In Vivo High-Frequency Ultrasound for the Characterization of Thrombi Associated with Aortic Aneurysms in an Experimental Mouse Model

The development of abdominal aortic aneurysm (AAA) associated thrombi plays an important role during the onset and progression of AAAs. The aim of this study was to evaluate the potential of high-frequency ultrasound for characterization of AAA associated thrombi in an apolipoprotein-E-deficient mouse-model. Ultrasound measurements were performed using a high-resolution ultrasound system (Vevo770, FUJIFILM VisualSonics, Inc., Toronto, ON, Canada) with a 30 MHz linear-array transducer (RMV707 B). Magnetic resonance imaging with a 3 Tesla scanner (Achieva MR system, Philips Healthcare, Best, The Netherlands) and a single-loop microscopy coil was performed as a reference standard. All stages of aneurysm development were evaluated by histologic analyses. The "signal-thrombus-matrix" to "signal-blood" ratio on high-frequency ultrasound measurements showed a strong correlation (R² = 0.81, p <0.05) with the state of extracellular matrix remodeling. Furthermore, size measurements derived from the high-frequency ultrasound correlated well with magnetic resonance imaging and histology. This study demonstrated that high-frequency ultrasound enables the reliable in vivo quantification of extracellular matrix remodeling at various stages of thrombus development, based on the thrombus echogenicity.

Contrast-enhanced magnetic resonance imaging for the detection of ruptured coronary plaques in patients with acute myocardial infarction

Purpose

X-ray coronary angiography (XCA) is the current gold standard for the assessment of lumen encroaching coronary stenosis but XCA does not allow for early detection of rupture-prone vulnerable plaques, which are thought to be the precursor lesions of most acute myocardial infarctions (AMI) and sudden death. The aim of this study was to investigate the potential of delayed contrast-enhanced magnetic resonance coronary vessel wall imaging (CE-MRCVI) for the detection of culprit lesions in the coronary arteries.

Methods

16 patients (13 male, age 61.9±8.6 years) presenting with sub-acute MI underwent CE-MRCVI within 24-72h prior to invasive XCA. CE-MRCVI was performed using a T1-weighted 3D gradient echo inversion recovery sequence (3D IR TFE) 40±4 minutes following the administration of 0.2 mmol/kg gadolinium-diethylenetriamine-pentaacetic acid (DTPA) on a 3T MRI scanner equipped with a 32-channel cardiac coil.

Results

14 patients were found to have culprit lesions (7x LAD, 1xLCX, 6xRCA) as identified by XCA. Quantitative CE-MRCVI correctly identified the culprit lesion location with a sensitivity of 79% and excluded culprit lesion formation with a specificity of 99%. The contrast to noise ratio (CNR) of culprit lesions (9.7±4.1) significantly exceeded CNR values of segments without culprit lesions (2.9±1.9, p<0.001).

Conclusion

CE-MRCVI allows the selective visualization of culprit lesions in patients immediately after myocardial infarction (MI). The pronounced contrast uptake in ruptured plaques may represent a surrogate biomarker of plaque activity and/or vulnerability.

In vivo MR-angiography for the assessment of aortic aneurysms in an experimental mouse model on a clinical MRI scanner: Comparison with high-frequency ultrasound and histology

BACKGROUND

MR-angiography currently represents one of the clinical reference-standards for the assessment of aortic-dimensions. For experimental research in mice, dedicated preclinical high-field MRI scanners are used in most studies. This type of MRI scanner is not available in most institutions. The aim of this study was to evaluate the potential of MR-angiography performed on a clinical MR scanner for the assessment of aortic aneurysms in an experimental mouse model, compared to a preclinical high-resolution ultrasound imaging system and histopathology.

METHODS

All in vivo MR imaging was performed with a clinical 3T MRI system (Philips Achieva) equipped with a clinical gradient system in combination with a single-loop surface-coil (47 mm). All MR sequences were based on clinically used sequences. For ultrasound, a dedicated preclinical high-resolution system (30 MHz linear transducer, Vevo770, VisualSonics) was used. All imaging was performed with an ApoE knockout mouse-model for aortic aneurysms. Histopathology was performed as reference-standard at all stages of aneurysm development.

RESULTS

MR-angiography on a clinical 3T system enabled the clear visualization of the aortic lumen and aneurysmal dilation at different stages of aneurysm development. A close correlation (R2 = 0.98; p < 0.001) with histological area measurements was found. Additionally, a good agreement between MR and ultrasound area measurements in systole (R2 = 0.91; p < 0.001) and diastole (R2 = 0.94; p < 0.001) were measured. Regarding interobserver reproducibility, MRI measurements yielded a smaller 95% confidence interval and a closer interreader correlation compared to ultrasound measurements (-0.37-0.46; R2 = 0.97 vs. -0.78-0.88; R2 = 0.87).

CONCLUSION

This study demonstrates that MR-angiography, performed on a clinical 3T MR scanner, enables the reliable detection and quantification of the aortic dilatation at different stages of aneurysm development in an experimental mouse model.

Influence of acquired obesity on coronary vessel wall late gadolinium enhancement in discordant monozygote twins

Objectives

The aim of this study was to investigate the impact of BMI on late gadolinium enhancement (LGE) of the coronary artery wall in identical monozygous twins discordant for BMI. Coronary LGE represents a useful parameter for the detection and quantification of atherosclerotic coronary vessel wall disease.

Methods

Thirteen monozygote female twin pairs (n = 26) with significantly different BMIs (>1.6 kg/m2) were recruited out of >10,000 twin pairs (TwinsUK Registry). A coronary 3D-T2prep-TFE MR angiogram and 3D-IR-TFE vessel wall scan were performed prior to and following the administration of 0.2 mmol/kg of Gd-DTPA on a 1.5 T MR scanner. The number of enhancing coronary segments and contrast to noise ratios (CNRs) of the coronary wall were quantified.

Results

An increase in BMI was associated with an increased number of enhancing coronary segments (5.3 ± 1.5 vs. 3.5 ± 1.6, p < 0.0001) and increased coronary wall enhancement (6.1 ± 1.1 vs. 4.8 ± 0.9, p = 0.0027) compared to matched twins with lower BMI.

Conclusions

This study in monozygous twins indicates that acquired factors predisposing to obesity, including lifestyle and environmental factors, result in increased LGE of the coronary arteries, potentially reflecting an increase in coronary atherosclerosis in this female study population.

Key points

• BMI-discordant twins allow the investigation of the influence of lifestyle factors independent from genetic confounders.

• Only thirteen obesity-discordant twins were identified underlining the strong genetic component of BMI.

• In female twins, a BMI increase is associated with increased coronary late gadolinium enhancement.

• Increased late gadolinium enhancement in the coronary vessel wall potentially reflects increased atherosclerosis.

Electronic supplementary material

The online version of this article (doi:10.1007/s00330-016-4616-8) contains supplementary material, which is available to authorized users.

Molecular imaging in cardiovascular diseases

Cardiovascular diseases remain the leading cause of morbidity and mortality in industrialized and developing countries. In clinical practice, the in-vivo identification of atherosclerotic lesions, which can lead to complications such as heart attack or stroke, remains difficult. Imaging techniques provide the reference standard for the detection of clinically significant atherosclerotic changes in the coronary and carotid arteries. The assessment of the luminal narrowing is feasible, while the differentiation of stable and potentially unstable or vulnerable atherosclerotic plaques is currently not possible using non-invasive imaging. With high spatial resolution and high soft tissue contrast, magnetic resonance imaging (MRI) is a suitable method for the evaluation of the thin arterial wall. In clinical practice, native MRI of the vessel wall already allows the differentiation and characterization of components of atherosclerotic plaques in the carotid arteries and the aorta. Additional diagnostic information can be gained by the use of non-specific MRI contrast agents. With the development of targeted molecular probes, that highlight specific molecules or cells, pathological processes can be visualized at a molecular level with high spatial resolution. In this review article, the development of pathophysiological changes leading to the development of the arterial wall are introduced and discussed. Additionally, principles of contrast enhanced imaging with non-specific contrast agents and molecular probes will be discussed and latest developments in the field of molecular imaging of the vascular wall will be introduced.

In vivo assessment of aortic aneurysm wall integrity using elastin-specific molecular magnetic resonance imaging

BACKGROUND

The incidence of abdominal aortic aneurysms (AAAs) has increased during the last decades. However, there is still controversy about the management of medium-sized AAAs. Therefore, novel biomarkers, besides aneurysmal diameter, are needed to assess aortic wall integrity and risk of rupture. Elastin is the key protein for maintaining aortic wall tensile strength and stability. The progressive breakdown of structural proteins, in particular, medial elastin, is responsible for the inability of the aortic wall to withstand intraluminal hemodynamic forces. Here, we evaluate the usefulness of elastin-specific molecular MRI for the in vivo characterization of AAAs.

METHODS AND RESULTS

To induce AAAs, ApoE(-/-) mice were infused with angiotensin-II. An elastin-specific magnetic resonance molecular imaging agent (ESMA) was administered after 1, 2, 3, and 4 weeks of angiotensin-II infusion to assess elastin composition of the aorta (n=8 per group). The high signal provided by ESMA allowed for imaging with high spatial resolution, resulting in an accurate assessment of ruptured elastic laminae and the compensatory expression of elastic fibers. In vivo contrast-to-noise ratios and R1-relaxation rates after ESMA administration were in good agreement with ex vivo histomorphometry (Elastica van Gieson stain) and gadolinium concentrations determined by inductively coupled plasma mass spectroscopy. Electron microscopy confirmed colocalization of ESMA with elastic fibers.

CONCLUSIONS

Changes in elastin content could be readily delineated and quantified at different stages of AAAs by elastin-specific molecular magnetic resonance imaging. ESMA-MRI offers potential for the noninvasive detection of the aortic rupture site prior to dilation of the aorta and the subsequent in vivo monitoring of compensatory repair processes during the progression of AAAs.

In vivo assessment of intraplaque and endothelial fibrin in ApoE(-/-) mice by molecular MRI

OBJECTIVE

Molecular magnetic resonance imaging (MRI) has emerged as a promising non-invasive modality to characterize atherosclerotic vessel wall changes on a morphological and molecular level. Intraplaque and endothelial fibrin has recently been recognized to play an important role in the progression of atherosclerosis. This study aimed to investigate the feasibility of intraplaque and endothelial fibrin detection using a fibrin-targeted contrast-agent, FTCA (EPIX Pharmaceuticals, Lexington, MA), in a mouse model of atherosclerosis.

METHODS

Male apolipoproteinE-knockout mice (ApoE(-/-)) were fed a high fat diet (HFD) for one to three months. MRI of the brachiocephalic artery was performed prior to and 90 min after the administration of FTCA (n=8 per group). Contrast to noise ratios (CNR) and longitudinal relaxation rates (R1) of plaques were determined and compared to ex vivo fibrin density measurements on immunohistological sections stained with a fibrin-specific antibody and gadolinium concentrations measured by inductively coupled mass spectroscopy (ICP-MS).

RESULTS

Molecular MRI after FTCA administration demonstrated a significant increase (p<0.05) in contrast agent uptake in brachiocephalic artery plaques. In vivo CNR measurements were in good agreement with ex vivo fibrin density measurements on immunohistochemistry (y=2.4x+11.3, R(2)=0.82) and ICP-MS (y=0.95x+7.1, R(2)=0.70). Late stage atherosclerotic plaques displayed the strongest increase in CNR, R1, ex vivo fibrin staining and gadolinium concentration (p<0.05).

CONCLUSION

This study demonstrated the feasibility of intraplaque and endothelial fibrin imaging using FTCA. Direct in vivo fibrin detection and quantification could be useful for characterization and staging of coronary and carotid atherosclerotic lesions, which may aid diagnosis and intervention.

Detection of intracoronary thrombus by magnetic resonance imaging in patients with acute myocardial infarction

BACKGROUND

Persistent intracoronary thrombus after plaque rupture is associated with an increased risk of subsequent myocardial infarction and mortality. Coronary thrombus is usually visualized invasively by x-ray coronary angiography. Non-contrast-enhanced T1-weighted magnetic resonance (MR) imaging has been useful for direct imaging of carotid thrombus and intraplaque hemorrhage by taking advantage of the short T1 of methemoglobin present in acute thrombus and intraplaque hemorrhage. The aim of this study was to investigate the use of non-contrast-enhanced MR for direct thrombus imaging (MRDTI) in patients with acute myocardial infarction.

METHODS AND RESULTS

Eighteen patients (14 men; age, 61±9 years) underwent MRDTI within 24 to 72 hours of presenting with an acute coronary syndrome before invasive x-ray coronary angiography; MRDTI was performed with a T1-weighted, 3-dimensional, inversion-recovery black-blood gradient-echo sequence without contrast administration. Ten patients were found to have intracoronary thrombus on x-ray coronary angiography (left anterior descending, 4; left circumflex, 2; right coronary artery, 4; and right coronary artery-posterior descending artery, 1), and 8 had no visible thrombus. We found that MRDTI correctly identified thrombus in 9 of 10 patients (sensitivity, 91%; posterior descending artery thrombus not detected) and correctly classified the control group in 7 of 8 patients without thrombus formation (specificity, 88%). The contrast-to-noise ratio was significantly greater in coronary segments containing thrombus (n=10) compared with those without visible thrombus (n=131; mean contrast-to-noise ratio, 15.9 versus 2.6; P<0.001).

CONCLUSION

Use of MRDTI allows selective visualization of coronary thrombus in a patient population with a high probability of intracoronary thrombosis.

Congenital heart disease: cardiovascular MR imaging by using an intravascular blood pool contrast agent

PURPOSE

To compare the image quality and diagnostic performance of a contrast agent-specific inversion-recovery (IR) steady-state free precession (SSFP) magnetic resonance (MR) imaging sequence performed by using an intravascular contrast agent (gadofosveset trisodium) with those of a commonly used T2-prepared SSFP sequence performed by using an extravascular (gadopentetate dimeglumine) and an intravascular (gadofosveset trisodium) contrast agent in patients with congenital heart disease (CHD).

MATERIALS AND METHODS

The local ethics committee and the United Kingdom Medicines and Healthcare products Regulatory Agency approved this study. Patient informed consent was obtained. Twenty-three patients with CHD were examined by using a 1.5-T MR imaging unit and a 32-channel coil. Gadopentetate dimeglumine and gadofosveset trisodium were used in the same patient on consecutive days. Vessel wall sharpness, contrast-to-noise ratios (CNRs), image quality, and diagnostic performance achieved by using the IR SSFP sequence with gadofosveset trisodium were compared with those achieved by using the T2-prepared SSFP sequence with gadopentetate dimeglumine and gadofosveset trisodium and with those achieved at respective contrast material-enhanced MR angiographic examinations. The Wilcoxon rank sum test was used to compare categoric variables; t tests were used to compare continuous variables.

RESULTS

Use of the IR SSFP sequence with gadofosveset trisodium significantly improved vessel wall sharpness, CNRs, and image quality (P < .05 for all) for all investigated intra- and extracardiac structures compared with the T2-prepared SSFP sequence with gadopentetate dimeglumine and gadofosveset trisodium and the respective contrast-enhanced MR angiographic examinations. With use of the IR SSFP sequence with gadofosveset trisodium, new, unsuspected diseases (five [22%] of 23) were diagnosed, while other diseases could be excluded (15 [65%] of 23). Information available from echocardiography (n = 23), conventional angiography (n = 4), and/or surgery (n = 1) confirmed all diagnoses.

CONCLUSION

IR SSFP with gadofosveset trisodium improved image quality and diagnostic performance, allowing a more accurate and complete assessment of cardiovascular anatomy in patients with CHD compared with T2-prepared SSFP with gadopentetate dimeglumine and gadofosveset trisodium and respective contrast-enhanced MR angiographic examinations.

Noninvasive assessment of atherosclerotic plaque progression in ApoE-/- mice using susceptibility gradient mapping

BACKGROUND

Macrophages have been identified as a major contributor to plaque development and destabilization in atherosclerosis. The aim of this study was to noninvasively assess uptake of citrate coated very small iron oxide particles at different stages of plaque development in the brachiocephalic artery of apoE(-/-) mice. Susceptibility gradient mapping (SGM) was applied to generate positive contrast images and to quantify iron oxide uptake.

METHODS AND RESULTS

ApoE(-/-) mice were fed a high-fat diet for 4, 8, or 12 weeks; 300 μmol Fe/kg was injected 24 and 48 hours before final MRI. Increasing very small iron oxide particle uptake was observed over the course of atherosclerotic plaque development. Simultaneous administration of pravastatin led to a significant decrease in very small iron oxide particle uptake, assessed by mass spectroscopy and histology. SGM-MRI allowed the generation of positive contrast images, and magnitudes (mT/m) of contrast enhancement in SG parameter maps significantly correlated with the absolute iron oxide content (R(2)=0.70, P<0.05) and the macrophage density (R(2)=0.71, P<0.05).

CONCLUSIONS

This study shows an increase in iron oxide uptake (measured by in vivo SGM-MRI, histology, and mass spectroscopy) with the progression of plaque development in an apoE(-/-) mouse model of accelerated atherosclerosis. Positive contrast provided by SGM-MRI allowed for a clear visualization of intraplaque iron oxide depositions, and magnitudes (mT/m) of contrast enhancement in SG parameter maps allowed for the quantification of intraplaque iron oxide particles.

Assessment of atherosclerotic plaque burden with an elastin-specific magnetic resonance contrast agent

Atherosclerosis and its consequences remain the main cause of mortality in industrialized and developing nations. Plaque burden and progression have been shown to be independent predictors for future cardiac events by intravascular ultrasound. Routine prospective imaging is hampered by the invasive nature of intravascular ultrasound. A noninvasive technique would therefore be more suitable for screening of atherosclerosis in large populations. Here we introduce an elastin-specific magnetic resonance contrast agent (ESMA) for noninvasive quantification of plaque burden in a mouse model of atherosclerosis. The strong signal provided by ESMA allows for imaging with high spatial resolution, resulting in accurate assessment of plaque burden. Additionally, plaque characterization by quantifying intraplaque elastin content using signal intensity measurements is possible. Changes in elastin content and the high abundance of elastin during plaque development, in combination with the imaging properties of ESMA, provide potential for noninvasive assessment of plaque burden by molecular magnetic resonance imaging (MRI).

Cardiovascular MRI in small animals

Imaging studies of cardiovascular disease in small rodents have become a prerequisite in preclinical cardiovascular research. Transgenic and gene-knockout models of cardiovascular diseases enables the investigation of the influence of single genes or groups of genes on disease pathogenesis. In addition, experimental and genetically altered models provide valuable in vivo platforms to investigate the efficacy of novel drugs and contrast agents. Owing to the excellent soft tissue contrast, high spatial and temporal resolution, as well as the tomographic nature of MRI, anatomy and function can be assessed with unique accuracy and reproducibility. Furthermore, using novel targeted MRI contrast agents, molecular changes associated with cardiovascular disease can be investigated in the same imaging session. This review focuses on recent advances in hardware, imaging sequences and probe design.