MR plaque imaging of the carotid artery

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

颈动脉粥样硬化斑块的MR成像

目的：探讨MR成像技术在颈动脉粥样硬化斑块检测中的应用，及其对斑块内成份进行定性分析的可能性。方法：采用3D-TOF、Double-IR(DIR)T1WI、T2WI、PDWI序列对11位高度疑诊颈动脉粥样硬化的患者及9位健康志愿者进行颈动脉成像。部分患者行彩超对照。结果：11例患者中9例可见明显颈动脉粥样硬化改变。TOF上表现为管腔内高信号血流中见低信号充盈缺损。DIR T1WI、T2WI、PDWI表现为管壁增厚，斑块呈等、低均匀信号或等、低、高混杂信号。与多普勒彩超对比，MRI对颈动脉粥样硬化的检出准确率较高。结论：MRI对颈动脉粥样硬化可达到满意的诊断，并提示MRI能分析斑块内成份，分析斑块成份及性质为临床选择治疗方案及估计预后提供有力的帮助。     

MR imaging of adventitial vasa vasorum in carotid atherosclerosis.

Vasa vasorum in the adventitia of atherosclerotic arteries may play a role in plaque progression. In this investigation, a method for characterizing vasa vasorum in the carotid artery is proposed, in which the perfusion properties of the adventitia are probed via dynamic contrast-enhanced (DCE) MRI. A parametric "vasa vasorum image" is automatically generated that depicts the plasma volume (vp) and transfer constant (K trans). The average K trans within the adventitia is proposed as a quantitative measurement related to the extent of the vasa vasorum. In 25 subjects with lesions meeting the requirements for carotid endarterectomy (CEA) significantly higher adventitial K trans of 0.155 +/- 0.045 min(-1) was observed, compared to 0.122 +/- 0.029 min(-1) in the remaining 20 subjects with moderate disease (P < 0.01). In the 25 subjects with endarterectomy specimens, histological evaluation showed that adventitial K trans was significantly correlated with the amount of neovasculature (R = 0.41; P = 0.04) and macrophages (R = 0.49; P = 0.01) in the excised plaque. In the remaining 20 subjects without histology, elevated adventitial K trans was significantly correlated with the log of C-reactive protein (CRP) levels (R = 0.57; P = 0.01) and was elevated in active smokers compared to nonsmokers (0.141 +/- 0.036 vs. 0.111 +/- 0.017 min(-1); P = 0.02). Because these factors are all associated with higher risk of atherosclerotic complications, these results suggest that adventitial K(trans) may be a marker of risk as well.     

Characterization of carotid atherosclerosis and detection of soft plaque with use of black-blood MR imaging

BACKGROUND AND PURPOSE: In the treatment of carotid atherosclerosis, the rate of stenosis and characteristics of plaque should be assessed to diagnose vulnerable plaques that increase the risk for cerebral infarction. We performed carotid black-blood (BB) MR imaging to diagnose plaque components and assess plaque hardness based on MR signals.MATERIALS AND METHODS: Three images of BB-MR imaging per plaque were obtained from 70 consecutive patients who underwent carotid endarterectomy (CEA) to generate T1- and T2-weighted images. To evaluate the relative signal intensity (rSI) of plaque components and the relationship between histologic findings and symptoms, we prepared sections at 2-mm intervals from 34 intact plaques. We then calculated the relative overall signal intensity (roSI) of 70 plaques to assess the relationship between MR signal intensity and plaque hardness and symptoms.RESULTS: The characteristics of rSI values on T1- and T2-weighted images of fibrous cap (FC), fibrosis, calcification, myxomatous tissue, lipid core (LC) with intraplaque hemorrhage (IPH), and LC without IPH differed. Symptomatic plaques were associated with FC disruption (P < .001) and LC with IPH (P < .05). The roSI on T1-weighted images was significantly higher for soft than nonsoft plaques. When the roSI cutoff value was set at 1.25 (mean of the roSI), soft plaques were diagnosed with 79.4% sensitivity and 84.4% specificity. The roSI was also significantly higher for symptomatic than for asymptomatic plaques. Soft and nonsoft plaques as well as symptomatic and asymptomatic plaques did not significantly differ on T2-weighted images.CONCLUSION: BB-MR imaging can diagnose plaque components and predict plaque hardness. This procedure provides useful information for planning therapeutic strategies of carotid atherosclerosis.

Carotid atherosclerosis accounts for a large proportion of the causes of cerebral infarction, and accurate diagnostic imaging of carotid stenosis is useful for clarification of the pathogenesis of cerebral infarction and planning of therapy. In the diagnostic imaging of carotid arterial lesions, luminography such as conventional angiography is generally performed to determine the rate of stenosis, and in randomized studies documenting the value of carotid endarterectomy (CEA) in the treatment of carotid atherosclerosis, therapeutic guidelines have been based on stenosis rate.^1–3 Recent studies have also shown the critical importance of diagnosing vulnerable plaques, which are associated with a higher risk for cerebral infarction, by imaging the carotid artery wall itself and determining plaque characteristics.^4,5 Therefore, less invasive and more accurate diagnostic modalities such as carotid ultrasonography (US) for plaque evaluation have considerable importance in the management of patients with carotid atherosclerosis. Carotid US has been widely applied to characterize atherosclerotic plaque, and the content of soft plaque (lipid and hemorrhage) is presently associated with echolucency.^6,7 Furthermore, accumulating evidence indicates that echolucent plaques represent biologically more active disease and are associated with the risk for future stroke.^8,9 In addition, although carotid artery stent placement (CAS) is becoming an increasingly popular alternative to CEA in the treatment of carotid stenosis, several reports have shown that soft plaques are associated with a high incidence of ischemic complication during CAS.^10–12 Therefore, accurate diagnosis of carotid soft plaque seems to be of paramount clinical importance. However, carotid US has some limitations because it is difficult to obtain full images on patients who have a short neck, high carotid bifurcation, or highly calcified plaques.¹³The chemical composition and physical properties of tissues can be determined by MR imaging, which indicates that this diagnostic technique should be useful in plaque characterization. Along with recent advances in imaging devices and techniques, many studies have documented the usefulness of high-resolution MR imaging in the diagnosis of plaque.^14–21 In addition to sorting plaque composition on the basis of MR signal intensity, if soft plaque can be differentiated from nonsoft plaque by overall plaque MR signal intensity, MR imaging will be a simple, objective, and useful method to diagnose carotid atherosclerosis. To our knowledge, however, few studies have closely assessed the MR imaging signals of plaque components by comparing CEA specimens with carotid MR imaging in vivo,²² and the findings on MR imaging of carotid soft plaque have not been described.Our study investigates the benefit of carotid black-blood (BB) MR imaging by evaluating the MR signal intensity of the components of carotid plaque and by detecting soft plaque on the basis of overall plaque MR signal intensity.     

对颈动脉斑块形成和不稳定因素的新认识及MR成像

动脉粥样硬化是诱发心脑血管事件的高危因素,如何对携带易损斑块的高危病人在缺血事件发生之前进行筛查和诊断成为目前影像学诊断的新挑战.综述对易损斑块形成及其不稳定因素的新观点以及MRI评价方法.     

对颈动脉斑块形成和不稳定因素的新认识及MR成像

动脉粥样硬化是诱发心脑血管事件的高危因素,如何对携带易损斑块的高危病人在缺血事件发生之前进行筛查和诊断成为目前影像学诊断的新挑战。综述对易损斑块形成及其不稳定因素的新观点以及MRI评价方法。     

运用3.0T磁共振探讨颈动脉与大脑中动脉斑块负荷的关系

目的应用3.0T磁共振研究急性脑缺血患者颈动脉和患侧大脑中动脉的斑块负荷,探究两者之间的关系。方法 50例经临床和MRI证实为急性前循环缺血的患者在发病1周内行颈动脉和患侧大脑中动脉磁共振多序列检查,测得各血管的管腔面积(LA)、管壁面积(WA)、血管总面积(TVA),并计算出管壁标准化指数(NWI=WA/TVA)。结果①同一患者两侧的颈动脉NWI之间差异无统计学意义(P>0.05);②大脑中动脉M1段的NWI(0.62±0.07)比同侧颈动脉的NWI(0.36±0.06)显著增高(P<0.01);③同侧颈动脉与大脑中动脉M1段的NWI间存在显著的相关性(左侧r=0.89,P<0.001;右侧r=0.79,P<0.001)。结论动脉粥样硬化是一种系统性病变,可以同时累及多个血管床,而急性脑缺血患者以大脑中动脉斑块负荷增加更为显著。     

Carotid artery atherosclerosis: in vivo morphologic characterization with gadolinium-enhanced double-oblique MR imaging initial results

In nine subjects with carotid atherosclerosis, double-oblique, contrast material-enhanced, double inversion-recovery, fast spin-echo magnetic resonance (MR) images were acquired through atheroma in the proximal internal carotid artery. Fibrocellular tissue within atheroma selectively enhanced 29% after administration of gadolinium-based contrast agent. Contrast enhancement helped discriminate fibrous cap from lipid core with a contrast-to-noise ratio as good as or better than that with T2-weighted MR images but with approximately twice the signal-to-noise ratio (postcontrast images, 36.6 +/- 3.6; T2-weighted images, 17.5 +/- 2.1; P <.001).     

Inflammation in carotid atherosclerotic plaque: a dynamic contrast-enhanced MR imaging study

PURPOSE: To prospectively evaluate if there is an association between plaque enhancement at magnetic resonance (MR) imaging and proinflammatory cardiovascular risk factors and plaque content. MATERIALS AND METHODS: This study was performed with informed consent, HIPAA compliance, and institutional review board approval. Contrast agent dynamics within carotid plaques were measured in 30 patients (29 men, one woman; mean age, 67.7 years +/- 10.7 [standard deviation]) who were scheduled to undergo carotid endarterectomy. Measurements were based on kinetic modeling of images obtained at 15-second intervals during which a gadolinium-based contrast agent was injected. The time-varying signal intensities within the plaques were used to estimate the fractional plasma volume (vp) and transfer constant (Ktrans) of contrast material into the extracellular space. Pearson correlation coefficients were computed between blinded MR measurements and histologic measurements of plaque composition, including macrophages, neovasculature, necrotic core, calcification, loose matrix, and dense fibrous tissue. Correlation coefficients or mean differences were computed regarding clinical markers of cardiovascular risk. RESULTS: Analyzable MR images and histologic results were obtained in 27 patients. Measurements of Ktrans correlated with macrophage (r = 0.75, P < .001), neovasculature (r = 0.71, P < .001), and loose matrix (r = 0.50, P = .01) content. Measurements of v(p) correlated with macrophage (r = 0.54, P = .004), neovasculature (r = 0.68, P < .001), and loose matrix (r = 0.42, P = .03) content. For clinical parameters, significant associations were correlated with Ktrans only, with decreased high-density lipoprotein levels (r = -0.66, P < .001) and elevated Ktrans measurements in smokers compared with nonsmokers (mean, 0.134 min(-1) vs 0.074 min(-1), respectively; P = .01). CONCLUSION: The correlations between Ktrans and histologic markers of inflammation suggest that Ktrans is a quantitative and noninvasive marker of plaque inflammation, which is further supported by the correlation of Ktrans with proinflammatory cardiovascular risk factors, decreased high-density lipoprotein levels, and smoking.     

High-resolution multicontrast-weighted MR imaging from human carotid endarterectomy specimens to assess carotid plaque components

The American Heart Association modified classification for atherosclerotic plaque lesions has defined vulnerable plaques as those prone to rupture. The aim of our study was to assess the sensitivity and specificity of 1.5-T magnetic resonance imaging (MRI) in the evaluation of the characteristics of plaque components. Twelve carotid endarterectomy specimens were imaged by ex-vivo high-resolution 1.5-T MRI. Thirty-four cross-section axial images were selected for pixel-by-pixel basis analysis to demonstrate the most significant tissue features. Data were then submitted for histopathological examination and each specimen analysed in the light of the histological components (lipid core, fibrous tissue, fibrous/loose connective tissue, calcifications). The overall sensitivity and specificity rates for each tissue type were, respectively, 92% and 74% for the lipid core, 82% and 94% for the fibrous tissue, 72% and 87% for the fibrous/loose connective tissue, and 98% and 99% for calcification. The use of 1.5-T MRI appears to be a reliable tool to characterise plaque components and could help in the screening of patients with high risk of plaque rupture. The possibility of applying MRI in clinical daily practice may change the non-invasive approach to carotid artery diagnostic imaging, thus allowing an early identification of patients with vulnerable plaques.     

应用高分辨力多对比加权MRI对人颈动脉内膜剥除术样本进行颈动脉粥样斑块成分的评定

美国心脏病学会修订的动脉粥样硬化斑块病变分类,把易损斑块定义为有破裂倾向斑块。研究旨在评价1．5T MR成像对于分析动脉粥样硬化斑块成分特征的敏感性和特异性。对12个颈动脉内膜剥除术的样本进行体外高分辨力1．5T MR成像。选择34个横断面图像进行以像素-像素为基础的分析,用以显示最显著的组织特征。     

Contrast-enhanced ultrasound imaging of the vasa vasorum of carotid artery plaque

The vasa vasorum of carotid artery plaque is a novel marker of accurately evaluating the vulnerability of carotid artery plaque, which was associated with symptomatic cerebrovascular and cardiovascular disease. The presence of ultrasound contrast agents in carotid artery plaque represents the presence of the vasa vasorum in carotid artery plaque because the ultrasound contrast agents are strict intravascular tracers. Therefore, contrast-enhanced ultrasound (CEUS) is a novel and safe imaging modality for evaluating the vasa vasorum in carotid artery plaque. However, there are some issues that needs to be assessed to embody fully the clinical utility of the vasa vasorum in carotid artery plaque with CEUS.     

MR imaging of atherosclerotic plaque

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

Feasibility of in vivo, multicontrast-weighted MR imaging of carotid atherosclerosis for multicenter studies

PURPOSE: To test the image quality (ImQ) and interscan coverage of MRI for measuring carotid atherosclerosis across multiple centers. MATERIALS AND METHODS: Thirty-nine subjects from five clinical sites (site 1: n=11; site 2: n=16; site 3: n=2; site 4: n=3; site 5: n=7) were imaged on GE 1.5T scanners using a standardized carotid imaging protocol with five weightings (T1, proton density (PD), T2, time-of-flight (TOF), and contrast-enhanced (CE) T1). MR technologists from the five sites received comprehensive protocol training. A maximum coverage of 24 mm (12 slices) was designed for each of four scans (baseline and at four, eight, and 13 weeks). The adequacy of coverage was calculated as the percentage of arteries with at least six slices matched across all four scans. ImQ was evaluated using an established five-point scale for each image. ImQ>or= 3 was considered acceptable for image analysis. RESULTS: Across five sites, the mean ImQ was 3.4-4.2 for T1W, 3.6-4.4 for CE-T1W, 3.4-4.2 for PDW, 3.3-4.2 for T2W, and 3.4-4.0 for TOF. The mean ImQ per site was 3.5-4.2. All sites generated at least six-slice coverage (mean=8.0-9.1) for all index carotid arteries. CONCLUSION: The ImQ and coverage values were comparable among clinical sites using a standardized carotid imaging protocol. With comprehensive protocol training, carotid MRI is technically feasible for use in multicenter studies.     

脑梗死与颈动脉粥样硬化斑块的关系

目的探讨脑梗死与颈动脉粥样硬化斑块的关系。方法对64例脑梗死患者和30例对照组进行颈动脉彩色多普勒超声检查。结果病例组斑块检出率79.7%,显著高于对照组斑块检出率16.7%(P<0.001)。颈动脉分叉处硬化斑块检出率约为55.4%,显著高于颈总动脉和颈内动脉斑块检出率(24.3%,20.3%)(P<0.001)。病例组中、重型患者软斑块检出率(28/45)显著高于轻型患者斑块检出率(3/19)(P<0.001)。结论脑梗死发病与颈动脉粥样硬化斑块相关。软斑块与脑梗死病情的轻重有一定关系。脑梗死患者颈动脉彩色多普勒超声检查有重要临床意义。     

Assessment of human atherosclerotic carotid plaque components with multisequence MR imaging: initial experience

PURPOSE: To prospectively determine, by using a stepwise logistic regression model, the optimal magnetic resonance (MR) weighting (ie, pulse sequence) combinations for plaque assessment and corresponding cutoff values of relative signal intensities (rSIs). MATERIALS AND METHODS: Institutional review board approval and patient consent were obtained. Eleven patients (seven men, four women; mean age +/- standard deviation, 68 years +/- 4) with symptomatic carotid disease and stenosis of more than 70% were investigated at MR imaging before carotid endarterectomy. The MR images were matched with histologic features of the endarterectomy specimens (reference standard). The rSIs (compared with that of muscle tissue) from regions of interest were assessed qualitatively and semiquantitatively. For all major components (calcification, lipid core, intraplaque hemorrhage, and fibrous tissue), optimal cutoff points for the rSIs were determined for five MR weightings by means of receiver operating characteristic curves. The best predicting combinations of these five dichotomized MR weightings were selected by means of stepwise logistic regression analysis. The potential sensitivity and specificity of MR imaging for vulnerable plaque with hemorrhage and/or lipid core were determined. RESULTS: The same optimal MR weighting combinations for identifying the four plaque components were found with qualitative and semiquantitative analysis. Sensitivity and specificity for vulnerable plaque were 93% (95% confidence interval: 77%, 99%) and 96% (95% confidence interval: 86%, 100%), respectively, for the qualitative analysis and 76% (95% confidence interval: 56%, 90%) and 100% (95% confidence interval: 93%, 100%) for the semiquantitative analysis. CONCLUSION: This study demonstrates the potential of a systematic approach of atherosclerotic plaque assessment with multisequence MR imaging by using the information provided from five different MR weightings in a stepwise logistic regression model.     

MR imaging of carotid webs

Purpose

We propose a magnetic resonance (MR) imaging protocol for the characterization of carotid web morphology, composition, and vessel wall dynamics. The purpose of this case series was to determine the feasibility of imaging carotid webs with MR imaging.

Methods

Five patients diagnosed with carotid web on CT angiography were recruited to undergo a 30-min MR imaging session. MR angiography (MRA) images of the carotid artery bifurcation were acquired. Multi-contrast fast spin echo (FSE) images were acquired axially about the level of the carotid web. Two types of cardiac phase resolved sequences (cineFSE and cine phase contrast) were acquired to visualize the elasticity of the vessel wall affected by the web.

Results

Carotid webs were identified on MRA in 5/5 (100%) patients. Multi-contrast FSE revealed vessel wall thickening and cineFSE demonstrated regional changes in distensibility surrounding the webs in these patients.

Conclusion

Our MR imaging protocol enables an in-depth evaluation of patients with carotid webs: morphology (by MRA), composition (by multi-contrast FSE), and wall dynamics (by cineFSE).