Imaging Of Atherosclerotic Plaques Research Articles

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.

Gadolinium-containing phosphatidylserine liposomes for molecular imaging of atherosclerosis

Exteriorized phosphatidylserine (PS) residues in apoptotic cells trigger rapid phagocytosis by macrophage scavenger receptor pathways. Mimicking apoptosis with liposomes containing PS may represent an attractive approach for molecular imaging of atherosclerosis. We investigated the utility of paramagnetic gadolinium liposomes enriched with PS (Gd-PS) in imaging atherosclerotic plaque. Gd-PS-containing Gd-conjugated lipids, fluorescent rhodamine, and PS were prepared and characterized. Cellular uptake in RAW macrophages (fluorescent uptake of rhodamine) was studied on a fluorescence plate reader, while Gd-PS-induced alteration in T1 relaxivity was evaluated using a 1.5 T MRI scanner. RAW cells demonstrate PS-dependent uptake of across a range of concentrations (2, 6, 12, and 20%) in comparison to control liposomes with no PS (0%). In vivo performance of Gd-PS was evaluated in the ApoE(-/-) mouse model by collection of serial T1 weighted gradient echo MR images using an 11.7 T MRI system and revealed rapid and significant enhancement of the aortic wall that was seen for at least 4 h after injection. Gd-PS-enriched liposomes enhance atherosclerotic plaque and colocalize with macrophages in experimental atherosclerosis.

Imaging atherosclerotic plaque composition with intracoronary optical coherence tomography.

Optical coherence tomography (OCT) allows highly accurate diagnosis of atherosclerotic plaques, including measurement of the thickness of fibrous caps, permitting an assessment of the risk of rupture. While the OCT image presents morphological information in highly resolved detail, it relies on interpretation by trained readers for the identification of tissue type. We developed a method for quantitative classification of atherosclerotic plaque constituents. The optical attenuation coefficient mu(t) distinguishes different tissue types: necrotic core and macrophage infiltration exhibit strong attenuation, mu(t)>/=10 mm(-1), while calcific and fibrous tissue have a lower mu(t) approximately 2-5 mm(-1). (Neth Heart J 2009;17:448-50.).

Contrast agent for MRI-based imaging of atherosclerotic plaques

Contrast agent for MRI-based imaging of atherosclerotic plaques

Dual targeting improves microbubble contrast agent adhesion to VCAM-1 and P-selectin under flow

To improve ultrasound contrast agents targeted to the adhesion molecules P-selectin and VCAM-1 for the purpose of molecular imaging of atherosclerotic plaques, perfluorocarbon-filled phospholipid microbubble contrast agents were coupled by a polyethylene glycol–biotin–streptavidin bridge with mAb MVCAM.A(429), a sialyl Lewis x polymer (PAA-sLe x), or both (dual). Approximately three hundred thousand antibody molecules were coupled to the surface of each microbubble. Recombinant mouse P-selectin and/or VCAM-1 coated on flow chambers showed saturation of binding at approximately 15 ng/μl, resulting in 800 and 1200 molecules/µm 2 for P-selectin and VCAM-1, respectively. Dual substrates coated with equal concentrations of P-selectin and VCAM-1 had site densities between 50 and 60% of single substrates. When microbubbles were perfused through flow chambers at 5 × 10 6 microbubbles/ml (wall shear stress from 1.5 to 6 dyn/cm 2) dual-targeted microbubbles adhered almost twice as efficiently as single-targeted microbubbles at 6 dyn/cm 2. The present study suggests that dual-targeted contrast agents may be useful for atherosclerotic plaque detection at physiologically relevant shear stresses.

Intracoronary thermography: Utility to detect vulnerable and culprit plaques in patients with coronary artery disease

Atherosclerosis is an inflammatory process, and vulnerable plaques are characterized by an increased inflammatory infiltrate that generates heat. Intracoronary thermography (ICT) is a catheter-based technique for the functional imaging of atherosclerotic plaques, with the ability to identify potential vulnerable and culprit plaques in patients with coronary artery disease (CAD). ICT is able to detect thermal heterogeneity, which has been shown to be present more often in unstable coronary plaques, and positively correlated to vulnerable plaque morphology characteristics and serum markers of systemic inflammation. ICT has also been shown to have a good predictive value for clinical events after percutaneous coronary intervention and has been used to assess response to statin therapy in patients with CAD. ICT has several important limitations and is yet to be validated in large prospective trials, but may be used in the future to detect vulnerable or culprit plaques and thus used to direct local and/or systemic therapy in patients with CAD.

Rapid-Clearance Iron Nanoparticles for Inflammation Imaging of Atherosclerotic Plaque: Initial Experience in Animal Model

To evaluate the use of a recently developed fast-clearing ultrasmall superparamagnetic iron oxide (USPIO) for detection of vascular inflammation in atherosclerotic plaque. The study protocol was approved by the animal experimentation ethics committee. A recently introduced USPIO, P904, and a reference-standard USPIO, ferumoxtran-10, were tested in a rabbit model of induced aortic atherosclerosis. In vivo magnetic resonance (MR) angiography and T2*-weighted plaque MR imaging were performed at baseline and after administration of P904 and ferumoxtran-10 (administered dose for both, 1000 micromol of iron per kilogram of body weight) in 26 hyperlipidemic New Zealand white rabbits. The variation in vessel wall area over time was evaluated with nonparametric testing. Ex vivo MR imaging findings were compared with iron content at linear regression analysis. With in vivo MR imaging, plaque analysis was possible as early as 24 hours after P904 injection. The authors observed a 27.75% increase in vessel wall area due to susceptibility artifacts on day 2 (P = .04) and a 38.81% increase on day 3 (P = .04) after P904 administration compared with a 44.5% increase in vessel wall area on day 7 (P = .04) and a 34.8% increase on day 10 (P = .22) after ferumoxtran-10 administration. These susceptibility artifacts were correlated with intraplaque iron uptake in the corresponding histologic slices. The number of pixels with signal loss on the ex vivo MR images was linearly correlated with the logarithm of the iron concentration (P = .0001; R(2) = 0.93). Plaque inflammation in rabbits can be detected earlier with P904 than with ferumoxtran-10 owing to the faster blood pharmacokinetics and the early uptake of P904 in the reticuloendothelial system. http://radiology.rsnajnls.org/cgi/content/full/252/2/401/DC1.

Morphological and biochemical imaging of coronary atherosclerotic plaques using optical coherence tomography and fluorescence lifetime imaging

Morphological and biochemical imaging of coronary atherosclerotic plaques using optical coherence tomography and fluorescence lifetime imaging

Contrast‐enhanced MRI of carotid atherosclerosis: Dependence on contrast agent

To investigate the dependence of contrast-enhanced magnetic resonance imaging (MRI) of carotid artery atherosclerotic plaque on the use of gadobenate dimeglumine versus gadodiamide. Fifteen subjects with carotid atherosclerotic plaque were imaged with 0.1 mmol/kg of each agent. For arteries with interpretable images, the areas of the lumen, wall, and necrotic core and overlying fibrous cap (when present) were measured, as were the percent enhancement and contrast-to-noise ratio (CNR). A kinetic model was applied to dynamic imaging results to determine the fractional plasma volume, v(p), and contrast agent transfer constant, K(trans). For 12 subjects with interpretable images, the agent used did not significantly impact any area measurements or the presence or absence of necrotic core (P > 0.1 for all). However, the percent enhancement was greater for the fibrous cap (72% vs. 54%; P < 0.05) necrotic core (51% vs. 42%; P = 0.12), and lumen (42% vs. 63%; P < 0.05) when using gadobenate dimeglumine, although no apparent difference in CNR was found. Additionally, K(trans) was lower when using gadobenate dimeglumine (0.0846 min(-1) vs. 0.101 min(-1); P < 0.01), although v(p) showed no difference (9.5% vs. 10.1%; P = 0.39). Plaque morphology measurements are similar with either contrast agent, but quantitative enhancement characteristics, such as percent enhancement and K(trans), differ.

Imagerie moléculaire de la plaque d’athérome vulnérable. Évaluation préclinique et clinique de traceurs radioactifs

Atherosclerotic cardiovascular diseases (CVD) are the leading cause of mortality worldwide, accounting for greater than 19.10 6 deaths annually. Despite major advances in the treatment of CVD, a high proportion of CVD victims die suddenly while being apparently healthy, the great majority of these accidents being due to the rupture or erosion of a vulnerable coronary atherosclerotic plaque. Indeed, an acute heart attack is the first symptom of atherosclerosis in as much as 50% of individuals with severe disease. A non-invasive imaging methodology allowing the early detection of vulnerable atherosclerosis in selected individuals prior to the occurrence of any symptom would therefore be of great public health benefit. Nuclear imaging could potentially allow the identification of vulnerable patients by non-invasive scintigraphic imaging following administration of a radiolabeled tracer. The development of radiolabeled probes that specifically bind to and allow the in vivo imaging of vulnerable atherosclerotic plaques is therefore the subject of intense ongoing experimental and clinical research. Radiotracers targeted at the inflammatory process seem particularly relevant and promising. Recently, macrophage targeting allowed the experimental in vivo detection of atherosclerosis using either SPECT or PET imaging. A few tracers have also been evaluated clinically. Targeting of apoptosis and macrophage metabolism both allowed the imaging of vulnerable atherosclerotic plaques in the carotid vessels of patients. However, nuclear imaging of vulnerable plaques at the level of the coronary arteries remains a challenging issue because of the small size of atherosclerotic lesions and of their vicinity with blood and the circulating tracer activity.

Gamma imaging of atherosclerotic plaques

Noninvasive gamma imaging of atherosclerotic plaques targets various metabolic aspects of atherogenesis. Vascular endothelial dysfunction or denudation resulting in the expression of adhesion molecules that attract inflammatory cells and increased vascular permeability has also been targeted. Enzymes and molecules associated with apoptosis of these inflammatory cells have provided additional targets, such as oxidized low-density lipoprotein, matrix metalloproteinases, and phosphatidylserine. In late atherosclerotic lesions, the lipid core as well as proliferating smooth muscle cells have been imaged successfully. Platelets and fibrin deposition may also be targeted to demonstrate thrombosis in plaque rupture or erosion. However, no unimodal approach can diagnose plaque vulnerability. It will require multi-modal, multitasking approaches for molecular diagnostic prediction of plaque vulnerability and impending rupture. Existing experimental and clinical gamma imaging applications in atherosclerotic lesion imaging are reviewed. Improvement by signal amplification to image small lesion and the concept of multimodal applications are introduced.

Novel clinical applications of state-of-the-art multi-slice computed tomography

Recent years have witnessed a rapid development of multi-slice computed tomography (MSCT) technology. The number of detector rows has increased from 4-slices to the current availability of 64-slice and even 320-slice systems. In addition, images are acquired with thinner slices and faster rotation times resulting in substantially improved image quality and diagnostic accuracy. Simultaneously, effective dose reduction acquisition techniques have been developed allowing considerable reduction of the radiation dose. Conceivably, these advancements may allow further expansion of the use of MSCT beyond the visual assessment of the presence or absence of significant coronary artery disease. Indeed, a particular advantage of the technique is that in addition to evaluation of the coronary arteries it also allows assessment of cardiac structures and function. The purpose of the current review is to discuss several novel applications of cardiac MSCT, including stenosis quantification, atherosclerotic plaque imaging and prognostification as well as imaging of left ventricular function, aortic and mitral valve anatomy using state-of-the-art technology.

Molecular imaging of atherosclerotic plaque targeted to oxidized LDL receptor LOX-1 using magnetic resonance

Molecular imaging of atherosclerotic plaque targeted to oxidized LDL receptor LOX-1 using magnetic resonance

Murine atherosclerotic plaque imaging with the USPIO Ferumoxtran-10

In this study we intended to image plaque inflammation in a murine model of atherosclerosis with MRI and Ferumoxtran-10 (Sinerem, Guerbet, France). 8 apoE-/- mice were injected 500 micromol Fe/kg or 1000 micromol Fe/kg Ferumoxtran-10. 2 apoE-/- mice were injected NaCl. After a post-contrast time of 24 to 336 hours the mice were scarificed and the aortas were imaged ex vivo. All measurements were performed on a 17.6 Tesla Bruker AVANCE 750WB MR scanner (Bruker, Germany). Spin-echo sequences and gradient-echo sequences with variable TE were performed and T2* maps were generated. Prussian-blue and hematoxilin-eosin histology were obtained afterwards and iron-uptake was quantified by counting iron positive areas. 2 apoE-/- mice were imaged in vivo before and 48 hours after 1000 micromol Fe/kg. Atheroma iron uptake was not elevated after 24 hours compared to controls. 48 hours after 1000 micromol Fe/kg but not 500 micromol Fe/kg histology revealed a 1.3- fold increase in plaque iron content compared to NaCl injected mice. Normalized T2*-times decreased from 0.86+/-0.02 in controls to 0.66+/-0.15 after a dose of 500 micromol Fe/ml and 0.59+/-0.14 in mice injected with 1000 micromol Fe/Kg (p=0.038). These results translated into a mean of 122% increase in CNR, as measured by in vivo MRI. We have demonstrated that Ferumoxtran-10 is taken up by atherosclerotic plaques in untreated apoE-/- mice and this alters plaque signal properties.

Atherosclerotic plaque imaging by carotid MRI

Carotid MRI (CMRI) is a valuable technique to examine the state of carotid atherosclerotic plaque in vivo. This article examines CMRI's current and future directions. These include technical requirements for implementing CMRI, as well as applications of CMRI, such as visualizing lesion types, plaque components, dynamic contrast imaging, comparison of stenosis and other measures of plaque burden, and ongoing knowledge gained from serial studies. Also, future directions of CMRI and how CMRI techniques can be transitioned into clinical practice are addressed.

Atherosclerotic plaque imaging by PET/CT; can inactive, active and mixed plaques be discerned?

Atherosclerosis is one of the leading causes ofmorbidity and mortality in the world. Rupture ofatherosclerotic plaques and formation of thrombi arethe primary mechanisms of myocardial infarction orcerebrovascularaccidents.Atherosclerosisisthereforea dynamic inflammatory disorder and the biologicalcomposition and inflammatory state of an atheroscle-rotic plaque, rather than the degree of stenosis or itssize, are the major determinants of acute clinicalevents [1, 2]. Noninvasive techniques to detectvulnerable atherosclerotic plaque are criticallyneeded. In recent years, radionuclide imaging [3–6],magnetic resonance imaging (MRI) [7–13], and com-puted tomography (CT) have been put forward as themost relevant imaging modalities to identify athero-sclerotic vascular disease and the vulnerable plaque[14–30]. Positron emission tomography with fluoro-deoxyglucose (FDG-PET) has become one of theadvanced imaging approaches for the detection ofvulnerable plaques [31]. Ogawa et al. [32] showed thatmacrophages are responsible for the accumulation ofFDG in atherosclerotic lesions whereby FDG uptake iscorrelated with macrophage density. Because vulner-able plaques are rich in macrophages, FDG imagingshould be useful for selective detection of suchplaques. In recent years, multislice CT has been shownto provide excellent anatomical and morphologicalimages both of the large vessels and the coronaryarteries [18]. In addition, differences in plaque com-position and distribution—both in acute coronarysyndromes and stable coronary artery disease—canadequately be visualized with multislice-CT [29, 30].Hybrid imaging with both PET and CT, being acombined functional and structural whole-body imagingmodality, could therefore offer the optimal potentialto characterize plaque composition [33, 34]. Tatsumiet al. [35] showed in 85 patients who underwent FDGPET/CT imaging that FDG uptake was commonlydepicted in the thoracic aortic wall. The FDG uptakesite was mostly distinct from the calcification site andwas located in areas of metabolic activity (i.e.,macrophages) of atherosclerotic changes. Ben-Haimet al. [36] evaluated FDG uptake in atheroscleroticplaques in 50 asymptomatic patients. Repeat PET/CTstudies were performed to assess changes in patternsof FDG uptake and CT calcifications. FDG-avid focirepresent a dynamic process of transient inflamma-tion, whereas CT calcifications indicate stableatherosclerosis. Hybrid imaging with PET/CT createstherefore incremental value over both imagingmodalities alone. Rudd et al. [37] showed thatatherosclerosis imaging with FDG-PET/CT wasuseful for tracking inflammation within plaques.

Identification of the target for therapeutic recombinant anti-apoB-100 peptide antibodies in human atherosclerotic lesions

PurposeAccumulation of oxidized LDL in the arterial wall is believed to play a key role in the development of atherosclerosis. Experimental studies have identified the presence of immune responses against epitopes in oxidized LDL that protects against atherosclerosis. We have produced human recombinant IgG against one of these epitopes (aldehyde-modified apoB-100 amino acids 661–680) and demonstrated that treatment with this human IgG1 2D03 antibody markedly reduces atherosclerosis in hypercholesterolemic mice. MethodsIn the present study, we screened a panel of 25 carotid plaques associated with clinical symptoms and 26 clinically silent plaques obtained at surgery for presence of the aldehyde-modified apoB-100 peptide defined by the 2D03 antibody and compared the expression of this epitope with other plaque constituents, plasma lipoproteins levels, plasma oxidized LDL and autoantibodies against apoB-100 peptides. ResultsWe demonstrated that the epitope is commonly expressed in human atherosclerotic plaques and that plaques associated with clinical symptoms have an almost three-fold higher content of this epitope (8.6±4.9% versus 22.1±12.2% immunostaining of total plaque area, p<0.0005). There was also a significant association between 2D03 epitope staining and the plaque content of cholesterol esters (r=0.43, p<0.05), whereas there was no association with plasma oxidized LDL and autoantibodies against apoB-100 peptides. ConclusionsBy demonstrating the presence of the 2D03 epitope in human atherosclerotic lesions our findings support the possibility that treatment with this antibody may have beneficial effects also in humans. Furthermore, they suggest the possibility to use these or other similar antibodies for diagnostic imaging of atherosclerotic plaques in humans.

Development of Receptor for Advanced Glycation End Products–Directed Imaging of Atherosclerotic Plaque in a Murine Model of Spontaneous Atherosclerosis

The receptor for advanced glycation end products (RAGE) is implicated in the development and progression of atherosclerosis. We tested the hypothesis that (99m)Tc-labeled anti-RAGE F(ab')(2) can be used as a noninvasive tool to image atherosclerotic lesions in apolipoprotein E-deficient (apoE(-/-)) mice. A sequence in the V-type Ig extracellular domain of RAGE was used to develop a peptide injected into rabbits; serum was retrieved, IgG prepared and affinity-purified, and pepsin-digested into F(ab')(2). Thirteen 6-week apoE(-/-) mice were fed a Western diet. At 20 weeks, 6 were injected with 15.2+/-1.9 MBq (350 to 411 microCi) (99m)Tc-labeled anti-RAGE F(ab')(2), 6 were injected with (99m)Tc-labeled control nonspecific IgG F(ab')(2), and 1 was injected with dual-labeled (99m)Tc and rhodamine anti-RAGE F(ab')(2). Four 20-week C57BL/6 mice were injected with (99m)Tc-labeled anti-RAGE F(ab')(2). All mice were imaged on a high resolution mini-gamma camera 4 hours after injection and euthanized. The aortic tree was dissected and photographed, and the proximal aorta was sectioned for staining after gamma scintillation counting. All 6 apoE(-/-) mice injected with (99m)Tc-labeled anti-RAGE F(ab')(2) fragments showed focal tracer uptake in the proximal aorta (mean %ID/g, 1.98%). Disease and antibody controls showed no focal tracer uptake in the thorax (%ID/g, <1.0%). Histological sections of the proximal aorta showed American Heart Association class III lesions with lipid laden macrophages, smooth muscle cells, and positive staining for RAGE. On immunofluorescence, RAGE colocalized with macrophages. These data show the feasibility of noninvasively imaging RAGE in atherosclerotic lesions in a murine model and confirm levels of RAGE expression sufficient to allow detection on in vivo imaging.

Fluorescence Reflectance Imaging of Macrophage-Rich Atherosclerotic Plaques Using an αvβ3 Integrin–Targeted Fluorochrome

Macrophages play an important role during the development and progression of atherosclerotic plaques. alphavbeta3 integrins are highly expressed by macrophages; thus, targeting alphavbeta3 may allow targeting of culprit macrophage-loaded atherosclerotic lesions in vivo. An alphavbeta3-targeted Arg-Gly-Asp (RGD) peptide was labeled with the cyanine 5.5 (Cy 5.5) dye and applied to image atherosclerotic plaques in apolipoprotein E-deficient mice. The peptide-dye conjugate binds to alphavbeta3 integrin-positive RAW264.7 macrophages with high affinity. Competition experiments confirmed binding specificity of the probe. A significant fluorochrome accumulation in atherosclerotic plaques was demonstrated 24 h after injection by fluorescence reflectance imaging, which was blocked with high efficiency by competition with the unlabeled peptide. Conversely, the nonconjugated dye revealed only a minor fluorescence signal in the plaques. Fluorescence microscopy revealed colocalization of the probe with macrophages in the plaque of a mouse model for accelerated atherosclerosis, which was corroborated in human carotid artery specimens. In addition to macrophage-associated signals, binding of the probe to the neointima or elastica of the arteries was observed. RGD-Cy 5.5, combined with near-infrared optical imaging methods, allows the specific imaging of alphavbeta3-integrin expression on macrophages recruited to vascular lesions and may serve to estimate macrophage-bound inflammatory activity of atherosclerotic lesions.

Coronary arterial atherosclerotic plaque imaging by contrast-enhanced computed tomography: Fantasy or reality?

Coronary arterial atherosclerotic plaque imaging by contrast-enhanced computed tomography: Fantasy or reality?