Imaging of the vulnerable plaque: noninvasive and invasive techniques

Positron emission tomography and its role in the assessment of vulnerable plaques in comparison to other imaging modalities

The diagnosis and management of vulnerable plaques are topics of high interest in the cardiovascular field. Although imaging techniques like computed tomography angiography (MCTA) and ultrasonography (USG) can structurally evaluate atherosclerotic plaques, they are limited in examining internal cellular processes. Positron emission tomography (PET) molecular imaging, on the other hand, can highlight these cellular processes, including inflammation, angiogenesis, and lipid oxidation. Magnetic resonance imaging (MRI) is also a valuable non-invasive imaging technique that can provide detailed anatomical and functional information on the cardiovascular system. In this review, we compare the advantages and drawbacks of MCTA, USG and MRI imaging techniques with PET molecular imaging in evaluating vulnerable plaques. PET imaging allows physicians to measure different pathophysiological events within the plaque using intravenous radiotracers, of which 18F-fluorodeoxyglucose (18F-FDG) is the most validated one. By using 18F-FDG, physicians can understand the formation of the plaque, assess the accumulation of macrophages, and predict major cardiovascular events. However, some limitations exist in using 18F-FDG, including myocardial uptake and low sensitivity in imaging coronary arteries. We also mention other radiotracers that can help in evaluating vulnerable plaques, including 18F-NaF. Although PET imaging is still challenging, it has shown promise in evaluating vulnerable plaques and could be used to intervene in high-risk patients before major cardiovascular events occur.

Simultaneous Noninvasive Detection and Therapy of Atherosclerosis Using HDL Coated Gold Nanorods

Cardiovascular disease (CVD) is a major cause of death and disability worldwide. A real need exists in the development of new, improved therapeutic methods for treating CVD, while major advances in nanotechnology have opened new avenues in this field. In this paper, we report the use of gold nanoparticles (GNPs) coated with high-density lipoprotein (HDL) (GNP-HDL) for the simultaneous detection and therapy of unstable plaques. Based on the well-known HDL cardiovascular protection, by promoting the reverse cholesterol transport (RCT), injured rat carotids, as a model for unstable plaques, were injected with the GNP-HDL. Noninvasive detection of the plaques 24 h post the GNP injection was enabled using the diffusion reflection (DR) method, indicating that the GNP-HDL particles had accumulated in the injured site. Pathology and noninvasive CT measurements proved the recovery of the injured artery treated with the GNP-HDL. The DR of the GNP-HDL presented a simple and highly sensitive method at a low cost, resulting in simultaneous specific unstable plaque diagnosis and recovery.

A Metabolic Intravascular Platform to Study FDG Uptake in Vascular Injury

PURPOSE

Metabolic alterations underlie many pathophysiological conditions, and their understanding is critical for the development of novel therapies. Although the assessment of metabolic changes in vivo has been historically challenging, recent developments in molecular imaging have allowed us to study novel metabolic research concepts directly in the living subject, bringing us closer to patients. However, in many instances, there is need for sensors that are in close proximity to the organ under investigation, for example to study vascular metabolism.

METHODS

In this study, we developed and validated a metabolic detection platform directly in the living subject under an inflammatory condition. The signal collected by a scintillating fiber is amplified using a photomultiplier tube and decodified by an in-house tunable analysis platform. For in vivo testing, we based our experiments on the metabolic characteristics of macrophages, cells closely linked to inflammation and avid for glucose and its analog ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). The sensor was validated in New Zealand rabbits, in which inflammation was induced by either a) high cholesterol (HC) diet for 16 weeks or b) vascular balloon endothelial denudation followed by HC diet.

RESULTS

There was no difference in weight, hemodynamics, blood pressure, or heart rate between the groups. Vascular inflammation was detected by the metabolic sensor (Inflammation: 0.60 ± 0.03 AU vs. control: 0.48 ± 0.03 AU, p = 0.01), even though no significant inflammation/atherosclerosis was detected by intravascular ultrasound, underscoring the high sensitivity of the system. These findings were confirmed by the presence of macrophages on ex vivo aortic tissue staining.

CONCLUSION

In this study, we validated a tunable very sensitive metabolic sensor platform that can be used for the detection of vascular metabolism, such as inflammation. This sensor can be used not only for the detection of macrophage activity but, with alternative probes, it could allow the detection of other pathophysiological processes.

Combining cholesterol-lowering strategies with imaging data: a visible benefit?

Coronary artery disease is characterised by the development of atherosclerotic plaques and is associated with significant morbidity and mortality on a global level. However, many patients with atherosclerosis are asymptomatic and the prediction of acute coronary events is challenging. The role of imaging studies in characterising plaque morphology and stability is emerging as a valuable prognostic tool, while providing evidence for the beneficial effects of cholesterol-lowering therapy on plaque burden. This review provides an overview of contemporary studies describing the value of imaging strategies for atherosclerotic plaques. Coronary angiography is commonly used in the clinical setting, but requires a significant radiation dose (similar to computed tomography). Magnetic resonance imaging evaluation of coronary vessels would avoid exposure to ionising radiation, but is not yet feasible due to motion artefacts. The roles of alternative imaging techniques, including grey-scale intravascular ultrasound, optical coherence tomography and near-infrared spectroscopy have emerged in recent years. In particular, grey-scale intravascular ultrasound has been effectively applied to detect changes in plaque burden and features of plaques predictive of rupture, as well as plaque characteristics during cholesterol-lowering therapy, providing novel insights into factors that may contribute to treatment effectiveness. Challenges and limitations to the use of imaging techniques are considered in this context, along with future imaging strategies.

Synthesis of Monocyte-targeting Peptide Amphiphile Micelles for Imaging of Atherosclerosis

Atherosclerosis is a major contributor to cardiovascular disease, the leading cause of death worldwide, which claims 17.3 million lives annually. Atherosclerosis is also the leading cause of sudden death and myocardial infarction, instigated by unstable plaques that rupture and occlude the blood vessel without warning. Current imaging modalities cannot differentiate between stable and unstable plaques that rupture. Peptide amphiphiles micelles (PAMs) can overcome this drawback as they can be modified with a variety of targeting moieties that bind specifically to diseased tissue. Monocytes have been shown to be early markers of atherosclerosis, while large accumulation of monocytes is associated with plaques prone to rupture. Hence, nanoparticles that can target monocytes can be used to discriminate different stages of atherosclerosis. To that end, here, we describe a protocol for the preparation of monocyte-targeting PAMs (monocyte chemoattractant protein-1 (MCP-1) PAMs). MCP-1 PAMs are self-assembled through synthesis under mild conditions to form nanoparticles of 15 nm in diameter with near neutral surface charge. In vitro, PAMs were found to be biocompatible and had a high binding affinity for monocytes. The methods described herein show promise for a wide range of applications in atherosclerosis as well as other inflammatory diseases.

Detection of gold nanorods uptake by macrophages using scattering analyses combined with diffusion reflection measurements as a potential tool for in vivo atherosclerosis tracking

In this study, we report a potential noninvasive technique for the detection of vulnerable plaques using scatter analyses with flow cytometry (FCM) method combined with the diffusion reflection (DR) method. The atherosclerotic plaques are commonly divided into two major categories: stable and vulnerable. The vulnerable plaques are rich with inflammatory cells, mostly macrophages (MΦ), which release enzymes that break down collagen in the cap. The detection method is based on uptake of gold nanorods (GNR) by MΦ. The GNR have unique optical properties that enable their detection using the FCM method, based on their scattering properties, and using the DR method, based on their unique absorption properties. This work demonstrates that after GNR labeling of MΦ, 1) the FCM scatter values increased up to 3.7-fold with arbitrary intensity values increasing from 1,110 to 4,100 and 2) the DR slope changed from an average slope of 0.196 (MΦ only) to an average slope of 0.827 (MΦ labeled with GNR) (P<0.001 for both cases). The combination of FCM and DR measurements provides a potential novel, highly sensitive, and noninvasive method for the identification of atherosclerotic vulnerable plaques, aimed to develop a potential tool for in vivo tracking.

Reproducibility of noncalcified coronary artery plaque burden quantification from coronary CT angiography across different image analysis platforms

OBJECTIVE

The objective of our study was to evaluate the reproducibility of noncalcified coronary artery plaque burden quantification from coronary CT angiography (CTA) across different commercial analysis platforms.

MATERIALS AND METHODS

For this study, 47 patients (36 men, 11 women; mean age ± SD, 62 ± 13 years) with noncalcified plaques on coronary CTA were included. Automated quantification of noncalcified coronary artery plaque volume was performed on identical datasets using three commercially available image analysis software platforms (software platforms 1-3). Identical tissue attenuation ranges between 0 and 50 HU for low-attenuation plaques and 50-130 HU for medium-attenuation plaques were consistently applied. Log volume data were compared with the Pearson correlation coefficient and Bland-Altman analysis.

RESULTS

Differences in plaque volume measurements on intraplatform repeat measurements were statistically insignificant (p = 0.923). At the low-attenuation threshold, software platform 3 had significantly higher log volumes (p < 0.001) than both software platforms 1 and 2 and software platform 1 had significantly higher log volumes than software platform 2 (p < 0.001). The results at the medium-attenuation level were identical except that the log volumes for software platforms 1 and 2 were not significantly different (p > 0.05) in the left anterior descending artery and left circumflex artery. The Pearson correlation coefficient was found to be 0.677 (p < 0.001; 95% CI, 0.608-0.735) between software platforms 1 and 2, 0.672 (p < 0.001; 95% CI, 0.603-0.732) between software platforms 1 and 3, and 0.550 (p < 0.001; 95% CI, 0.463-0.627) between software platforms 2 and 3.

CONCLUSION

Currently available noncalcified plaque quantification software provides good intraplatform reproducibility but poor interplatform reproducibility. Serial or comparative assessments require evaluation using the same software. Industry standards should be developed to enable reproducible assessments across manufacturers.

Plaque stability and the southern European paradox

Differences between European countries in coronary heart disease mortality were initially described in the 20th century, and albeit less dramatic than first reported, these differences remain substantial. Three main hypotheses have been proposed to explain the so-called "Mediterranean paradox": a) underestimation of coronary heart disease mortality due to methodological flaws; b) the "lag time" hypothesis, and c) the traditional Mediterranean diet and lifestyle. In this manuscript we present and discuss another possible explanation for the Mediterranean paradox related to the higher prevalence and and incidence of stable atheromatous plaques in this area.

Intracranial atherosclerotic plaque enhancement in patients with ischemic stroke

BACKGROUND AND PURPOSE

Inflammation of an atherosclerotic plaque is a well-known risk factor in the development of ischemic stroke and myocardial infarction. MR imaging is capable of characterizing inflammation by assessing plaque enhancement in both extracranial carotid arteries and coronary arteries. Our goal was to determine whether enhancing intracranial atherosclerotic plaque was present in the vessel supplying the territory of infarction by using high-resolution vessel wall MR imaging.

MATERIALS AND METHODS

High-resolution vessel wall 3T MR imaging studies performed in 29 patients with ischemic stroke and intracranial vascular stenoses were reviewed for presence and strength of plaque enhancement.

RESULTS

Sixteen patients were studied during the acute phase (<4 weeks from acute stroke), 5 patients in the subacute phase (4-12 weeks), and 8 patients in the chronic phase (>12 weeks) of the ischemic injury. In all of the acute phase patients, atherosclerotic plaque in the vessel supplying the stroke territory demonstrated strong enhancement. There was a trend of decreasing enhancement as the time of imaging relative to the ischemic event increased.

CONCLUSIONS

Strong pathologic enhancement of intracranial atherosclerotic plaque was seen in all patients imaged within 4 weeks of ischemic stroke in the vessel supplying the stroke territory. The strength and presence of enhancement of the atherosclerotic plaque decreased with increasing time after the ischemic event. These findings suggest a relationship between enhancing intracranial atherosclerotic plaque and acute ischemic stroke.

A historical perspective towards a non-invasive treatment for patients with atherosclerosis

The history of atherosclerosis and cardiovascular disease dates back to ancient times. From the teachings of Galen to the response-to-injury hypothesis of Russel Ross, we have now arrived at the concept of the vulnerable plaque. Next to the development of new treatment options for patients with atherosclerosis, also novel diagnostic imaging techniques have been developed to visualise the arterial wall and to characterise plaque composition. In this article the historical context of atherosclerosis and the attempts towards a noninvasive therapy for patients with atherosclerotic diseases are described. (Neth Heart J 2009;17:140-4.).

Prevention of 7β-hydroxycholesterol-induced cell death by mangafodipir is mediated through lysosomal and mitochondrial pathways

Mangafodipir, a MRI contrast agent, has been used as a viability marker in patients with myocardial infarction and showed vascular relaxation effect. It confers myocardial protection against oxidative stress. However mechanisms underlying such protection have not yet been investigated. In this investigation we first studied whether mangafodipir inhibits apoptosis induced by 7beta-hydroxycholesterol (7betaOH), a cytotoxic cholesterol oxidation product found in atherosclerotic lesions in humans and in heart of ethanol-fed rats. We then focused on whether mangafodipir influences the production of reactive oxygen species, lysosomal and mitochondrial membrane permeabilities in the cell model. Our results revealed that pre-treatment with mangafodipir (400 microM) protected against cellular reactive oxygen species production, apoptosis, and permeabilization of lysosomal and mitochondrial membranes induced by 7betaOH. In conclusion, a novel effect of mangafodipir on 7betaOH-induced apoptosis is via reduction of cellular reactive oxygen species and stabilization of lysosomal and mitochondrial membranes. This is the first report to show the additional cytoprotective effect of mangafodipir, which may suggest possible use of the drug.

Comprehensive noninvasive arterial vascular evaluation

Noninvasive evaluation of the health of the vasculature is an increasingly important approach to identifying individuals at risk for cardiovascular morbid events. Vascular health assessment involves functional and structural examination of the large and small arteries. Endothelial dysfunction, which alters small artery function, is a key contributor to progressive structural vascular changes. Ultrasonography of conduit arteries and aortic pulse-wave velocity provide information limited to structural changes in large arteries. Comprehensive assessment of the large and small arteries is best carried out by pulse-contour analysis. This can provide insight into functional and structural abnormalities in the small as well as the large arteries, and may be useful in monitoring response to therapy.