Future imaging of atherosclerosis: molecular imaging of coronary atherosclerosis with (18)F positron emission tomography

Atorvastatin Promotes Macrocalcification, But Not Microcalcification in Atherosclerotic Rabbits: An 18F-NaF PET/CT Study

INTRODUCTION

Our study aimed to investigate the effect of atorvastatin on plaque calcification by matching the results obtained by 18F-sodium fluoride (18F-NaF) positron emission tomography (PET)/computed tomography (CT) with data from histologic sections.

METHODS AND RESULTS

The rabbits were divided into 2 groups as follows: an atherosclerosis group (n = 10) and an atorvastatin group (n = 10). All rabbits underwent an abdominal aortic operation and were fed a high-fat diet to induce atherosclerosis. Plasma samples were used to analyze serum inflammation markers and blood lipid levels. 18F-NaF PET/CT scans were performed twice. The plaque area, macrophage number and calcification were measured, and the data from the pathological sections were matched with the 18F-NaF PET/CT scan results. The mean standardized uptake value (0.725 ± 0.126 vs. 0.603 ± 0.071, P < 0.001) and maximum standardized uptake value (1.024 ± 0.116 vs. 0.854 ± 0.091, P < 0.001) significantly increased in the atherosclerosis group, but only slightly increased in the atorvastatin group (0.616 ± 0.103 vs. 0.613 ± 0.094, P = 0.384; 0.853 ± 0.099 vs.0.837 ± 0.089, P < 0.001, respectively). The total calcium density was significantly increased in rabbits treated with atorvastatin compared with rabbits not treated with atorvastatin (1.64 ± 0.90 vs. 0.49 ± 0.35, P < 0.001), but the microcalcification level was significantly lower. There were more microcalcification deposits in the areas with increased radioactive uptake of 18F-NaF.

CONCLUSIONS

Our study suggests that the anti-inflammatory activity of atorvastatin may promote macrocalcification but not microcalcification within atherosclerotic plaques. 18F-NaF PET/CT can detect plaque microcalcifications.

Molecular Imaging and Non-molecular Imaging of Atherosclerotic Plaque Thrombosis

Thrombosis in the context of atherosclerosis typically results in life-threatening consequences, including acute coronary events and ischemic stroke. As such, early detection and treatment of thrombosis in atherosclerosis patients is essential. Clinical diagnosis of thrombosis in these patients is typically based upon a combination of imaging approaches. However, conventional imaging modalities primarily focus on assessing the anatomical structure and physiological function, severely constraining their ability to detect early thrombus formation or the processes underlying such pathology. Recently, however, novel molecular and non-molecular imaging strategies have been developed to assess thrombus composition and activity at the molecular and cellular levels more accurately. These approaches have been successfully used to markedly reduce rates of atherothrombotic events in patients suffering from acute coronary syndrome (ACS) by facilitating simultaneous diagnosis and personalized treatment of thrombosis. Moreover, these modalities allow monitoring of plaque condition for preventing plaque rupture and associated adverse cardiovascular events in such patients. Sustained developments in molecular and non-molecular imaging technologies have enabled the increasingly specific and sensitive diagnosis of atherothrombosis in animal studies and clinical settings, making these technologies invaluable to patients' health in the future. In the present review, we discuss current progress regarding the non-molecular and molecular imaging of thrombosis in different animal studies and atherosclerotic patients.

Quantitative thoracic aorta calcification assessment by 18F-NaF PET/CT and its correlation with atherosclerotic cardiovascular disorders and increasing age

OBJECTIVES

We aimed to assess the correlation between age and cardiovascular risk factors with NaF-PET/CT imaging in the thoracic aorta (TA).

METHODS

In this prospective study, 80 healthy controls and 44 patients with chest pain underwent NaF-PET/CT imaging, and three segments of the aorta (ascending, arch, and descending) were examined. Average SUVmax, SUVmean, and Alavi-Carlsen Score (ACS) were calculated in each segment and the entire vessel. The degree of NaF uptake in controls and patients and its correlation with age were determined. Multivariate linear regression and logistic regression models were employed to determine the predictabilities of Framingham Risk Score (FRS) and unfavorable cardiovascular disease (CVD) risk profile by these measurements.

RESULTS

Average SUVmax, average SUVmean, and ACS were significantly higher in patients than in controls, and all correlated well with age. The correlation of average SUVmean with age was significant in both controls (r = 0.32, p = 0.04) and patients (r = 0.64, p < 0.001). ACS of the entire TA was a stronger predictor of FRS compared with average SUVmax and average SUVmean (adjusted R² = 0.38, standardized β = 0.58, p < 0.001). ACS was a significant predictor of unfavorable CVD risk profile as compared with other values (odds ratio = 1.006, 95% CI = 1.000-1.013, p = 0.05).

CONCLUSIONS

Active calcification in TA correlates with age, and its correlation is higher among subjects with CVD risk factors. Global assessment (ACS) can predict unfavorable CVD risk profile. These data provide evidence for the potential role of NaF in assessing micro-calcification in arteries and its relations to cardiovascular events.

KEY POINTS

• Global micro-calcification in the thoracic aorta as measured by NaF-PET/CT imaging correlates with increasing age. • The extent of the correlation was higher among patients with cardiovascular disease (CVD) risk factors. • These data provide evidence for the potential role of NaF in assessing active calcification in arteries and its relations to cardiovascular events.

Pet tracers for vulnerable plaque imaging

Most of the acute ischemic events, such as acute coronary syndromes and stroke, are attributed to vulnerable plaques. These lesions have common histological and pathophysiological features, including inflammatory cell infiltration, neo-angiogenesis, remodelling, haemorrhage predisposition, thin fibrous cap, large lipid core, and micro-calcifications. Early detection of the presence of a plaque prone to rupture could be life-saving for the patient; however, vulnerable plaques usually cause non-haemodynamically significant stenosis, and anatomical imaging techniques often underestimate, or may not even detect, these lesions. Although ultrasound techniques are currently considered as the "first-line" examinations for the diagnostic investigation and treatment monitoring in patients with atherosclerotic plaques, positron emission tomography (PET) imaging could open new horizons in the assessment of atherosclerosis, given its ability to visualize metabolic processes and provide molecular-functional evidence regarding vulnerable plaques. Moreover, modern hybrid imaging techniques, combining PET with computed tomography or magnetic resonance imaging, can evaluate simultaneously both functional and morphological parameters of the atherosclerotic plaques, and are expected to significantly expand their clinical role in the future. This review summarizes current research on the PET imaging of the vulnerable atherosclerotic plaques, outlining current and potential applications in the clinical setting.

18Fluoride-based molecular imaging of coronary atherosclerosis in HIV infected patients

BACKGROUND AND AIMS

Molecular imaging with ¹⁸Fluorodeoxyglucose (FDG) and ¹⁸F-sodium-fluoride (NaF) captures arterial inflammation and micro-calcification and can reveal potentially unstable atherosclerotic plaques.

METHODS

We performed FDG and NaF PET/CT imaging in two clinically similar cohorts of patients living with HIV (PLWH) with no symptomatic cardiovascular disease. The prevalence and intensity of coronary artery uptake of each tracer, measured as target-to-background ratio (TBR), were assessed in patients at low and high cardiovascular risk.

RESULTS

Ninety-three PLWH were submitted to PET/CT imaging with FDG (N = 43) and NaF (N = 50); 42% were at low and 58% at high cardiovascular risk. The intensity of uptake and multivessel coronary artery uptake were significantly higher with NaF than FDG both in low and high-risk patients. When each ¹⁸F-tracer was tested in low and high-risk patients, an equal proportion of subjects showed no vessel, single and multivessel NaF uptake; the same was true for no and single vessel uptake of FDG (no multivessel FDG uptake was noted). Waist circumference, CRP, D-dimer, HIV duration and treatment with nucleoside reverse transcriptase inhibitors were associated with high NaF uptake in univariable analyses; D-dimer remained significant in multivariable analyses (OR = 1.05; p=0.02). There were no significant associations with FDG uptake.

CONCLUSIONS

The prevalence of coronary artery uptake was higher with NaF compared to FDG both in high and low risk patients, hence microcalcification imaging may be a more sensitive tool to detect coronary atherosclerosis than inflammation imaging. However, the uptake of each ¹⁸Fluoride tracer was similar between low and high-risk subjects, and this underscores the discordance between clinical and imaging based risk assessment. Future investigation should address the prognostic significance of NaF coronary artery uptake.

Current approach to the diagnosis of atherosclerotic coronary artery disease: more questions than answers

Despite its commonality in routine clinical practice, the approach to a diagnosis of atherosclerotic coronary artery disease remains complex and, in part, contentious. The traditional dogma linking ischaemia to hard clinical outcomes has been questioned and reframed over the years; rather than being a predictor of hard clinical outcomes, the degree of ischaemia may simply be a marker of atherosclerotic disease burden. A renewed interest in the imaging of plaque burden has spawned the contemporary role of CT imaging for not only diagnosis and prognosis, but also for dictating downstream management. As the technology develops and evidence expands, decisions on investigative modalities remain centred around patient factors, local availability, test performance and cost. This review summarizes the available methods for diagnosis in the symptomatic patient and provides an overview of the current evidence behind functional and anatomical approaches.

Clinical use of cardiac PET/MRI: current state-of-the-art and potential future applications

Combined PET/MRI is a novel imaging method integrating the advances of functional and morphological MR imaging with PET applications that include assessment of myocardial viability, perfusion, metabolism of inflammatory tissue and tumors, as well as amyloid deposition imaging. As such, PET/MRI is a promising tool to detect and characterize ischemic and non-ischemic cardiomyopathies. To date, the greatest benefit may be expected for diagnostic evaluation of systemic diseases and cardiac masses that remain unclear in cardiac MRI, as well as for clinical and scientific studies in the setting of ischemic cardiomyopathies. Diagnosis and therapeutic monitoring of cardiac sarcoidosis has the potential of a possible 'killer-application' for combined cardiac PET/MRI. In this article, we review the current evidence and discuss current and potential future applications of cardiac PET/MRI.

Positron emission tomography imaging of coronary atherosclerosis

Inflammation has a central role in the progression of coronary atherosclerosis. Recent developments in cardiovascular imaging with the advent of hybrid positron emission tomography have provided a window into the molecular pathophysiology underlying coronary plaque inflammation. Using novel radiotracers targeted at specific cellular pathways, the potential exists to observe inflammation, apoptosis, cellular hypoxia, microcalcification and angiogenesis in vivo. Several clinical studies are now underway assessing the ability of this hybrid imaging modality to inform about atherosclerotic disease activity and the prediction of future cardiovascular risk. A better understanding of the molecular mechanisms governing coronary atherosclerosis may be the first step toward offering patients a more stratified, personalized approach to treatment.