurysms.

In vivo detection of inflammation using pegylated iron oxide particles targeted at E-selectin: a multimodal approach using MR imaging and EPR spectroscopy

OBJECTIVES

Ultrasmall particles of iron oxide (USPIO) possess superparamagnetic properties and are used as negative contrast agent in magnetic resonance imaging (MRI) because of their strong T(2) and T(2)* effects. Besides this method, electron paramagnetic resonance (EPR) offers the unique capability to quantify these particles. The objective of this study was to evaluate a molecular marker for non invasive diagnosis and monitoring of inflammation. During inflammation cell adhesion molecules such as E-selectin are expressed on the endothelial cell surface. An E-selectin ligand was coupled to pegylated USPIO particles.

MATERIALS AND METHODS

Inflammation was induced by intramuscular injection of Freund's Complete Adjuvant in male NMRI mice. After intravenous injection of grafted or ungrafted USPIO particles, iron concentration in inflamed muscles was quantified ex vivo by X-band EPR. Particle accumulation was also assessed in vivo by L-Band EPR, as well as by T(2)-weighted MRI.

RESULTS

We determined the mean iron oxide concentration in inflamed muscles after injection of grafted or ungrafted UPSIO particles, which was 0.8% and 0.4% of the initially injected dose, respectively. By L-band EPR, we observed that the concentration of the grafted USPIO particles in inflamed muscles was twice higher than for the ungrafted particles. Using MRI experiments, a higher signal loss was clearly observed in the inflamed muscle when grafted particles were injected in comparison with the ungrafted particles.

CONCLUSION

Even taking into account a non specific accumulation of iron oxides, the targeting of USPIO particles with E-selectin ligands significantly improved the sensitivity of detection of inflamed tissues.

Coronary MR imaging: lumen and wall

Coronary MR imaging is a promising noninvasive technique for the combined assessment of coronary artery anatomy and function. Anomalous coronary arteries and aneurysms can reliably be assessed in clinical practice using coronary MR imaging and the presence of significant left main or proximal multivessel coronary artery disease detected. Technical challenges that need to be addressed are further improvements in motion suppression and abbreviated scanning times aimed at improving spatial resolution and patient comfort. The development of new and specific contrast agents, high-field MR imaging with improved spatial resolution, and continued progress in MR imaging methods development will undoubtedly lead to further progress toward the noninvasive and comprehensive assessment of coronary atherosclerotic disease.

Serial contrast-enhanced cardiac magnetic resonance imaging demonstrates regression of hyperenhancement within the coronary artery wall in patients after acute myocardial infarction

OBJECTIVES

Our aim was to determine whether serial contrast-enhanced cardiac magnetic resonance (CE-CMR) is useful for the characterization of tissue signal changes within the coronary vessel wall in patients after acute myocardial infarction (AMI).

BACKGROUND

Inflammation plays a key role in the development of AMI. CE-CMR of the vessel wall has been found useful for the characterization of inflammatory tissue signal changes in patients with carotid artery stenosis, giant cell arteritis, or Takayasu's arteritis; however, it has never been serially performed in the coronary artery wall in patients with acute and chronic myocardial infarction using a gadolinium-based contrast medium and compared with systemic markers of inflammation.

METHODS

CE-CMR using a T1-weighted 3-dimensional gradient echo inversion recovery sequence of the coronary artery wall and 0.2 mmol/kg of gadolinium-diethylenetriaminepentaacetic acid was performed in 10 patients with AMI 6 days and 3 months after coronary intervention and in 9 subjects without coronary artery disease on invasive coronary angiography. Contrast-to-noise ratio (CNR) within the coronary artery wall was quantified in comparison with blood signal.

RESULTS

Patients with AMI demonstrated a significantly increased coronary vessel wall enhancement 6 days after infarction compared with normal subjects (CNR 7.8 +/- 4.4 vs. 5.3 +/- 3.2, p < 0.001). Three months after infarction, CNR decreased to 6.5 +/- 4.7 (p < 0.03). This decrease paralleled declines in C-reactive protein. Angiographically normal segments showed no contrast changes, but CNR significantly decreased in stenotic segments, from 10.9 +/- 3.8 to 6.8 +/- 5.0 (p < 0.002), resulting in a reduction of enhanced segments from 70% to 25% (p < 0.01).

CONCLUSIONS

Serial CE-CMR identified changes in spatial extent and intensity of coronary contrast enhancement in patients after AMI. This technique may be useful for the characterization of transient coronary tissue signal changes, which may represent edema or inflammation during the post-infarction phase. In addition, CE-CMR may offer the potential for visualization of inflammatory activity in atherosclerosis associated with acute coronary syndromes.

Inflammation imaging in atherosclerosis

Inflammation is important at many stages of atherosclerotic plaque development. We highlight several imaging modalities that can quantify the degree of plaque inflammation noninvasively. Imaging of this type might allow testing of novel antiatherosclerosis drugs, identification of patients at risk of plaque rupture, and deeper insight into the biology of the disease. The imaging modalities are discussed in relation to their potential use in these areas.

Coronary computed tomography and magnetic resonance imaging

Cardiac computed tomography and magnetic resonance are relatively new imaging modalities that can exceed the ability of established imaging modalities to detect present pathology or predict patient outcomes. Coronary calcium scoring may be useful in asymptomatic patients at intermediate risk. Computed tomographic coronary angiography is a first-line indication to evaluate congenitally abnormal coronary arteries and, along with stress magnetic resonance myocardial perfusion imaging, is useful in symptomatic patients with nondiagnostic conventional stress tests. Cardiac magnetic resonance is indicated for visualizing cardiac structure and function, and delayed enhancement magnetic resonance is a first-line indication for assessing myocardial viability. Imaging plaque and molecular mechanisms related to plaque rupture holds great promise for the presymptomatic detection of patients at risk for coronary events but is not yet suitable for routine clinical use.

Fast low-angle positive contrast steady-state free precession imaging of USPIO-labeled macrophages: theory and in vitro experiment

The feasibility of imaging macrophages labeled with ultrasmall superparamagnetic iron-oxide nanoparticles (USPIO) with fast low-angle positive contrast steady-state free precession (FLAPS) was investigated through theory and in vitro experiment. Human macrophage cells were labeled with USPIO and imaged at 1.5 T. The metric "visibility," which combines magnitude and spatial extent of positive contrast, was used to evaluate the images. Negative contrast steady-state free precession (SSFP) and gradient-echo (GRE) imaging were also evaluated. Positive contrast was observed for relatively high concentrations of labeled cells for flip angles less than alpha=25 degrees . Theoretical and experimental results indicate that positive visibility (VIS(POS)) was maximized at alpha=10 degrees and 15 degrees. Low flip angle SSFP also provided negative contrast comparable to standard SSFP and GRE imaging. Results suggest that USPIO-labeled macrophages are capable of producing the conditions necessary for positive contrast with FLAPS at clinical field strength (1.5 T) and resolution (0.8x0.8x3 mm(3)).

The ATHEROMA (Atorvastatin Therapy: Effects on Reduction of Macrophage Activity) Study. Evaluation using ultrasmall superparamagnetic iron oxide-enhanced magnetic resonance imaging in carotid disease

OBJECTIVES

The aim of this study was to evaluate the effects of low-dose (10 mg) and high-dose (80 mg) atorvastatin on carotid plaque inflammation as determined by ultrasmall superparamagnetic iron oxide (USPIO)-enhanced carotid magnetic resonance imaging (MRI). The hypothesis was that treatment with 80 mg atorvastatin would demonstrate quantifiable changes in USPIO-enhanced MRI-defined inflammation within the first 3 months of therapy.

BACKGROUND

Preliminary studies indicate that USPIO-enhanced MRI can identify macrophage infiltration in human carotid atheroma in vivo and hence may be a surrogate marker of plaque inflammation.

METHODS

Forty-seven patients with carotid stenosis >40% on duplex ultrasonography and who demonstrated intraplaque accumulation of USPIO on MRI at baseline were randomly assigned in a balanced, double-blind manner to either 10 or 80 mg atorvastatin daily for 12 weeks. Baseline statin therapy was equivalent to 10 mg of atorvastatin or less. The primary end point was change from baseline in signal intensity (DeltaSI) on USPIO-enhanced MRI in carotid plaque at 6 and 12 weeks.

RESULTS

Twenty patients completed 12 weeks of treatment in each group. A significant reduction from baseline in USPIO-defined inflammation was observed in the 80-mg group at both 6 weeks (DeltaSI 0.13; p = 0.0003) and at 12 weeks (DeltaSI 0.20; p < 0.0001). No difference was observed with the low-dose regimen. The 80-mg atorvastatin dose significantly reduced total cholesterol by 15% (p = 0.0003) and low-density lipoprotein cholesterol by 29% (p = 0.0001) at 12 weeks.

CONCLUSIONS

Aggressive lipid-lowering therapy over a 3-month period is associated with significant reduction in USPIO-defined inflammation. USPIO-enhanced MRI methodology may be a useful imaging biomarker for the screening and assessment of therapeutic response to "anti-inflammatory" interventions in patients with atherosclerotic lesions. (Effects of Atorvastatin on Macrophage Activity and Plaque Inflammation Using Magnetic Resonance Imaging [ATHEROMA]; NCT00368589).

Cellular magnetic resonance imaging: potential for use in assessing aspects of cardiovascular disease

There is rapidly increasing interest in the use of magnetic resonance imaging (MRI) to track cell migration in vivo. Iron oxide MR contrast agents can be detected at micromolar concentrations of iron, and offer sufficient sensitivity for T2*-weighted imaging. Cellular MRI shows potential for assessing aspects of cardiovascular disease. Labeling in vivo and tracking macrophages using iron oxide nanoparticles has been a goal for cellular MRI because macrophages play a pivotal role in the pathophysiology of many human diseases, including atherosclerosis. Cellular MRI has also been using to track transplanted therapeutic cells in myocardial regeneration. This review looked at iron oxide nanoparticles, methods of cell labeling, image acquisition techniques and limitations encountered for visualization. Particular attention was paid to stem cells and macrophages for the cardiovascular system.

Quantification of inflammation within rabbit atherosclerotic plaques using the macrophage-specific CT contrast agent N1177: a comparison with 18F-FDG PET/CT and histology

Macrophages play a key role in atherosclerotic plaque rupture. The iodine-based contrast agent N1177 accumulates in macrophages, allowing for their detection with CT. In this study, we tested whether the intensity of enhancement detected with CT in the aortic wall of rabbits injected with N1177 correlated with inflammatory activity evaluated with (18)F-FDG PET/CT and macrophage density on histology.

METHODS

Atherosclerotic plaques were induced in the aorta of New Zealand White rabbits (n = 7) by a repeated balloon injury (4 wk apart) and 4 mo of hyperlipemic diet. Noninjured rabbits, fed a chow diet, were used as controls (n = 3). A CT scan of the aorta (n = 10) was acquired in each rabbit before, during, and at 2 h after intravenous injection of N1177 (250 mg of iodine/kg). One week later, the same rabbits underwent PET/CT 3 h after injection of (18)F-FDG (37 MBq/kg [1 mCi/kg]). CT enhancement was calculated as the difference in aortic wall densities between images obtained before and images obtained at 2 h after injection of N1177. Mean standardized uptake values were measured on PET axial slices of the aorta in regions of interest encompassing the vessel wall. Macrophage density was measured by immunohistology (anti-RAM-11 antibody) on corresponding aortic cross-sections.

RESULTS

N1177-enhanced CT measured stronger enhancement in the aortic wall of atherosclerotic rabbits than in control rabbits (10.0 +/- 5.2 vs. 2.0 +/- 2.1 Hounsfield units, respectively; P < 0.05). After the injection of (18)F-FDG, PET detected higher standardized uptake values in the aortic wall of atherosclerotic rabbits than in control rabbits (0.61 +/- 0.12 vs. 0.21 +/- 0.02; P < 0.05). The intensity of enhancement in the aortic wall measured with CT after injection of N1177 correlated with (18)F-FDG uptake on PET/CT (r = 0.61, P < 0.001) and macrophage density on immunohistology (r = 0.63, P < 0.001).

CONCLUSION

The intensity of enhancement detected with CT in the aortic wall of rabbits injected with N1177 correlates with intense uptake of (18)F-FDG measured with PET and with macrophage density on histology, suggesting a role for N1177 in noninvasive identification of high-risk atherosclerotic plaques with CT.

Coronary CT angiography (CCTA) and advances in CT plaque imaging

The goal of this review is to highlight current advances in the non-invasive detection of clinically significant atherosclerotic disease including the so-called vulnerable plaque with computed tomography. Atherosclerotic disease encompasses stages of plaque progression, stabilization, and even regression. Traditionally, the focus of diagnostic imaging has been the detection of lumen-occluding atheroma. However, advances in our understanding of the pathophysiology of atherosclerotic plaque have shown that, in certain stages of plaque progression, plaque is "vulnerable" and able to cause acute coronary syndromes despite "non-significant" vascular occlusion at baseline. This provides a rationale to improve our non-invasive imaging technology. Presented here are improvements in soft-tissue resolution with technical advancements as well as contrast-enhancement and lately even nanotechnology-based technology which are geared to detect the clinically elusive vulnerable plaque and provide an opportunity for preventative therapy.

In vivo imaging of stem cells and Beta cells using direct cell labeling and reporter gene methods

Cellular transplantation therapy offers a means to stimulate cardiovascular repair either by direct (graft-induced) or indirect (host-induced) tissue regeneration or angiogenesis. Typically, autologous or donor cells of specific subpopulations are expanded exogenously before administration to enrich the cells most likely to participate in tissue repair. In animal models of cardiovascular disease, the fate of these exogenous cells can be determined using histopathology. Recently, methods to label cells with contrast agents or transduce cells with reporter genes to produce imaging beacons has enabled the serial and dynamic assessment of the survival, fate, and engraftment of these cells with noninvasive imaging. Although cell tracking methods for cardiovascular applications have been most studied in stem or progenitor cells, research in tracking of whole islet transplants and particularly insulin producing beta cells has implications to the cardiovascular community attributable to the vascular changes associated with diabetes mellitus. In this review article, we will explore some of the state-of-the art methods for stem, progenitor, and beta cell tracking.

Nanomedicine: perspective and promises with ligand-directed molecular imaging

Molecular imaging and targeted drug delivery play an important role toward personalized medicine, which is the future of patient management. Of late, nanoparticle-based molecular imaging has emerged as an interdisciplinary area, which shows promises to understand the components, processes, dynamics and therapies of a disease at a molecular level. The unprecedented potential of nanoplatforms for early detection, diagnosis and personalized treatment of diseases, have found application in every biomedical imaging modality. Biological and biophysical barriers are overcome by the integration of targeting ligands, imaging agents and therapeutics into the nanoplatform which allow for theranostic applications. In this article, we have discussed the opportunities and potential of targeted molecular imaging with various modalities putting a particular emphasis on perfluorocarbon nanoemulsion-based platform technology.

Contrast agents for MRI

Molecular imaging is a rapidly growing field with the potential to revolutionize cardiovascular medicine by shifting diagnostic focus from functional abnormalities which occur late in a disease process to the biochemical events which precipitate the earliest stages of disease. MRI is a modality well suited to this task as it allows a variety of contrast mechanisms for detection of epitopes of interest, as well as high-resolution anatomical localization and functional information. In this review, we discuss the widerange of available molecular MRI contrast agents and their application to diseases such as atherosclerosis, thrombus imaging, and stem cell tracking, along with opportunities for molecularly targeted drug delivery.

MR-optical imaging of cardiovascular molecular targets

Our understanding of the intricate inflammation biology underlying atherosclerosis is rapidly progressing. Molecular imaging strategies, harnessing this body of knowledge, have been developed to visualize some key cellular and molecular events in plaque evolution and vulnerability. Here, we discuss recent advances in magnetic resonance and fluorescence imaging of key biomarkers including adhesion molecules, inflammatory cells, and enzyme activity. We discuss strengths and limitations of respective imaging technologies, and comment on the potential of multi-modality imaging approaches.

Cardiovascular diseases as target for imaging

Recent pathophysiological findings have lead to new concepts to identify patients at risk for cardiovascular disease using systemic serum markers or new imaging methodology. New probe technology and progress in imaging techniques have set the base for development of molecular imaging concepts in the cardiovascular systems. The aim of these new imaging techniques is the detection of active biological processes in cardiovascular systems combining specific probes with contrast agents for MRI, SPECT or PET. There are promising strategies mostly in preclinical tests, which will prove clinical applicability in the near future.

Imaging circulating cells and lymphoid tissues with iron oxide nanoparticles

The use of nanometer-sized iron oxide nanoparticles and micron-sized iron oxide particles as magnetic resonance (MR) contrast agents has garnered a high degree of interest in diverse areas of biology and medicine. Applications such as cell tracking, molecular imaging, gene detection, and lymphography are being explored to provide insight into disease mechanisms, monitor therapeutic efficacy, and facilitate diagnostic imaging. What makes iron oxide so appealing is a number of favorable properties including high detectability by MR, biodegradability and low toxicity. Here we describe the recent progress on the use of magnetic nanoparticles in imaging circulating cells and lymphoid tissues. The study of the lymph system and the biodistribution of various circulating immune cells is important in the diagnosis, prognosis, and treatment of a wide range of diseases and is expected to have a profound effect on patient outcome.

Nanotechnology in medical imaging: probe design and applications

Nanoparticles have become more and more prevalent in reports of novel contrast agents, especially for molecular imaging, the detection of cellular processes. The advantages of nanoparticles include their potency to generate contrast, the ease of integrating multiple properties, lengthy circulation times, and the possibility to include high payloads. As the chemistry of nanoparticles has improved over the past years, more sophisticated examples of nano-sized contrast agents have been reported, such as paramagnetic, macrophage targeted quantum dots or alpha(v)beta(3)-targeted, MRI visible microemulsions that also carry a drug to suppress angiogenesis. The use of these particles is producing greater knowledge of disease processes and the effects of therapy. Along with their excellent properties, nanoparticles may produce significant toxicity, which must be minimized for (clinical) application. In this review we discuss the different factors that are considered when designing a nanoparticle probe and highlight some of the most advanced examples.

Imaging of the vulnerable plaque: noninvasive and invasive techniques

In a large proportion of previously asymptomatic individuals, sudden coronary death or acute myocardial infarction occurs as the first manifestation of coronary atherosclerosis. Imaging of coronary atheromatous plaques has traditionally centered on assessing the degree of luminal stenosis. The angiographic techniques that are routinely used to identify stenotic atherosclerotic lesions are unable to identify high-risk plaques; plaques prone to rupture and cause a cardiovascular event. This is partly due to the fact that the majority of culprit lesions that produce acute cardiovascular syndromes are not severely stenotic, possibly due to significant positive remodeling and reduced protective collateral circulation as well as because the risk of plaque rupture is more closely related to plaque content than plaque size. Recently, the focus of new imaging techniques is to identify the high risk plaques; the "vulnerable plaques." In this review, we will refer to the noninvasive and invasive techniques that can detect the vulnerable plaque.

Photothermal response of superparamagnetic iron oxide nanoparticles

BACKGROUND AND OBJECTIVE

Superparamagnetic iron oxide nanoparticles have been used as MRI contrast agents in medical imaging. The purpose of this study was to explore the photothermal response of superparamagnetic iron oxide nanoparticles for biomedical applications.

STUDY DESIGN/MATERIALS AND METHODS

Absorbance, temperature increase, and optical path length change of solutions of superparamagnetic iron oxide nanoparticles, SPIO and MION, in response to a 532 nm pulsed laser irradiation were measured.

RESULTS

Both SPIO and MION showed absorption at 532 nm, temperature increase, and optical path length change. SPIO and MION underwent selective heating due to absorption of laser energy (532 nm).

CONCLUSION

Temperature increase and optical path length change of SPIO and MION in response to 532 nm pulsed laser irradiation demonstrate a potential application of these particles in biomedical imaging. For further study, additional experiments applying the photothermal response of SPIO and MION in tissues are required.

Multimodality nanotracers for cardiovascular applications

Targeted imaging and therapeutics is becoming a field of prime importance in the study and treatment of cardiovascular disease; it promises to enable early diagnosis, promote improved understanding of pathology, and offer a way to improve therapeutic efficacy. Agents, particularly for cardiovascular disease, have been reported to permit the in vivo imaging, by multiple modalities, of macrophages, vascular targets such as vascular cell adhesion molecule 1, and markers for angiogenesis such as alpha(v)beta(3) integrin. In this Article, we first discuss the general concept of multimodality nanoparticles and then focus in greater depth on their clinical application for molecular imaging and therapy. Lastly, several examples of cardiovascular applications are discussed, including combined imaging and therapy approaches.

Evaluation of neovessels in atherosclerotic plaques of rabbits using an albumin-binding intravascular contrast agent and MRI

PURPOSE

To test whether B-22956/1, a novel intravascular contrast agent with a high affinity to serum albumin (Bracco Imaging SpA.), allowed quantifying neovessel and macrophage density in atherosclerotic plaques of rabbits using MRI.

MATERIALS AND METHODS

A T1-weighted MRI of the aorta was acquired in 10 rabbits (7 atherosclerotic and 3 control rabbits) before and up to 2 h after intravenous injection of 100 mumol/kg of Gd-DTPA or 75 mumol/kg of B-22956/1. Plaque enhancement was measured at different time points. Immunohistochemistry was performed using anti-CD 31 antibodies and anti-RAM 11 antibodies to correlate to neovessel and macrophage density, respectively.

RESULTS

MRI showed a significant plaque enhancement 2 h after B-22956/1 versus Gd-DTPA in the atherosclerotic group (39.75% versus 9.5%; P < 0.0001. Early atherosclerotic plaques (n = 146) enhancement positively correlates with neovessel density on corresponding histological sections (r = 0.42; P < 0.01). Enhancement of atherosclerotic plaques 2 h after injection of B-22956/1 correlated with macrophage density (r = 0.71; P < 0.01).

CONCLUSION

Enhancement of atherosclerotic plaques with MRI correlated with neovessel density at early time points after the injection of B-22956/1 and with macrophage density, at later time points. Hence, B-22956/1-enhanced MRI represents a promising imaging technique for the identification of "high-risk" plaques.

Noninvasive detection of macrophage-rich atherosclerotic plaque in hyperlipidemic rabbits using "positive contrast" magnetic resonance imaging

OBJECTIVES

This study was designed to identify macrophage-rich atherosclerotic plaque noninvasively by imaging the tissue uptake of long-circulating superparamagnetic nanoparticles with a positive contrast off-resonance imaging sequence (inversion recovery with ON-resonant water suppression [IRON]).

BACKGROUND

The sudden rupture of macrophage-rich atherosclerotic plaques can trigger the formation of an occlusive thrombus in coronary vessels, resulting in acute myocardial infarction. Therefore, a noninvasive technique that can identify macrophage-rich plaques and thereby assist with risk stratification of patients with atherosclerosis would be of great potential clinical utility.

METHODS

Experiments were conducted on a clinical 3-T magnetic resonance imaging (MRI) scanner in 7 heritable hyperlipidemic and 4 control rabbits. Monocrystalline iron-oxide nanoparticles (MION)-47 were administrated intravenously (2 doses of 250 mumol Fe/kg), and animals underwent serial IRON-MRI before injection of the nanoparticles and serially after 1, 3, and 6 days.

RESULTS

After administration of MION-47, a striking signal enhancement was found in areas of plaque only in hyperlipidemic rabbits. The magnitude of enhancement on magnetic resonance images had a high correlation with the number of macrophages determined by histology (p < 0.001) and allowed for the detection of macrophage-rich plaque with high accuracy (area under the curve: 0.92, SE: 0.04, 95% confidence interval: 0.84 to 0.96, p < 0.001). No significant signal enhancement was measured in remote areas without plaque by histology and in control rabbits without atherosclerosis.

CONCLUSIONS

Using IRON-MRI in conjunction with superparamagnetic nanoparticles is a promising approach for the noninvasive evaluation of macrophage-rich, vulnerable plaques.

Off-resonance angiography: a new method to depict vessels--phantom and rabbit studies

PURPOSE

To evaluate the utility of inversion recovery with on-resonant water suppression (IRON) in combination with injection of the long-circulating monocrystalline iron oxide nanoparticle (MION)-47 for contrast material-enhanced magnetic resonance (MR) angiography.

MATERIALS AND METHODS

Experiments were approved by the institutional animal care committee. Eleven rabbits were imaged at baseline before injection of a contrast agent and then serially 5-30 minutes, 2 hours, 1 day, and 3 days after a single intravenous bolus injection of 80 micromol of MION-47 per kilogram of body weight (n = 6) or 250 micromol/kg MION-47 (n = 5). Conventional T1-weighted MR angiography and IRON MR angiography were performed on a clinical 3.0-T imager. Signal-to-noise and contrast-to-noise ratios were measured in the aorta of rabbits in vivo. Venous blood was obtained from the rabbits before and after MION-47 injection for use in phantom studies.

RESULTS

In vitro blood that contained MION-47 appeared signal attenuated on T1-weighted angiograms, while characteristic signal-enhanced dipolar fields were observed on IRON angiograms. In vivo, the vessel lumen was signal attenuated on T1-weighted MR angiograms after MION-47 injection, while IRON supported high intravascular contrast by simultaneously providing positive signal within the vessels and suppressing background tissue (mean contrast-to-noise ratio, 61.9 +/- 12.4 [standard deviation] after injection vs 1.1 +/- 0.4 at baseline, P < .001). Contrast-to-noise ratio was higher on IRON MR angiograms than on conventional T1-weighted MR angiograms (9.0 +/- 2.5, P < .001 vs IRON MR angiography) and persisted up to 24 hours after MION-47 injection (76.2 +/- 15.9, P < .001 vs baseline).

CONCLUSION

IRON MR angiography in conjunction with superparamagnetic nanoparticle administration provides high intravascular contrast over a long time and without the need for image subtraction.