18F-Sodium Fluoride Positron Emission Tomography/Computed Tomography in Ex Vivo Human Coronary Arteries With Histological Correlation

Noninvasive Atherosclerotic Phenotyping: The Next Frontier into Understanding the Pathobiology of Coronary Artery Disease

PURPOSE OF REVIEW

Despite recent advances, coronary artery disease remains one of the leading causes of mortality worldwide. Noninvasive imaging allows atherosclerotic phenotyping by measurement of plaque burden, morphology, activity and inflammation, which has the potential to refine patient risk stratification and guide personalized therapy. This review describes the current and emerging roles of advanced noninvasive cardiovascular imaging methods for the assessment of coronary artery disease.

RECENT FINDINGS

Cardiac computed tomography enables comprehensive, noninvasive imaging of the coronary vasculature, and is used to assess luminal stenoses, coronary calcifications, and distinct adverse plaque characteristics, helping to identify patients prone to future events. Novel software tools, implementing artificial intelligence solutions, can automatically quantify and characterize atherosclerotic plaque from standard computed tomography datasets. These quantitative imaging biomarkers have been shown to improve patient risk stratification beyond clinical risk scores and current clinical interpretation of cardiac computed tomography. In addition, noninvasive molecular imaging in higher risk patients can be used to assess plaque activity and plaque thrombosis. Noninvasive imaging allows unique insight into the burden, morphology and activity of atherosclerotic coronary plaques. Such phenotyping of atherosclerosis can potentially improve individual patient risk prediction, and in the near future has the potential for clinical implementation.

Coronary Atherosclerotic Plaque Activity and Risk of Myocardial Infarction

BACKGROUND

Total coronary atherosclerotic plaque activity across the entire coronary arterial tree is associated with patient-level clinical outcomes.

OBJECTIVES

We aimed to investigate whether vessel-level coronary atherosclerotic plaque activity is associated with vessel-level myocardial infarction.

METHODS

In this secondary analysis of an international multicenter study of patients with recent myocardial infarction and multivessel coronary artery disease, we assessed vessel-level coronary atherosclerotic plaque activity using coronary 18F-sodium fluoride positron emission tomography to identify vessel-level myocardial infarction.

RESULTS

Increased 18F-sodium fluoride uptake was found in 679 of 2,094 coronary arteries and 414 of 691 patients. Myocardial infarction occurred in 24 (4%) vessels with increased coronary atherosclerotic plaque activity and in 25 (2%) vessels without increased coronary atherosclerotic plaque activity (HR: 2.08; 95% CI: 1.16-3.72; P = 0.013). This association was not demonstrable in those treated with coronary revascularization (HR: 1.02; 95% CI: 0.47-2.25) but was notable in untreated vessels (HR: 3.86; 95% CI: 1.63-9.10; Pinteraction = 0.024). Increased coronary atherosclerotic plaque activity in multiple coronary arteries was associated with heightened patient-level risk of cardiac death or myocardial infarction (HR: 2.43; 95% CI: 1.37-4.30; P = 0.002) as well as first (HR: 2.19; 95% CI: 1.18-4.06; P = 0.013) and total (HR: 2.50; 95% CI: 1.42-4.39; P = 0.002) myocardial infarctions.

CONCLUSIONS

In patients with recent myocardial infarction and multivessel coronary artery disease, coronary atherosclerotic plaque activity prognosticates individual coronary arteries and patients at risk for myocardial infarction.

Aortic sodium [18F]fluoride uptake following endovascular aneurysm repair

OBJECTIVE

In patients with abdominal aortic aneurysms, sodium [18F]fluoride positron emission tomography identifies aortic microcalcification and disease activity. Increased uptake is associated with aneurysm expansion and adverse clinical events. The effect of endovascular aneurysm repair (EVAR) on aortic disease activity and sodium [18F]fluoride uptake is unknown. This study aimed to compare aortic sodium [18F]fluoride uptake before and after treatment with EVAR.

METHODS

In a preliminary proof-of-concept cohort study, preoperative and post-operative sodium [18F]fluoride positron emission tomography-computed tomography angiography was performed in patients with an infrarenal abdominal aortic aneurysm undergoing EVAR according to current guideline-directed size treatment thresholds. Regional aortic sodium [18F]fluoride uptake was assessed using aortic microcalcification activity (AMA): a summary measure of mean aortic sodium [18F]fluoride uptake.

RESULTS

Ten participants were recruited (76±6 years) with a mean aortic diameter of 57±2 mm at time of EVAR. Mean time from EVAR to repeat scan was 62±21 months. Prior to EVAR, there was higher abdominal aortic AMA when compared with the thoracic aorta (AMA 1.88 vs 1.2; p<0.001). Following EVAR, sodium [18F]fluoride uptake was markedly reduced in the suprarenal (ΔAMA 0.62, p=0.03), neck (ΔAMA 0.72, p=0.02) and body of the aneurysm (ΔAMA 0.69, p=0.02) while it remained unchanged in the thoracic aorta (ΔAMA 0.11, p=0.41).

CONCLUSIONS

EVAR is associated with a reduction in AMA within the stented aortic segment. This suggests that EVAR can modify aortic disease activity and aortic sodium [18F]fluoride uptake is a promising non-invasive surrogate measure of aneurysm disease activity.

Coronary Atherosclerotic Plaque Activity and Future Coronary Events

Importance

Recurrent coronary events in patients with recent myocardial infarction remain a major clinical problem. Noninvasive measures of coronary atherosclerotic disease activity have the potential to identify individuals at greatest risk.

Objective

To assess whether coronary atherosclerotic plaque activity as assessed by noninvasive imaging is associated with recurrent coronary events in patients with myocardial infarction.

Design, Setting, and Participants

This prospective, longitudinal, international multicenter cohort study recruited participants aged 50 years or older with multivessel coronary artery disease and recent (within 21 days) myocardial infarction between September 2015 and February 2020, with a minimum 2 years' follow-up.

Intervention

Coronary 18F-sodium fluoride positron emission tomography and coronary computed tomography angiography.

Main Outcomes and Measures

Total coronary atherosclerotic plaque activity was assessed by 18F-sodium fluoride uptake. The primary end point was cardiac death or nonfatal myocardial infarction but was expanded during study conduct to include unscheduled coronary revascularization due to lower than anticipated primary event rates.

Results

Among 2684 patients screened, 995 were eligible, 712 attended for imaging, and 704 completed an interpretable scan and comprised the study population. The mean (SD) age of participants was 63.8 (8.2) years, and most were male (601 [85%]). Total coronary atherosclerotic plaque activity was identified in 421 participants (60%). After a median follow-up of 4 years (IQR, 3-5 years), 141 participants (20%) experienced the primary end point: 9 had cardiac death, 49 had nonfatal myocardial infarction, and 83 had unscheduled coronary revascularizations. Increased coronary plaque activity was not associated with the primary end point (hazard ratio [HR], 1.25; 95% CI, 0.89-1.76; P = .20) or unscheduled revascularization (HR, 0.98; 95% CI, 0.64-1.49; P = .91) but was associated with the secondary end point of cardiac death or nonfatal myocardial infarction (47 of 421 patients with high plaque activity [11.2%] vs 19 of 283 with low plaque activity [6.7%]; HR, 1.82; 95% CI, 1.07-3.10; P = .03) and all-cause mortality (30 of 421 patients with high plaque activity [7.1%] vs 9 of 283 with low plaque activity [3.2%]; HR, 2.43; 95% CI, 1.15-5.12; P = .02). After adjustment for differences in baseline clinical characteristics, coronary angiography findings, and Global Registry of Acute Coronary Events score, high coronary plaque activity was associated with cardiac death or nonfatal myocardial infarction (HR, 1.76; 95% CI, 1.00-3.10; P = .05) but not with all-cause mortality (HR, 2.01; 95% CI, 0.90-4.49; P = .09).

Conclusions and Relevance

In this cohort study of patients with recent myocardial infarction, coronary atherosclerotic plaque activity was not associated with the primary composite end point. The findings suggest that risk of cardiovascular death or myocardial infarction in patients with elevated plaque activity warrants further research to explore its incremental prognostic implications.

Imaging of atherosclerosis with [64Cu]Cu-DOTA-TATE in a translational head-to-head comparison study with [18F]FDG, and Na[18F]F in rabbits

Atherosclerosis is a chronic inflammatory disease of the larger arteries that may lead to cardiovascular events. Identification of patients at highest risk of cardiovascular events is challenging, but molecular imaging using positron emission tomography (PET) may prove useful. The aim of this study was to evaluate and compare head-to-head three different PET tracers. Furthermore, tracer uptake is compared to gene expression alterations of the arterial vessel wall. Male New Zealand White rabbits (control group; n = 10, atherosclerotic group; n = 11) were used for the study. Vessel wall uptake was assessed with the three different PET tracers: [¹⁸F]FDG (inflammation), Na[¹⁸F]F (microcalcification), and [⁶⁴Cu]Cu-DOTA-TATE (macrophages), using PET/computed tomography (CT). Tracer uptake was measured as standardized uptake value (SUV), and arteries from both groups were analyzed ex vivo by autoradiography, qPCR, histology, and immunohistochemistry. In rabbits, the atherosclerotic group showed significantly higher uptake of all three tracers compared to the control group [¹⁸F]FDG: SUV_mean 1.50 ± 0.11 versus 1.23 ± 0.09, p = 0.025; Na[¹⁸F]F: SUV_mean 1.54 ± 0.06 versus 1.18 ± 0.10, p = 0.006; and [⁶⁴Cu]Cu-DOTA-TATE: SUV_mean 2.30 ± 0.27 versus 1.65 ± 0.16; p = 0.047. Of the 102 genes analyzed, 52 were differentially expressed in the atherosclerotic group compared to the control group and several genes correlated with tracer uptake. In conclusion, we demonstrated the diagnostic value of [⁶⁴Cu]Cu-DOTA-TATE and Na[¹⁸F]F for identifying atherosclerosis in rabbits. The two PET tracers provided information distinct from that obtained with [¹⁸F]FDG. None of the three tracers correlated significantly to each other, but [⁶⁴Cu]Cu-DOTA-TATE and Na[¹⁸F]F uptake both correlated with markers of inflammation. [⁶⁴Cu]Cu-DOTA-TATE was higher in atherosclerotic rabbits compared to [¹⁸F]FDG and Na[¹⁸F]F.

18F-Sodium Fluoride PET/CT in Assessing Valvular Heart and Atherosclerotic Diseases

Aortic valve stenosis is the most common valvular disease in Western countries, while atherosclerotic cardiovascular disease is the foremost cause of death and disability worldwide. Valve degeneration and atherosclerosis are mediated by inflammation and calcification and inevitably progress over time. Computed tomography can visualise the later stages of macroscopic calcification but fails to assess the early stages of microcalcification and cannot differentiate active from burnt out disease states. Molecular imaging has the ability to provide complementary information related to disease activity, which may allow us to detect disease early, to predict disease progression and to monitor preventive or therapeutic strategies for in both aortic stenosis and atherosclerosis. PET/CT is a non-invasive imaging technique that enables visualization of ongoing molecular processes within small structures, such as the coronary arteries or heart valves. ¹⁸F-sodium fluoride (¹⁸F-NaF) binds hydroxyapatite deposits in the extracellular matrix, with preferential binding to newly developing deposits of microcalcification, which provides an assessment of calcification activity. In recent years, ¹⁸F-NaF has attracted the attention of many research groups and has been evaluated in several pathological cardiovascular processes. Histologic validation of the ¹⁸F-NaF PET signal in valvular disease and atherosclerosis has been reported in multiple independent studies. The selective high-affinity binding of ¹⁸F-NaF to microscopic calcified deposits (beyond the resolution of μCT) has been demonstrated ex vivo, as well as its ability to distinguish between areas of macro- and active microcalcification. In addition, prospective clinical studies have shown that baseline ¹⁸F-NaF uptake in patients with aortic stenosis and mitral annular calcification is correlated with subsequent calcium deposition and valvular dysfunction after a follow-up period of 2 years. In patients with surgical bioprosthetic aortic valves but without morphological criteria for prosthetic degeneration, increased ¹⁸F-NaF uptake at baseline was associated with subsequent bioprosthetic degeneration over time. Similar data were obtained in a cohort of patients with transcatheter aortic valve implantation. Furthermore, several studies have confirmed the association of coronary ¹⁸F-NaF uptake with adverse atherosclerotic plaque features, active disease and future disease progression. ¹⁸F-NaF uptake is also associated with future fatal or nonfatal myocardial infarction in patients with established coronary artery disease. The link between ¹⁸F-NaF uptake and active atherosclerotic disease has not only been demonstrated in the coronary arteries, but also in peripheral arterial disease, abdominal aortic aneurysms and carotid atherosclerosis. It can be assumed that ¹⁸F-NaF PET/CT will strengthen the diagnostic toolbox of practitioners in the coming years. Indeed, there is a strong medical need to diagnose degenerative valvular disease and to detect active atherosclerotic disease states. Finally, the use of ¹⁸F-NaF as a biomarker to monitor the efficacy of drug therapies in preventing these pathological processes is attractive. In this review, we consider the role of ¹⁸F-NaF PET/CT imaging in cardiac valvular diseases and atherosclerosis.

A review of cardio-pulmonary microvascular dysfunction in pulmonary hypertension

Microvascular dysfunction progressing to pulmonary hypertension can be a primary cause of right ventricular failure or a secondary cause because of an underlying systemic illness. Little is known regarding the etiology and epidemiology of coronary microvascular dysfunction in pulmonary hypertension. Despite this limitation, its presence has been described in patients with pulmonary hypertension. This review focuses on the pathogenesis of cardiac and pulmonary microvascular dysfunction in pulmonary hypertension. Additionally, this review provides a contemporary assessment on the diagnosis and treatment of microvascular dysfunction in patients in pulmonary hypertension. This topic is important to raise awareness of microvascular dysfunction in the coronary and pulmonary circulation, so that future studies will investigate its impact on the pulmonary hypertension patient cohort.

In Vivo Coronary 18F-Sodium Fluoride Activity: Correlations With Coronary Plaque Histological Vulnerability and Physiological Environment

OBJECTIVES

This prospective study aimed to evaluate the associations between in vivo coronary ¹⁸F-sodium fluoride (¹⁸F-NaF) positron emission tomography (PET)/computed tomography (CT) activity and ex vivo histological characteristics, to determine whether coronary ¹⁸F-NaF activity is a novel biomarker of plaque pathological vulnerability, and to explore the underlying physiological environment of ¹⁸F-NaF adsorption to vascular microcalcification.

BACKGROUND

¹⁸F-NaF PET/CT is a promising new approach for assessing microcalcification in vascular plaque.

METHODS

Patients with coronary artery disease (CAD) underwent coronary computed tomography angiography (CTA) and ¹⁸F-NaF PET/CT. Histological vulnerability and immunohistochemical characteristics were evaluated in coronary endarterectomy (CE) specimens from patients who underwent coronary artery bypass grafting with adjunctive CE. Correlations between in-vivo coronary ¹⁸F-NaF activity with coronary CTA adverse plaque features and with ex vivo CE specimen morphological features, CD68 expression, inflammatory cytokines expression (tumor necrosis factor-α, interleukin-1β), osteogenic differentiation cytokines expression (osteopontin, runt-related transcription factor 2, osteocalcin) were evaluated. High- and low- to medium-risk plaques were defined by standard pathological classification.

RESULTS

A total of 55 specimens were obtained from 42 CAD patients. Coronary ¹⁸F-NaF activity of high-risk specimens was significantly higher than low- to medium-risk specimens (median [25th-75th percentile]: 1.88 [1.41-2.54] vs 1.12 [0.91-1.54]; P < 0.001). Coronary ¹⁸F-NaF activity showed high discriminatory accuracy in identifying high-risk plaque (AUC 0.80). Coronary CTA adverse plaque features (positive remodeling, low-attenuation plaque, remodeling index), histologically vulnerable features (large necrotic core, thin-fibro cap, microcalcification), CD68 expression, tumor necrosis factor-α expression, and interleukin-1β expression correlated with coronary ¹⁸F-NaF activity (all P < 0.05). No significant association between coronary ¹⁸F-NaF activity and osteogenic differentiation cytokines was found (all P > 0.05).

CONCLUSIONS

Coronary ¹⁸F-NaF activity was associated with histological vulnerability, CD68 expression, inflammatory cytokines expression, but not with osteogenic differentiation cytokines expression. ¹⁸F-NaF PET/CT imaging may provide a powerful tool for detecting high-risk coronary plaque and could improve the risk stratification of CAD patients.

Lipids and cardiovascular calcification: contributions to plaque vulnerability

PURPOSE OF REVIEW

Cardiovascular calcification, a common feature of atherosclerotic lesions, has long been known to associate with cardiovascular risk. The roles of lipoproteins in atherosclerosis are also established, and lipid-modifying therapies have shown capacity for plaque regression. However, the association of lipid-modifying therapies with calcification is more complex, and currently no medical therapies have been found to reverse or attenuate calcification in patients. In this review, we summarize recent developments in our understanding of the interplay between lipids and cardiovascular calcification, as well as new imaging modalities for assessing calcified atherosclerotic plaque vulnerability.

RECENT FINDINGS

Recent clinical studies have highlighted the associations of lipoprotein subtypes, such as low-density and high-density lipoprotein particles, as well as lipoprotein (a) [Lp(a)], with coronary calcification and calcific aortic valve disease. Further, evidence continues to emerge for the utility of fused 18F-sodium fluoride positron-emission tomographic and computed tomographic (18F-NaF PET/CT) imaging in characterizing the microarchitecture and vulnerability of atherosclerotic plaque, in both humans and animal models.

SUMMARY

The relationship between lipids and cardiovascular calcification is complex, and new imaging techniques, such as 18F-NaF PET/CT imaging, may allow for better identification of disease-modifying therapies and prediction of calcified plaque progression and stability to help guide clinical management.

Analysis of 18F-Sodium Fluoride Positron Emission Tomography Signal Sources in Atherosclerotic Minipigs Shows Specific Binding of 18F-Sodium Fluoride to Plaque Calcifications

OBJECTIVE

¹⁸F-sodium fluoride (¹⁸F-NaF) positron emission tomography (PET) imaging is thought to visualize active atherosclerotic plaque calcification. This is supported by the binding of ¹⁸F-NaF to plaque calcification ex vivo, but no prior studies have examined binding of ¹⁸F-NaF to human-like plaque in vivo. Our aim was to validate the specificity of ¹⁸F-NaF PET for plaque calcifications in atherosclerotic minipigs. Approach and Results: Gain-of-function PCSK9^D374Y (proprotein convertase/subtilisin kexin type 9) transgenic Yucatan minipigs (n=4) were fed high-fat diet for 2.5 years to develop atherosclerosis and then subjected to ¹⁸F-NaF PET/computed tomography imaging. The heart, aorta, and iliac arteries were immediately re-scanned ex vivo after surgical extraction. Lesions from the abdominal aorta, iliac arteries, and coronary arteries were cryo-sectioned for autoradiography. Histological plaque characteristics, PET/computed tomography signal, and autoradiography were linked through regression and co-localization analysis. Arterial ¹⁸F-NaF PET signal had intensities comparable to clinical scans and colocalized moderately with calcification detected by computed tomography. Histological analysis showed calcification spanning from microcalcifications near lipid pools and necrotic core to more homogenous macrocalcifications. Comparison with arteries from autopsy cases confirmed the resemblance in localization and appearance with early human plaque calcification. Regression analysis in the abdominal aorta showed correlations with calcified plaque but could not rule out contributions from noncalcified plaque. This was resolved by autoradiography, which showed specific accumulation in plaque calcifications in all examined arteries. In the context of porcine abdominal aorta, ¹⁸F-NaF PET imaging was, however, less accurate than computed tomography for detecting small calcifications.

CONCLUSIONS

¹⁸F-NaF accumulates specifically in calcifications of atherosclerotic plaques in vivo.

Coronary Microvascular Dysfunction

[Figure: see text].

Autoradiographical assessment of inflammation-targeting radioligands for atherosclerosis imaging: potential for plaque phenotype identification

PURPOSE

Many radioligands have been developed for the visualization of atherosclerosis by targeting inflammation. However, interpretation of in vivo signals is often limited to plaque identification. We evaluated binding of some promising radioligands in an in vitro approach in atherosclerotic plaques with different phenotypes.

METHODS

Tissue sections of carotid endarterectomy tissue were characterized as early plaque, fibro-calcific plaque, or phenotypically vulnerable plaque. In vitro binding assays for the radioligands [111In]In-DOTATATE; [111In]In-DOTA-JR11; [67Ga]Ga-Pentixafor; [111In]In-DANBIRT; and [111In]In-EC0800 were conducted, the expression of the radioligand targets was assessed via immunohistochemistry. Radioligand binding and expression of radioligand targets was investigated and compared.

RESULTS

In sections characterized as vulnerable plaque, binding was highest for [111In]In-EC0800; followed by [111In]In-DANBIRT; [67Ga]Ga-Pentixafor; [111In]In-DOTA-JR11; and [111In]In-DOTATATE (0.064 ± 0.036; 0.052 ± 0.029; 0.011 ± 0.003; 0.0066 ± 0.0021; 0.00064 ± 0.00014 %Added activity/mm2, respectively). Binding of [111In]In-DANBIRT and [111In]In-EC0800 was highest across plaque phenotypes, binding of [111In]In-DOTA-JR11 and [67Ga]Ga-Pentixafor differed most between plaque phenotypes. Binding of [111In]In-DOTATATE was the lowest across plaque phenotypes. The areas positive for cells expressing the radioligand's target differed between plaque phenotypes for all targets, with lowest percentage area of expression in early plaque sections and highest in phenotypically vulnerable plaque sections.

CONCLUSIONS

Radioligands targeting inflammatory cell markers showed different levels of binding in atherosclerotic plaques and among plaque phenotypes. Different radioligands might be used for plaque detection and discerning early from vulnerable plaque. [111In]In-EC0800 and [111In]In-DANBIRT appear most suitable for plaque detection, while [67Ga]Ga-Pentixafor and [111In]In-DOTA-JR11 might be best suited for differentiation between plaque phenotypes.

The Molecular Basis of Predicting Atherosclerotic Cardiovascular Disease Risk

Atherosclerotic cardiovascular disease (ASCVD) proceeds through a series of stages: initiation, progression (or regression), and complications. By integrating known biology regarding molecular signatures of each stage with recent advances in high-dimensional molecular data acquisition platforms (to assay the genome, epigenome, transcriptome, proteome, metabolome, and gut microbiome), snapshots of each phase of atherosclerotic cardiovascular disease development can be captured. In this review, we will summarize emerging approaches for assessment of atherosclerotic cardiovascular disease risk in humans using peripheral blood molecular signatures and molecular imaging approaches. We will then discuss the potential (and challenges) for these snapshots to be integrated into a personalized movie providing dynamic readouts of an individual's atherosclerotic cardiovascular disease risk status throughout the life course.

Atherosclerosis Imaging with 18F-Sodium Fluoride PET

The evidence on atherosclerosis imaging with ¹⁸F-sodium-fluoride (NaF) positron emission tomography (PET) is hotly debated because of the different patient characteristics, methodology, vascular beds, etc. in reported studies. This review is a continuation of a previous review on this topic, which covered the period 2010-2018. The purpose was to examine whether some of the most important questions that the previous review had left open had been elucidated by the most recent literature. Using principles of a systematic review, we ended analyzing 25 articles dealing with the carotids, coronary arteries, aorta, femoral, intracranial, renal, and penile arteries. The knowledge thus far can be summarized as follows: by targeting active arterial microcalcification, NaF uptake is considered a marker of early stage atherosclerosis, is age-dependent, and consistently associated with cardiovascular risk. Longitudinal studies on NaF uptake, conducted in the abdominal aorta only, showed unchanged uptake in postmenopausal women for nearly four years and varying uptake in prostate cancer patients over 1.5 years, despite constant or increasing calcium volume detected by computed tomography (CT). Thus, uncertainty remains about the transition from active arterial wall calcification marked by increased NaF uptake to less active or consolidated calcification detected by CT. The question of whether early-phase atherosclerosis and calcification can be modified remains also unanswered due to lack of intervention studies.

An Update on [18F]Fluoride PET Imaging for Atherosclerotic Disease

Atherosclerosis is the leading cause of life-threatening morbidity and mortality, as the rupture of atherosclerotic plaques leads to critical atherothrombotic events such as myocardial infarction and ischemic stroke, which are the 2 most common causes of death worldwide. Vascular calcification is a complicated pathological process involved in atherosclerosis, and microcalcifications are presumed to increase the likelihood of plaque rupture. Despite many efforts to develop novel non-invasive diagnostic modalities, diagnostic techniques are still limited, especially before symptomatic presentation. From this point of view, vulnerable plaques are a direct target of atherosclerosis imaging. Anatomic imaging modalities have the limitation of only visualizing macroscopic structural changes, which occurs in later stages of disease, while molecular imaging modalities are able to detect microscopic processes and microcalcifications, which occur early in the disease process. Na[¹⁸F]-fluoride positron emission tomography/computed tomography could allow the early detection of plaque instability, which is deemed to be a primary goal in the prevention of cardiac or brain ischemic events, by quantifying the microcalcifications within vulnerable plaques and evaluating the atherosclerotic disease burden.

18F-Sodium Fluoride (18F-NaF) for Imaging Microcalcification Activity in the Cardiovascular System

Accumulating preclinical and clinical evidence suggests that calcification is one of the body's primary responses to injury and a key pathological feature of cardiovascular disease. Calcification activity can now be imaged using 18F-sodium fluoride (18F-NaF) positron emission tomography (PET) in combination with either computed tomography or magnetic resonance. These techniques allow visualization of calcification activity and, therefore, provide different information to the established macroscopic calcium imaged with computed tomography. Indeed, 18F-NaF PET has been used to investigate a wide range of valvular conditions, including aortic stenosis, mitral annular calcification, and bioprosthetic valve disease, as well as vascular conditions, including abdominal aortic aneurysm disease, coronary, and carotid atherosclerosis, peripheral vascular disease, and erectile dysfunction. In this brief review, we will focus on how 18F-NaF PET has improved our pathophysiological understanding of cardiovascular calcification and how it can be used as a marker of vascular calcification, providing a useful tool that can be utilized in clinical trials investigating the prediction of both disease progression and clinical events. Finally, we will discuss how 18F-NaF might be employed clinically to improve patient assessment and to guide decision-making.

X-ray Micro-Computed Tomography: An Emerging Technology to Analyze Vascular Calcification in Animal Models

Vascular calcification describes the formation of mineralized tissue within the blood vessel wall, and it is highly associated with increased cardiovascular morbidity and mortality in patients with chronic kidney disease, diabetes, and atherosclerosis. In this article, we briefly review different rodent models used to study vascular calcification in vivo, and critically assess the strengths and weaknesses of the current techniques used to analyze and quantify calcification in these models, namely 2-D histology and the o-cresolphthalein assay. In light of this, we examine X-ray micro-computed tomography (µCT) as an emerging complementary tool for the analysis of vascular calcification in animal models. We demonstrate that this non-destructive technique allows us to simultaneously quantify and localize calcification in an intact vessel in 3-D, and we consider recent advances in µCT sample preparation techniques. This review also discusses the potential to combine 3-D µCT analyses with subsequent 2-D histological, immunohistochemical, and proteomic approaches in correlative microscopy workflows to obtain rich, multifaceted information on calcification volume, calcification load, and signaling mechanisms from within the same arterial segment. In conclusion we briefly discuss the potential use of µCT to visualize and measure vascular calcification in vivo in real-time.