Characterization of aortic root atherosclerosis in ApoE knockout mice: high-resolution in vivo and ex vivo MRM with histological correlation.

In vivo, cardiac-gated, black-blood, and ex vivo magnetic resonance microscopy (MRM) images of the aortic root, and histopathology data were obtained from 12 transgenic and wild-type (WT) mice. MRM was performed using a black-blood imaging spin-echo sequence with upstream and downstream in-flow saturation pulses to obtain aortic root images in three contrast techniques: proton density-weighted (PDW), T(1)- (T(1)W), and T(2)-weighted (T(2)W). Aortic wall thickness and area were measured and correlated with histopathology data (R > 0.90). Ex vivo lesion components (lipid core, fibrous tissue, and cell tissue) were identified and characterized by differing image contrast in PDW, T(1)W, and T(2)W MRM, and by histopathology. The differences between WT and transgenic mice for maximal wall thickness and area were statistically significant (P < 0.05). This study demonstrates the feasibility of in vivo murine aortic root lesion assessment and ex vivo plaque characterization by MRM.     

Comparison of 2D and multislab 3D magnetic resonance techniques for measuring carotid wall volumes

Purpose

To compare a multislab three‐dimensional volume‐selective fast spin‐echo (FSE) magnetic resonance (MR) sequence with a routine two‐dimensional FSE sequence for quantification of carotid wall volume.

Materials and Methods

One hundred normal subjects (50 men, mean age 44.6 years) underwent carotid vessel wall MR using 2D and 3D techniques. Carotid artery total vessel volume, lumen volume, wall volume, and wall/outer wall (W/OW) ratio were measured over 20 contiguous slices. Two‐ (2D) and three‐dimensional (3D) results were compared.

Results

The mean difference between 2D and 3D datasets (as a percentage of the mean absolute value) was 1.7% for vessel volume, 4.9% for lumen volume, 4.7% for wall volume, and 5.8% for W/OW ratio. There was good correlation between 2D and 3D models for total vessel volume (R² = 0.93, P < 0.001), lumen area (R² = 0.92, P < 0.001), and wall volume (R² = 0.77, P < 0.001). The correlation for the W/OW ratio was weaker (R² = 0.30; P < 0.001). The signal‐to‐noise ratio (SNR) for the 3D technique was 2.1‐fold greater than for the 2D technique (P < 0.001). When using the 3D sequence, scan time was reduced by 63%.

Conclusion

Multislab volume selective 3D FSE carotid arterial wall imaging performs similarly to a conventional 2D technique, but with over twice the SNR and substantially reduced scan time. J. Magn. Reson. Imaging 2008;28:1476–1482. © 2008 Wiley‐Liss, Inc.     

MRI measurements of carotid plaque in the atherosclerosis risk in communities (ARIC) study: Methods,reliability and descriptive statistics

Purpose:

To measure carotid plaque components using MRI and estimate reliability in the population‐based Atherosclerosis Risk in Communities (ARIC) study.

Materials and Methods:

Contrast‐enhanced high‐resolution (0.51 × 0.58 × 2 mm³) MRI images were acquired through internal (ICA) and common carotid arteries (CCA) of 2066 ARIC participants at four sites. Sixty‐one exams were repeated and 164 pairs had repeated interpretations. Plaque component thicknesses, areas and volumes over eight slices (1.6‐cm segment) were measured. Intraplaque hemorrhage was recorded. Reliability was evaluated by intraclass correlations and κ statistics.

Results:

There were 1769 successful MRI exams (mean age 71 years; 57% females; 81% white; 19% African‐Americans). Repeat scan reliability was highest for CCA lumen area (0.94) and maximum wall thickness (0.89), ICA lumen area (0.89) and maximum wall thickness (0.77) and total wall volume (0.79), and lowest for small structures—core volume (0.30) and mean cap thickness (0.38). Overall reliability was primarily related to reader variability rather than scan acquisition. K's for presence of core, calcification and hemorrhage were fair to good. White men had the thickest plaques (average maximum ICA wall thickness = 2.3 mm) and the most cores (34%).

Conclusion:

The most important limiting factor for MRI measurements of plaque components is reader variability. Measurement error depends largely on the analyzed structure's size. J. Magn. Reson. Imaging 2010; 31: 406–415. © 2010 Wiley‐Liss, Inc.     

Quantification of human atherosclerotic plaques using spatially enhanced cluster analysis of multicontrast-weighted magnetic resonance images.

One of the current limitations of magnetic resonance imaging (MRI) is the lack of an objective method to classify plaque components. Here we present a cluster analysis technique that can objectively quantify and classify MR images of atherosclerotic plaques. We obtained three-dimensional (3D) images from 12 human coronary artery specimens on a 9.4T imaging system using multicontrast-weighted fast spin-echo (T1-, proton density-, and T2-weighted) imaging with an isotropic voxel size of 39 micro. Spatially enhanced cluster analysis (SECA) was performed on multicontrast MR images, and the resulting segmentation was evaluated against histological tracings. To visualize the overall structure of plaques, the MR images were rendered in 3D. The specimens exhibited lesions of American Heart Association (AHA) plaque classification types I-VI. Both MR images and histological sections were independently reviewed, categorized, and compared. Overall, the classification obtained from the cluster-analyzed MR and histopathology images showed very good agreement for all AHA types (92%, Cohen's kappa = 0.89, P < 0.0001). All plaque types were identified and quantified by SECA with a high degree of correlation between cluster-analyzed MR and manually traced histopathology data. MRI combined with SECA provides an objective method for atherosclerotic plaque component characterization and quantification.     

High-resolution MRI with cardiac and respiratory gating allows for accurate in vivo atherosclerotic plaque visualization in the murine aortic arch.

Genetically engineered mouse models provide enormous potential for investigation of the underlying mechanisms of atherosclerotic disease, but noninvasive imaging methods for analysis of atherosclerosis in mice are currently limited. This study aimed to demonstrate the feasibility of MRI to noninvasively visualize atherosclerotic plaques in the thoracic aorta in mice deficient in apolipoprotein-E, who develop atherosclerotic lesions similar to those observed in humans. To freeze motion, MR data acquisition was both ECG- and respiratory-gated. T(1)-weighted MR images were acquired with TR/TE approximately 1000/10 ms. Spatial image resolution was 49 x 98 x 300 micro m(3). MRI revealed a detailed view of the lumen and the vessel wall of the entire thoracic aorta. Comparison of MRI with corresponding cross-sectional histopathology showed excellent agreement of aortic vessel wall area (r = 0.97). Hence, noninvasive MRI should allow new insights into the mechanisms involved in progression and regression of atherosclerotic disease.     

Continuous arterial spin-labeling perfusion magnetic resonance imaging of the human testis

RATIONALE AND OBJECTIVES: The purpose of this study was to determine if continuous arterial spin-labeling perfusion magnetic resonance imaging could be used to detect testicular perfusion in human subjects. MATERIALS AND METHODS: Continuous arterial spin-labeling magnetic resonance perfusion imaging was performed in seven normal male volunteers and in one patient with a painful scrotum following vasectomy. RESULTS: Normal testicular blood flow was demonstrated in 14 of 14 normal testes in seven volunteers, as well as in two normally perfused testes in the post-vasectomy patient. A change in the steady state magnetization was observed in all of the normally perfused testes of the seven volunteers. CONCLUSION: It is possible to detect blood flow to the normally perfused testes using noninvasive spin-labeling perfusion magnetic resonance imaging. This modality could potentially, in future investigations, be used to image patients with suspected testicular torsion and resultant testicular ischemia.     

Ex vivo identification of atherosclerotic plaque calcification by a 31P solid-state magnetic resonance imaging technique.

Calcified tissue is a common component of atherosclerotic plaques, and occurs most often in mature plaques. The process of calcification is a poorly understood risk factor that may contribute to a plaque's vulnerability to sudden rupture. In this study a solid-state imaging sequence, termed single-point imaging (SPI), was used to observe calcification directly in ex vivo atherosclerotic plaques. Standards were used to validate the ability of (31)P SPI to detect and differentiate calcification from crystalline cholesterol, phospholipids, and other plaque components. After suitable experimental parameters were found, human carotid specimens obtained by endarterectomy were imaged ex vivo by (31)P solid-state imaging and standard (1)H methods. In contrast to (1)H imaging methods, (31)P imaging detected only the calcification in the plaque.     

MRI detects increased coronary wall thickness in asymptomatic individuals: the multi-ethnic study of atherosclerosis (MESA)

PURPOSE: To evaluate the use of coronary wall MRI as a measure of atherosclerotic disease burden in an asymptomatic population free of clinical cardiovascular disease. Coronary wall magnetic resonance imaging (MRI) is a noninvasive method for evaluation of arterial wall remodeling associated with atherosclerosis. MATERIALS AND METHODS: Asymptomatic participants of the Multi-Ethnic Study of Atherosclerosis (MESA) study were studied using black blood MRI. MRI-assessed coronary wall thickness was compared with computed tomography calcium score, carotid intimal-medial thickness, and risk factors for coronary artery disease. RESULTS: Eighty-eight arterial segments were evaluated in 38 MESA participants (mean age, 61.3+/-8.7 years). The maximum coronary wall thickness was greater for participants with two or more cardiovascular risk factors than for those with one or no risk factors (2.59+/-0.33 mm vs. 2.36+/-0.30 mm, respectively, P=0.05.) For participants with zero calcium score, the mean and maximum coronary wall thickness for subjects with two or more risk factors for coronary artery disease were greater than the wall thickness for subjects with one or no risk factors (mean thickness: 1.95+/-0.17 mm vs. 1.7+/-0.19 mm; maximum thickness: 2.67+/-0.24 mm vs. 2.32+/-0.27 mm, respectively, P<0.05). Subjects with increased carotid intimal-medial thickness also had increased coronary artery wall thickness (P<0.05). CONCLUSION: Coronary artery wall MRI detects increased coronary wall thickness in asymptomatic individuals with subclinical markers of atherosclerotic disease and in individuals with zero calcium score.     

In vivo detection of hemorrhage in human atherosclerotic plaques with magnetic resonance imaging

PURPOSE: To investigate the performance of high-resolution T1-weighted (T1w) turbo field echo (TFE) magnetic resonance imaging (MRI) for the identification of the high-risk component intraplaque hemorrhage, which is described in the literature as a troublesome component to detect. MATERIALS AND METHODS: An MRI scan was performed preoperatively on 11 patients who underwent carotid endarterectomy because of symptomatic carotid disease with a stenosis larger than 70%. A commonly used double inversion recovery (DIR) T1w turbo spin echo (TSE) served as the T1w control for the T1w TFE pulse sequence. The MR images were matched slice by slice with histology, and the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of the MR images were calculated. Additionally, two readers, who were blinded for the histological results, independently assessed the MR slices concerning the presence of intraplaque hemorrhage. RESULTS: More than 80% of the histological proven intraplaque hemorrhage could be detected using the TFE sequence with a high interobserver agreement (Kappa = 0.73). The TFE sequence proved to be superior to the TSE sequence concerning SNR and CNR, but also in the qualitative detection of intraplaque hemorrhage. The false positive TFE results contained fibrous tissue and were all located outside the main plaque area. CONCLUSION: The present study shows that in vivo high-resolution T1w TFE MRI can identify the high-risk component intraplaque hemorrhage with a high detection rate in patients with symptomatic carotid disease. Larger clinical trials are warranted to investigate whether this technique can identify patients at risk for an ischemic attack.     

颅内动脉粥样硬化的高分辨力血管壁成像特征及临床应用

颅内动脉粥样硬化是缺血性脑卒中最主要的原因。高分辨力磁共振血管壁成像（VM-MRI）可以直接对血管壁进行成像,评价血管狭窄程度、血管重构、斑块负荷、斑块成分与分布,被认为是唯一能对颅内血管壁进行成像的无创检查技术。近年来颅内VW-MRI发展迅速,现就VW-MRI技术、VW-MRI对颅内动脉粥样硬化斑块的评价及其临床应用做一综述。     

In vivo accuracy of multisequence MR imaging for identifying unstable fibrous caps in advanced human carotid plaques

PURPOSE: To evaluate the in vivo accuracy of a multisequence MRI technique for prospectively identifying one feature of the vulnerable plaque-an unstable fibrous cap-in human carotid atherosclerosis. MATERIALS AND METHODS: The carotid arteries of 18 endarterectomy patients were preoperatively imaged in a 1.5 T scanner using a multisequence protocol that generated four contrast weightings (3D time of flight (ToF), T1, proton density (PD), and T2) at each slice location. With the use of previously published MR criteria, the images of the vessel wall were first examined for evidence of an unstable fibrous cap. The imaging findings were then correlated with the histology from the surgical specimens. RESULTS: A blinded review of the MR findings with the histologic state of the fibrous cap revealed that 1). assessing the preoperative appearance of the fibrous cap has a high test sensitivity (0.81) and specificity (0.90) for identifying an unstable cap in vivo; and 2). the availability of different contrast weightings facilitated image interpretation when intimal calcifications or flow artifacts obscured the lumen surface. CONCLUSION: Multisequence MRI can accurately characterize the in vivo state of the fibrous cap. This finding supports the use of these noninvasive techniques to prospectively identify vulnerable plaques.     

Imaging inflammation: direct visualization of perivascular cuffing in EAE by magnetic resonance microscopy

PURPOSE: To determine if the architectural features revealed by magnetic resonance microscopy (MRM) allow one to detect microscopic abnormalities associated with neuroinflammation in fixed brain sections from animals with experimental allergic encephalomyelitis (EAE), an animal model for multiple sclerosis (MS). MATERIALS AND METHODS: Imaging was performed at the Center for In Vivo Microscopy (CIVM) using a 9.4-Tesla, 89-mm bore, superconducting magnet with actively shielded gradients capable of 850 mT/m. A number of MR contrasts and spatial resolutions were explored. RESULTS: The assessment of EAE brain showed that it is possible to visualize perivascular cuffing in vitro by MRM on three-dimensional T1 proton stains. CONCLUSION: Inflammatory cell infiltration is a prerequisite for the development of lesions in EAE and MS. Thus, the ability to directly detect individual perivascular cuffs of inflammation may provide a useful means of monitoring the time course of inflammatory events, as conventional histopathological scoring of perivascular cuffs is utilized, but in the absence of sectioning and staining.     

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.     

Magnetic resonance microscopy quantifies the disease progression in Marfan syndrome mice

PURPOSE: To use noninvasive magnetic resonance microscopy (MRM) to examine aneurysmal disease in the mouse model of Marfan syndrome (MFS). MATERIALS AND METHODS: A total of eight wild-type (WT) and MFS mice were imaged using MRM; four of them at three different time points over an 8-week period and the remaining animals were imaged at one time point. The maximal cross-sectional area of the aorta was measured by manual tracing and by automated means from combined cardiac and respiratory-gated bright-blood images. Relationships between aortic size and age and the differences between WT and mutant mice aortic size were established. RESULTS: Maximal cross-sectional aortic areas differed significantly (P < 0.05) between WT and mutant mice for all time points, with MFS mice having larger aortic size. There was a positive correlation between aortic size and age in MFS mice (r = 0.80) with a significant increase from the 14th to the 22nd week (P < 0.05). CONCLUSION: MRM detected the differences in aortic size between WT and mutant mice over time, demonstrating a potential for noninvasive technique for the assessment of potential therapies in MFS mice.     

Myocardial function in infarcted and remote regions early after infarction in man: Assessment by magnetic resonance tagging and strain analysis

Early after infarction in the perfusion bed of the left anterior descending coronary artery, cine MRI with spatial modulation of magnetization (SPAMM) tagging (7-mm grid) was used for short- and long-axis cardiac imaging. Two-dimensional strain analysis of triangular finite elements was performed between end-diastole and end-systole. Patients (n = 10) were compared with age-matched healthy subjects (n = 8). The anteroseptal region at midventricular level was considered representative for “infarcted” and the posterolateral region at basal level was considered “remote”. The left ventricular end-diastolic volume index was larger in the patients (69 ± 15 ml/m² versus 56 ± 4 ml/m², P < 0.05). Short-axis images showed in the infarcted region a decrease of first principal strain (greatest systolic lengthening: 1.10 ±. 06 versus 1.27 ± 0.04, P < 0.0001), and in the remote region an increase (1.48 ± 0.11 versus 1.36 ± 0.07, P < 0.025). The lateral and inferior ventricular regions at mid- and basal levels were found to function normally. Long-axis images yielded similar results. Early after infarction, regions with dysfunction, normal function, and hyperfunction can be delineated with MR tagging. The compensatory increased contraction in the remote region is possibly triggered by the Frank-Starling mechanism.     

Diagnostic value of the flow profile in the distal descending aorta by phase-contrast magnetic resonance for predicting severe coarctation of the aorta

Purpose:

To compare aortic flow profiles at the level of the proximal descending (PDAo) and distal descending aorta (DDAo) in patients investigated for coarctation of the aorta (CoA), and compare their respective diagnostic value for predicting severe CoA. Diastolic flow decay in the PDAo predicts severe CoA, but flow measurements at this level are limited by flow turbulence, aliasing, and stent‐related artifacts.

Materials and Methods:

We studied 49 patients evaluated for CoA with phase contrast magnetic resonance imaging (PC‐MRI). Parameters of diastolic flow decay in the PDAo and DDAo were compared. Their respective diagnostic value was compared with the standard reference of transcatheter peak gradient ≥20 mmHg.

Results:

Flow measurement in the PDAo required repeated acquisition with adjustment of encoding velocity or location of the imaging plane in 69% of patients; measurement in the DDAo was achieved in single acquisition in all cases. Parameters of diastolic flow decay in the PDAo and DDAo, including rate‐corrected (RC) deceleration time and RC flow deceleration yielded a good correlation (r = 0.78; P < 0.01, and r = 0.92; P < 0.01), and a similar diagnostic value for predicting severe CoA. The highest diagnostic accuracy was achieved by RC deceleration time at DDAo (sensitivity 85%, specificity 85%).

Conclusion:

Characterization of aortic flow profiles at the DDAo offers a quick and reliable noninvasive means of assessing hemodynamically significant CoA. J. Magn. Reson. Imaging 2011;33:1440–1446. © 2011 Wiley‐Liss, Inc.     

Motion suppression improves quantification of rat liver volume in vivo by magnetic resonance imaging

In response to the presence of certain compounds, rat liver weight can increase. Under the assumption that the liver density does not change, the liver volume will increase as well. To develop the capability to monitor this process noninvasively over time, we used liver volumes determined from MR images to estimate the in vivo liver volumes and weights of normal rats. We acquired multislice, spin-echo images from 18 rats using several protocols for suppression of motion artifacts. We found that volumes determined from data obtained using a combination of gradient moment mulling and respiratory gating, or a combination of signal averaging and “retarded” (after the π pulse) phase-encoding, produced the most accurate estimates of in vivo liver volume and weight.     

Evaluation of neovessels in atherosclerotic plaques of rabbits using an albumin-binding intravascular contrast agent and MRI

Purpose

To test whether B‐22956/1, a novel intravascular contrast agent with a high affinity to serum albumin (Bracco Imaging SpA.), allowed quantifying neovessel and macrophage density in atherosclerotic plaques of rabbits using MRI.

Materials and Methods

A T1‐weighted MRI of the aorta was acquired in 10 rabbits (7 atherosclerotic and 3 control rabbits) before and up to 2 h after intravenous injection of 100 μmol/kg of Gd‐DTPA or 75 μmol/kg of B‐22956/1. Plaque enhancement was measured at different time points. Immunohistochemistry was performed using anti‐CD 31 antibodies and anti‐RAM 11 antibodies to correlate to neovessel and macrophage density, respectively.

Results

MRI showed a significant plaque enhancement 2 h after B‐22956/1 versus Gd‐DTPA in the atherosclerotic group (39.75% versus 9.5%; P < 0.0001. Early atherosclerotic plaques (n = 146) enhancement positively correlates with neovessel density on corresponding histological sections (r = 0.42; P < 0.01). Enhancement of atherosclerotic plaques 2 h after injection of B‐22956/1 correlated with macrophage density (r = 0.71; P < 0.01).

Conclusion

Enhancement of atherosclerotic plaques with MRI correlated with neovessel density at early time points after the injection of B‐22956/1 and with macrophage density, at later time points. Hence, B‐22956/1‐enhanced MRI represents a promising imaging technique for the identification of “high‐risk” plaques. J. Magn. Reson. Imaging 2008;27:1406–1411. © 2008 Wiley‐Liss, Inc.