Magnetic resonance imaging identifies the fibrous cap in atherosclerotic abdominal aortic aneurysm

Current Imaging Strategies in Patients with Abdominal Aortic Aneurysms

BACKGROUND

 An abdominal aortic aneurysm (AAA) is defined as a localized dilatation of the abdominal aorta of ≥ 3 cm. With a prevalence of 4-8 %, AAA is one of the most common vascular diseases in Western society. Radiological imaging is an elementary component in the diagnosis, monitoring, and treatment planning of AAA patients.

METHOD

 This is a narrative review article on preoperative imaging strategies of AAA, incorporating expert opinions based on the current literature and standard-of-care practices from our own center. Examples are provided to illustrate clinical cases from our institution.

RESULTS AND CONCLUSION

 Radiological imaging plays a pivotal role in the initial diagnosis and monitoring of patients with AAA. Ultrasound is the mainstay imaging modality for AAA screening and surveillance. Contrast-enhanced CT angiography is currently considered the gold standard for preoperative imaging and image-based treatment planning in AAA repair. New non-contrast MR angiography techniques are robustly applicable and allow precise determination of aortic diameters, which is of critical importance, particularly with regard to current diameter-based surgical treatment guidelines. 3D imaging with multiplanar reformation and automatic centerline positioning enables more accurate assessment of the maximum aortic diameter. Modern imaging techniques such as 4D flow MRI have the potential to further improve individualized risk stratification in patients with AAA.

KEY POINTS

  · Ultrasound is the mainstay imaging modality for AAA screening and monitoring. · Contrast-enhanced CT angiography is the gold standard for preoperative imaging in AAA repair. · Non-contrast MR angiography allows for accurate monitoring of aortic diameters in AAA patients. · Measurement of aortic diameters is more accurate with 3D-CT/MRI compared to ultrasound. · Research seeks new quantitative imaging biomarkers for AAA risk stratification, e. g., using 4D flow MRI.

Survival intravascular photoacoustic imaging of lipid-rich plaque in cholesterol fed rabbits

Intravascular photoacoustic (IVPA) imaging is a promising modality for quantitative assessment of lipid-laden atherosclerotic plaques. Yet, survival IVPA imaging of the same plaque in the same animal is not demonstrated. Here, using a sheathed IVUS/PA catheter of 0.9 mm in diameter, we demonstrate MRI-guided survival IVPA imaging of same plaque in an aorta of a well-established rabbit model mimicking atherosclerosis in human patients. The IVUS/PA results were confirmed by histology. These advances open the opportunity to evaluate the effectiveness of a therapy that aims to reduce the size of atherosclerotic plaques and demonstrates the potential of translating the IVPA catheter into clinic for detection of lipid-rich plaques that are at high risk for thrombosis.

Semiautomatic Volumetry of Low Attenuation of Thoracic Aortic Plaques on Curved Planar Reformations Using MDCT Angiographic Data with 0.5 mm Collimation

To evaluate the relationship of aortic low attenuation plaque volume (LAPV) on multidetector computed tomography (MDCT) with the abdominal aortic aneurysm (AAA), the coronary arterial disease (CAD, ≥50% stenosis), severe (≥90% stenosis) CAD, hypertension, and long-term (≥10 years) hypertension. Curved planar reformations (CPR) of three segments (the ascending, the arch, and the upper descending aorta) of the thoracic aorta were generated with attenuation-dependent color codes to measure LAPV with 0~29 HU and total noncalcified plaque volume (TNPV) with 0~150 HU in 95 patients. Correlation coefficients were employed to assess the impact of each LAPV and TNPV on AAA, CAD, severe CAD, hypertension, and long-term hypertension. Each Mean LAPV/cm and TNPV/cm was statistically greater in the aortic arch than the ascending (p < 0.001 on each) or the proximal descending segment (p < 0.001 on each). LAPV in the aortic arch has moderate correlations with AAA, severe CAD, and long-term hypertension (r = 0.643, 0.639, 0.662, resp.). Plaque volumes in each aortic segment can be measured clinically and the increasing LAPV in the arch may be a significant factor associated with the development of severe atherosclerosis underlying AAA, severe CAD, and long-term hypertension.

Comparison of in vivo targeting ability between cRGD and collagen-targeting peptide conjugated nano-carriers for atherosclerosis

Atherosclerosis plaque is a major cause of cardiovascular diseases across the globe and a silent killer. There are no physical symptoms of the disease in its early stage and current diagnostic techniques cannot detect the small plaques effectively or safely. Plaques formed in blood vessels can cause serious clinical problems such as impaired blood flow or sudden death, regardless of their size. Thus, detecting early stage of plaques is especially more important to effectively reduce the risk of atherosclerosis. Nanoparticle based delivery systems are recognized as a promising option to fight against this disease, and various targeting ligands are typically used to improve their efficiency. So, the choice of appropriate targeting ligand is a crucial factor for optimal targeting efficiency. cRGD peptide and collagen IV targeting peptide, which binds with the α_vβ₃ integrin overexpressed in the neovasculature of the plaque and collagen type IV present in the plaque, respectively, are frequently used for the targeting of nanoparticles. However, at present no study has directly compared these two peptides. Therefore, in this study, we have prepared cRGD or collagen IV targeting (Col IV-tg-) peptide conjugated and iron oxide nanoparticle (IONP) loaded Pluronic based nano-carriers for systemic comparison of their targeting ability towards in vivo atherosclerotic plaque in Apolipoprotein E deficient (Apo E^-/-) mouse model. Nano-carriers with similar size, surface charge, and IONP loading content but with different targeting ligands were analyzed through in vitro and in vivo experiments. Near infrared fluorescence imaging and magnetic resonance imaging techniques as well as Prussian blue staining were used to compare the accumulation of different ligand conjugated nano-caariers in the aorta of atherosclerotic mice. Our results indicate that cRGD based targeting is more efficient than Col IV-tg-peptide in the early stage of atherosclerosis.

Aortic atheroma as a source of stroke - assessment of embolization risk using 3D CMR in stroke patients and controls

BACKGROUND

It was our purpose to identify vulnerable plaques in the thoracic aorta using 3D multi-contrast CMR and estimate the risk of cerebral embolization using 4D flow CMR in cryptogenic stroke patients and controls.

METHODS

One hundred patients (40 with cryptogenic stroke, 60 ophthalmologic controls matched for age, sex and presence of hypertension) underwent a novel 3D multi-contrast (T1w, T2w, PDw) CMR protocol at 3 Tesla for plaque detection and characterization within the thoracic aorta, which was combined with 4D flow CMR for mapping potential embolization pathways. Plaque morphology was assessed in consensus reading by two investigators and classified according to the modified American-Heart-Association (AHA) classification of atherosclerotic plaques.

RESULTS

In the thoracic aorta, plaques <4 mm thickness were found in a similar number of stroke patients and controls [23 (57.5%) versus 33 (55.0%); p = 0.81]. However, plaques ≥4 mm were more frequent in stroke patients [22 (55.0%) versus 10 (16.7%); p < 0.001]. Of those patients with plaques ≥4 mm, seven (17.5%) stroke patients and two (3.3%) controls (p < 0.001) had potentially vulnerable AHA type VI plaques. Six stroke patients with vulnerable AHA type VI plaques ≥4 mm had potential embolization pathways connecting the plaque, located in the aortic arch (n = 3) and proximal descending aorta (n = 3), with the individual territory of stroke, which made them the most likely source of stroke in those patients.

CONCLUSIONS

Our findings underline the significance of ≥4 mm thick and vulnerable plaques in the aortic arch and descending aorta as a relevant etiology of stroke.

CLINICAL TRIAL REGISTRATION

Unique identifier: DRKS00006234 ; date of registration: 11/06/2014.

Multi-contrast and three-dimensional assessment of the aortic wall using 3T MRI

OBJECTIVES

To develop a 3D-multi-contrast MRI protocol allowing for high resolution imaging of the wall and of atheroma in the thoracic aorta.

METHODS

Eleven healthy volunteers and eleven acute stroke patients with aortic plaques detected by TEE underwent MRI at 3T. The MRI-protocol consisted of a T1w-bright-blood, a T2w- and a PDw-black-blood sequence (spatial resolution=1.15mm³). Image quality was assessed by two blinded investigators using a 3-point score and intra- and inter-rater agreement was tested. In patients, atherosclerotic plaques were graded according to the modified American Heart Association (AHA) classification.

RESULTS

Total examination time was 35:42±7:48min in volunteers and 41:07±3:15min in patients. Image quality was graded with the highest score in 80-94% of T1w, 89-96% of T2w and 79-86% of PDw datasets. Intra- and inter-rater reliability regarding image quality grading was high. Five stroke patients showed AHA type III lesions, three had AHA type VII and two had type VIII plaques. One patient had a vulnerable appearing AHA VI plaque.

CONCLUSIONS

3D-multi-contrast MR-imaging of the aorta was performed with high image quality and in reasonable time. It allows evaluation of atherosclerotic plaque composition throughout the aortic arch and can be used to identify vulnerable plaques in acute stroke patients.

Evidence of deregulated cholesterol efflux in abdominal aortic aneurysm

Previous studies indicated that lipids may be associated with abdominal aortic aneurysm (AAA); however the molecular mechanism involved is unclear. Our study aimed to investigate the expression pattern of cholesterol efflux related proteins in AAA. Liver X receptors (LXRα and LXRβ), ATP-binding-cassette transporter A1 (ABCA1), Apolipoprotein AI (ApoAI), smooth muscle α-actin (α-SM) and vimentin expression levels were evaluated in human AAA, atherosclerotic (ATH) and normal abdominal aortic tissues. We found significant differences in LXRα, LXRβ and ABCA1 mRNA expression levels between AAA, ATH and normal whole aortic tissues and also within the AAA, ATH and normal "intima-media" layers. Specifically, LXRα, LXRβ and ABCA1 mRNA levels were decreased in AAA compared to ATH-whole tissues, as well as in AAA "intima-media" compared to ATH and normal "intima-media" layers. Moreover, immunohistochemical evaluation revealed that LXRα and ABCA1 immunoreactivities (IR) were reduced in the AAA media compared to the normal and ATH media layers and that they were also reduced in the intima layer of AAA and ATH tissues, whereas ApoAI-IR was increased in the AAA and ATH aortic walls compared to normal pointing to possible deregulation of the cholesterol efflux mechanism in AAA. Furthermore, double staining for vimentin and α-SM showed vimentin expression in the intima and inner media layer of AAA with sparse vimentin positive SMCs designating possible SMCs phenotype switch from contractile to synthetic form. In addition, histochemical analysis showed excessive lipid accumulation in the AAA wall, while co-staining using Oil Red O with α-SM or CD68 revealed lipid accumulation in SMCs and macrophages, respectively. Our study provides novel evidence for impaired cholesterol efflux in AAA associated with lipid accumulation in SMCs and macrophages, as well as switch of SMCs phenotype from contractile to synthetic form.

Monitoring the biological activity of abdominal aortic aneurysms Beyond Ultrasound

Abdominal aortic aneurysms (AAAs) are an important cause of morbidity and, when ruptured, are associated with >80% mortality. Current management decisions are based on assessment of aneurysm diameter by abdominal ultrasound. However, AAA growth is non-linear and rupture can occur at small diameters or may never occur in those with large AAAs. There is a need to develop better imaging biomarkers that can identify the potential risk of rupture independent of the aneurysm diameter. Key pathobiological processes of AAA progression and rupture include neovascularisation, necrotic inflammation, microcalcification and proteolytic degradation of the extracellular matrix. These processes represent key targets for emerging imaging techniques and may confer an increased risk of expansion or rupture over and above the known patient-related risk factors. Magnetic resonance imaging, using ultrasmall superparamagnetic particles of iron oxide, can identify and track hotspots of macrophage activity. Positron emission tomography, using a variety of targeted tracers, can detect areas of inflammation, angiogenesis, hypoxia and microcalcification. By going beyond the simple monitoring of diameter expansion using ultrasound, these cellular and molecular imaging techniques may have the potential to allow improved prediction of expansion or rupture and to better guide elective surgical intervention.

Gadospin F-enhanced magnetic resonance imaging for diagnosis and monitoring of atherosclerosis: validation with transmission electron microscopy and x-ray fluorescence imaging in the apolipoprotein e-deficient mouse

The aim of this study was to investigate the feasibility of noninvasive monitoring of plaque burden in apolipoprotein E-deficient (ApoE-/-) mice by Gadospin F (GDF)-enhanced magnetic resonance imaging (MRI). Gadolinium uptake in plaques was controlled using transmission electron microscopy (TEM) and x-ray fluorescence (XRF) microscopy. To monitor the progression of atherosclerosis, ApoE-/- (n  =  5) and wild-type (n  =  2) mice were fed a Western diet and imaged at 5, 10, 15, and 20 weeks. Contrast-enhanced MRI was performed at 7 T Clinscan (Bruker, Ettlingen, Germany) before and 2 hours after intravenous injection of GDF (100 μmol/kg) to determine the blood clearance. Plaque size and contrast to noise ratio (CNR) were calculated for each time point using region of interest measurements to evaluate plaque progression. Following MRI, aortas were excised and GDF uptake was cross-validated by TEM and XRF microscopy. The best signal enhancement in aortic plaque was achieved 2 hours after application of GDF. No signal differences between pre- and postcontrast MRI were detectable in wild-type mice. We observed a gradual and considerable increase in plaque CNR and size for the different disease stages. TEM and XRF microscopy confirmed the localization of GDF within the plaque. GDF-enhanced MRI allows noninvasive and reliable estimation of plaque burden and monitoring of atherosclerotic progression in vivo.

Coronary plaque characterization using CT

OBJECTIVE. In this article, we review the histopathologic classification of coronary atherosclerotic plaques and describe the possibilities and limitations of CT regarding the evaluation of coronary artery plaques. CONCLUSION. The composition of atherosclerotic plaques in the coronary arteries displays substantial variability and is associated with the likelihood for rupture and downstream ischemic events. Accurate identification and quantification of coronary plaque components on CT is challenging because of the limited temporal, spatial, and contrast resolutions of current scanners. Nonetheless, CT may provide valuable information that has potential for characterization of coronary plaques. For example, the extent of calcification can be determined, lipid-rich lesions can be separated from more fibrous ones, and positive remodeling can be identified.

MRI using ultrasmall superparamagnetic particles of iron oxide in patients under surveillance for abdominal aortic aneurysms to predict rupture or surgical repair: MRI for abdominal aortic aneurysms to predict rupture or surgery-the MA(3)RS study

Introduction

Population screening for abdominal aortic aneurysms (AAA) halves the associated mortality and has led to the establishment of national screening programmes. Prediction of aneurysm growth and rupture is challenging and currently relies on serial diameter measurements with ultrasound. Recently, a novel MRI-based technique using ultrasmall superparamagnetic particles of iron oxide (USPIO) has demonstrated considerable promise as a method of identifying aneurysm inflammation and expansion.

Methods and analysis

The MA³RS study is a prospective observational multicentre cohort study of 350 patients with AAA in three centres across Scotland. All participants will undergo MRI with USPIO and aneurysm expansion will be measured over 2 years with CT in addition to standard clinical ultrasound surveillance. The relationship between mural USPIO uptake and subsequent clinical outcomes, including expansion, rupture and repair, will be evaluated and used to determine whether the technique augments standard risk prediction markers. To ensure adequate sensitivity to answer the primary question, we need to observe 130 events (composite of rupture or repair) with an estimated event rate of 41% over 2 years of follow-up. The MA³RS study is currently recruiting and expects to report in 2017.

Discussion

This is the first study to evaluate the use of USPIO-enhanced MRI to provide additional information to aid risk prediction models in patients with AAA. If successful, this study will lay the foundation for a large randomised controlled trial targeted at applying this technique to determine clinical management.

Trial registration number

Current Controlled Trials: ISRCTN76413758.

Bad aorta

As its outcomes improve, cardiac surgery has been performed on more and more cases which were previously considered to be difficult to deal with. However, there are still a number of problems to be solved regarding surgery on patients with severe sclerotic lesions in the ascending aorta, which we collectively call "bad aorta". Concerning a preoperative assessment of the ascending aorta, our report revealed no relationship between the severity of calcification detected with a preoperative non-enhanced CT and the aortic lesion found during the surgery. Meanwhile, an intraoperative epiaortic ultrasound enables us to make high-quality evaluations of the aorta without imposing much burden on the patient. This modality may be essential for cardiac surgery. As for surgical management for bad aorta, quite a few methods have been reported to this point, but the overall operative mortality rate and cerebrovascular accident rate are relatively high, at a little <10 %, respectively. With the recent cross-clamping method under short-term total circulatory arrest (TCA), however, the results are much better; these rates total around 5 %. Further improvement is expected in the outcome of cardiac surgery on bad aorta cases by establishing a modality to evaluate sclerotic lesions in the ascending aorta with epiaortic ultrasound and by selecting a proper procedure for each case.

A multimodality vascular imaging phantom of an abdominal aortic aneurysm with a visible thrombus

PURPOSE

With the continuous development of new stent grafts and implantation techniques, it has now become technically feasible to treat abdominal aortic aneurysms (AAA) with challenging anatomy using endovascular repair with standard, fenestrated, or branched stent-grafts. In vitro experimentations are very useful to improve stent-graft design and conformability or imaging guidance for stent-graft delivery or follow-up. Vascular replicas also help to better understand the limitation of endovascular approaches in challenging anatomy and possibly improve surgical planning or training by practicing high risk clinical procedures in the laboratory to improve outcomes in the operating room. Most AAA phantoms available have a very basic anatomy, which is not representative of the clinical reality. This paper presents a method of fabrication of a realistic AAA phantom with a visible thrombus, as well as some mechanical properties characterizing such phantom.

METHODS

A realistic AAA geometry replica of a real patient anatomy taken from a multidetector computed tomography (CT) scan was manufactured. To demonstrate the multimodality imaging capability of this new phantom with a thrombus visible in magnetic resonance (MR) angiography, CT angiography (CTA), digital subtraction angiography (DSA), and ultrasound, image acquisitions with all these modalities were performed by using standard clinical protocols. Potential use of this phantom for stent deployment was also tested. A rheometer allowed defining hyperelastic and viscoelastic properties of phantom materials.

RESULTS

MR imaging measurements of SNR and CNR values on T1 and T2-weighted sequences and MR angiography indicated reasonable agreement with published values of AAA thrombus and abdominal components in vivo. X-ray absorption also lay within normal ranges of AAA patients and was representative of findings observed on CTA, fluoroscopy, and DSA. Ultrasound propagation speeds for developed materials were also in concordance with the literature for vascular and abdominal tissues.

CONCLUSIONS

The mimicked abdominal tissues, AAA wall, and surrounding thrombus were developed to match imaging features of in vivo MR, CT, and ultrasound examinations. This phantom should be of value for image calibration, segmentation, and testing of endovascular devices for AAA endovascular repair.

Statins reduce extensive aortic atheromas in patients with abdominal aortic aneurysms

OBJECTIVE

Statins have been used widely to reduce dyslipidemia and recently have been reported to have pleiotropic effects such as plaque reduction and stabilization. This study retrospectively evaluated the regression of extensive thoracic atheromas ("shaggy aorta") in abdominal aortic aneurysm (AAA) patients who underwent contrast-enhanced computed tomography (CECT) before and after statin administration.

MATERIALS AND METHODS

CECT was used to examine thoracic aortas of 29 patients (statin group; n = 22, non-statin group; n = 7) with extensive atheromas from the ostium of the left subclavian artery to that of the more proximal renal artery. Extensive thoracic atheroma was defined by: (1) thickness >5 mm, (2) involved circumference of thoracic aorta >50%, and (3) length >30 mm. The areas of atheroma (cm(2)) were measured before and after administration of statins, and the atheroma reduction ratio (ARR) was evaluated.

RESULTS

The area of atheroma decreased after administration of statins, and the ARR was significant (P <0.01). The ARR increased with all cases in non-statin group. No complications associated with extensive atheroma were observed during the follow-up period.

CONCLUSION

This pilot study indicates statins can reduce extensive thoracic atheromas and lower lipid concentrations.

Molecular MRI of atherosclerosis

Despite advances in prevention, risk assessment and treatment, coronary artery disease (CAD) remains the leading cause of morbidity and mortality in Western countries. The lion's share is due to acute coronary syndromes (ACS), which are predominantly triggered by plaque rupture or erosion and subsequent coronary thrombosis. As the majority of vulnerable plaques does not cause a significant stenosis, due to expansive remodeling, and are rather defined by their composition and biological activity, detection of vulnerable plaques with x-ray angiography has shown little success. Non-invasive vulnerable plaque detection by identifying biological features that have been associated with plaque progression, destabilization and rupture may therefore be more appropriate and may allow earlier detection, more aggressive treatment and monitoring of treatment response. MR molecular imaging with target specific molecular probes has shown great promise for the noninvasive in vivo visualization of biological processes at the molecular and cellular level in animals and humans. Compared to other imaging modalities; MRI can provide excellent spatial resolution; high soft tissue contrast and has the ability to simultaneously image anatomy; function as well as biological tissue composition and activity.

Molecular imaging of experimental abdominal aortic aneurysms

Current laboratory research in the field of abdominal aortic aneurysm (AAA) disease often utilizes small animal experimental models induced by genetic manipulation or chemical application. This has led to the use and development of multiple high-resolution molecular imaging modalities capable of tracking disease progression, quantifying the role of inflammation, and evaluating the effects of potential therapeutics. In vivo imaging reduces the number of research animals used, provides molecular and cellular information, and allows for longitudinal studies, a necessity when tracking vessel expansion in a single animal. This review outlines developments of both established and emerging molecular imaging techniques used to study AAA disease. Beyond the typical modalities used for anatomical imaging, which include ultrasound (US) and computed tomography (CT), previous molecular imaging efforts have used magnetic resonance (MR), near-infrared fluorescence (NIRF), bioluminescence, single-photon emission computed tomography (SPECT), and positron emission tomography (PET). Mouse and rat AAA models will hopefully provide insight into potential disease mechanisms, and the development of advanced molecular imaging techniques, if clinically useful, may have translational potential. These efforts could help improve the management of aneurysms and better evaluate the therapeutic potential of new treatments for human AAA disease.

Ruptured abdominal aortic aneurysm diagnosed through non-contrast MRI

Rupture of an aneurysm is a rare complication although it is considered a common cause of death. Some of these patients present with the classic triad of symptoms such as abdominal pain, pulsatile abdominal mass and shock. Most symptoms are misleading and will only present as vague abdominal pain. Here we describe one such patient with an unusual presentation of a misleading abdominal mass which was eventually diagnosed as a ruptured abdominal aortic aneurysm after an emergency MRI.

Usefulness of cardiovascular magnetic resonance imaging of the superficial femoral artery for screening patients with diabetes mellitus for atherosclerosis

Cardiovascular magnetic resonance (CMR) of the superficial femoral artery (SFA) allows direct and noninvasive visualization of atherosclerotic plaque burden. We examined atherosclerosis in 3 groups of patients without history or symptoms of peripheral arterial disease with varying expected burdens: those with diabetes mellitus (DM) and known coronary artery disease (CAD) (n = 24), those with DM and a high prevalence of CAD risk factors (n = 20), and controls of similar age without DM or CAD and few CAD risk factors (n = 15). We also assessed the diagnostic accuracy of this technique to differentiate among these 3 groups. T1-weighted spin-echocardiographic images were used to measure mean wall thickness (WT) and total wall volume indexed to total vessel volume. Diagnostic accuracy was assessed by area under receiver operating characteristics curve analysis. Patients with DM plus risk factors and DM plus CAD had higher mean WT (1.28 and 1.37 mm) and mean indexed wall volume (0.53 and 0.56) compared to controls (mean WT 1.16 mm and mean indexed wall volume 0.45; p <0.010 for all comparisons). Mean WT and indexed wall volume showed good diagnostic accuracy in discriminating controls from those with DM plus CAD (areas under curve 0.85 and 0.87, respectively, p <0.001), whereas only indexed wall volume discriminated DM plus risk factors from controls (area under curve 0.82, p <0.001). Neither could discriminate between DM plus risk factors and DM plus CAD. In conclusion, patients with DM plus risk factors and DM plus CAD had significantly greater atherosclerotic burden in the SFA on CMR imaging than controls of similar age, with good diagnostic accuracy in differentiating these groups. The high reproducibility and reliability of CMR of the SFA may facilitate improved assessment of atherosclerosis prevalence and progression/regression in studies of novel therapies.

Is aortic atherothrombotic disease detected using multidetector-row CT associated with an increased risk of early ischemic lesion recurrence after acute ischemic stroke?

BACKGROUND AND PURPOSE

Multidetector-row CT (MDCT) is emerging as a new tool for diagnosing aortic atherothrombotic disease (AAD). We elucidated whether MDCT-detected AAD is associated with an increased risk of early ischemic lesion recurrence on diffusion-weighted MRI after ischemic stroke.

METHODS

A consecutive series of patients with acute ischemic stroke confirmed using diffusion-weighted MRI who were hospitalized within 48 hours after symptom onset and underwent MDCT were identified in a prospective stroke registry database. AAD on MDCT was defined as the presence of plaque formation that was noncalcified and ≥4 mm thick, ulcerative, or soft and thrombosed (vulnerable) in the proximal aortic arch. Ischemic lesion recurrence on diffusion-weighted MRI was defined as the occurrence of any new lesion separate from the index lesion on follow-up diffusion-weighted MRI performed within 14 days after symptom onset.

RESULTS

A total of 138 patients was selected. MDCT detected AAD in 24 of 138 (17.4%); ≥4 mm thickness in 17 of 138 (12.3%); ulcerated plaque in 20 of 138 (14.5%); and vulnerable plaque in 16 of 138 (11.6%). With respect to diffusion-weighted MRI lesion recurrence, the crude ORs (95% CIs) were as follows: AAD, 3.56 (1.43-8.89); vulnerable plaque, 3.21 (1.11-9.30); ulcerated plaque, 3.37 (1.27-8.95); and ≥4 mm thickness of the noncalcified plaque, 4.23 (1.11-16.19). These results remained significant after adjustments for potential confounders were made.

CONCLUSIONS

This study shows that AAD detected by MDCT increases the risk of early ischemic lesion recurrence after acute ischemic stroke, thus supporting the role of MDCT in diagnosing AAD and assessing its contribution to recurrence.

Accelerated aortic imaging using small field of view imaging and electrocardiogram-triggered quadruple inversion recovery magnetization preparation

PURPOSE

To accelerate and optimize black blood properties of the quadruple inversion recovery (QIR) technique for imaging the abdominal aortic wall.

MATERIALS AND METHODS

QIR inversion delays were optimized for different heart rates in simulations and phantom studies by minimizing the steady state magnetization of blood for T(1) = 100-1400 ms. To accelerate and improve black blood properties of aortic vessel wall imaging, the QIR prepulse was combined with zoom imaging and (a) "traditional" and (b) "trailing" electrocardiogram (ECG) triggering. Ten volunteers were imaged pre- and post-contrast administration using a conventional ECG-triggered double inversion recovery (DIR) and the two QIR implementations in combination with a zoom-TSE readout.

RESULTS

The QIR implemented with "trailing" ECG-triggering resulted in consistently good blood suppression as the second inversion delay was timed during maximum systolic flow in the aorta. The blood signal-to-noise ratio and vessel wall to blood contrast-to-noise ratio, vessel wall sharpness, and image quality scores showed a statistically significant improvement compared with the traditional QIR implementation with and without ECG-triggering.

CONCLUSION

We demonstrate that aortic vessel wall imaging can be accelerated with zoom imaging and that "trailing" ECG-triggering improves black blood properties of the aorta which is subject to motion and variable blood flow during the cardiac cycle.

Coronary artery distensibility assessed by 3.0 Tesla coronary magnetic resonance imaging in subjects with and without coronary artery disease

Coronary vessel distensibility is reduced with atherosclerosis and normal aging, but direct measurements have historically required invasive measurements at cardiac catheterization. Therefore, we sought to assess coronary artery distensibility noninvasively using 3.0 Telsa coronary magnetic resonance imaging (MRI) and to test the hypothesis that this noninvasive technique can detect differences in coronary distensibility between healthy subjects and those with coronary artery disease (CAD). A total of 38 healthy, adult subjects (23 men, mean age 31 ± 10 years) and 21 patients with CAD, diagnosed using x-ray angiography (11 men, mean age 57 ± 6 years) were studied using a commercial whole-body MRI system. In each subject, the proximal segment of a coronary artery was imaged for the cross-sectional area measurements using cine spiral MRI. The distensibility (mm Hg(-1) × 10(3)) was determined as (end-systolic lumen area - end-diastolic lumen area)/(pulse pressure × end-diastolic lumen area). The pulse pressure was calculated as the difference between the systolic and diastolic brachial blood pressure. A total of 34 healthy subjects and 19 patients had adequate image quality for coronary area measurements. Coronary artery distensibility was significantly greater in the healthy subjects than in those with CAD (mean ± SD 2.4 ± 1.7 mm Hg(-1) × 10(3) vs 1.1 ± 1.1 mm Hg(-1) × 10(3), respectively, p = 0.007; median 2.2 vs 0.9 mm Hg(-1) × 10(3)). In a subgroup of 10 patients with CAD, we found a significant correlation between the coronary artery distensibility measurements assessed using MRI and x-ray coronary angiography (R = 0.65, p = 0.003). In a group of 10 healthy subjects, the repeated distensibility measurements demonstrated a significant correlation (R = 0.80, p = 0.006). In conclusion, 3.0-Tesla MRI, a reproducible noninvasive method to assess human coronary artery vessel wall distensibility, is able to detect significant differences in distensibility between healthy subjects and those with CAD.

Imaging of atherosclerosis: magnetic resonance imaging

Atherosclerosis and its thrombotic complications are the major cause of morbidity and mortality in the industrialized countries. Despite advances in our understanding of the pathophysiology, pathogenesis, and new treatment modalities, the absence of an adequate non-invasive imaging tool for early detection limits both the prevention and treatment of patients with various degrees and anatomical localizations of atherothrombotic disease. An ideal clinical imaging modality for atherosclerotic vascular disease should be safe, inexpensive, non-invasive or minimally invasive, accurate, and reproducible, and the results should correlate with the extent of atherosclerotic disease and have high predictive values for future clinical events. High-resolution magnetic resonance imaging (MRI) has emerged as the most promising technique for studying atherothrombotic disease in humans in vivo. Most importantly, MRI allows for the characterization of plaque composition, i.e. the discrimination of lipid core, fibrosis, calcification, and intraplaque haemorrhage deposits. Magnetic resonance imaging also allows for the detection of arterial thrombi and in defining thrombus age. Magnetic resonance imaging has been used to monitor plaque progression and regression in several animal models of atherosclerosis and in humans. Emerging MRI techniques capable of imaging biological processes, including inflammation, neovascularization, and mechanical forces, may aid in advancing our understanding of the atherothrombotic disease. Advances in diagnosis do prosper provided they march hand-in-hand with advances in treatment. We stand at the threshold of accurate non-invasive assessment of atherosclerosis. Thus, MRI opens new strategies ranging from screening of high-risk patients for early detection and treatment as well as monitoring of the target lesions for pharmacological intervention. Identification of subclinical atherosclerosis and early treatment initiation has the potential to surpass conventional risk factor assessment and management in terms of overall impact on cardiovascular morbidity and mortality. Such strategy is currently under clinical investigation.

Diagnostic and therapeutic strategies for small abdominal aortic aneurysms

Abdominal aortic aneurysms (AAA) affect 5% of the population in developed countries and are characterized by progressive aortic dilatation with an unpredictable time course. This condition is more common in men than in women, and in smokers than in nonsmokers. If left untreated, AAA can result in aortic rupture and death. Pathologically, aortic extracellular matrix degradation, inflammation, and neovascularization are hallmarks of AAA. Diagnosis of AAA and subsequent surveillance utilize established aortic imaging methods, such as ultrasound, CT, and MRI. More-speculative diagnostic approaches include molecular and cellular imaging methods that interrogate the underlying pathological processes at work within the aneurysm. In this Review, we explore the current diagnostic and therapeutic strategies for the management of AAA. We also describe the diagnostic potential of new imaging techniques and therapeutic potential of new treatments for the management of small AAA.

Abdominal aortic aneurysm growth predicted by uptake of ultrasmall superparamagnetic particles of iron oxide: a pilot study

BACKGROUND

Abdominal aortic aneurysms are a major cause of death. Prediction of aneurysm expansion and rupture is challenging and currently relies on the simple measure of aneurysm diameter. Using MRI, we aimed to assess whether areas of cellular inflammation correlated with the rate of abdominal aortic aneurysm expansion.

METHODS AND RESULTS

Stable patients (n=29; 27 male; age, 70±5 years) with asymptomatic abdominal aortic aneurysms (4.0 to 6.6 cm) were recruited from a surveillance program and imaged using a 3-T MRI scanner before and 24 to 36 hours after administration of ultrasmall superparamagnetic particles of iron oxide (USPIO). The change in T2* value on T2*-weighted imaging was used to detect accumulation of USPIO within the abdominal aortic aneurysm. Histological examination of aneurysm tissue confirmed colocalization and uptake of USPIO in areas with macrophage infiltration. Patients with distinct mural uptake of USPIO had a 3-fold higher growth rate (n=11, 0.66 cm/y; P=0.020) than those with no (n=6, 0.22 cm/y) or nonspecific USPIO uptake (n=8, 0.24 cm/y) despite having similar aneurysm diameters (5.4±0.6, 5.1±0.5, and 5.0±0.5 cm, respectively; P>0.05). In 1 patient with an inflammatory aneurysm, there was a strong and widespread uptake of USPIO extending beyond the aortic wall.

CONCLUSIONS

Uptake of USPIO in abdominal aortic aneurysms identifies cellular inflammation and appears to distinguish those patients with more rapidly progressive abdominal aortic aneurysm expansion. This technique holds major promise as a new method of risk-stratifying patients with abdominal aortic aneurysms that extends beyond the simple anatomic measure of aneurysm diameter. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT00794092.

Molecular imaging with targeted contrast agents

Molecular imaging with targeted contrast agents by magnetic resonance imaging (MRI) allows for the noninvasive detection and characterization of biological changes on a molecular level. In this article, the principles of molecular MRI and its applications in cardiovascular diseases are reviewed. First, basic properties of positive and negative contrast agents are introduced and their effect on signal generation in a magnetic field is described. In the next part, different types of MRI scanners and the influence of field strength on signal properties of contrast agents for molecular imaging are discussed. Additionally, the assessment, analysis, and quantification of the changes in T1 and T2* relaxation time induced by the different molecular contrast agents are reviewed. Finally, the basic mechanisms of targeting of imaging probes on a molecular level and recent applications of molecular MRI in cardiovascular disease are reviewed.

Current techniques for MR imaging of atherosclerosis

Vessel wall imaging of large vessels has the potential to identify culprit atherosclerotic plaques that lead to cardiovascular events. Comprehensive assessment of atherosclerotic plaque size, composition, and biological activity is possible with magnetic resonance imaging (MRI). Magnetic resonance imaging of the atherosclerotic plaque has demonstrated high accuracy and measurement reproducibility for plaque size. The accuracy of in vivo multicontrast MRI for identification of plaque composition has been validated against histological findings. Magnetic resonance imaging markers of plaque biological activity such as neovasculature and inflammation have been demonstrated. In contrast to other plaque imaging modalities, MRI can be used to study multiple vascular beds noninvasively over time. In this review, we compare the status of in vivo plaque imaging by MRI to competing imaging modalities. Recent MR technological improvements allow fast, accurate, and reproducible plaque imaging. An overview of current MRI techniques required for carotid plaque imaging including hardware, specialized pulse sequences, and processing algorithms are presented. In addition, the application of these techniques to coronary, aortic, and peripheral vascular beds is reviewed.

Screening for atherosclerotic plaques in the abdominal aorta in high-risk patients with multicontrast-weighted MRI: a prospective study at 3.0 and 1.5 tesla

OBJECTIVE

This prospective study compares MRI of atherosclerotic plaque in the abdominal aorta at 3 T with that at 1.5 T in patients suffering from hereditary hyperlipidaemia, a major risk factor for atherosclerosis.

METHODS

MRI of the abdominal aorta at 1.5 and 3 T was performed in 21 patients (mean age 58 years). The study protocol consisted of proton density (PD), T(1), T(2) and fat-saturated T(2) weighted black blood images of the abdominal aorta in corresponding orientation. Two independent radiologists performed image rating. First, image quality was rated on a five-point scale. Second, atherosclerotic plaques were scored according to the modified American Heart Association (AHA) classification and analysed for field strength-related differences. Weighted κ statistics were calculated to assess interobserver agreement.

RESULTS

Interobserver agreement was substantial for nearly all categories. MRI at 3 T offered superior image quality in all contrast weightings, most significantly in T(1) and T(2) weighted techniques. Plaque burden in the study collective was unexpectedly moderate. The majority of plaques were classified as AHA III lesions; no lesions were classified above AHA V. There was no significant influence of the field strength regarding the AHA classification.

CONCLUSION

Abdominal aortal plaque screening is basically feasible at both field strengths, whereas the image quality is rated superior at 3 T. However, the role of the method in clinical practice remains uncertain, since substantial findings in the high-risk collective were scarce.

Magnetic resonance imaging of carotid atherosclerotic plaque in clinically suspected acute transient ischemic attack and acute ischemic stroke

BACKGROUND

Carotid atherosclerotic plaque rupture is thought to cause transient ischemic attack (TIA) and ischemic stroke (IS). Pathological hallmarks of these plaques have been identified through observational studies. Although generally accepted, the relationship between cerebral thromboembolism and in situ atherosclerotic plaque morphology has never been directly observed noninvasively in the acute setting.

METHODS AND RESULTS

Consecutive acutely symptomatic patients referred for stroke protocol magnetic resonance imaging/angiography underwent additional T1- and T2-weighted carotid bifurcation imaging with the use of a 3-dimensional technique with blood signal suppression. Two blinded reviewers performed plaque gradings according to the American Heart Association classification system. Discharge outcomes and brain magnetic resonance imaging results were obtained. Image quality for plaque characterization was adequate in 86 of 106 patients (81%). Eight TIA/IS patients with noncarotid pathogenesis were excluded, yielding 78 study patients (38 men and 40 women with a mean age of 64.3 years, SD 14.7) with 156 paired watershed vessel/cerebral hemisphere observations. Thirty-seven patients had 40 TIA/IS events. There was a significant association between type VI plaque (demonstrating cap rupture, hemorrhage, and/or thrombosis) and ipsilateral TIA/IS (P<0.001). A multiple logistic regression model including standard Framingham risk factors and type VI plaque was constructed. Type VI plaque was the dominant outcome-associated observation achieving significance (P<0.0001; odds ratio, 11.66; 95% confidence interval, 5.31 to 25.60).

CONCLUSIONS

In situ type VI carotid bifurcation region plaque identified by magnetic resonance imaging is associated with ipsilateral acute TIA/IS as an independent identifier of events, thereby supporting the dominant disease pathophysiology.

Gadolinium Enhanced MR Coronary Vessel Wall Imaging at 3.0 Tesla

Purpose. We evaluated the influence of the time between low-dose gadolinium (Gd) contrast administration and coronary vessel wall enhancement (LGE) detected by 3T magnetic resonance imaging (MRI) in healthy subjects and patients with coronary artery disease (CAD). Materials and Methods. Four healthy subjects (4 men, mean age 29 ± 3 years and eleven CAD patients (6 women, mean age 61 ± 10 years) were studied on a commercial 3.0 Tesla (T) whole-body MR imaging system (Achieva 3.0 T; Philips, Best, The Netherlands). T1-weighted inversion-recovery coronary magnetic resonance imaging (MRI) was repeated up to 75 minutes after administration of low-dose Gadolinium (Gd) (0.1 mmol/kg Gd-DTPA). Results. LGE was seen in none of the healthy subjects, however in all of the CAD patients. In CAD patients, fifty-six of 62 (90.3%) segments showed LGE of the coronary artery vessel wall at time-interval 1 after contrast. At time-interval 2, 34 of 42 (81.0%) and at time-interval 3, 29 of 39 evaluable segments (74.4%) were enhanced. Conclusion. In this work, we demonstrate LGE of the coronary artery vessel wall using 3.0 T MRI after a single, low-dose Gd contrast injection in CAD patients but not in healthy subjects. In the majority of the evaluated coronary segments in CAD patients, LGE of the coronary vessel wall was already detectable 30–45 minutes after administration of the contrast agent.

Aortic atheroma and adverse cerebral outcome: risk, diagnosis, and management options

Aortic atheromatous disease is a common finding in the patient presenting for cardiac surgery. Adverse neurologic outcome has been closely linked to the extent of aortic atherosclerosis. In order to optimize perioperative outcomes, the location and severity of disease needs accurate characterization using multimodal techniques. Although various preoperative radiographic techniques have variably identified patients with significant atheroma, intraoperative echocardiographic imaging has proven most useful in localizing and characterizing the degree of aortic atheroma. Epiaortic assessment of the ascending aorta has been utilized in guiding surgical modifications and interventions aimed at reducing the risk of neurologic injury. Although no particular technique has been definitely studied, avoidance of the identifiable atheromatous aortic region has been a main feature of the various modifications employed to optimize neurologic outcome after cardiac surgery.

Complex atheromatosis of the aortic arch in cerebral infarction

In many stroke patients it is not possible to establish the etiology of stroke. However, in the last two decades, the use of transesophageal echocardiography in patients with stroke of uncertain etiology reveals atherosclerotic plaques in the aortic arch, which often protrude into the lumen and have mobile components in a high percentage of cases. Several autopsy series and retrospective studies of cases and controls have shown an association between aortic arch atheroma and arterial embolism, which was later confirmed by prospectively designed studies. The association with ischemic stroke was particularly strong when atheromas were located proximal to the ostium of the left subclavian artery, when the plaque was ≥ 4 mm thick and particularly when mobile components are present. In these cases, aspirin might not prevent adequately new arterial ischemic events especially stroke. Here we review the evidence of aortic arch atheroma as an independent risk factor for stroke and arterial embolism, including clinical and pathological data on atherosclerosis of the thoracic aorta as an embolic source. In addition, the impact of complex plaques (≥ 4 mm thick, or with mobile components) on increasing the risk of stroke is also reviewed. In non-randomized retrospective studies anticoagulation was superior to antiplatelet therapy in patients with stroke and aortic arch plaques with mobile components. In a retrospective case-control study, statins significantly reduced the relative risk of new vascular events. However, given the limited data available and its retrospective nature, randomized prospective studies are needed to establish the optimal secondary prevention therapeutic regimens in these high risk patients.

The association of lesion eccentricity with plaque morphology and components in the superficial femoral artery: a high-spatial-resolution, multi-contrast weighted CMR study

BACKGROUND

Atherosclerotic plaque morphology and components are predictors of subsequent cardiovascular events. However, associations of plaque eccentricity with plaque morphology and plaque composition are unclear. This study investigated associations of plaque eccentricity with plaque components and morphology in the proximal superficial femoral artery using cardiovascular magnetic resonance (CMR).

METHODS

Twenty-eight subjects with an ankle-brachial index less than 1.00 were examined with 1.5 T high-spatial-resolution, multi-contrast weighted CMR. One hundred and eighty diseased locations of the proximal superficial femoral artery (about 40 mm) were analyzed. The eccentric lesion was defined as [(Maximum wall thickness- Minimum wall thickness)/Maximum wall thickness] >or= 0.5. The arterial morphology and plaque components were measured using semi-automatic image analysis software.

RESULTS

One hundred and fifteen locations were identified as eccentric lesions and sixty-five as concentric lesions. The eccentric lesions had larger wall but similar lumen areas, larger mean and maximum wall thicknesses, and more calcification and lipid rich necrotic core, compared to concentric lesions. For lesions with the same lumen area, the degree of eccentricity was associated with an increased wall area. Eccentricity (dichotomous as eccentric or concentric) was independently correlated with the prevalence of calcification (odds ratio 3.78, 95% CI 1.47-9.70) after adjustment for atherosclerotic risk factors and wall area.

CONCLUSIONS

Plaque eccentricity is associated with preserved lumen size and advanced plaque features such as larger plaque burden, more lipid content, and increased calcification in the superficial femoral artery.

[Diagnosing stroke aetiologies. Morphologic and functional analysis of the aorta and carotid arteries by MRI]

Magnetic resonance imaging allows detailed visualization of the thoracic aorta and is not limited by air artefacts or insonation angles like transoesophageal echocardiography (TEE). Thus the aortic arch can be investigated with higher accuracy, and additional embolic high-risk sources such as complex plaques can be additionally detected by MRI in patients with cryptogenic stroke. Furthermore, MRI provides exact 3D plaque localisation and can be combined with multidirectional 3D MRI velocity mapping. In this way, previously not demonstrable retrograde flow paths originating at complex descending aortic plaques reaching the supra-aortic great arteries can be identified as the probable stroke mechanism in certain patients. The same technique can also be applied to the carotid arteries. This allows analysing the complex 3D helical flow within the internal carotid artery as well as measuring absolute flow velocities and wall shear stress in combination with data on vessel anatomy derived from conventional MR angiography. It is the purpose of this work to describe the state of the art of these modern MR imaging techniques and their potential to identify potential stroke mechanisms, and to analyse the particular role of individual haemodynamic factors on the development of local atherosclerosis.

Hemorrhage and large lipid-rich necrotic cores are independently associated with thin or ruptured fibrous caps: an in vivo 3T MRI study

OBJECTIVE

Histological studies suggest associations between hemorrhage and large lipid-rich/necrotic cores with thin or ruptured fibrous caps in advanced atherosclerosis. We investigated these associations in carotid arteries with mild to severe stenosis by in vivo 3T MRI.

METHODS AND RESULTS

Seventy-seven patients with >or=50% carotid stenosis in at least one side by duplex ultrasound underwent bilateral multi-contrast carotid MRI scans. Measurements for wall and lipid-rich/necrotic core sizes, presence of hemorrhage, and fibrous cap status (classified as intact thick, intact thin or ruptured) were recorded. Arteries with poor image quality, occlusion, or no detectable lipid-rich/necrotic core were excluded. For the 798 MRI slices included, multivariate ordinal regression analysis demonstrated larger %lipid-rich/necrotic core (odds ratio for 10% increase, 1.49; P=0.02) and presence of hemorrhage (odds ratio, 5.91; P<0.001) were independently associated with a worse (intact thin or ruptured) stage of fibrous cap status. For artery-based multivariate analysis, a larger maximum %lipid-rich/necrotic core and presence of hemorrhage independently associated with worse fibrous cap status (P<0.001, for both). No hemorrhage was detected in arteries with thick fibrous caps.

CONCLUSIONS

Hemorrhage and larger %lipid-rich/necrotic core were independently associated with a thin or ruptured fibrous cap status at an early to advanced stage of carotid atherosclerosis.

MR molecular imaging of thrombus: development and application of a Gd-based novel contrast agent targeting to P-selectin

Molecular imaging of thrombus formation at initial stage requires a robust thrombus-specific contrast agent with high sensitivity. In this study, we report a novel P-selectin-targeted paramagnetic molecular imaging agent and the agent's potential to sensitively detect occult microthrombi on the intimal surface of endothelium. Platelet clots and blood clots targeted in vitro with paramagnetic nanoparticles presented a highly detectable, homogeneous T1-weighted contrast enhancement that was improved with increasing gadolinium level. In vivo contrast enhancement under part of circulation conditions was assessed in dogs. The micro-thrombi around the femoral vein of dog demonstrated higher signal intensities than the control clots and the adjacent muscle. Histology was performed on regions likely to contain thrombus as indicated by MRI. These results suggest that molecular imaging of P-selectin-targeted paramagnetic nanoparticles can provide sensitive detection and localization of P-selectin and may allow for early, direct identification of microthrombi, leading to early diagnosis.

Improved characterization of atherosclerotic plaques by gadolinium contrast during intravascular magnetic resonance imaging of human arteries

OBJECTIVES

To determine whether gadolinium-DTPA (Gd-DTPA) facilitates discrimination of fibrous, lipid or calcified constituents during intravascular magnetic resonance imaging (IVMRI) of human atherosclerotic arteries.

BACKGROUND

Atherosclerotic plaques that cause fatal thrombosis due to rupture have high content of lipid relative to fibrous tissue. We recently demonstrated that IVMRI identifies lipid, fibrous, and calcified components within atherosclerotic human arteries with favorable sensitivity and specificity. Gd-DTPA, a T1-shortening agent, selectively amplifies the signal from fibrous tissue on T1 weighted (T1w) surface MRI.

METHODS

A 0.030 in. diameter receiver coil coupled to a 1.5T MR scanner was positioned in iliac arteries of nine subjects with atherosclerosis. Previously validated multi-parametric analysis of T1w and moderate T2w images identified 137 fibrous, lipid and calcified regions of interest within 37 arterial segments. T1w imaging was repeated following 0.1 mmol/kg IV Gd-DTPA infusion.

RESULTS

Computer-derived mean gray value in fibrous regions increased by 34.2% with Gd-DTPA (95% CI 24.3-43.5%, p=0.0001) while lipid and calcified regions showed only a non-significant increase of 4.3% (95% CI -0.6 to 9.2%, p=0.0825) and 3.8% (95% CI -1.1 to 7.7%, p=0.103), respectively. The increase in mean gray value with Gd-DTPA was greater for fibrous than for lipid or calcified regions (p=0.0001).

CONCLUSIONS

Gd-DTPA selectively enhances signal intensity of fibrous constituents during IVMRI of human atherosclerotic arteries and thus identifies key tissue characteristics associated with plaque stability. These findings have important implications for the assessment of plaque-stabilizing therapies and ultimately for reducing cardiovascular events.