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

The utility of superparamagnetic contrast agents in MRI: theoretical consideration and applications in the cardiovascular system

This review will discuss the in vivo physical chemical relaxation properties of superparamagnetic iron oxide particles. Various parameters such as size, magnetization, compartmentalization and water exchange effects and how these alter the behavior of the iron oxide particles in an in vitro vs an in vivo situation with special reference to the cardiovascular system will be exemplified. Furthermore, applications using iron oxide particles for vascular, perfusion and viability imaging as well as assessment of the inflammatory status of a given tissue will be discussed.

Molecular, cellular and functional imaging of atherothrombosis

Recent years have seen a dramatic expansion in our knowledge of the events of atherogenesis and in the availability of drugs that can retard the progression - and even induce the regression - of this disease process. Our understanding has been advanced considerably by developments in genetics and molecular biology and by the use of genetically modified mouse models that have provided key mechanistic insights. Increasingly sophisticated imaging techniques will capitalize on these advances by bringing forward diagnosis, enhancing disease characterization and providing more precise evaluation of the effects of treatment. In this review, techniques for imaging atherosclerosis and thrombosis will be discussed. Particular attention will be given to magnetic resonance imaging techniques that enable lesion characterization and allow the targeted imaging of cells, molecules and biological processes. Emphasis is given to the potential contribution of magnetic resonance imaging methods to therapeutic monitoring, drug delivery and drug discovery.

Applications of ultrasmall superparamagnetic iron oxide contrast agents in the MR study of animal models

Ultrasmall superparamagnetic iron oxide nanoparticles have been widely used during the past decade as MR intravascular contrast agents in the study of animal models. Such agents enhance both T1 and T2/T2* relaxation, although for animal studies it is the later type of enhancement that is most commonly exploited. Their strong microscopic intravascular susceptibility effect enables the local blood volume distribution to be mapped in various organs. High spatial resolution and sensitivity can be achieved, because the long half-life of these agents in blood, combined with anesthetization, permits steady-state measurements over extended periods. This capability has been utilized to study the cerebrovascular blood volume distributions and their changes in normal, activated, pathologic and pharmacologically or genetically modified states, particularly in rodent animal models. It has also been applied to study blood volume changes in other tissues, such as the myocardium. The relaxation rate shifts Delta R2 and Delta R2* induced by iron oxide agents may differ depending on certain morphological characteristics of the microvascular network, and sensitive Delta R2 and Delta R2* mapping can potentially provide, in addition to blood volume, measurement of other important microvascular parameters such as blood vessel density and size. This work aims to review the applications of ultrasmall superparamagnetic iron oxide contrast agents in MR animal studies, with an emphasis on the investigation of microvascular parameters.

Macrophage Imaging in Central Nervous System and in Carotid Atherosclerotic Plaque Using Ultrasmall Superparamagnetic Iron Oxide in Magnetic Resonance Imaging

The long blood circulating time and the progressive macrophage uptake in inflammatory tissues of ultrasmall superparamagnetic iron oxide (USPIO) particles are 2 properties of major importance for magnetic resonance imaging (MRI) pathologic tissue characterization. This article reviews the proof of principle of applications such as imaging of carotid atherosclerotic plaque, stroke, brain tumor characterization, or multiple sclerosis. In the human carotid artery, USPIO accumulation in activated macrophages induced a focal drop in signal intensity compared with preinfusion MRI. The USPIO signal alterations observed in ischemic areas of stroke patients is probably related to the visualization of inflammatory macrophage recruitment into human brain infarction since animal experiments in such models demonstrated the internalization of USPIO into the macrophages localized in these areas. In brain tumors, USPIO particles which do not pass the ruptured blood-brain barrier at early times postinjection can be used to assess tumoral microvascular heterogeneity. Twenty-four hours after injection, when the cellular phase of USPIO takes place, the USPIO tumoral contrast enhancement was higher in high-grade than in low-grade tumors. Several experimental studies and a pilot multiple sclerosis clinical trial in 10 patients have shown that USPIO contrast agents can reveal the presence of inflammatory multiple sclerosis lesions. The enhancement with USPIO does not completely overlap with the gadolinium chelate enhancement. While the proof of concept that USPIO can visualize macrophage infiltrations has been confirmed in animals and patients in several applications (carotid atherosclerotic lesions, stroke, brain tumors and multiple sclerosis), larger prospective clinical studies are needed to demonstrate the clinical benefit of using USPIO as an MRI in vivo surrogate marker for brain inflammatory diseases.

Detection of synovial macrophages in an experimental rabbit model of antigen-induced arthritis: ultrasmall superparamagnetic iron oxide-enhanced MR imaging

PURPOSE

To evaluate intravenously administered ultrasmall superparamagnetic iron oxide (USPIO) as a marker of macrophage activity in an experimental rabbit model of antigen-induced arthritis.

MATERIALS AND METHODS

Unilateral arthritis was induced by means of intraarticular injection of methylated bovine serum albumin in 10 knees of 10 rabbits that had been presensitized to the same antigen. The contralateral knees in these rabbits, as well as six knees in three other rabbits, served as controls. After onset of arthritis, all knees were imaged prior to and 24 hours after administration of USPIO. The magnetic resonance (MR) imaging protocol included T1-weighted spin-echo, T2-weighted fast spin-echo, T2*-weighted gradient-echo, and short inversion time inversion-recovery sequences. Images were analyzed quantitatively and qualitatively with regard to signal characteristics and pattern. MR findings were correlated with histopathologic findings. Wilcoxon signed rank test was used to compare results of signal-to-noise ratio calculations before and after USPIO administration.

RESULTS

All knees with intraarticular injection of antigen suspension developed unilateral arthritis, whereas no signs of arthritis occurred in the control knees. On USPIO-enhanced images obtained 24 hours after contrast agent administration, significant T1 (P =.03) and more predominantly T2* (P =.02) and T2 effects (P =.01) were evident in the synovium of all 10 arthritic knees, which reflected USPIO uptake by macrophages in the synovial tissue. To a lesser extent, T2* effects were present also within the joint effusion (P =.01). No significant changes in signal characteristics were detected in the 10 nonarthritic knees in the antigen-injected group or the six knees in the control group (P =.06-.91). Histologic examination confirmed uptake of iron in the macrophages of arthritic knees. Changes in MR signal characteristics within the arthritic synovium and synovial effusion were visually detectable after intravenous administration of USPIO.

CONCLUSION

MR imaging at 1.5 T can depict USPIO uptake in phagocytic-active macrophages in an antigen-induced arthritis animal model.

Magnetic resonance imaging and computed tomography in assessment of atherosclerotic plaque

The two most promising noninvasive imaging modalities for the study of atherosclerosis are magnetic resonance imaging (MRI) and computed tomography (CT). Both have been shown to be capable of imaging vessel wall structures and differentiating various stages of atherosclerotic wall changes. MRI has been applied in various in vivo human studies to image atherosclerotic plaques in coronary arteries, carotid arteries, and aorta. The latest generation of multidetector row computed tomography (MDCT) systems allows for the noninvasive characterization of different plaque components in various vascular structures. MDCT allows evaluation of the whole arterial vasculature. In addition, MDCT has the ability to visualize the vessel wall and to give a quantitative measurement of calcified and noncalcified plaque. Using either technique, the repeatable, noninvasive study of atherosclerotic disease during its natural history and after therapeutic intervention will enhance our understanding of disease progression and regression. MDCT and MRI, therefore, may help in selecting appropriate treatments.

Capacity of human monocytes to phagocytose approved iron oxide MR contrast agents in vitro

To evaluate the capacity of human monocytes to phagocytose various approved iron oxide based magnetic resonance (MR) contrast agents and to optimize in vitro labeling of these cells. Human monocytes were incubated with two superparamagnetic iron oxide particles (SPIO) as well as two ultrasmall SPIO (USPIO) at varying iron oxide concentrations and incubation times. Iron uptake in monocytes was proven by histology, quantified by atomic emission absorption spectrometry and depicted with T2* weighted fast field echo (FFE) MR images at 1.5 T. Additionally, induction of apoptosis in iron oxide labeled monocytes was determined by YO-PRO-1 staining. Cellular iron uptake was significantly (P<0.01) higher after incubation with SPIO compared with USPIO. For SPIO, the iron oxide uptake was significantly (P<0.01) higher after incubation with the ionic Ferucarbotran as compared with the non-ionic Ferumoxides. Efficient cell labeling was achieved after incubation with Ferucarbotran at concentrations > or = 500 microg Fe/ml and incubation times > or = 1 h, resulting in a maximal iron oxide uptake of up to 50 pg Fe/cell without impairment of cell viability. In vitro labeling of human monocytes for MR imaging is most effectively obtained with the approved SPIO Ferucarbotran. Potential subsequent in vivo cell tracking applications comprise, e.g. specific targeting of inflammatory processes.

Quand, comment et pourquoi réaliser une imagerie des artères des membres inférieurs ?

Noninvasive screening of lower limb arterial disease has long been performed using color Doppler ultrasonography, whereas surgical or endovascular treatment planning relied on conventional angiography. With continued improvements of noninvasive imaging modalities, it is now possible to image the entire lower limb vasculature without arterial catheterization. Multidetector row helical CT angiography has the advantage of visualizing the arterial lumen and arterial wall calcifications, and nephrotoxicity is reduced by decreasing amounts of contrast medium. Three-dimensional MR angiography is a safe procedure, with high contrast sensitivity, and has recently benefited from step table technology that allows a single injection of contrast medium. The literature shows that both helical CT and MR angiography have high levels of accuracy, but outcome studies of their respective role in the setting of acute and chronic lower limb ischemia are lacking. These noninvasive techniques should play a major role in two main situations: first, in critical ischemia where therapeutic planning should ideally be achieved non invasively; second, in the follow-up of treated patients presenting with recurrent ischemia.

Macrophage endocytosis of superparamagnetic iron oxide nanoparticles: mechanisms and comparison of ferumoxides and ferumoxtran-10

RATIONALE AND OBJECTIVES

Superparamagnetic iron oxides (SPIO) used as magnetic resonance (MR) contrast agents undergo specific uptake by macrophages. The purpose of this study was first to determine the mechanism of macrophage uptake for Ferumoxides by using competition experiments with specific ligands of scavenger receptors SR-A (I/II) and second, to evaluate and compare the internalization of 2 different contrast agents, Ferumoxides (SPIO) and Ferumoxtran-10 (USPIO: ultrasmall superparamagnetic iron oxide) using macrophages obtained by chemical activation of human monocytic cells.

METHODS

Ferumoxides and Ferumoxtran-10 are 2 MR contrast agents, composed of dextran-coated iron oxide nanoparticles. The endocytosis pathway of Ferumoxides was studied using competition experiments on mouse peritoneal macrophages in the presence of specific ligands of scavenger receptors SR-A (types I and II): polyinosinic acid and fucoidan. In vitro assays using THP-1 (human promonocyte) cells activated into macrophages were performed in the presence of the 2 superparamagnetic nanoparticles. The cellular uptake was determined by measuring the iron content using ICP-AES (inductively coupled plasma-atomic emission spectrometry) and by Prussian blue staining.

RESULTS

In the presence of polyinosinic acid or fucoidan, the endocytosis of Ferumoxides by mouse peritoneal macrophages was inhibited. This inhibition was obtained using 10 microg/mL of scavenger receptor ligands at a concentration of 62.5 microg Fe/mL of SPIO, and a dose-dependent relationship was observed. Without competitors, the percentage of uptake of Ferumoxides by mouse peritoneal macrophages ranged between 3 and 8%. On the human activated monocyte THP-1 cell assay, Ferumoxides underwent a higher macrophage uptake (between 1.1 and 3%) compared with Ferumoxtran-10 (between 0.03 and 0.12%). This difference is attributed to the larger size of Ferumoxides nanoparticles.

CONCLUSIONS

Competition experiments indicate that the cellular uptake of Ferumoxides involves scavenger receptor SR-A-mediated endocytosis. The comparison between Ferumoxides and Ferumoxtran-10 confirms that macrophage uptake of iron oxide nanoparticles depends mainly on the size of these contrast agents.

MR angiography of the extracranial circulation

In this article, the authors present a brief history of MR angiography (MRA) of the neck with emphasis on the techniques developed, particularly recent ones, to improve image quality. The goal of MRA is to eventually replace catheter angiography. The use of MRA, particularly contrast enhanced MRA with regards to pathology (atherosclerotic disease/plaque formation, dissection and post-traumatic aneurysm) involving the extra-cranial carotid and vertebral arteries is addressed. The authors also comment on computed tomographic angiography and sonography and how they compare with contrast enhanced MRA when pertinent.

Superparamagnetic iron oxide nanoparticles: nodal metastases and beyond

Superparamagnetic iron oxide (SPIO) nanoparticles are unique MR contrast agents and are of great interest for their multiple potentials. SPIO nanoparticles have a higher diagnostic accuracy for detecting metastatic lymph nodes than conventional MR studies, particularly in head and neck. The impact of this unique MR contrast agent on treatment decision of patients with head and neck cancer needs to be investigated in comparison with contrast-enhanced CT. As MR technology advances, the accuracy of SPIO nanoparticles for detection of metastasis certainly improves; thus, 1 day we may be able to reliably detect metastases in stage N0 patients, so that treatment strategy is established for each individual patient. This article presents physiologic properties of SPIO, technical considerations and diagnostic accuracy for imaging with SPIO, and other potential applications of SPIO agents.

Complementary results of computed tomography and magnetic resonance imaging of the heart and coronary arteries: a review and future outlook

MR and CT imaging are emerging as promising complementary imaging modalities in the primary diagnosis of CAD and for the detection of subclinical atherosclerotic disease. For the detection or exclusion of significant CAD, both cardiac CT (including coronary calcium screening and non-invasive coronary angiography), and cardiac MRI (using stress function and stress perfusion imaging) are becoming widely available for routine clinical evaluation. Their high negative predictive value, especially when combining two or more of these modalities, allows the exclusion of significant CAD with high certainty, provided that patients are selected appropriately. The primary goal of current investigations using this combined imaging approach is to reduce the number of unnecessary diagnostic coronary catheterizations, and not to replace cardiac catheterization altogether. For the diagnosis of obstructive coronary atherosclerosis and for screening for subclinical disease, CT and MRI have shown potential to directly image the atherosclerotic lesion, measure atherosclerotic burden, and characterize the plaque components. The information obtained may be used to assess progression and regression of atherosclerosis and may open new areas for diagnosis, prevention, and treatment of coronary atherosclerosis. Further clinical investigation is needed to define the technical requirements for optimal imaging, develop accurate quantitative image analysis techniques, outline criteria for image interpretation, and define the clinical indications for both MR or CT imaging. Additional studies are also needed to address the cost effectiveness of such a combined approach versus other currently available imaging modalities.

Coronary artery magnetic resonance angiography

Coronary magnetic resonance angiography (coronary MRA) continues to advance rapidly from both a technical and clinical perspective. Coronary MRA has benefited directly from improvements in spatial resolution, contrast definition, and advances in motion correction, which have furthered its routine use in evaluating coronary artery bypass grafts and anomalous coronary arteries. Work in refining the techniques for more accurate identification of coronary artery disease (CAD) continues, with advances in navigator-gated and breath-hold motion correction techniques, novel k-space strategies (e.g., spiral and radial k-space filling), development and application of intravascular contrast agents, and imaging at higher field strengths. Ultimately, these developments may lead to the routine application of coronary MRA as a screening tool for CAD. This article reviews the development of coronary MRA, discusses the requirements and tools necessary for optimal visualization of the coronary arteries, and describes the application of coronary MRA to acquired and congenital CAD.

Imaging of the vulnerable plaque: new modalities

Atherosclerosis is currently considered to be an inflammatory and thus a systemic disease affecting multiple arterial beds. Recent advances in intravascular imaging have shown multiple sites of atherosclerotic changes in coronary arterial wall. Traditionally, angiography has been used to detect and characterize atherosclerotic plaque in coronary arteries, but recently it has been found that plaques that are not significantly stenotic on angiography cause acute myocardial infarction. As a result, newer imaging and diagnostic modalities are required to predict which of the atherosclerotic plaque are prone to rupture and hence distinguish "stable" and "vulnerable" plaques. Intravascular ultrasound can identify multiple plaques that are not seen on coronary angiography. Thermography has shown much promise and is based on the concept that the inflammatory plaques are associated with increased temperature and can also identify "vulnerable patients." Of all these newer modalities, magnetic resonance imaging has shown the most promise in identification and characterization of vulnerable plaques. In this article, we review the newer coronary artery imaging modalities and discuss the limitations of traditional coronary angiography.

Molecular imaging, targeted therapeutics, and nanoscience

The recent emergence of "molecular imaging" as an academic discipline has set the stage for an evolutionary leap in diagnostic imaging. Recent advances in nuclear, ultrasound, optical, and magnetic resonance imaging have generated interest in molecular imaging across all modalities and across various academic, industrial, and governmental agencies. In this perspective, examples of the progress and the prospects for the future of molecular imaging and linked targeted therapeutics are reviewed.

Imaging in the evaluation of the patient with suspected acute coronary syndrome

Patients presenting to the emergency department with chest pain have a common problem. Definitive diagnosis at presentation is difficult due to limitations of the initial evaluation, and, thus, the majority of patients are admitted. Recognition of these limitations has driven the investigation of alternative evaluation techniques and protocols to attempt to improve diagnostic sensitivity without increasing overall costs. Acute myocardial perfusion imaging has been a highly valuable technique for risk stratification of intermediate to low-risk patients with chest pain. However, for a variety of reasons, it has not been widely embraced. In the past few years, alternative techniques have been investigated for use in the diagnosis of acute coronary syndromes in the acute setting. Coronary calcium scoring and cardiac magnetic resonance imaging show promise as new tools in the armamentarium for acute coronary syndromes. The challenge now lays in developing a strategy that uses these and future techniques most appropriately to support optimal medical decision making.

Imaging of carotid artery disease: from luminology to function?

There have been tremendous advances in our ability to image atheromatous disease, particularly in the carotid artery, which is accessible and large enough to image. The repertoire of methodology available is growing, giving anatomical information on luminal narrowing which is approaching the level at which conventional carotid angiography will become very uncommon as CT and contrast-enhanced MR angiographic techniques become the norm. More exciting is the tentative ability to perform functional plaque imaging addressing enhancement patterns and macrophage activity using MR or positron-emission tomography techniques. These techniques, once rigorously evaluated, may, in addition to complex mathematical modelling of plaque, eventually allow us to assess true plaque risk. Time will best judge whether we will be able to move from the use of simple luminology to assessment of plaque function.

High-resolution magnetic resonance imaging at 2 Tesla: potential for atherosclerotic lesions exploration in the apolipoprotein E knockout mouse

INTRODUCTION

The aim of the present study was to evaluate the potential of high-resolution MRI at 2 Tesla (T) for direct noninvasive imaging of the aortic wall in a mouse model of atherosclerosis.

MATERIAL AND METHODS

A specific mouse antenna was developed and sequence parameters were adjusted. T(1)- and T2-weighted images of abdominal aorta were obtained at 2 T with a spatial resolution of 86 x 86 x 800 microm3 in vivo. With a dedicated small coil, ex vivo MRI of the aorta was performed with a spatial resolution of 54 x 54 x 520 microm3.

RESULTS

In vivo, the aortic wall was clearly defined on T(2)-weighted images in 15 of 16 mice: along the aorta the lumen circumference ranged from 1.07 to 3.61 mm and mean wall thickness from 0.11 to 0.67 mm. In vivo measurements of plaque distribution were confirmed by ex vivo MR imaging and by histology, with a good correlation with histology regarding lumen circumference (r = 0.94) and wall thickness (r = 0.97).

CONCLUSION

Magnetic resonance imaging at 2 T to analyze in vivo atherosclerotic lesions in mice is possible with a spatial resolution of 86 x 86 x 800 microm3 and thus can be used for noninvasive follow-up in evaluation of new drugs.

Integration of vascular biology and magnetic resonance imaging in the understanding of atherothrombosis and acute coronary syndromes

The interaction between the vulnerable atherosclerotic plaque prone to disruption and thrombus formation is the cornerstone of acute coronary syndrome (ACS). Although distinct from one another, the atherosclerotic and thrombotic processes appear to be interdependent, hence the term atherothrombosis. Inflammation is a crucial common pathophysiological mechanism. Overall, the association of plaque vulnerability and ACS has been well documented. Given the multifactorial origin of atherothrombosis the best preventive approach should be aggressive management of all the risk factors. New interventions should be directed toward decreasing vulnerability of the lesions thereby decreasing the risk of ACS.

The impact of spatial resolution and respiratory motion on MR imaging of atherosclerotic plaque

PURPOSE

To examine the impact of spatial resolution and respiratory motion on the ability to accurately measure atherosclerotic plaque burden and to visually identify atherosclerotic plaque composition.

MATERIALS AND METHODS

Numerical simulations of the Bloch equations and vessel wall phantom studies were performed for different spatial resolutions by incrementally increasing the field of view. In addition, respiratory motion was simulated based on a measured physiologic breathing pattern.

RESULTS

While a spatial resolution of > or = 6 pixels across the wall does not result in significant errors, a resolution of < or = 4 pixels across the wall leads to an overestimation of > 20%. Using a double-inversion T2-weighted turbo spin echo sequence, a resolution of 1 pixel across equally thick tissue layers (fibrous cap, lipid, smooth muscle) and a respiratory motion correction precision (gating window) of three times the thickness of the tissue layer allow for characterization of the different coronary wall components.

CONCLUSIONS

We found that measurements in low-resolution black blood images tend to overestimate vessel wall area and underestimate lumen area.

Tissue-specific MR contrast agents

The purpose of this review is to outline recent trends in contrast agent development for magnetic resonance imaging. Up to now, small molecular weight gadolinium chelates are the workhorse in contrast enhanced MRI. These first generation MR contrast agents distribute into the intravascular and interstitial space, thus allowing the evaluation of physiological parameters, such as the status or existence of the blood-brain-barrier or the renal function. Shortly after the first clinical use of paramagnetic metallochelates in 1983, compounds were suggested for liver imaging and enhancing a cardiac infarct. Meanwhile, liver specific contrast agents based on gadolinium, manganese or iron become reality. Dedicated blood pool agents will be available within the next years. These gadolinium or iron agents will be beneficial for longer lasting MRA procedures, such as cardiac imaging. Contrast enhanced lymphography after interstitial or intravenous injection will be another major step forward in diagnostic imaging. Metastatic involvement will be seen either after the injection of ultrasmall superparamagnetic iron oxides or dedicated gadolinium chelates. The accumulation of both compound classes is triggered by an uptake into macrophages. It is likely that similar agents will augment MRI of atheriosclerotic plaques, a systemic inflammatory disease of the arterial wall. Thrombus-specific agents based on small gadolinium labeled peptides are on the horizon. It is very obvious that the future of cardiovascular MRI will benefit from the development of new paramagnetic and superparamagnetic substances. The expectations for new tumor-, pathology- or receptor-specific agents are high. However, is not likely that such a compound will be available for daily routine MRI within the next decade.

Multislice computed tomography of the coronary arteries

Multislice spiral CT (MSCT) of the coronary arteries is developing rapidly as a noninvasive method for assessing coronary artery anatomy and for calculating various parameters of cardiac function. This article reviews the issues involved in MSCT coronary angiography and the methods used to overcome them. The current clinical applications for MSCT coronary angiography are summarized with reference to published clinical studies. The capabilities of CT are compared with those of MRI. Both modalities have a wide range of indications in the noninvasive diagnostic assessment of the heart and partly overlap in their clinical application.

Vascular MRI in the diagnosis and therapy of the high risk atherosclerotic plaque

Disruption of a high risk plaque is known as the primary cause of cardiovascular events. Characterization of arterial wall components has become an essential adjunct in the identification of patients with plaques prone to rupture. Magnetic Resonance Imaging (MRI) has been revealed as one of the noninvasive tools possibly capable of identifying and characterizing high risk atherosclerotic plaque. MRI may facilitate diagnosis, and guide and serially monitor interventional and pharmacological treatment of atherosclerotic disease. In addition, it permits the simultaneous assessment of the anatomy, morphology, and hemodynamics for the study of flow-induced atherogenesis. It possibly will identify asymptomatic patients with subclinical atherosclerosis. This has potential significance for the improvement of strategies in primary and secondary prevention.

Functional-morphologic MR imaging with ultrasmall superparamagnetic particles of iron oxide in acute and chronic soft-tissue infection: study in rats

PURPOSE

To investigate the use of magnetic resonance (MR) imaging enhanced with ultrasmall superparamagnetic particles of iron oxide (USPIO) to identify acute, early chronic, and late chronic abscess formation in an experimental model of soft-tissue abscess.

MATERIALS AND METHODS

Experimental soft-tissue infection in 15 rats was imaged with an MR imaging unit on days 1 and 2 (acute), days 5 and 6 (early chronic), and days 8 and 9 (late chronic) after inoculation of the infectious agent. All animals were imaged without contrast enhancement and immediately and 24 hours after USPIO administration. MR and histopathologic findings were compared. The changes in relative signal intensity (SI) and in the extent and pattern of contrast enhancement (macrophage distribution) between the animal groups were analyzed. Statistical testing was performed with Kruskal-Wallis analysis of variance and the chi2 test.

RESULTS

At 24 hours after USPIO administration, the relative SI of the abscess wall and the relative macrophage extent were 0.50 (0.33-0.73) and 1.03 (0.90-1.08), respectively, for acute infection; 0.11 (0.10-0.18) and 0.94 (0.93-1.01) for early chronic infection; and 0.53 (0.44-0.58) and 0.80 (0.77-0.83) for late chronic infection. The changes in enhancement pattern (P <.001), relative SI (P <.001), and relative macrophage extent (P <.05) with time were significant.

CONCLUSION

The macrophage distribution pattern increases the specificity of MR findings in chronic infection and allows differentiation between areas with active inflammation and areas of reparative granulation tissue.

New balloon-thermography catheter for in vivo temperature measurements in human coronary atherosclerotic plaques: a novel approach for thermography?

Although ex vivo studies showed marked thermal heterogeneity in atheromatic plaques, in in vivo human studies trivial temperature variations are recorded due to the cooling effect of blood flow. We investigated a new balloon-thermogaphy catheter for temperature measurements during coronary flow interruption. A thermistor probe is positioned at the distal segment of the catheter. At the opposite site of the thermistor, a balloon is placed. By inflation of the balloon, coronary flow is interrupted. Ten patients with effort angina were studied. Coronary flow velocity was continuously recorded. Temperature was recorded at the proximal vessel wall and at the lesion before, during, and after complete interruption of blood flow by inflation of the balloon. DeltaTp was assigned as the difference between the background temperature and the maximal temperature during and after balloon inflation. DeltaTl was assigned as the difference between the atherosclerotic plaque and the proximal vessel wall. The procedure was not complicated. DeltaTp during and after balloon inflation was 0.01 +/- 0.01 degrees C and -0.003 +/- -0.01 degrees C (P < 0.001), respectively. DeltaTl was 0.07 +/- 0.04 degrees C at baseline, 0.17 +/- 0.06 degrees C (59.3% +/- 11.8% increase) during, and 0.07 +/- 0.05 degrees C after flow interruption (P < 0.001). DeltaTl was greater than DeltaTp during and after impairment of flow (P < 0.001). In vivo atherosclerotic plaque temperature recording seems to be feasible with this new balloon-thermography catheter. This device may introduce a new approach for the detection of thermal heterogeneity in plaques by addressing the issue of cooling effect of blood flow.

New understanding, diagnosis, and prognosis of atherothrombosis and the role of imaging

Despite crucial advances in our knowledge of the pathologic mechanisms and the availability of effective diagnostic and treatment modalities, coronary atherothrombosis remains the most frequent cause of ischemic heart disease. Plaque disruption with superimposed thrombosis is the main cause of unstable angina, myocardial infarction, and sudden death. New findings have recently introduced exciting concepts that could have major impact on the treatment of the atherothrombotic disease. We will discuss the mechanisms that lead to the development of atherothrombosis and those responsible for the acute coronary syndromes, as well as some of the concepts derived from in vivo observations using new imaging technologies (eg, high-resolution magnetic resonance imaging).

MR imaging for the noninvasive assessment of atherothrombotic plaques

Imaging methods to quantify the progression and regression of atherosclerosis could play a strong role in the management of patients. High-resolution, noninvasive MR imaging may provide exhaustive 3D anatomical information about the lumen and the vessel wall. Furthermore, MR imaging has the ability to characterize plaque composition and microanatomy and therefore to identify lesions vulnerable to rupture or erosion. The high resolution of MR imaging and the development of sophisticated contrast agents offer the promise of molecular in vivo molecular imaging of the plaque. This may aid early intervention (eg, lipid-lowering drug regiments) in both primary and secondary treatment of vascular disease in all arterial beds.

MR imaging with ultrasmall superparamagnetic iron oxide particles in experimental soft-tissue infections in rats

PURPOSE

To investigate the feasibility of macrophage magnetic resonance (MR) imaging in rats by using an experimental soft-tissue infection model.

MATERIALS AND METHODS

Thirteen rats with unilateral calf-muscle infection were imaged with a 4.7-T MR imager at an early chronic stage of infection (day 4 before contrast material injection, days 4-7 after injection). Eleven animals were imaged before and 3 and 24 hours after intravenous application of ultrasmall superparamagnetic iron oxide (USPIO), and eight animals were additionally imaged 48 hours and three animals 72 hours after USPIO application. Two infected rats served as controls. T1- and T2-weighted spin-echo and T2*-weighted gradient-echo sequences were applied. All animals were sacrificed, and histopathologic findings were correlated with findings on MR images. Electron microscopy was performed in two rats. For quantitative analysis, signal intensities on T2*-weighted images and T2 values on T2 maps were measured within regions of interest, and the temporal variation was analyzed by using the signed rank test.

RESULTS

Visualization of USPIO-loaded macrophages was most sensitive with a T2*-weighted sequence. USPIO distribution pattern and quantitative analysis of T2 and T2* effects 3 hours after USPIO application were significantly different (P <.05) from those at 24 and 48 hours, reflecting the dynamic transit of the particle accumulation from the intravascular to the intracellular compartment by means of macrophage phagocytosis. Local signal intensity alterations could be correlated with iron-loaded macrophages at histopathologic examination.

CONCLUSION

Activated macrophages in acute soft-tissue infection can be labeled with USPIOs and detected with MR imaging because of susceptibility effects.

Magnetic resonance imaging to identify the high-risk plaque

Atherosclerotic plaque stability depends on 3 factors: (1) lipid core, (2) fibrous cap and its thickness, and (3) inflammation within the cap. Magnetic resonance imaging (MRI) is a noninvasive technique that can provide information on these plaque components using a variety of pulse sequences. Assessment of plaque volume and tissue components and the efficacy of lipid-lowering therapy has been performed in human aorta and carotid arteries. Imaging the coronary wall for plaque burden is a novel application of MRI. Newer approaches also include intravascular and transesophageal MRI techniques. Several emerging MR contrast agents being tested in animal models hold promise for targeted imaging of plaque. MRI is a powerful noninvasive imaging tool with high spatial resolution that continues to prove its value in determining atherosclerotic plaque volume and tissue components.

Vulnerable atherosclerotic plaque: emerging challenge for animal models

A recent shift in the clinical paradigm of acute coronary syndromes led to a burst of activity in developing animal models related to plaque vulnerability. In the present review, animal models of spontaneous and induced plaque rupture, thrombosis, and hemorrhage and "vulnerability endpoints" in conventional models of atherosclerosis are discussed. These endpoints include readouts related to biomechanical properties of the plaques, collagen turnover, underlying inflammation, and lipid accumulation. Challenges in model validation are emphasized. Development of new animal models and new tools of monitoring plaque vulnerability will facilitate design of plaque-stabilizing therapies.

Use of a blood-pool contrast agent for MR-guided vascular procedures: feasibility of ultrasmall superparamagnetic iron oxide particles

RATIONALE AND OBJECTIVES

The purpose of this study was to examine the dose dependency of the intravascular signal intensity after injection of ultrasmall superparamagnetic iron oxide (USPIO) particles (SH U 555 C) in a rabbit model studied with a low-field-strength magnetic resonance (MR) imaging system. The results were used to facilitate MR-guided vascular procedures in a pig.

MATERIALS AND METHODS

All experiments were performed at 0.2 T. To determine the optimum USPIO (or SH U 555 C) dose for intravascular interventions, the authors acquired coronal three-dimensional MR angiographic images in 12 rabbits after injection of four dose levels (10, 20, 30, and 40 micromol of iron per kilogram body weight). The intraaortic signal intensities were measured in user-defined regions of interest. For numerical analysis, signal intensity enhancement was computed. Subsequently MR image-guided procedures were performed in USPIO-enhanced vessels in one pig.

RESULTS

The signal intensity evaluation shows a clear-cut dose dependence in both early and late phases after administration of SH U 555 C. A high-spatial-resolution MR angiogram acquired 20 minutes after injection yielded the best results with the highest dose (40 micromol of iron per kilogram); at that dose, intravascular enhancement was sufficient for vascular procedures for 60 minutes after injection.

CONCLUSION

SH U 555 C is a promising contrast agent for MR angiography and MR-guided vascular procedures in an open low-field-strength MR imager.

Noncoronary and coronary atherothrombotic plaque imaging and monitoring of therapy by MRI

In the future, the use of imaging methods to quantify the progression and regression of atherosclerosis could play a strong role in the management of patients. High-resolution, noninvasive MRI may provide exhaustive 3-D anatomic information about the lumen and the vessel wall. Furthermore, MRI has the ability to characterize plaque composition and microanatomy and therefore to identify lesions vulnerable to rupture or erosion. The high resolution of MRI and the development of sophisticated contrast agents offer the promise of molecular in vivo molecular imaging of the plaque. This may aid early intervention (e.g., lipid lowering drug regiments) in both primary and secondary treatment of vascular disease in all arterial beds.

MR imaging of atherosclerotic plaque

MRI is a powerful noninvasive imaging tool with high spatial resolution that continues to prove its value in determining atherosclerotic plaque size, volume, and tissue components. Multispectral MRI sequences have been validated to characterize atherosclerotic plaque components in animals; they have recently been applied to human aorta and carotid artery and are being used to identify the vulnerable plaque. The ability to measure wall thickness in human coronary artery wall has been realized. Future developments may allow plaque characterization in the coronary arteries with surface coil imaging, but intravascular MRI may play an important role in this regard. Novel contrast agents for identifying inflammation and thrombus within atherosclerotic plaque will aid in the identification of higher-risk atherosclerotic disease. Lastly, MRI has progressed to the point where it can be used in serial studies of atherosclerotic plaque progression and regression in the face of therapeutic intervention. MRI will continue to evolve an important role in imaging of atherosclerotic plaque.

Iron-oxide-enhanced magnetic resonance imaging of atherosclerotic plaques: postmortem analysis of accuracy, inter-observer agreement, and pitfalls

INTRODUCTION

Contrast-enhanced magnetic resonance (MR) imaging using ultra small superparamagnetic iron oxide (USPIO) particles is a new noninvasive modality for imaging inflammatory atherosclerotic plaques. We determined the accuracy, interobserver agreement, and potential sources of error of this technique by means of postmortem MR imaging of aortic preparations.

MATERIAL AND METHODS

Anesthetized atherosclerotic Watanabe heritable hyperlipidemic (WHHL) rabbits were studied after administration of different dosages of intravenous USPIO (DDM 43/34, IDF Berlin, Germany) and different postcontrast time intervals. A (n = 5) received 0 micromol Fe/kg. B (n = 5) received 50 micromol Fe/kg, 8-hour postcontrast interval. C (n = 5) received 50 micromol, 24 hours. D received 200 micromol, 48 hours. The aortas were removed and 3-mm segments prepared for postmortem examination by MR imaging using a T2-weighted gradient-echo sequence (TR/TE/FA; 41 milliseconds/11 milliseconds/15 degrees ), radiography (mammography), and histology (iron staining). USPIO accumulation was defined as the presence of 20 iron-positive cells per microscopic view (x100 magnification). Two independent readers analyzed the MR images and rated their confidence level for a positive MRI finding, defined as a focal signal loss, on a 5-point scale. The results were evaluated by receiver-operator characteristic (ROC) analysis.

RESULTS

Of a total of 621 vessel segments technically acceptable for evaluation, 534 were histologically negative and 87 positive. Accuracy, expressed as the area under the ROC curve, was 0.85 for reader 1 and 0.88 for reader 2. Interobserver agreement was 0.67. False-positive findings were established by at least one reader for 121 of the 621 segments, false-negative findings for only 15 segments. Calcifications and mural thrombi were identified as potential sources of error of the method.

CONCLUSION

Postmortem USPIO-enhanced MR imaging of atherosclerotic plaques showed a high accuracy and good interobserver agreement in the animal model used here. Further optimization of the method should aim at reducing the rather high percentage of false-positive results.

Non-invasive imaging of plaque vulnerability: an important tool for the assessment of agents to stabilise atherosclerotic plaques

Disruption of a vulnerable atherosclerotic plaque is well-recognised as the primary cause of stroke, non-fatal myocardial infarction and sudden cardiac death. Novel therapeutic agents are being developed to stabilise such plaques. The initial evaluation of these drugs would be facilitated by the use of non-invasive imaging techniques to identify vulnerable plaque and document serial changes in plaque stability. The aim of this review is to explain the characteristics of the leading non-invasive imaging modalities and discuss their role in examining the vulnerable plaque. This knowledge will be extremely important for physicians and scientists involved in the clinical evaluation of novel agents to stabilise the vulnerable plaque.