Transesophageal magnetic resonance imaging of the aortic arch and descending thoracic aorta in patients with aortic atherosclerosis

Vessel wall MR imaging of aortic arch, cervical carotid and intracranial arteries in patients with embolic stroke of undetermined source: A narrative review

Despite advancements in multi-modal imaging techniques, a substantial portion of ischemic stroke patients today remain without a diagnosed etiology after conventional workup. Based on existing diagnostic criteria, these ischemic stroke patients are subcategorized into having cryptogenic stroke (CS) or embolic stroke of undetermined source (ESUS). There is growing evidence that in these patients, non-cardiogenic embolic sources, in particular non-stenosing atherosclerotic plaque, may have significant contributory roles in their ischemic strokes. Recent advancements in vessel wall MRI (VW-MRI) have enabled imaging of vessel walls beyond the degree of luminal stenosis, and allows further characterization of atherosclerotic plaque components. Using this imaging technique, we are able to identify potential imaging biomarkers of vulnerable atherosclerotic plaques such as intraplaque hemorrhage, lipid rich necrotic core, and thin or ruptured fibrous caps. This review focuses on the existing evidence on the advantages of utilizing VW-MRI in ischemic stroke patients to identify culprit plaques in key anatomical areas, namely the cervical carotid arteries, intracranial arteries, and the aortic arch. For each anatomical area, the literature on potential imaging biomarkers of vulnerable plaques on VW-MRI as well as the VW-MRI literature in ESUS and CS patients are reviewed. Future directions on further elucidating ESUS and CS by the use of VW-MRI as well as exciting emerging techniques are reviewed.

Immersed boundary-finite element model of fluid-structure interaction in the aortic root

It has long been recognized that aortic root elasticity helps to ensure efficient aortic valve closure, but our understanding of the functional importance of the elasticity and geometry of the aortic root continues to e-volve as increasingly detailed in vivo imaging data become available. Herein, we describe a fluid-structure interaction model of the aortic root, including the aortic valve leaflets, the sinsuses of Valsalva, the aortic annulus, and the sinotubular junction, that employs a version of Peskin's immersed boundary (IB) method with a finite element (FE) description of the structural elasticity. As in earlier work, we use a fiber-based model of the valve leaflets, but this study extends earlier IB models of the aortic root by employing an incompressible hyperelastic model of the mechanics of the sinuses and ascending aorta using a constitutive law fit to experimental data from human aortic root tissue. In vivo pressure loading is accounted for by a backward displacement method that determines the unloaded configurations of the root model. Our model yields realistic cardiac output at physiological pressures, with low transvalvular pressure differences during forward flow, minimal regurgitation during valve closure, and realistic pressure loads when the valve is closed during diastole. Further, results from high-resolution computations indicate that although the detailed leaflet and root kinematics show some grid sensitivity, our IB model of the aortic root nonetheless produces essentially grid-converged flow rates and pressures at practical grid spacings for the high-Reynolds number flows of the aortic root. These results thereby clarify minimum grid resolutions required by such models when used as stand-alone models of the aortic valve as well as when used to provide models of the outflow valves in models of left ventricular fluid dynamics.

Identification of vulnerable atherosclerotic plaques

There has been great interest in the possibility of identifying plaques that might be the site of future acute coronary events. These plaques are termed vulnerable and the majority are lipid-rich with an abundance of inflammatory cells and a thin fibrous cap. Several techniques developed to identify these plaques are in various stages of development and in the near future, one might employ a strategy to potentially identify and therapeutically modify such lesions during percutaneous intervention to avoid future acute events. Although this approach of identifying the vulnerable plaque seems promising, there are significant potential limitations. The natural history of a vulnerable plaque is unknown and clinical trials utilizing this strategy of identification and therapeutic intervention are lacking. Moreover, in any given patient, multiple vulnerable plaques are likely to be present. This article reviews some of the techniques for identifying a vulnerable plaque and discusses the potential advantages and limitations of this strategy.

Impaired aortic distensibility in hypothyroid patients with negative coronary angiograms: Simultaneous evaluation with 128-slice multiple row detector computed tomography

Objective

To determine the relationship between arterial distensibility and hypothyroidism by simultaneous assessment of coronary angiography and aortic distensibility, measured using 128-slice retrospective electrocardiogram-gated multiple row detector computed tomography (MDCT).

Methods

Untreated hypothyroid patients and age-matched euthyroid patients, who underwent 128-slice retrospective electrocardiogram-gated MDCT to evaluate the coronary arteries, were assessed. Intraluminal aortic areas in systolic and diastolic phases of the cardiac cycle were measured with multiphase reformation at predefined locations including the ascending aorta, proximal descending aorta and distal descending aorta in the widest axial plane.

Results

The study included 21 hypothyroid and 21 euthyroid patients. Patients with hypothyroidism exhibited decreased aortic distensibility between each location in the thoracic aorta (ascending aorta, proximal descending aorta and distal descending aorta) compared with euthryoid patients.

Conclusions

Hypothyroidism was associated with preclinical vascular alteration, characterized by impaired aortic distensibility at each location of the thoracic aorta, which has been shown to be related to increased cardiovascular risk. Monitoring of aortic distensibility is relevant for evaluating coronary artery disease progression and treatment choices.

Immersed boundary model of aortic heart valve dynamics with physiological driving and loading conditions

The immersed boundary (IB) method is a mathematical and numerical framework for problems of fluid–structure interaction, treating the particular case in which an elastic structure is immersed in a viscous incompressible fluid. The IB approach to such problems is to describe the elasticity of the immersed structure in Lagrangian form, and to describe the momentum, viscosity, and incompressibility of the coupled fluid–structure system in Eulerian form. Interaction between Lagrangian and Eulerian variables is mediated by integral equations with Dirac delta function kernels. The IB method provides a unified formulation for fluid–structure interaction models involving both thin elastic boundaries and also thick viscoelastic bodies. In this work, we describe the application of an adaptive, staggered-grid version of the IB method to the three-dimensional simulation of the fluid dynamics of the aortic heart valve. Our model describes the thin leaflets of the aortic valve as immersed elastic boundaries, and describes the wall of the aortic root as a thick, semi-rigid elastic structure. A physiological left-ventricular pressure waveform is used to drive flow through the model valve, and dynamic pressure loading conditions are provided by a reduced (zero-dimensional) circulation model that has been fit to clinical data. We use this model and method to simulate aortic valve dynamics over multiple cardiac cycles. The model is shown to approach rapidly a periodic steady state in which physiological cardiac output is obtained at physiological pressures. These realistic flow rates are not specified in the model, however. Instead, they emerge from the fluid–structure interaction simulation.

Interventional loopless antenna at 7 T

The loopless antenna magnetic resonance imaging detector is comprised of a tuned coaxial cable with an extended central conductor that can be fabricated at submillimeter diameters for interventional use in guidewires, catheters, or needles. Prior work up to 4.7 T suggests a near-quadratic gain in signal-to-noise ratio with field strength and safe operation at 3 T. Here, for the first time, the signal-to-noise ratio performance and radiofrequency safety of the loopless antenna are investigated both theoretically, using the electromagnetic method-of-moments, and experimentally in a standard 7 T human scanner. The results are compared with equivalent 3 T devices. An absolute signal-to-noise ratio gain of 5.7 ± 1.5-fold was realized at 7 T vs. 3 T: more than 20-fold higher than at 1.5 T. The effective field-of-view area also increased approximately 10-fold compared with 3 T. Testing in a saline gel phantom suggested that safe operation is possible with maximum local 1-g average specific absorption rates of <12 W kg(-1) and temperature increases of <1.9°C, normalized to a 4 W kg(-1) radiofrequency field exposure at 7 T. The antenna did not affect the power applied to the scanner's transmit coil. The signal-to-noise ratio gain enabled magnetic resonance imaging microscopy at 40-50 μm resolution in diseased human arterial specimens, offering the potential of high-resolution large-field-of-view or endoscopic magnetic resonance imaging for targeted intervention in focal disease.

Interventional MRI: tapering improves the distal sensitivity of the loopless antenna

The "loopless antenna" is an interventional MRI detector consisting of a tuned coaxial cable and an extended inner conductor or "whip". A limitation is the poor sensitivity afforded at, and immediately proximal to, its distal end, which is exacerbated by the extended whip length when the whip is uniformly insulated. It is shown here that tapered insulation dramatically improves the distal sensitivity of the loopless antenna by pushing the current sensitivity toward the tip. The absolute signal-to-noise ratio is numerically computed by the electromagnetic method-of-moments for three resonant 3-T antennae with no insulation, uniform insulation, and with linearly tapered insulation. The analysis shows that tapered insulation provides an approximately 400% increase in signal-to-noise ratio in trans-axial planes 1 cm from the tip and a 16-fold increase in the sensitive area as compared to an equivalent, uniformly insulated antenna. These findings are directly confirmed by phantom experiments and by MRI of an aorta specimen. The results demonstrate that numerical electromagnetic signal-to-noise ratio analysis can accurately predict the loopless detector's signal-to-noise ratio and play a central role in optimizing its design. The manifold improvement in distal signal-to-noise ratio afforded by redistributing the insulation should improve the loopless antenna's utility for interventional MRI.

The performance of interventional loopless MRI antennae at higher magnetic field strengths

Interventional, "loopless antenna" MRI detectors are currently limited to 1.5 T. This study investigates whether loopless antennae offer signal-to-noise ratio (SNR) and field-of-view (FOV) advantages at higher fields, and whether device heating can be controlled within safe limits. The absolute SNR performance of loopless antennae from 0.5 to 5 T is investigated both analytically, using electromagnetic (EM) dipole antenna theory, and numerically with the EM method of moments, and found to vary almost quadratically with field strength depending on the medium's electrical properties, the noise being dominated by direct sample conduction losses. The prediction is confirmed by measurements of the absolute SNR of low-loss loopless antennae fabricated for 1.5, 3, and 4.7 T, immersed in physiologically comparable saline. Gains of 3.8 +/- 0.2- and 9.7 +/- 0.3-fold in SNR, and approximately 10- and 50-fold gains in the useful FOV area are observed at 3 and 4.7 T, respectively, compared to 1.5 T. Heat testing of a 3 T biocompatible nitinol-antenna fabricated with a redesigned decoupling circuit shows maximum heating of approximately 1 degrees C for MRI operating at high MRI exposure levels. Experiments in the rabbit aorta confirm the SNR and FOV advantages of the 3 T antenna versus an equivalent commercial 1.5 T device in vivo. This work is the first to study the performance of experimental internal MRI detectors above 1.5 T. The large SNR and FOV gains realized present a major opportunity for high-resolution imaging of vascular pathology and MRI-guided intervention.

Imaging of atherosclerosis in WHHL rabbits using high-resolution ultrasound

Watanabe heritable hyperlipidemic (WHHL) rabbits provide an animal model of hypercholesterolemia and atherosclerotic progression. However, a large individual variation in plaque progression rate calls for serial investigations, as do treatment studies. In contrast to histopathology, transthoracic ultrasound imaging of the aortic arch is a noninvasive technique suitable for repeated investigations. We studied 34 WHHL rabbits by both techniques. Ultrasound correctly interpreted plaque morphology compared with histopathology of the same spot (location verified by needle puncture). Intima media thickness (IMT) measured by the two methods agreed well. Ultrasonic values were similar to the histopathologic average circumferential values when these were corrected for postmortem shrinkage. Finally, the transthoracic ultrasound technique demonstrated a significant increase in IMT over a 15-week period (p = 0.0002). We conclude that transthoracic ultrasound of aortic arch IMT in WHHL rabbits is a reliable and feasible technique for studies of plaque progression and the evaluation of interventions.

Imaging of atherosclerosis using magnetic resonance: state of the art and future directions

Atherosclerosis is the leading cause of morbidity and mortality in industrialized societies, and its incidence is projected to increase in the future. Because the atherosclerotic process begins in the vessel wall, the focus of cardiovascular imaging is shifting from the arterial lumen to imaging of the vessel wall, with the goal of detecting preclinical atherosclerosis. MRI, because of its high resolution, three-dimensional capabilities, noninvasive nature, and capacity for soft tissue characterization, is emerging as an important modality to assess the atherosclerotic plaque burden in the arterial wall and can monitor atherosclerosis in different arterial beds, including the carotid arteries, aorta, and more recently, the coronary arteries. Furthermore, it has also been successfully utilized to monitor plaque regression following therapeutic interventions. Finally, the emergence of high-resolution MRI and development of sophisticated contrast agents offers tremendous promise for in vivo molecular imaging of the atherosclerotic plaque.

Esophageal Varices: Evaluation with Transesophageal MR Imaging—Initial Experience

PURPOSE

To prospectively use transesophageal magnetic resonance (MR) imaging to determine the morphologic and hemodynamic characteristics of esophageal varices.

MATERIALS AND METHODS

The study was approved by the ethics committee. All patients gave written informed consent. Forty-two patients (29 men, 13 women; mean age, 58 years +/- 11 [standard deviation]) with esophageal varices that were recently demonstrated at endoscopy were included in the study. MR imaging was performed by using a receiver probe that was placed in the esophagus. Black-blood T2-weighted MR images were obtained with cardiac triggering and navigator gating of the right hemidiaphragm. On these images, the maximal diameter, minimal diameter, and surface area of the largest esophageal varix were measured. Periesophageal and paraesophageal varices were recorded. A hemodynamic examination was performed in the last 21 patients to undergo MR imaging, which was used to obtain measurements of flow velocity and rate before and after intravenous injection of 50 mug of octreotide or a placebo. A Kruskal-Wallis test was used to assess differences in the diameter and surface area of the varices according to endoscopic grade. Hemodynamic changes observed after octreotide or placebo injection were compared by using an analysis of variance and a 95% confidence interval.

RESULTS

Periesophageal varices were observed in 36 patients, and paraesophageal varices were observed in 32 patients. The minimal diameter, maximal diameter, and surface area of the esophageal varices at MR imaging differed significantly according to endoscopic grade (P < .001). In the periesophageal varices, the velocity and flow changes caused by octreotide differed significantly from those caused by the placebo (P < .001). A decrease in velocity (mean velocity change, -2.766 cm.sec(-1)) and flow (mean flow change, -0.455 mL.sec(-1)) was noted after octreotide injection, but no significant change in velocity (mean velocity change, 0.252 cm.sec(-1)) or flow (mean flow change, 0.018 mL.sec(-1)) was noted after placebo injection. The surface area of the varices did not change significantly after octreotide (mean change, -0.771 mm2) or placebo (mean change, -0.015 mm2) injection.

CONCLUSION

Transesophageal MR imaging is a feasible method to assess the morphologic and hemodynamic characteristics of esophageal varices before and after pharmacologic treatment.

Delayed Contrast-Enhanced MRI of the Aortic Wall in Takayasu's Arteritis: Initial Experience

OBJECTIVE

Delayed contrast-enhanced MRI is increasingly being used for cardiac viability imaging. Takayasu's arteritis is a rare inflammatory disorder of unknown cause that affects the aorta, its major branches, and the pulmonary artery; it is characterized by inflammation and fibrosis in the arterial wall. We report our initial experience with seven patients (six women, one man; age range, 25-62 years) with delayed (20 min) gadolinium-enhanced MRI (inversion recovery prepared gated fast gradient-echo pulse sequence) in patients with known Takayasu's arteritis.

CONCLUSION

Patients with Takayasu's arteritis (particularly those with abnormal laboratory values) have evidence of delayed hyper-enhancement on delayed contrast-enhanced MRI. Thus, delayed contrast-enhanced MRI might be a useful technique to identify inflammation in arterial wall.

Magnetic resonance imaging in the assessment of coronary artery disease

Magnetic resonance imaging (MRI) is gaining importance in cardiology as the noninvasive test of choice for patients with a multitude of cardiovascular problems. Recently, cardiovascular MRI has emerged as an important noninvasive diagnostic modality in the assessment of coronary artery disease. Because of its superior spatial resolution, integration of qualitative and quantitative methodology, and excellent reproducibility, MRI has advantages over conventional noninvasive modalities currently used in the evaluation of coronary artery disease. This article reviews the rapidly expanding recent literature that has now established cardiovascular MRI as an ideal choice in the evaluation of myocardial ischemia (including dobutamine cine MRI and vasodilator perfusion MRI techniques). We further discuss the role of delayed contrast-enhanced MRI and low-dose dobutamine cine MRI for evaluation of myocardial viability. Comparisons with more established techniques, such as dobutamine stress echocardiography, single-photon emission computed tomography perfusion imaging, and positron emission tomography, are reviewed.

Does shear stress modulate both plaque progression and regression in the thoracic aorta?

OBJECTIVES

The purpose of this study was to investigate the role of shear stress (SS) in plaque regression.

BACKGROUND

A condition favorable to the development of atherosclerotic lesions is low oscillating SS. In the descending thoracic aorta, the relationship between plaque distribution and SS has never been characterized. The regression of plaque as the result of lipid-lowering therapy is associated with reverse atherogenic mechanisms. Therefore, we investigated the role of SS in plaque regression. Magnetic resonance imaging (MRI) provides a unique opportunity to noninvasively study morphology and hemodynamics.

METHODS

Cross-sectional images of atherosclerotic plaques in the descending thoracic aorta of 10 asymptomatic, hypercholesteremic patients were acquired at baseline and 24 months after starting lipid-lowering therapy by using a black-blood sequence on a 1.5-T clinical MRI system (5 mm x 780 microm x 780 microm). Average wall thickness (WT) was derived per quadrant. The aorta was subdivided in segments 2 cm in length starting 1 cm from the aortic arch.

RESULTS

Average WT decreased with increasing distance from the arch (3.0 +/- 0.7 mm vs. 2.5 +/- 0.3 mm; p < 0.05) and showed a helical pattern from the proximal to distal segments. Phase-contrast MRI was performed in the thoracic aorta of eight healthy volunteers to derive typical average SS distribution. Shear stress predicted the location of WT (r(2) = 0.29, p < 0.05) but did not predict plaque regression. The best predictor of plaque regression was baseline WT.

CONCLUSIONS

Our data showing an association between WT and average low SS locations support the role of local hemodynamics in the development of atherosclerotic lesions in descending thoracic aorta. Furthermore, SS does not seem to be the major predictor for plaque regression by lipid-lowering interventions. Therefore, our data suggest that other mechanisms are involved in the lipid-reversal mechanism.

Cardiovascular magnetic resonance imaging: current and emerging applications

Magnetic resonance (MR) imaging is gaining importance in cardiology as the newest, most complex, and rapidly emerging noninvasive test of choice for patients with a multitude of cardiovascular problems. It has long been recognized to provide an accurate and reliable means of assessing the function and anatomy of the heart and great vessels, but its emerging role as one of the dominant imaging modalities in other aspects of cardiology such as perfusion imaging, atherosclerosis imaging, and coronary artery imaging cannot be understated. As MR technology evolves, newer therapeutic applications are also being developed, including specific MR-compatible catheters for electrophysiology studies/ablation as well as interventional cardiology related procedures, which may alter the way we practice cardiology in the future. Also, MR is entering an important phase in its evolution, with an anticipated exponential growth in its current applications and through the development of newer molecular imaging applications. It is anticipated that such developments will be coupled to the utilization of molecular markers to index biologic processes to allow for their in vivo visualization. This combination of biochemical markers and imaging methodology will also usher in an era of molecular imaging during which much progress in the diagnosis and treatment of cardiovascular disease is anticipated.

Statin-Induced Cholesterol Lowering and Plaque Regression After 6 Months of Magnetic Resonance Imaging–Monitored Therapy

Background— Statin therapy reduces adverse outcomes, with a minimal decrease in vessel stenosis. Magnetic resonance imaging (MRI) noninvasively detects atherosclerotic plaque (AP) reduction. We hypothesized that statin-induced AP regression can be monitored by MRI and detected earlier than previously reported and is significantly associated with its lipid-lowering effect.

Methods and Results— APs in thoracic aorta were measured by combined surface/transesophageal MRI in 27 patients (treated with simvastatin 20 to 80 mg daily) before and after 6 months of therapy. AP volume and luminal dimensions were measured from 6 cross sections used to construct a 2.4-cm 3D volume of the aorta that included plaque and lumen. Method reproducibility was studied in 10 patients imaged twice, 1 week apart. AP volume was reduced from 3.3±0.1.4 to 2.9±1.4 cm 3 at 6 months ( P <0.02), whereas luminal volume increase was less accentuated (from 12.0±3.9 to 12.2±3.7 cm 3 , P <0.06). LDL cholesterol decreased by 23% (from 125±32 to 97±27 mg/dL, P <0.05) in 6 months. AP regression (plaque volume/area reduction) was significantly related to LDL cholesterol reduction ( P <0.02 and P <0.005, respectively), and luminal volume increase was inversely related to LDL cholesterol reduction ( P <0.04). Plaque volume measurement was highly reproducible (intraclass correlation R =0.98 and variability=4.8%). Intraobserver (0.91) and interobserver (0.81) concordances were documented for plaque volume assessment.

Conclusions— AP regression and reverse remodeling can be detected accurately by MRI 6 months after statin therapy initiation, and it is strongly associated with LDL cholesterol reduction.

Magnetic resonance imaging and computed tomography in assessment of atherosclerotic plaque

The two most promising noninvasive imaging modalities for the study of atherosclerosis are magnetic resonance imaging (MRI) and computed tomography (CT). Both have been shown to be capable of imaging vessel wall structures and differentiating various stages of atherosclerotic wall changes. MRI has been applied in various in vivo human studies to image atherosclerotic plaques in coronary arteries, carotid arteries, and aorta. The latest generation of multidetector row computed tomography (MDCT) systems allows for the noninvasive characterization of different plaque components in various vascular structures. MDCT allows evaluation of the whole arterial vasculature. In addition, MDCT has the ability to visualize the vessel wall and to give a quantitative measurement of calcified and noncalcified plaque. Using either technique, the repeatable, noninvasive study of atherosclerotic disease during its natural history and after therapeutic intervention will enhance our understanding of disease progression and regression. MDCT and MRI, therefore, may help in selecting appropriate treatments.

Using MRI to Assess Aortic Wall Thickness in the Multiethnic Study of Atherosclerosis: Distribution by Race, Sex, and Age

OBJECTIVE

Understanding the determinants of subclinical atherosclerosis may aid in elucidating the pathogenesis of atherosclerosis and guide prevention strategies. In this pilot study, we investigated the role of aortic wall thickness as a measure of subclinical atherosclerosis, assessed a method by which to measure aortic wall thickness using MRI, and attempted to define differences in aortic wall thickness by patient race, sex, and age. SUBJECTS AND METHODS. In this prospective study, 196 participants (99 black, 97 white; 98 men, 98 women) were selected from the Multiethnic Study of Atherosclerosis, which consists of participants 45-84 years old without clinical cardiovascular disease, who were recruited from six study centers in the United States. We performed fast spin-echo double inversion recovery MRI to measure thoracic aortic wall thickness. We tested interobserver agreement using the intraclass correlation coefficient, for sex and race differences in wall thickness using the Mann-Whitney test, and for associations between age and wall thickness using linear regression.

RESULTS

Reproducibility was excellent for measurements of average and maximal wall thickness on MRI. Average and maximal wall thickness increased with age (p < 0.001 and p = 0.002, respectively). Men had greater mean average wall thickness (2.32 vs 2.11 mm, p = 0.028) and mean maximal wall thickness (3.85 vs 3.31 mm, p = 0.010) than women. Blacks had greater mean maximal wall thickness than whites (3.74 vs 3.42 mm, p = 0.023).

CONCLUSION

MRI is a feasible method to measure aortic wall thickness with high interobserver agreement. Aortic wall thickness increases with age and also varies by race and sex.

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

BACKGROUND

MRI can distinguish components of atherosclerotic plaque. We hypothesized that contrast enhancement with gadolinium-DTPA (Gd-DTPA) could aid in the differentiation of plaque components in abdominal aortic aneurysm (AAA).

METHODS AND RESULTS

Twenty-three patients (19 males, age 70+/-8 years) with AAA underwent MRI on a 1.5-T clinical scanner 3+/-3 days before surgical grafting. T1- and T2-weighted (W) black blood spin echo imaging was performed in 1 axial slice, and the T1-W imaging was repeated after a Gd-DTPA-enhanced 3D magnetic resonance angiogram. A section of the aorta at the site of imaging was resected at surgery for histopathologic examination of tissue components and inflammatory cells. Signal-to-noise and contrast-to-noise ratios (CNR) were measured in visualized plaque components from multispectral MRI, and percent enhancement after contrast on T1-W imaging was calculated. The kappa value for agreement between pathology and MRI for the number of tissue components was 0.785. T2-W imaging identified thrombus as regions of high signal and lipid core as low signal, with a CNR of 6.43+/-3.41. Nine patients had a fibrous cap pathologically, which was visualized as a discrete area of uniform increased signal on T2-W imaging with a CNR of 4.52+/-1.93 compared with lipid core. Within the cap, the percent enhancement after Gd-DTPA on T1-W imaging was 91+/-63%.

CONCLUSIONS

Higher signal on T2-W MRI identifies the fibrous cap and thrombus within AAA. Contrast enhancement improves delineation of the fibrous cap. The addition of contrast to MRI plaque imaging may enhance identification of vulnerable plaque.

Intravascular extended sensitivity (IVES) MRI antennas

The design and application of an intravascular extended sensitivity (IVES) MRI antenna is described. The device is a loopless antenna design that incorporates both an insulating, dielectric coating and a winding of the antenna whip into a helical shape. Because this antenna produces a broad region of high SNR and also allows for imaging near the tip of the device, it is useful for imaging long, luminal structures. To elucidate the design and function of this device, the effects of both insulation and antenna winding were characterized by theoretical and experimental studies. Insulation broadens the longitudinal region over which images can be collected (i.e., along the lumen of a vessel) by increasing the resonant pole length. Antenna winding, conversely, allows for imaging closer to the tip of the antenna by decreasing the resonant pole length. Over a longitudinal region of 20 cm, the IVES imaging antenna described here produces a system SNR of approximately 40,000/r (mL(-1)Hz(1/2)), where r is the radial distance from the antenna axis in centimeters. As opposed to microcoil antenna designs, these antennas do not require exact positioning and allow for imaging over broad tissue regions. While focusing on the design of the IVES antenna, this work also serves to enhance our overall understanding of the properties and behavior of the loopless antenna design.

Vascular MRI in the diagnosis and therapy of the high risk atherosclerotic plaque

Disruption of a high risk plaque is known as the primary cause of cardiovascular events. Characterization of arterial wall components has become an essential adjunct in the identification of patients with plaques prone to rupture. Magnetic Resonance Imaging (MRI) has been revealed as one of the noninvasive tools possibly capable of identifying and characterizing high risk atherosclerotic plaque. MRI may facilitate diagnosis, and guide and serially monitor interventional and pharmacological treatment of atherosclerotic disease. In addition, it permits the simultaneous assessment of the anatomy, morphology, and hemodynamics for the study of flow-induced atherogenesis. It possibly will identify asymptomatic patients with subclinical atherosclerosis. This has potential significance for the improvement of strategies in primary and secondary prevention.

Magnetic resonance imaging to identify the high-risk plaque

Atherosclerotic plaque stability depends on 3 factors: (1) lipid core, (2) fibrous cap and its thickness, and (3) inflammation within the cap. Magnetic resonance imaging (MRI) is a noninvasive technique that can provide information on these plaque components using a variety of pulse sequences. Assessment of plaque volume and tissue components and the efficacy of lipid-lowering therapy has been performed in human aorta and carotid arteries. Imaging the coronary wall for plaque burden is a novel application of MRI. Newer approaches also include intravascular and transesophageal MRI techniques. Several emerging MR contrast agents being tested in animal models hold promise for targeted imaging of plaque. MRI is a powerful noninvasive imaging tool with high spatial resolution that continues to prove its value in determining atherosclerotic plaque volume and tissue components.

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.

Age and sex distribution of subclinical aortic atherosclerosis: a magnetic resonance imaging examination of the Framingham Heart Study

Autopsy data demonstrate a correlation between subclinical aortic atherosclerosis and cardiovascular disease. Therefore, noninvasive cardiovascular magnetic resonance (CMR) of subclinical atherosclerosis may provide a novel measure of cardiovascular risk, but it has not been applied to an asymptomatic population-based cohort to establish age- and sex-specific normative data. Participants in the Framingham Heart Study offspring cohort who were free of clinically apparent coronary disease were randomly sampled from strata of sex, quartiles of age, and quintiles of Framingham Coronary Risk Score. Subjects (n=318, aged 60+/-9 years, range 36 to 78 years, 51% women) underwent ECG-gated T2-weighted black-blood thoracoabdominal aortic CMR scanning. CMR evidence of aortic atherosclerosis was noted in 38% of the women and 41% of the men. Plaque prevalence and all measures of plaque burden increased with age group and were greater in the abdomen than in the thorax for both sexes and across all age groups. In addition, the Framingham Coronary Risk Score was significantly correlated with all plaque prevalence and burden measures for women but only for men after age adjustment. These noninvasive CMR data extend the prior autopsy-based prevalence estimates of subclinical atherosclerosis and may help to lay the foundation for future studies of risk stratification and treatment of affected individuals.

Possibilities of transesophageal MRI for assessment of aortic disease: a review

The thoracic aortic wall is a common site of atherosclerotic plaque in humans. Tools for serial, non-invasive assessment of these plaques are of value for addressing gaps in our basic understanding of the biology of plaque rupture and its relationship to atherosclerotic disease progression as well as for monitoring response to anti-atherosclerotic interventions in therapeutic clinical trials. Common approaches to assessment of the wall of the thoracic aorta in vivo are limited. Here we discuss some of the challenges and limitations encountered by conventional techniques and review a novel approach, transesophageal MRI (TEMRI). Initial experiences in applying the TEMRI approach to assessment of aortic morphology and pathology are discussed.