High resolution FDG-microPET of carotid atherosclerosis: plaque components underlying enhanced FDG uptake

Epidemiology, Pathophysiology, and Imaging of Atherosclerotic Intracranial Disease

Intracranial atherosclerotic disease (ICAD) is one of the most common causes of stroke worldwide. Among people with stroke, those of East Asia descent and non-White populations in the United States have a higher burden of ICAD-related stroke compared with Whites of European descent. Disparities in the prevalence of asymptomatic ICAD are less marked than with symptomatic ICAD. In addition to stroke, ICAD increases the risk of dementia and cognitive decline, magnifying ICAD societal burden. The risk of stroke recurrence among patients with ICAD-related stroke is the highest among those with confirmed stroke and stenosis ≥70%. In fact, the 1-year recurrent stroke rate of >20% among those with stenosis >70% is one of the highest rates among common causes of stroke. The mechanisms by which ICAD causes stroke include plaque rupture with in situ thrombosis and occlusion or artery-to-artery embolization, hemodynamic injury, and branch occlusive disease. The risk of stroke recurrence varies by the presumed underlying mechanism of stroke, but whether techniques such as quantitative magnetic resonance angiography, computed tomographic angiography, magnetic resonance perfusion, or transcranial Doppler can help with risk stratification beyond the degree of stenosis is less clear. The diagnosis of ICAD is heavily reliant on lumen-based studies, such as computed tomographic angiography, magnetic resonance angiography, or digital subtraction angiography, but newer technologies, such as high-resolution vessel wall magnetic resonance imaging, can help distinguish ICAD from stenosing arteriopathies.

Imaging Carotid Plaque Inflammation Using Positron Emission Tomography: Emerging Role in Clinical Stroke Care, Research Applications, and Future Directions

Atherosclerosis is a chronic systemic inflammatory condition of the vasculature and a leading cause of stroke. Luminal stenosis severity is an important factor in determining vascular risk. Conventional imaging modalities, such as angiography or duplex ultrasonography, are used to quantify stenosis severity and inform clinical care but provide limited information on plaque biology. Inflammatory processes are central to atherosclerotic plaque progression and destabilization. 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is a validated technique for quantifying plaque inflammation. In this review, we discuss the evolution of FDG-PET as an imaging modality to quantify plaque vulnerability, challenges in standardization of image acquisition and analysis, its potential application to routine clinical care after stroke, and the possible role it will play in future drug discovery.

Motion detection and correction for carotid MRI using a markerless optical system

PURPOSE

Motion related artifact is a challenge for MRI, especially when imaging regions like the carotid artery where complex motion (abrupt and bulk motion) may occur. This study aims to develop a non-contact motion detection and correction system for carotid MRI using a markerless optical tracking system.

METHODS

The proposed markerless optical tracking system consisted of a cross-line laser, an MRI-compatible camera and plastic holders mounted inside the scanner bore. The neck motion of the subject can be captured by monitoring the change of the projected laser position in real-time. The system was used to correct both abrupt motion and bulk motion for carotid MRI. The abrupt motion (e.g. coughing) was compensated by discarding the corrupted k-space lines and re-estimating the missing lines using SPIRiT algorithm. The bulk motion was corrected by phase adjustment of k-space lines according to the measured 1D-translational bulk motion (along anterior-posterior direction) and optimized in-plane translation parameters. Ten volunteers underwent carotid MRI with real-time neck motion detection and retrospective motion correction. Artery sharpness, vessel wall thickness and overall image quality score were compared between the motion-corrupted image and motion-corrected images of different correction strategies.

RESULTS

Both the abrupt motion and the bulk motion during carotid scanning were successfully detected and corrected. The results of ten volunteers demonstrated significant improvement in carotid artery sharpness, vessel wall thickness measurement, and overall image quality score using the proposed markerless optical tracking system and motion correction strategies.

CONCLUSION

The proposed markerless structured light based motion detection and correction system can sensitively detect both abrupt and bulk motion during carotid MR scans. By correcting for both abrupt and bulk motion, vessel wall delineation was improved in carotid MR images, which could potentially facilitate carotid plaque identification and atherosclerosis diagnosis in the future.

[18F]FDG and [18F]NaF as PET markers of systemic atherosclerosis progression: A longitudinal descriptive imaging study in patients with type 2 diabetes mellitus

BACKGROUND

While [18F]-fluordeoxyglucose ([18F]FDG) uptake is associated with arterial inflammation, [18F]-sodium fluoride ([18F]NaF) is a marker for arterial micro-calcification. We aimed to investigate the prospective correlation between both PET markers over time and whether they are prospectively ([18F]FDG) and retrospectively ([18F]NaF) related to progression of systemic arterial disease in a longitudinal study in patients with type 2 diabetes mellitus (T2DM).

METHODS

Baseline [18F]FDG PET/Low Dose (LD) Computed Tomography (CT) scans of ten patients with early T2DM without cardiovascular history (70% men, median age 63 years) were compared with five-year follow-up [18F]NaF/LDCT scans. Systemic activity was expressed as mean target-to-background ratio (meanTBR) by dividing the maximal standardized uptake value (SUVmax) of ten arteries by SUVmean of the caval vein. CT-assessed macro-calcifications were scored visually and expressed as calcified plaque (CP) score. Arterial stiffness was assessed with carotid-femoral pulse wave velocity (PWV). Five-year changes were expressed absolutely with delta (Δ) and relatively with %change.

RESULTS

Baseline meanTBR[18F]FDG was strongly correlated with five-year follow-up meanTBR[18F]NaF (r = 0.709, P = .022). meanTBR[18F]NaF correlated positively with ΔCPscore, CPscore at baseline, and follow-up (r = 0.845, P = .002 and r = 0.855, P = .002, respectively), but not with %change in CPscore and PWV.

CONCLUSION

This proof-of-concept study demonstrated that systemic arterial inflammation is an important pathogenetic factor in systemic arterial micro-calcification development.

Advances in Radiopharmaceutical Sciences for Vascular Inflammation Imaging: Focus on Clinical Applications

Biomedical imaging technologies offer identification of several anatomic and molecular features of disease pathogenesis. Molecular imaging techniques to assess cellular processes in vivo have been useful in advancing our understanding of several vascular inflammatory diseases. For the non-invasive molecular imaging of vascular inflammation, nuclear medicine constitutes one of the best imaging modalities, thanks to its high sensitivity for the detection of probes in tissues. 2-[¹⁸F]fluoro-2-deoxy-d-glucose ([¹⁸F]FDG) is currently the most widely used radiopharmaceutical for molecular imaging of vascular inflammatory diseases such as atherosclerosis and large-vessel vasculitis. The combination of [¹⁸F]FDG and positron emission tomography (PET) imaging has become a powerful tool to identify and monitor non-invasively inflammatory activities over time but suffers from several limitations including a lack of specificity and avid background in different localizations. The use of novel radiotracers may help to better understand the underlying pathophysiological processes and overcome some limitations of [¹⁸F]FDG PET for the imaging of vascular inflammation. This review examines how [¹⁸F]FDG PET has given us deeper insight into the role of inflammation in different vascular pathologies progression and discusses perspectives for alternative radiopharmaceuticals that could provide a more specific and simple identification of pathologies where vascular inflammation is implicated. Use of these novel PET tracers could lead to a better understanding of underlying disease mechanisms and help inform the identification and stratification of patients for newly emerging immune-modulatory therapies. Future research is needed to realize the true clinical translational value of PET imaging in vascular inflammatory diseases.

Recommendations for the Assessment of Carotid Arterial Plaque by Ultrasound for the Characterization of Atherosclerosis and Evaluation of Cardiovascular Risk: From the American Society of Echocardiography

Atherosclerotic plaque detection by carotid ultrasound provides cardiovascular disease risk stratification. The advantages and disadvantages of two-dimensional (2D) and three-dimensional (3D) ultrasound methods for carotid arterial plaque quantification are reviewed. Advanced and emerging methods of carotid arterial plaque activity and composition analysis by ultrasound are considered. Recommendations for the standardization of focused 2D and 3D carotid arterial plaque ultrasound image acquisition and measurement for the purpose of cardiovascular disease stratification are formulated. Potential clinical application towards cardiovascular risk stratification of recommended focused carotid arterial plaque quantification approaches are summarized.

Carotid plaque imaging profiling in subjects with risk factors (diabetes and hypertension)

Carotid artery stenosis (CAS) due to the presence of atherosclerotic plaque (AP) is a frequent medical condition and a known risk factor for stroke, and it is also known from literature that several risk factors promote the AP development, in particular aging, smoke, male sex, hypertension, hyperlipidemia, smoke, diabetes type 1 and 2, and genetic factors. The study of carotid atherosclerosis is continuously evolving: even if the strategies of treatment still depends mainly on the degree of stenosis (DoS) determined by the plaque, in the last years the attention has moved to the study of the plaque components in order to identify the so called "vulnerable" plaque: features like the fibrous cap status and thickness, the volume of the lipid-rich necrotic core and the presence of intraplaque hemorrhage (IPH) are risk factors for plaque rupture, that can be studied with modern imaging techniques. The aim of this review is to give a general overview of the principle histological and imaging features of the subcomponent of carotid AP (CAP), focalizing in particular on the features of CAP of patients affected by hypertension and diabetes (in particular type 2 diabetes mellitus).

Carotid plaque imaging and the risk of atherosclerotic cardiovascular disease

Carotid artery plaque is a measure of atherosclerosis and is associated with future risk of atherosclerotic cardiovascular disease (ASCVD), which encompasses coronary, cerebrovascular, and peripheral arterial diseases. With advanced imaging techniques, computerized tomography (CT) and magnetic resonance imaging (MRI) have shown their potential superiority to routine ultrasound to detect features of carotid plaque vulnerability, such as intraplaque hemorrhage (IPH), lipid-rich necrotic core (LRNC), fibrous cap (FC), and calcification. The correlation between imaging features and histological changes of carotid plaques has been investigated. Imaging of carotid features has been used to predict the risk of cardiovascular events. Other techniques such as nuclear imaging and intra-vascular ultrasound (IVUS) have also been proposed to better understand the vulnerable carotid plaque features. In this article, we review the studies of imaging specific carotid plaque components and their correlation with risk scores.

Immunometabolism orchestrates training of innate immunity in atherosclerosis

Atherosclerosis is characterized by a persistent, low-grade inflammation of the arterial wall. Monocytes and monocyte-derived macrophages play a pivotal role in the various stages of atherosclerosis. In the past few years, metabolic reprogramming has been identified as an important controller of myeloid cell activation status. In addition, metabolic and epigenetic reprogramming are key regulatory mechanisms of trained immunity, which denotes the non-specific innate immune memory that can develop after brief stimulation of monocytes with microbial or non-microbial stimuli. In this review, we build the case that metabolic reprogramming of monocytes and macrophages, and trained immunity in particular, contribute to the pathophysiology of atherosclerosis. We discuss the specific metabolic adaptations, including changes in glycolysis, oxidative phosphorylation, and cholesterol metabolism, that have been reported in atherogenic milieus in vitro and in vivo. In addition, we will focus on the role of these metabolic pathways in the development of trained immunity.

A Clinical Role of PET in Atherosclerosis and Vulnerable Plaques?

Atherosclerosis is a chronic and most often progressive disease with a long clinically apparently silent period, and can become unstable at any time, due to a plaque rupture or erosion, leading to an acute atherothrombotic event. Atherosclerosis has a progression rate that is highly variable among patients and in the same patient. The progression of atherosclerotic plaque from asymptomatic to symptomatic phase depends on its structure and composition in which inflammation plays an essential role. Prototype of the ruptured plaque contains a large, soft, lipid-rich necrotic core with intraplaque hemorrhage that accounts for more than half of the volume of the plaque covered by a thin and inflamed fibrous cap with few smooth muscle cells, and a heavy infiltrate of inflammatory cells. Noninvasive imaging modalities might provide an assessment of the atherosclerotic disease process through the exploration of these plaque features. Computed tomography angiography and magnetic resonance imaging can characterize plaque morphology, whereas molecular imaging, owing to the high sensitivity of nuclear medicine for the detection of radiopharmaceuticals in tissues, allows to explore plaque biology. During the last 2 decades, FDG-PET imaging has also emerged as a powerful tool to explore noninvasively inflammatory activities in atherosclerotic plaques providing new insights on the evolution of metabolic activities in the vascular wall over time. This review highlights the role of PET imaging for the exploration of metabolic activities in atherosclerotic plaques. It will resume the evidence that have been gathered from clinical studies using FDG-PET and will discuss the perspectives of new radiopharmaceuticals for vulnerable plaque imaging.

Dual-Tracer Positron-Emission Tomography for Identification of Culprit Carotid Plaques and Pathophysiology In Vivo

BACKGROUND

Inflammation and microcalcification are interrelated processes contributing to atherosclerotic plaque vulnerability. Positron-emission tomography can quantify these processes in vivo. This study investigates (1) ¹⁸F-fluorodeoxyglucose (FDG) and ¹⁸F-sodium fluoride (NaF) uptake in culprit versus nonculprit carotid atheroma, (2) spatial distributions of uptake, and (3) how macrocalcification affects this relationship.

METHODS

Individuals with acute ischemic stroke with ipsilateral carotid stenosis of ≥50% underwent FDG-positron-emission tomography and NaF-positron-emission tomography. Tracer uptake was quantified using maximum tissue-to-background ratios (TBR_max) and macrocalcification quantified using Agatston scoring.

RESULTS

In 26 individuals, median most diseased segment TBR_max (interquartile range) was higher in culprit than in nonculprit atheroma for both FDG (2.08 [0.52] versus 1.89 [0.40]; P<0.001) and NaF (2.68 [0.63] versus 2.39 [1.02]; P<0.001). However, whole vessel TBR_max was higher in culprit arteries for FDG (1.92 [0.41] versus 1.71 [0.31]; P<0.001) but not NaF (1.85 [0.28] versus 1.79 [0.60]; P=0.10). NaF uptake was concentrated at carotid bifurcations, while FDG was distributed evenly throughout arteries. Correlations between FDG and NaF TBR_max differed between bifurcations with low macrocalcification (r_s=0.38; P<0.001) versus high macrocalcification (r_s=0.59; P<0.001).

CONCLUSIONS

This is the first study to demonstrate increased uptake of both FDG and NaF in culprit carotid plaques, with discrete distributions of pathophysiology influencing vulnerability in vivo. These findings have implications for our understanding of the natural history of the disease and for the clinical assessment and management of carotid atherosclerosis.

Trained Immunity: An Underlying Driver of Inflammatory Atherosclerosis

Atherosclerosis, a chronic inflammatory disease of the arterial wall, is among the leading causes of morbidity and mortality worldwide. The persistence of low-grade vascular inflammation has been considered to fuel the development of atherosclerosis. However, fundamental mechanistic understanding of the establishment of non-resolving low-grade inflammation is lacking, and a large number of atherosclerosis-related cardiovascular complications cannot be prevented by current therapeutic regimens. Trained immunity is an emerging new concept describing a prolonged hyperactivation of the innate immune system after exposure to certain stimuli, leading to an augmented immune response to a secondary stimulus. While it exerts beneficial effects for host defense against invading pathogens, uncontrolled persistent innate immune activation causes chronic inflammatory diseases. In light of the above, the long-term over-activation of the innate immune system conferred by trained immunity has been recently hypothesized to serve as a link between non-resolving vascular inflammation and atherosclerosis. Here, we provide an overview of current knowledge on trained immunity triggered by various exogenous and endogenous inducers, with particular emphasis on its pro-atherogenic effects and the underlying intracellular mechanisms that act at both the cellular level and systems level. We also discuss how trained immunity could be mechanistically linked to atherosclerosis from both preclinical and clinical perspectives. This review details the mechanisms underlying the induction of trained immunity by different stimuli, and highlights that the intracellular training programs can be different, though partly overlapping, depending on the stimulus and the biological system. Thus, clinical investigation of risk factor specific innate immune memory is necessary for future use of trained immunity-based therapy in atherosclerosis.

Molecular and Nonmolecular Magnetic Resonance Coronary and Carotid Imaging

Atherosclerosis is the leading cause of cardiovascular morbidity and mortality. Over the past 2 decades, increasing research attention is converging on the early detection and monitoring of atherosclerotic plaque. Among several invasive and noninvasive imaging modalities, magnetic resonance imaging (MRI) is emerging as a promising option. Advantages include its versatility, excellent soft tissue contrast for plaque characterization and lack of ionizing radiation. In this review, we will explore the recent advances in multicontrast and multiparametric imaging sequences that are bringing the aspiration of simultaneous arterial lumen, vessel wall, and plaque characterization closer to clinical feasibility. We also discuss the latest advances in molecular magnetic resonance and multimodal atherosclerosis imaging.

Molecular imaging of carotid artery atherosclerosis with PET: a systematic review

PURPOSE

To conduct a systematic review of articles on PET imaging of carotid atherosclerosis with emphasis on clinical usefulness and comparison with other imaging modalities.

METHODS

Research articles reporting carotid artery PET imaging with different radiotracers until 30 November 2018 were systematically searched for in Medline/PubMed, Scopus, Embase, Google Scholar, and Cochrane Library. Duplicates were removed, and editorials, case studies, and investigations on feasibility or reproducibility of PET imaging and of patients with end-stage diseases or immunosuppressive medications were omitted. After quality assessment of included articles using Joanna Briggs Institute checklists, all eligible articles were reviewed.

RESULTS

Of 1718 primary hits, 53 studies comprising 4472 patients, aged 47-91 years (78.8% males), were included and grouped under the following headlines: diagnostic performance, risk factors, laboratory findings, imaging modalities, and treatment. ¹⁸F-fluorodeoxyglucose (FDG) (49/53) and ¹⁸F-sodium fluoride (NaF) (5/53) were the most utilized tracers to visualize carotid wall inflammation and microcalcification, respectively. Higher carotid FDG uptake was demonstrated in patients with than without symptomatic carotid atherosclerosis. Normal carotid arteries presented with the lowest FDG uptake. In symptomatic atherosclerosis, carotid arteries ipsilateral to a cerebrovascular event had higher FDG uptake than the contralateral carotid artery. FDG uptake was significantly associated with age, male gender, and body mass index in healthy individuals, and in addition with arterial hypertension, hypercholesterolemia, and diabetes mellitus in patients. Histological assessment indicated a strong correlation between microcalcification and NaF uptake in symptomatic patients. Histological evidence of calcification correlated inversely with FDG uptake, which was associated with increased macrophage and CD68 count, both accounting for increased local inflammatory response.

CONCLUSION

FDG-PET visualizes the inflammatory part of carotid atherosclerosis enabling risk stratification to a certain degree, whereas NaF-PET seems to indicate long-term consequences of ongoing inflammation by demonstrating microcalcification allowing discrimination of atherosclerotic from normal arteries and suggesting clinically significant carotid atherosclerosis.

Monocyte and Macrophage Dynamics in the Cardiovascular System: JACC Macrophage in CVD Series (Part 3)

It has long been recognized that the bone marrow is the primary site of origin for circulating monocytes that may later become macrophages in atherosclerotic lesions. However, only in recent times has the complex relationship among the bone marrow, monocytes/macrophages, and atherosclerotic plaques begun to be understood. Moreover, the systemic nature of these interactions, which also involves additional compartments such as extramedullary hematopoietic sites (i.e., spleen), is only just becoming apparent. In parallel, progressive advances in imaging and cell labeling techniques have opened new opportunities for in vivo imaging of monocyte/macrophage trafficking in atherosclerotic lesions and at the systemic level. In this Part 3 of a 4-part review series covering the macrophage in cardiovascular disease, the authors intersect systemic biology with advanced imaging techniques to explore monocyte and macrophage dynamics in the cardiovascular system, with an emphasis on how events at the systemic level might affect local atherosclerotic plaque biology.

Nuclear Imaging

Noninvasive imaging technologies offer to identify several anatomic and molecular features of high-risk plaques. For the noninvasive molecular imaging of atherosclerotic plaques, nuclear medicine constitutes one of the best imaging modalities, thanks to its high sensitivity for the detection of probes in tissues. 18F-fluorodeoxyglucose (FDG) is currently the most widely used radiopharmaceutical for molecular imaging of atherosclerotic plaques with positron emission tomography. The intensity of FDG uptake in the vascular wall correlates closely with the degree of macrophage infiltration in atherosclerotic plaques. FDG positron emission tomographic imaging has become a powerful tool to identify and monitor noninvasively inflammatory activities in atherosclerotic plaques over time. This review examines how FDG positron emission tomographic imaging has given us deeper insight into the role of inflammation in atherosclerotic plaque progression and discusses perspectives for alternative radiopharmaceuticals to FDG that could provide a more specific and simple identification of high-risk lesions and help improve risk stratification of atherosclerotic patients.

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Imaging biomarkers of vulnerable carotid plaques for stroke risk prediction and their potential clinical implications

Stroke represents a massive public health problem. Carotid atherosclerosis plays a fundamental part in the occurence of ischaemic stroke. European and US guidelines for prevention of stroke in patients with carotid plaques are based on quantification of the percentage reduction in luminal diameter due to the atherosclerotic process to select the best therapeutic approach. However, better strategies for prevention of stroke are needed because some subtypes of carotid plaques (eg, vulnerable plaques) can predict the occurrence of stroke independent of the degree of stenosis. Advances in imaging techniques have enabled routine characterisation and detection of the features of carotid plaque vulnerability. Intraplaque haemorrhage is accepted by neurologists and radiologists as one of the features of vulnerable plaques, but other characteristics-eg, plaque volume, neovascularisation, and inflammation-are promising as biomarkers of carotid plaque vulnerability. These biomarkers could change current management strategies based merely on the degree of stenosis.

PET Imaging of Atherosclerotic Disease: Advancing Plaque Assessment from Anatomy to Pathophysiology

Atherosclerosis is a leading cause of morbidity and mortality. It is now widely recognized that the disease is more than simply a flow-limiting process and that the atheromatous plaque represents a nidus for inflammation with a consequent risk of plaque rupture and atherothrombosis, leading to myocardial infarction or stroke. However, widely used conventional clinical imaging techniques remain anatomically focused, assessing only the degree of arterial stenosis caused by plaques. Positron emission tomography (PET) has allowed the metabolic processes within the plaque to be detected and quantified directly. The increasing armory of radiotracers has facilitated the imaging of distinct metabolic aspects of atherogenesis and plaque destabilization, including macrophage-mediated inflammatory change, hypoxia, and microcalcification. This imaging modality has not only furthered our understanding of the disease process in vivo with new insights into mechanisms but has also been utilized as a non-invasive endpoint measure in the development of novel treatments for atherosclerotic disease. This review provides grounding in the principles of PET imaging of atherosclerosis, the radioligands in use and in development, its research and clinical applications, and future developments for the field.