Imaging Of Atherosclerotic Plaques Research Articles

Superparamagnetic iron oxide binding and uptake as imaged by magnetic resonance is mediated by the integrin receptor Mac-1 (CD11b/CD18): Implications on imaging of atherosclerotic plaques

Introduction Superparamagnetic iron oxide nanoparticles (SPIONs) have been successfully used for magnetic resonance imaging (MRI) of atherosclerotic plaques. Endocytosis into monocytes/macrophages has been proposed as the mechanism for SPION uptake, but a specific receptor has not been identified yet. A potential candidate is the versatile integrin Mac-1 (CD11b/CD18, αMβ2), which is involved in leukocyte adhesion, complement activation and phagocytosis. Methods and results Intracellular SPION-accumulation was confirmed in cultured human monocytes using immunohistochemistry and iron staining. Recombinant cells expressing Mac-1 in different activation states as well as human monocytes with or without PMA stimulation were incubated either with an unspecific IgG or a CD11b-blocking antibody. Thereafter, cells were incubated with FITC-labeled amino-covered SPIONs or ferumoxtran-10 SPIONs and signal intensity was quantified by flow cytometry. Depending on the activation status of Mac-1, a significant increase in SPION binding/uptake was observed, independent on surface coating. Furthermore, SPION binding/uptake was significantly reduced after CD11b blockade. Results were confirmed in recombinant cells incubated with amino-PVA SPIONs and ferumoxtran-10, using T2 *-weighted 3T MRI. Conclusion The integrin Mac-1 is directly involved in SPION binding/uptake. Thus, monocytes abundantly expressing Mac-1 and especially activated monocytes expressing activated Mac-1 may be useful vehicles for high resolution MRI labeling of atherosclerotic plaques.

Imaging of vulnerable atherosclerotic plaques with FDG-microPET: No FDG accumulation

Background Non-invasive methods of evaluating atherosclerosis in humans and experimental animals are needed. Studies indicate that FDG-PET has a potential to detect vulnerable, inflamed atherosclerotic lesions. Methods Nine atherosclerotic apoE-deficient mice were PET scanned. Four to determine optimal timing for imaging, and five post mortem after 1 h redistribution of FDG and again after sequential removal of the interscapular brown fat and the atherosclerotic aortic arch. Uptake in various tissues in fasting ( n = 13) and non-fasting ( n = 7) apoE-deficient mice, including atherosclerotic and non-atherosclerotic aorta, was measured. Finally, accelerated atherosclerosis was induced by carotid ligation ( n = 12), and FDG-uptake was measured. Results FDG accumulation initially thought to correspond to the atherosclerotic aortic arch was recorded. Removal of interscapular brown fat, but not atherosclerotic aortic arch, removed the signal. The aortic arch accumulated less FDG than the non-atherosclerotic thoracic aorta both in fasting (ratio 0.5, p = 0.008) and non-fasting (ratio 0.33, p = 0.02) conditions. Carotid atherosclerosis likewise failed to increase FDG-uptake compared to the non-ligated artery (ratio 1.03). Conclusion Spontaneously developed advanced atherosclerotic lesions in aorta were, paradoxically, associated with reduced FDG uptake, and accelerated carotid atherosclerosis also failed to increase FDG-uptake. The results seriously question the potential of FDG-PET for imagining of advanced, vulnerable atherosclerotic lesions.

The ACTIVATE study: lessons for the future of atherosclerotic therapy

The use of medical therapies that target established atherosclerotic risk factors has had a profound impact on the incidence of cardiovascular events. However, the majority of clinical events are not prevented. This highlights the need to identify new therapeutic targets to achieve further clinical benefit. A number of pathological events within the arterial wall, which contribute to atheroma formation, represent potential targets for therapeutic intervention. The use of surrogate markers of clinical efficacy has become increasingly popular in the assessment of novel therapies. Imaging of atherosclerotic plaques and their progression by intravascular ultrasound has been used in a number of studies to evaluate therapeutic strategies. Using this analytical approach, in the recently reported acyl cholesteryly acyl transferase intravascular atherosclerosis treatment evaluation (ACTIVATE) study, the experimental acyl-coenzyme:cholesterol acyltransferase inhibitor, pactimibe, was reported to have a detrimental ...

Atherosclerosis Neovascularization and Imaging

Neovascularization in atherosclerotic plaques is particularly prominent in complicated lesions, and has been recently identified as a marker of plaque vulnerability. This observation has led to a growing interest in the development of imaging techniques with the ability to visualize and quantify the extent of plaque neovascularization. Such feature may play an important role in identifying those lesions more prone to destabilization and rupture, and in the guidance and monitoring of therapeutic interventions. Several modalities have emerged as potential candidates for imaging neovessels in atherosclerotic lesions. They include magnetic resonance imaging, x-ray computed tomography, positron emission tomography, single photon emission computed tomography, ultrasound, or near-infrared optical imaging. These techniques differ in their achievable spatial and temporal resolution, availability, cost, reproducibility, degree of intrusiveness, capability to image atherosclerotic plaques in various vascular territories and ability to discern different plaque components, specifically the presence of neovessels. Molecular imaging, a rapidly evolving multidisciplinary field devoted to the visualization of specific physiopathologic processes at the cellular or molecular level, appears particularly well suited for this purpose because of its ability to target and visualize individual molecules specific to neoangiogenesis. In this manuscript we will review current evidence on the potential application of the various modalities, with a particular emphasis in molecular imaging.

Intravascular Ultrasound in Cardiovascular Medicine

Case presentation: A 38-year-old man underwent angiography 2 weeks after a non–ST-segment myocardial infarction. He was overweight and prehypertensive, with a blood pressure of 134/86 mm Hg. His biochemical parameters included low-density lipoprotein cholesterol (LDL-C) 127 mg/dL, high-density lipoprotein cholesterol (HDL-C) 38 mg/dL, and C-reactive protein (CRP) 3.2 mg/L. Angiography revealed mild disease throughout the coronary arteries and a hazy appearance that involved the proximal left anterior descending coronary artery. Intravascular ultrasound (IVUS) imaging revealed diffuse and extensive atherosclerosis with evidence of plaque rupture at multiple sites (Figure 1). In the presence of no significant luminal stenoses, the patient was treated medically with aspirin and low-dose statin therapy. Figure 1. Illustrative example of coronary angiography of the left anterior descending artery that appears to be free of irregularities, apart from a hazy appearance in the proximal segment. A representative cross-sectional tomographic image obtained by IVUS examination (top right) within the proximal segment (white arrow on angiogram) revealed substantial atheroma within the arterial wall. The bottom right segment illustrates a tomographic image of atheroma containing significant ulceration (5 o’clock) at the site of the gold arrow on the angiogram. The application of IVUS in this case highlights a number of important points with regard to the natural history of atherosclerosis and its modification by use of established medical therapies. The ability to image the entire arterial wall represents a significant advantage over coronary angiography. In the presence of minimal luminal stenoses, IVUS imaging in patients with coronary symptoms typically reveals extensive and diffuse atherosclerosis.1 Application of serial IVUS imaging in prospective clinical trials has enabled a greater understanding of the impact of antiatherosclerotic interventions on patients with established coronary artery disease (CAD). Ultrasonic coronary imaging has revealed essential details about the natural history of atherosclerosis. Atheroma formation begins at a …

Magnetic resonance imaging of experimental atherosclerotic plaque: Comparison of two ultrasmall superparamagnetic particles of iron oxide

To evaluate a new ultrasmall superparamagnetic particles of iron oxide (USPIO) compound, ferumoxytol, as a marker of macrophage activity in atherosclerotic plaques and to compare it to ferumoxtran-10. Ten mature heritable hyperlipidemic (WHHL) female Watanabe rabbits served as the animal model for atherosclerosis, four coeval female New Zealand White (NZW) rabbits were the control group. Five WHHL and two NZW received a single intravenous injection (250 micromol/kg) of either ferumoxtran-10 or ferumoxytol and were subjected to daily MR examinations on a 1.5T whole body scanner for the next five days. Development of signal intensity changes and susceptibility effects was assessed. Statistical analysis was based on a nonparametric Wilcoxon-Mann-Whitney-U test by using a P value at the 0.05 significance level. On day 5, the rabbits were sacrificed and the aorta was referred to histopathology, distribution of iron particles in the vessel wall was analyzed. MRI was feasible in all animals. Three days after injection of ferumoxytol the highest luminal signal intensity measurements were observed in the ferumoxytol group; the highest measurements were five days after injection in the ferumoxtran-10 group (P < 0.05). In the WHHL, susceptibility effects presented as homogeneous dark lines parallel to the aortic wall after ferumoxytol and spotted areas void of signal after ferumoxtran-10. None of these findings were observed in the NZW control groups. Ferumoxtran-10 and ferumoxytol at a respective dose of 250 mumol/kg appear well suited for atherosclerotic plaque detection with MRI in experimental atherosclerosis. Ferumoxytol warrants further analysis in humans.

Evaluation of99mTc labeled diadenosine tetraphosphate as an atherosclerotic plaque imaging agent in experimental models

The potential of 99mTc labeled P1, P4-di (adenosine-5')-tetraphosphate (Ap4A) for imaging experimental atherosclerotic plaques was evaluated in New Zealand white (NZW) rabbits. To label the 99mTc to Ap4A, stannous tartrate solution was used. 99mTc-Ap4A was purified on a Sephadex G-25 column. The radiochemistry purities of 99mTc-Ap4A were 85% to 91%. Biodistribution study revealed 99mTc-Ap4A cleared from blood rapidly. Thirty min after 99mTc-Ap4A administrated on NZW atherosclerotic rabbits, lesion to blood (target/blood, T/B) ratio was 3.17 +/- 1.27, and lesions to normal (target/non-target, T/NT) ratio was 5.23 +/- 1.87. Shadows of atherosclerotic plaques were clearly visible on radioautographic film. Aortas with atherosclerotic plaques also could be seen on ex vivo gamma camera images. Atherosclerotic abdominal aortas were clearly visible on in vivo images 15 min to 3 h after 99mTc-Ap4A administration. 99mTc-labeled Ap4A can be used for rapid noninvasive detection of experimental atherosclerotic plaque.

Multicontrast-weighted magnetic resonance imaging of atherosclerotic plaques at 3.0 and 1.5 Tesla: ex-vivo comparison with histopathologic correlation

The purpose was to analyze magnetic resonance (MR) plaque imaging at 3.0 Tesla and 1.5 Tesla in correlation with histopathology. MR imaging (MRI) of the abdominal aorta and femoral artery was performed on seven corpses using T1-weighted, T2-weighted, and PD-weighted sequences at 3.0 and 1.5 Tesla. Cross-sectional images at the branching of the inferior mesenteric artery and the profunda femoris were rated with respect to image quality. Corresponding cross sections of the imaged vessels were obtained at autopsy. The atherosclerotic plaques in the histological slides and MR images were classified according to the American Heart Association (AHA) and analyzed for differences. MRI at 3.0 Tesla offered superior depiction of arterial wall composition in all contrast weightings, rated best for T2-weighted images. Comparing for field strength, the highest differences were observed in T1-weighted and T2-weighted techniques (both P< or =0.001), with still significant differences in PD-weighted sequence (P< or =0.005). The majority of plaques were histologically classified as calcified plaques. In up to 21% of the cases, MRI at both field strengths detected signal loss characteristic of calcification although calcified plaque was absent in histology. MRI at 3.0 Tesla offers superior plaque imaging quality compared with 1.5 Tesla, but further work is necessary to determine whether this translates in superior diagnostic accuracy.

Inflammatory Biomarkers in Acute Coronary Syndromes

The inflammatory etiology of atherosclerosis has prompted a search for biomarkers of inflammation that predict risk for coronary artery disease and its sequelae. Within the acute coronary syndromes (ACS), inflammatory biomarkers may provide independent information regarding pathophysiology, prognosis, and optimal therapeutic strategies. On the basis of the hypothesis that different pathophysiological processes provide nonoverlapping information regarding risk stratification and disease management, this review series addresses biomarkers for each step in the inflammatory process that leads to ACS. Part I reviewed cytokines; this part reviews acute-phase reactants and biomarkers of endothelial cell activation; subsequent parts will address biomarkers of oxidative stress, angiogenesis, extracellular matrix degradation, and platelet activation. In the acute-phase response, cytokines drive production of acute-phase reactants, defined by >25% change in circulating concentration during an inflammatory response. These markers may also remain elevated chronically because of continuing inflammatory stimuli (Table). View this table: Inflammatory Biomarkers in ACS ### C-Reactive Protein C-reactive protein (CRP) is a pentraxin acute-phase protein, members of which are evolutionarily conserved in most vertebrates.1 Hepatocytes and possibly smooth muscle cells and macrophages transcriptionally activate production of CRP in response to inflammatory cytokines, including interleukin-1 and interleukin-6.2 CRP is a robust clinical marker because of its stability, reproducible results, and ease of assay. Although it was originally proposed as a nonspecific marker of inflammation, several reports suggest that CRP may play a direct pathophysiological role in the development and progression of atherosclerosis. Proposed mechanisms include induction of endothelial dysfunction,3 promotion of foam cell formation,4 inhibition of endothelial progenitor cell survival and differentiation,5 and activation of complement in atherosclerotic plaque intima6 and ischemic myocardium.7 Patients with ACS have elevations in CRP in association with their presenting symptoms. There appears to be a bimodal CRP response among patients with ACS. In some patients, CRP may remain elevated for …

Vulnerable Plaque Detection by 3.0 Tesla Magnetic Resonance Imaging

A clinical case report is presented on a 76-year-old man who volunteered for a 3.0 T magnetic resonance (MR) carotid protocol. The subject was referred for carotid endarterectomy and histology was performed on the ex vivo specimen and compared with the in vivo images. The 3.0 and 1.5 T (obtained for comparison) MR protocol consisted of 2-dimensional (2D) and 3-dimensional (3D) multicontrast bright and black blood imaging for detecting the lumen and vessel wall. The combination of multicontrast black blood transverse images and the 3D time of flight transverse images provided visualization of a narrowed internal carotid artery lumen 4 mm above of the bifurcation and the presence of a complex atherosclerotic plaque containing a large lipid pool, calcification, and intact fibrous cap. Quantitative comparisons including vessel lumen and plaque area, signal-to-noise (SNR) and contrast-to-noise (CNR) ratios were obtained for 1.5 and 3.0 T image data. Plaque composition was verified with histology. Macrophages were also detected in the shoulders of the plaque as demonstrated by CD68 staining and corresponded with a small hyperintense area in the T2W images at 3.0 T, but not observed in comparable 1.5 T images. High field 3.0 T multicontrast MRI of atherosclerotic plaque has been validated with histology comparison and provides improved detection of complex atherosclerotic plaque with increased SNR and CNR compared with 1.5 T. Further studies validating contrast mechanisms of plaque at 3.0 T are required, but atherosclerotic plaque imaging has clear benefit from application at the higher magnetic field strength.

Bright and black blood imaging of the carotid bifurcation at 3.0 T

Purpose The aim of this study was to evaluate our preliminary experience at 3.0 T with imaging of the carotid bifurcation in healthy and atherosclerotic subjects. Application at 3.0 T is motivated by the signal-to-noise gain for improving spatial resolution and reducing signal averaging requirements. Materials and methods We utilized a dual phased array coil and applied 2D, 3D time of flight (TOF) and turbo spin echo (TSE) sequences with comparison of two lumen signal suppression methods for black blood (BB) TSE imaging including double inversion preparation (DIR) and spatial presaturation pulses. The signal-to-noise ratios (SNR) of healthy carotid vessel walls were compared in 2D and 3D BB TSE acquisitions. The bright and black blood multi-contrast exam was demonstrated for a complex carotid plaque. Results Contrast-to-noise (CNR) greater than 150 was achieved between the lumen and suppressed background for 3D TOF. For BB, both methods provided sufficient lumen signal suppression but slight residual flow artifacts remained at the bifurcation level. As expected 3D TSE images had higher SNR compared to 2D, but increased motion sensitivity is a significant issue for 3D at high field. For multi-contrast imaging of atherosclerotic plaque, fibrous, calcified and lipid components were resolved. The CNR ratio of fibrous (bright on PDW, T2W) and calcified (dark in T1W, T2W, PDW) plaque components was maximal in the T2W images. The 3D TOF angiogram indicating a 40% stenosis was complemented by 3D multi-planar reformat of BB images that displayed plaque extent. Detection of intimal thickening, the earliest change associated with atherosclerotic progression was observed in BB PDW images at 3.0 T. Conclusions High SNR and CNR images have been demonstrated for the healthy and diseased carotid. Improvements in RF coils along with pulse sequence optimization, and evaluation of endogenous and exogenous contrast mechanisms will further enhance carotid imaging at 3.0 T.

Correlation of cortical thickness and interfragmentary compression in intramedullary osteosynthesis: an experimental study

Cardiovascular diseases such as heart attack and stroke are often caused by rupture of a vulnerable atherosclerotic plaque. The mechanisms governing plaque progression and causing plaque rupture are not fully understood. 3D multi-component FSI models for atherosclerotic plaques based on in vivo/ex vivo MRI images were introduced to investigate plaque progression and rupture. MRI images of human carotid atherosclerotic plaques were acquired using multi-contrast techniques. Both artery wall and plaque components were assumed to be hyperelastic, isotropic, incompressible and homogeneous. The modified Mooney-Rivlin model was used for all the materials with parameters selected to fit experimental data. Blood flow was assumed to be laminar, Newtonian, viscous and incompressible. The incompressible Navier-Stokes equations were used as the governing equations. The fully coupled 3D multicomponent FSI models were solved using ADINA (ADINA R&D, Watertown, MA, USA) to perform flow and stress/strain analysis and seek critical information which may be related to plaque progression and rupture. It is wellknown that low and oscillating flow wall shear stresses play an important role in atherosclerotic plaque initiation and progression. For advanced atherosclerotic plaques, our results indicate that flow wall shear stress is considerably higher in the stenotic region because of lumen narrowing. While the low and oscillating flow shear stress hypothesis correctly identifies locations of intimal thickening, it cannot explain why advanced plaques continue to grow under high flow shear stress conditions. Positive and negative correlations of flow shear stress and wall maximum principal stress with respect to wall thickness were found from our investigations which lead to a possible new hypothesis for intermediate and advanced plaque progression: Low structure stress has positive correlation with plaque wall thickness, and may create favorable mechanical conditions in the plaque for intermediate and advanced plaque progression. Acknowledgement: This research was supported by NIH grant R01 EB004759.

Molecular Beacons Illuminate Subcellular Events

Building on decades of technological advances and imaging experience, during which noninvasive imaging of cardiac structure and function, myocardial perfusion and viability, and noncoronary vascular pathology have become intimately entwined in routine clinical practice, advanced imaging techniques are now poised to deliver noninvasive coronary arteriograms and to delineate the coronary artery wall in exquisite detail. At the same time that these highly promising methods for anatomic imaging at the macroscopic level are creating great excitement among physicians, their patients, and the public, a quieter imaging revolution is underway with steady progress in detecting and tracking fundamental biological processes at the cellular and subcellular levels. In the era of genomic research, molecular biology, and stem cell therapies, these methods have great potential to accelerate understanding of basic pathophysiological processes in animals and humans and to develop new tools for early diagnosis and drug development. Cardiovascular molecular imaging is taking hold. See p 1800 Molecular imaging is the science of visually representing, characterizing, and quantifying cellular/subcellular biological processes in intact organisms. These processes include gene expression, protein–protein interaction, signal transduction, cellular metabolism, and both intracellular and intercellular trafficking. Molecular imaging has the potential to quantify these events, to monitor multiple events simultaneously, to localize these events in 3 dimensions, and to monitor these events serially. Thus, biological processes can be identified and studied in time and space, providing “4-dimensional” information. The emergence of molecular imaging has coincided with and has been made possible by the enormous progress in molecular biology, cell biology, and transgenic animal models, as well as the development of imaging probes that are specific, reproducible, and quantifiable.1 Molecular imaging was once the sole domain of nuclear medicine, with tracers developed for both positron emission tomography (PET) and single photon emission computed tomography (SPECT). Examples include antimyosin antibodies for …

Does shear stress modulate both plaque progression and regression in the thoracic aorta?: Human study using serial magnetic resonance imaging

The purpose of this study was to investigate the role of shear stress (SS) in plaque regression. A condition favorable to the development of atherosclerotic lesions is low oscillating SS. In the descending thoracic aorta, the relationship between plaque distribution and SS has never been characterized. The regression of plaque as the result of lipid-lowering therapy is associated with reverse atherogenic mechanisms. Therefore, we investigated the role of SS in plaque regression. Magnetic resonance imaging (MRI) provides a unique opportunity to noninvasively study morphology and hemodynamics. Cross-sectional images of atherosclerotic plaques in the descending thoracic aorta of 10 asymptomatic, hypercholesteremic patients were acquired at baseline and 24 months after starting lipid-lowering therapy by using a black-blood sequence on a 1.5-T clinical MRI system (5 mm x 780 microm x 780 microm). Average wall thickness (WT) was derived per quadrant. The aorta was subdivided in segments 2 cm in length starting 1 cm from the aortic arch. Average WT decreased with increasing distance from the arch (3.0 +/- 0.7 mm vs. 2.5 +/- 0.3 mm; p < 0.05) and showed a helical pattern from the proximal to distal segments. Phase-contrast MRI was performed in the thoracic aorta of eight healthy volunteers to derive typical average SS distribution. Shear stress predicted the location of WT (r(2) = 0.29, p < 0.05) but did not predict plaque regression. The best predictor of plaque regression was baseline WT. Our data showing an association between WT and average low SS locations support the role of local hemodynamics in the development of atherosclerotic lesions in descending thoracic aorta. Furthermore, SS does not seem to be the major predictor for plaque regression by lipid-lowering interventions. Therefore, our data suggest that other mechanisms are involved in the lipid-reversal mechanism.

Cylindrical meanderline radiofrequency coil for intravascular magnetic resonance studies of atherosclerotic plaque

In order to improve the performance of magnetic resonance imaging and spectroscopy of atherosclerotic plaque the potential use of novel radiofrequency coil structures with sensitive detection volumes tailored to the geometry of the arterial wall was investigated. It was found that a cylindrical meanderline (zig-zag) coil design provides a sensitive volume that is restricted to a cylindrical shell, thereby maximizing the filling factor and signal-to-noise ratio for plaques while reducing the intense blood signal. The cylindrical meanderline coil has the added advantages of an open interior, which allows for unimpeded blood flow during scanning, and the potential to be expanded against the walls of the artery, thereby stabilizing the coil against the pulsatile blood flow and minimizing motion artifacts. The performance of cylindrical meanderline coils with theoretical simulations of the electromagnetic fields as well as with experimental images of test objects (phantoms) and human endarterectomy surgical specimens is demonstrated. This radically new RF coil geometry offers the potential to improve the efficiency of MR data acquisition in medical applications in which curved surfaces or slabs contain the material of interest.

Imaging of atherosclerotic plaque

The behavior and composition of coronary atherosclerotic plaques are ultimately responsible for the threat of acute ischemic events in patients with coronary artery disease. Different imaging modalities have been developed over the last several years in order to better characterize the atherosclerotic plaque and attempt to predict those in peril of complication. Since its implementation into cardiovascular medicine, nearly 40 years ago, coronary angiography has been the mainstay of identifying hemodynamically stenotic lesions. Further investigation into imaging modalities have suggested, however, that the degree of stenosis is only one of several factors influencing a plaque's tendency to rupture. Recent advances in imaging modalities, including invasive and non-invasive studies, have allowed us to examine the histological components that comprise these plaques. Specific information such as variations in temperature, plaque stiffness and calcification level is currently being researched as well as biological and chemical markers. The ultimate goal is to visualize the plaque and its characteristics, stratify its risk for acute events, be able to apply this modality to the general population of cardiac patients, while exposing the patient to minimal risk and having adequate positive and negative predictive values. This manuscript will review the more recent data concerning these interventions and their individual characteristics.

Molecular imaging of atherosclerotic plaques with technetium-99m-labelled antisense oligonucleotides

The purpose of this study was to visualise experimental atherosclerotic lesions using radiolabelled antisense oligonucleotides (ASONs). Atherosclerosis was induced in New Zealand White rabbits fed 1% cholesterol for approximately 60 days. In vivo and ex vivo imaging was performed in atherosclerotic rabbits and normal control rabbits after i.v. injection of 92.5+/-18.5 MBq (99m)Tc-labelled ASON or (99m)Tc-labelled sense oligonucleotides. Immediately after the in vivo imaging, the animals were sacrificed and ex vivo imaging of the aortic specimens was performed. Biodistribution of radiolabelled c-myc ASON was evaluated in vivo in atherosclerotic rabbits. Planar imaging revealed accumulation of (99m)Tc-labelled c-myc ASON in atherosclerotic lesions along the artery wall. Ex vivo imaging further demonstrated that the area of activity accumulation matched the area of atherosclerotic lesions. In contrast, no atherosclerotic lesions were found in the vessel wall and no positive imaging results were obtained in animals of the control group. This molecular imaging approach has potential for non-invasive imaging of atherosclerotic plaques at an early stage.

Molecular Imaging of Atherosclerotic Plaques Using a Human Antibody Against the Extra-Domain B of Fibronectin

Current imaging modalities of human atherosclerosis, such as angiography, ultrasound, and computed tomography, visualize plaque morphology. However, methods that provide insight into plaque biology using molecular tools are still insufficient. The extra-domain B (ED-B) is inserted into the fibronectin molecule by alternative splicing during angiogenesis and tissue remodeling but is virtually undetectable in normal adult tissues. Angiogenesis and tissue repair are also hallmarks of advanced plaques. For imaging atherosclerotic plaques, the human antibody L19 (specific against ED-B) and a negative control antibody were labeled with radioiodine or infrared fluorophores and injected intravenously into atherosclerotic apolipoprotein E-null (ApoE-/-) or normal wild-type mice. Aortas isolated 4 hours, 24 hours, and 3 days after injection exhibited a selective and stable uptake of L19 when using radiographic or fluorescent imaging. L19 binding was confined to the plaques as assessed by fat staining. Comparisons between fat staining and autoradiographies 24 hours after 125I-labeled L19 revealed a significant correlation (r=0.89; P<0.0001). Minimal antibody uptake was observed in normal vessels from wild-type mice receiving the L19 antibody and in atherosclerotic vessels from ApoE-/- mice receiving the negative control antibody. Immunohistochemical studies revealed increased expression of ED-B not only in murine but also in human plaques, in which it was found predominantly around vasa vasorum and plaque matrix. In summary, we demonstrate selective targeting of atheromas in mice using the human antibody to the ED-B domain of fibronectin. Thus, our findings may set the stage for antibody-based molecular imaging of atherosclerotic plaques in the intact organism.

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.

A method to create reference maps for evaluation of ultrasound images of carotid atherosclerotic plaque

Ten formalin-fixed atherosclerotic carotid plaques removed by endarterectomy were molded into rectangular agar blocks containing fiducial markers on the top surface. Plaque and fiducial markers were imaged with 3-D multiangle ultrasound (US) spatial compounding as well as planar X ray. Subsequently, the blocks were decalcified, sliced, photographed and analyzed histologically. This gave a total of 123 slices. The plaque regions of the photographs were outlined and the outline adjusted to partly compensate for occasional displacement during slicing. Inside this outline, the material constitutions were found by incorporating the histologic information. From this set, slices with 1. too much tissue displacement due to cutting or 2. lack of identification of calcification as found by x ray, were removed. This resulted in 53 reference maps. The material types identified covered soft tissues, fibrous tissue, calcified tissue and unidentified tissues. The 53 reference maps can be used for direct automated quantitative comparison with US images. (E-mail: jw@oersted.dtu.dk)