Comparison of gadofluorine-M and Gd-DTPA for noninvasive staging of atherosclerotic plaque stability using MRI

Mass Spectrometry Imaging of atherosclerosis-affine Gadofluorine following Magnetic Resonance Imaging

Molecular imaging of atherosclerosis by Magnetic Resonance Imaging (MRI) has been impaired by a lack of validation of the specific substrate responsible for the molecular imaging signal. We therefore aimed to investigate the additive value of mass spectrometry imaging (MSI) of atherosclerosis-affine Gadofluorine P for molecular MRI of atherosclerotic plaques. Atherosclerotic Ldlr^−/− mice were investigated by high-field MRI (7 T) at different time points following injection of atherosclerosis-affine Gadofluorine P as well as at different stages of atherosclerosis formation (4, 8, 16 and 20 weeks of HFD). At each imaging time point mice were immediately sacrificed after imaging and aortas were excised for mass spectrometry imaging: Matrix Assisted Laser Desorption Ionization (MALDI) Imaging and Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) imaging. Mass spectrometry imaging allowed to visualize the localization and measure the concentration of the MR imaging probe Gadofluorine P in plaque tissue ex vivo with high spatial resolution and thus adds novel and more target specific information to molecular MR imaging of atherosclerosis.

Molecular imaging of myocardial infarction with Gadofluorine P - A combined magnetic resonance and mass spectrometry imaging approach

Background

Molecular MRI is becoming increasingly important for preclinical research. Validation of targeted gadolinium probes in tissue however has been cumbersome up to now. Novel methodology to assess gadolinium distribution in tissue after in vivo application is therefore needed.

Purpose

To establish combined Magnetic Resonance Imaging (MRI) and Mass Spectrometry Imaging (MSI) for improved detection and quantification of Gadofluorine P deposition in scar formation and myocardial remodeling.

Materials and methods

Animal studies were performed according to institutionally approved protocols. Myocardial infarction was induced by permanent ligation of the left ascending artery (LAD) in C57BL/6J mice. MRI was performed at 7T at 1 week and 6 weeks after myocardial infarction. Gadofluorine P was used for dynamic T₁ mapping of extracellular matrix synthesis during myocardial healing and compared to Gd-DTPA. After in vivo imaging contrast agent concentration as well as distribution in tissue were validated and quantified by spatially resolved Matrix-Assisted Laser Desorption Ionization (MALDI) MSI and Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS) imaging.

Results

Both Gadofluorine P enhancement as well as local tissue content in the myocardial scar were highest at 15 minutes post injection. R₁ values increased from 1 to 6 weeks after MI (1.62 s⁻¹ vs 2.68 s⁻¹, p = 0.059) paralleled by an increase in Gadofluorine P concentration in the infarct from 0.019 mM at 1 week to 0.028 mM at 6 weeks (p = 0.048), whereas Gd-DTPA enhancement showed no differences (3.95 s⁻¹ vs 3.47 s⁻¹, p = 0.701). MALDI-MSI results were corroborated by elemental LA-ICP-MS of Gadolinium in healthy and infarcted myocardium. Histology confirmed increased extracellular matrix synthesis at 6 weeks compared to 1 week.

Conclusion

Adding quantitative MSI to MR imaging enables a quantitative validation of Gadofluorine P distribution in the heart after MI for molecular imaging.

Contrast agents for cardiovascular magnetic resonance imaging: an overview

Contrast agents for Cardiovascular Magnetic Resonance (CMR) play a major role in research and clinical cardiology.

Safety Evaluation and Imaging Properties of Gadolinium-Based Nanoparticles in nonhuman primates

In this article, we report the safety evaluation of gadolinium-based nanoparticles in nonhuman primates (NHP) in the context of magnetic resonance imaging (MRI) studies in atherosclerosis bearing animals and healthy controls. In healthy NHP, the pharmacokinetics and toxicity profiles demonstrated the absence of dose, time, and sex-effects, as well as a suitable tolerance of intravenous administration of the nanoparticles. We investigated their imaging properties for arterial plaque imaging in a standard diet or a high cholesterol diet NHP, and compared their characteristics with clinically applied Gd-chelate. This preliminary investigation reports the efficient and safe imaging of atherosclerotic plaques.

Noninvasive diagnosis of vulnerable coronary plaque

Myocardial infarction and sudden cardiac death are frequently the first manifestation of coronary artery disease. For this reason, screening of asymptomatic coronary atherosclerosis has become an attractive field of research in cardiovascular medicine. Necropsy studies have described histopathological changes associated with the development of acute coronary events. In this regard, thin-cap fibroatheroma has been identified as the main vulnerable coronary plaque feature. Hence, many imaging techniques, such as coronary computed tomography, cardiac magnetic resonance or positron emission tomography, have tried to detect noninvasively these histomorphological characteristics with different approaches. In this article, we review the role of these diagnostic tools in the detection of vulnerable coronary plaque with particular interest in their advantages and limitations as well as the clinical implications of the derived findings.

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

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

Visualizing the atherosclerotic plaque: a chemical perspective

Atherosclerosis is the major underlying pathologic cause of coronary artery disease. An early detection of the disease can prevent clinical sequellae such as angina, myocardial infarction, and stroke. The different imaging techniques employed to visualize the atherosclerotic plaque provide information of diagnostic and prognostic value. Furthermore, the use of contrast agents helps to improve signal-to-noise ratio providing better images. For nuclear imaging techniques and optical imaging these agents are absolutely necessary. We report on the different contrast agents that have been used, are used or may be used in future in animals, humans, or excised tissues for the distinct imaging modalities for atherosclerotic plaque imaging.

MR imaging of the arterial vessel wall: molecular imaging from bench to bedside

Cardiovascular diseases remain the leading cause of morbidity and mortality in the Western world and developing countries. In clinical practice, in vivo characterization of atherosclerotic lesions causing myocardial infarction, ischemic stroke, and other complications remains challenging. Imaging methods, limited to the assessment luminal stenosis, are the current reference standard for the assessment of clinically significant coronary and carotid artery disease and the guidance of treatment. These techniques do not allow distinction between stable and potentially vulnerable atherosclerotic plaque. Magnetic resonance (MR) imaging is a modality well suited for visualization and characterization of the relatively thin arterial vessel wall, because it allows imaging with high spatial resolution and excellent soft-tissue contrast. In clinical practice, atherosclerotic plaque components of the carotid artery and aorta may be differentiated and characterized by using unenhanced vessel wall MR imaging. Additional information can be gained by using clinically approved nonspecific contrast agents. With the advent of targeted MR contrast agents, which enhance specific molecules or cells, pathologic processes can be visualized at a molecular level with high spatial resolution. In this article, the pathophysiologic changes of the arterial vessel wall underlying the development of atherosclerosis will be first reviewed. Then basic principles and properties of molecular MR imaging contrast agents will be introduced. Additionally, recent advances in preclinical molecular vessel wall imaging will be reviewed. Finally, the clinical feasibility of arterial vessel wall imaging at unenhanced and contrast material-enhanced MR imaging of the aortic, carotid, and coronary vessel wall will be discussed.

Molecular imaging of inflammation in atherosclerosis

Acute rupture of vulnerable plaques frequently leads to myocardial infarction and stroke. Within the last decades, several cellular and molecular players have been identified that promote atherosclerotic lesion formation, maturation and plaque rupture. It is now widely recognized that inflammation of the vessel wall and distinct leukocyte subsets are involved throughout all phases of atherosclerotic lesion development. The mechanisms that render a stable plaque unstable and prone to rupture, however, remain unknown and the identification of the vulnerable plaque remains a major challenge in cardiovascular medicine. Imaging technologies used in the clinic offer minimal information about the underlying biology and potential risk for rupture. New imaging technologies are therefore being developed, and in the preclinical setting have enabled new and dynamic insights into the vessel wall for a better understanding of this complex disease. Molecular imaging has the potential to track biological processes, such as the activity of cellular and molecular biomarkers in vivo and over time. Similarly, novel imaging technologies specifically detect effects of therapies that aim to stabilize vulnerable plaques and silence vascular inflammation. Here we will review the potential of established and new molecular imaging technologies in the setting of atherosclerosis, and discuss the cumbersome steps required for translating molecular imaging approaches into the clinic.

Combined MR, fluorescence and histology imaging strategy in a human breast tumor xenograft model

Applications of molecular imaging in cancer and other diseases frequently require the combination of in vivo imaging modalities, such as MR and optical imaging, with ex vivo optical, fluorescence, histology and immunohistochemical imaging to investigate and relate molecular and biological processes to imaging parameters within the same region of interest. We have developed a multimodal image reconstruction and fusion framework that accurately combines in vivo MRI and MRSI, ex vivo brightfield and fluorescence microscopic imaging and ex vivo histology imaging. Ex vivo brightfield microscopic imaging was used as an intermediate modality to facilitate the ultimate link between ex vivo histology and in vivo MRI/MRSI. Tissue sectioning necessary for optical and histology imaging required the generation of a three-dimensional reconstruction module for two-dimensional ex vivo optical and histology imaging data. We developed an external fiducial marker-based three-dimensional reconstruction method, which was able to fuse optical brightfield and fluorescence with histology imaging data. The registration of the three-dimensional tumor shape was pursued to combine in vivo MRI/MRSI and ex vivo optical brightfield and fluorescence imaging data. This registration strategy was applied to in vivo MRI/MRSI, ex vivo optical brightfield/fluorescence and histology imaging datasets obtained from human breast tumor models. Three-dimensional human breast tumor datasets were successfully reconstructed and fused with this platform.

Gadolinium-Based Contrast Agents for Vessel Wall Magnetic Resonance Imaging (MRI) of Atherosclerosis

Cardiovascular disease due to atherosclerosis is the number one killer in the Western world, and threatens to become the major cause of morbidity and mortality worldwide. It is therefore paramount to develop non-invasive methods for the detection of high-risk, asymptomatic individuals before the onset of clinical symptoms or events. In the recent past, great strides have been made in the understanding of the pathological mechanisms involved in the atherosclerotic cascade down to the molecular details. This has allowed the development of contrast agents that can aid in the in vivo characterization of these processes. Gadolinium chelates are among the contrast media most commonly used in MR imaging. Originally used for MR angiography for the detection and quantification of vascular stenosis, more recently they have been applied to improve characterization of atherosclerotic plaques. In this manuscript, we will briefly review gadolinium-chelates (Gd) based contrast agents for non-invasive MR imaging of atherosclerosis. We will first describe Gd-based non-targeted FDA approved agents, used routinely in clinical practice for the evaluation of neovascularization in other diseases. Secondly, we will describe non-specific and specific targeted contrast agents, which have great potential for dissecting specific biological processes in the atherosclerotic cascade. Lastly, we will briefly compare Gd-based agents to others commonly used in MRI and to other imaging modalities.

Cardiovascular magnetic resonance imaging in small animals

Noninvasive imaging studies involving small animals are becoming increasingly important in preclinical pharmacological, genetic, and biomedical cardiovascular research. Especially small animal magnetic resonance imaging (MRI) using high field and clinical MRI systems has gained significant importance in recent years. Compared to other imaging modalities, like computer tomography, MRI can provide an excellent soft tissue contrast, which enables the characterization of different kinds of tissues without the use of contrast agents. In addition, imaging can be performed with high spatial and temporal resolution. Small animal MRI cannot only provide anatomical information about the beating murine heart; it can also provide functional and molecular information, which makes it a unique imaging modality. Compared to clinical MRI examinations in humans, small animal MRI is associated with additional challenges. These included a smaller size of all cardiovascular structures and a up to ten times higher heart rate. Dedicated small animal monitoring devices make a reliable cardiac triggering and respiratory gating feasible. MRI in combination with molecular probes enables the noninvasive imaging of biological processes at a molecular level. Different kinds of iron oxide or gadolinium-based contrast agents can be used for this purpose. Compared to other molecular imaging modalities, like single photon emission computed tomography (SPECT) and positron emission tomography (PET), MRI can also provide imaging with high spatial resolution, which is of high importance for the assessment of the cardiovascular system. The sensitivity for detection of MRI contrast agents is however lower compared to sensitivity of radiation associated techniques like PET and SPECT. This chapter is divided into the following sections: (1) "Introduction," (2) "Principals of Magnetic Resonance Imaging," (3) "MRI Systems for Preclinical Imaging and Experimental Setup," and (4) "Cardiovascular Magnetic Resonance Imaging."

Imaging assessment of cardiovascular disease in systemic lupus erythematosus

Systemic lupus erythematosus is a multisystem, autoimmune disease known to be one of the strongest risk factors for atherosclerosis. Patients with SLE have an excess cardiovascular risk compared with the general population, leading to increased cardiovascular morbidity and mortality. Although the precise explanation for this is yet to be established, it seems to be associated with the presence of an accelerated atherosclerotic process, arising from the combination of traditional and lupus-specific risk factors. Moreover, cardiovascular-disease associated mortality in patients with SLE has not improved over time. One of the main reasons for this is the poor performance of standard risk stratification tools on assessing the cardiovascular risk of patients with SLE. Therefore, establishing alternative ways to identify patients at increased risk efficiently is essential. With recent developments in several imaging techniques, the ultimate goal of cardiovascular assessment will shift from assessing symptomatic patients to diagnosing early cardiovascular disease in asymptomatic patients which will hopefully help us to prevent its progression. This review will focus on the current status of the imaging tools available to assess cardiac and vascular function in patients with SLE.

Magnetic resonance enhancement pattern and diagnostic accuracy of gadofluorine M in a rabbit VX2 tumor model: Comparison with gadopentetate dimeglumine

OBJECTIVE

The purpose of this study was to evaluate the enhancement pattern and the diagnostic accuracy of gadofluorine M in comparison with gadopentetate dimeglumine in a rabbit VX2 tumor model.

MATERIALS AND METHODS

Thirteen rabbits with experimentally induced VX2 carcinomas in the thighs underwent sequential T1-weighted enhancement MR imaging using a 3.0T MR imager, first with gadopentetate dimeglumine, and then 24 (n=4) or 4h (n=9) later with gadofluorine M. In 4 rabbits with 13 tumors, the time-percentage enhancement (PE; i.e., percentage of signal intensity increase) curve was obtained for up to 24h for each contrast agent. In 9 rabbits with 49 tumors (random numbers of VX2 tumors were inoculated at random sites in the thigh), 3 readers unaware of the histopathologic results interpreted the MR images and determined the number and conspicuity level of the detected tumors. The reference standard was the histopathology of the specimen.

RESULTS

The time-to-peak PE for gadopentetate dimeglumine was 1min and gadopentetate dimeglumine showed a rapid washout pattern. The time-to-peak PE for gadofluorine M was 30min and gadofluorine M showed a plateau enhancement pattern for up to 24h. The peak PE of gadofluorine M was approximately twice that of the same dose of gadopentetate dimeglumine (108.2±14.8 vs. 51.5±24.0). The sensitivities for detecting VX2 tumors by 3 readers were 89.8% (44/49), 85.7% (42/49), and 95.9% (47/49) for gadopentetate dimeglumine-enhanced MR imaging, and 87.8% (43/49), 89.8% (44/49), and 89.8% (44/49) for gadofluorine M-enhanced MR imaging. No significant differences in the sensitivities existed between the two contrast agents for any reader. However, the conspicuity level of tumors was superior with gadofluorine M-enhanced MR imaging for two readers and similar for the other reader.

CONCLUSION

Gadofluorine M showed strong and plateau enhancement of tumors for up to 24h. In the reader study, gadofluorine M showed better conspicuity for VX2 tumors than gadopentetate dimeglumine, but had a similar sensitivity.

Molecular imaging of macrophage protease activity in cardiovascular inflammation in vivo

Macrophages contribute pivotally to cardiovascular diseases (CVD), notably to atherosclerosis. Imaging of macrophages in vivo could furnish new tools to advance evaluation of disease and therapies. Proteolytic enzymes serve as key effectors of many macrophage contributions to CVD. Therefore, intravital imaging of protease activity could aid evaluation of the progress and outcome of atherosclerosis, aortic aneurysm formation, or rejection of cardiac allografts. Among the large families of proteases, matrix metalloproteinases (MMPs) and cysteinyl cathepsins have garnered the most interest because of their participation in extracellular matrix remodelling. These considerations have spurred the development of dedicated imaging agents for protease activity detection. Activatable fluorescent probes, radiolabelled inhibitors, and nanoparticles are currently under exploration for this purpose. While some agents and technologies may soon see clinical use, others will require further refinement. Imaging of macrophages and protease activity should provide an important adjunct to understanding pathophysiology in vivo, evaluating the effects of interventions, and ultimately aiding clinical care.