¹⁸F-NaF Uptake by Atherosclerotic Plaque on PET/CT Imaging: Inverse Correlation Between Calcification Density and Mineral Metabolic Activity

18F-NaF uptake on vascular PET imaging in symptomatic versus asymptomatic atherosclerotic disease: A meta-analysis

INTRODUCTION

18F-sodium fluoride (NaF) positron-emission tomography (PET) is increasingly being used to measure microcalcification in atherosclerotic disease in vivo. Correlations have been drawn between sodium fluoride uptake and the presence of high-risk plaque features, as well as its association with clinical atherosclerotic sequelae. The aim of this study was to perform a meta-analysis of NaF uptake on PET imaging and its relation to symptomatic and asymptomatic disease.

METHODS

A systematic review was performed according to PRISMA guidelines, via searching the Ovid MEDLINE, Ovid Embase, Cochrane Library, PubMed, Scopus, and Web of Science Core Collection databases up to May 2024. The search strategy included the terms 'NaF', 'PET', and 'plaque', and all studies with data regarding the degree of microcalcification, as measured by 18F-NaF uptake in symptomatic and asymptomatic atherosclerotic plaques, were included. Analysis involved calculating mean differences between uptake values and comparison using a random-effects model.

RESULTS

A total of 16 articles, involving 423 participants, were included in the meta-analysis (10 carotid artery studies, five coronary artery studies, and one in peripheral vascular disease). Comparing 18F-NaF uptake in symptomatic versus asymptomatic atherosclerotic plaques, a mean difference of 0.43 (95% CI 0.29 to 0.57; p < 0.0001, I2 = 65%) was noted in studies comparing symptomatic and asymptomatic plaques in the same participant, with a significant difference in effect based on arterial territory studied (χ2 = 12.68, p = 0.0018). In studies of participants with and without symptomatic disease, there was no significant difference between symptomatic and asymptomatic plaques (mean difference 0.27, 95% CI -0.26 to 0.80, p = 0.28, I2 = 85%).

CONCLUSIONS

PET imaging using 18F-NaF can detect differences in microcalcification between symptomatic and asymptomatic atherosclerotic plaques within, but not between, individuals, and thus, is a marker of symptomatic disease. The standardization of 18F-NaF PET imaging protocols, and its future use as a risk stratification tool or outcome measure, requires further study. (PROSPERO Registration ID: CRD42023451363).

Novel imaging modalities for the identification of vulnerable plaques

Atherosclerosis is a slow, progressive disease that is closely associated with major adverse cardiovascular events. Early diagnosis and risk assessment of atherosclerosis can effectively improve the prognosis and reduce the occurrence of adverse cardiovascular events in the later stage. A variety of invasive and non-invasive imaging modalities are important tools for diagnosing lesions, monitoring the efficacy of treatments, and predicting associated risk events. This review mainly introduces the four commonly used non-invasive imaging modalities in clinical practice and intravascular imaging such as optical coherence tomography, intravascular ultrasound imaging, and near-infrared spectroscopy, compares the advantages and disadvantages in the diagnosis of vulnerable plaques, and briefly summarizes the new progressions of each.

Bilateral Carotid Artery Molecular Calcification Assessed by [18F] Fluoride PET/CT: Correlation with Cardiovascular and Thromboembolic Risk Factors

Atherosclerosis, a leading cause of mortality and morbidity worldwide, involves inflammatory processes that result in plaque formation and calcification. The early detection of the molecular changes underlying these processes is crucial for effective disease management. This study utilized positron emission tomography/computed tomography (PET/CT) with [18F] sodium fluoride (NaF) as a tracer to visualize active calcification and inflammation at the molecular level. Our aim was to investigate the association between cardiovascular risk factors and [18F] NaF uptake in the left and right common carotid arteries (LCC and RCC). A cohort of 102 subjects, comprising both at-risk individuals and healthy controls, underwent [18F] NaF PET/CT imaging. The results revealed significant correlations between [18F] NaF uptake and cardiovascular risk factors such as age (β = 0.005, 95% CI 0.003-0.008, p < 0.01 in LCC and β = 0.006, 95% CI 0.004-0.009, p < 0.01 in RCC), male gender (β = -0.08, 95% CI -0.173--0.002, p = 0.04 in LCC and β = -0.13, 95% CI -0.21--0.06, p < 0.01 in RCC), BMI (β = 0.02, 95% CI 0.01-0.03, p < 0.01 in LCC and β = 0.02, 95% CI 0.01-0.03, p < 0.01 in RCC), fibrinogen (β = 0.006, 95% CI 0.0009-0.01, p = 0.02 in LCC and β = 0.005, 95% CI 0.001-0.01, p = 0.01), HDL cholesterol (β = 0.13, 95% CI 0.04-0.21, p < 0.01 in RCC only), and CRP (β = -0.01, 95% CI -0.02-0.001, p = 0.03 in RCC only). Subjects at risk showed a higher [18F] NaF uptake compared to healthy controls (one-way ANOVA; p = 0.02 in LCC and p = 0.04 in RCC), and uptake increased with estimated cardiovascular risk (one-way ANOVA, p < 0.01 in LCC only). These findings underscore the potential of [18F] NaF PET/CT as a sensitive tool for the early detection of atherosclerotic plaque, assessment of cardiovascular risk, and monitoring of disease progression. Further research is needed to validate the technique's predictive value and its potential impact on clinical outcomes.

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.

Emerging PET Tracers in Cardiac Molecular Imaging

¹⁸F-fluorodeoxyglucose (FDG) and ¹⁸F-sodium fluoride (NaF) represent emerging PET tracers used to assess atherosclerosis-related inflammation and molecular calcification, respectively. By localizing to sites with high glucose utilization, FDG has been used to assess myocardial viability for decades, and its role in evaluating cardiac sarcoidosis has come to represent a major application. In addition to determining late-stage changes such as loss of perfusion or viability, by targeting mechanisms present in atherosclerosis, PET-based techniques have the ability to characterize atherogenesis in the early stages to guide intervention. Although it was once thought that FDG would be a reliable indicator of ongoing plaque formation, micro-calcification as portrayed by NaF-PET/CT appears to be a superior method of monitoring disease progression. PET imaging with NaF has the additional advantage of being able to determine abnormal uptake due to coronary artery disease, which is obscured by physiologic myocardial activity on FDG-PET/CT. In this review, we discuss the evolving roles of FDG, NaF, and other PET tracers in cardiac molecular imaging.

Vulnerable Plaque Imaging

Atherosclerotic plaques progress as a result of inflammation. Both invasive and noninvasive imaging techniques have been developed to identify and characterize plaque as vulnerable (more likely to rupture and cause a clinical event). Imaging techniques to identify vulnerable include identifying vessels with focal subendothelial collections of I) inflammatory cells; II) lipid/ fatty acid; III) local regions of hypoxia; IV) local expression of angiogenesis factors; V) local expression of protease; VI) intravascular foci of thrombus; hemorrhage (most often seen in the aftermath of a clinical event); VII) apoptosis and VIII) microcalcification. This review provides an overview of atherosclerotic plaque progression and tracers which can visualize specific molecules associated with vulnerability.

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.

Longitudinal analysis of atherosclerotic plaques evolution: an 18F-NaF PET/CT study

PURPOSE

18F-NaF-PET/CT can detect mineral metabolism within atherosclerotic plaques. To ascertain whether their 18F-NaF uptake purports progression, this index was compared with subsequent morphologic evolution.

METHODS

71 patients underwent two consecutive 18F-NaF-PET/CTs (PET1/PET2). In PET1, non-calcified 18F-NaF hot spots were identified in the abdominal aorta. Their mean/max HU was compared with those of a non-calcified control region (CR) and with corresponding areas in PET2. A target-to-background ratio (TBR), mean density (HU), and calcium score (CS) were calculated on calcified atherosclerotic plaques in PET1 and compared with those in PET2. A VOI including the entire abdominal aorta was drawn; mean TBR and total CS were calculated on PET1 and compared with those PET2.

RESULTS

Hot spots in PET1 (N = 179) had a greater HU than CR (48 ± 8 vs 37 ± 9, P < .01). Mean hot spots HU increased to 59 ± 12 in PET2 (P < .001). New calcifications appeared at the hot spots site in 73 cases (41%). Baseline atherosclerotic plaque's (N = 375) TBR was proportional to percent HU and CS increase (P < .01 for both). Aortic CS increased (P < .001); the whole-aorta TBR in PET1 correlated with the CS increase between the baseline and the second PET/CT (R = .63, P < .01).

CONCLUSIONS

18F-NaF-PET/CT depicts the early stages of plaques development and tracks their evolution over time.

Advances in positron emission tomography tracers related to vascular calcification

Microcalcification, a type of vascular calcification, increases the instability of plaque and easily leads to acute clinical events. Positron emission tomography (PET) is a new examination technology with significant advantages in identifying vascular calcification, especially microcalcification. The use of the ¹⁸F-NaF is undoubtedly the benchmark, and other PET tracers related to vascular calcification are also currently in development. Despite all this, a large number of studies are still needed to further clarify the specific mechanisms and characteristics. This review aimed at providing a summary of the application and progress of different PET tracers and also the future development direction.

Molecular imaging in atherosclerosis

Purpose

As atherosclerosis is a  prominent cause of morbidity and mortality, early detection of atherosclerotic plaques is vital to prevent complications. Imaging plays a significant role in this goal. Molecular imaging and structural imaging detect different phases of atherosclerotic progression. In this review, we explain the relation between these types of imaging with the physiopathology of plaques, along with their advantages and disadvantages. We also discuss in detail the most commonly used positron emission tomography (PET) radiotracers for atherosclerosis imaging.

Method

A comprehensive search was conducted to extract articles related to imaging of atherosclerosis in PubMed, Google Scholar, and Web of Science. The obtained papers were reviewed regarding precise relation with our topic. Among the search keywords utilized were "atherosclerosis imaging", "atherosclerosis structural imaging", "atherosclerosis CT scan" "positron emission tomography", "PET imaging", "18F-NaF", "18F-FDG", and "atherosclerosis calcification."

Result

Although structural imaging such as computed tomography (CT) offers essential information regarding plaque structure and morphologic features, these modalities can only detect macroscopic alterations that occur later in the disease’s progression, when the changes are frequently irreversible. Molecular imaging modalities like PET, on the other hand, have the advantage of detecting microscopic changes and allow us to treat these plaques before irreversible changes occur. The two most commonly used tracers in PET imaging of atherosclerosis are 18F-sodium fluoride (18F-NaF) and 18F-fluorodeoxyglucose (18F-FDG). While there are limitations in the use of 18F-FDG for the detection of atherosclerosis in coronary arteries due to physiological uptake in myocardium and high luminal blood pool activity of 18F-FDG, 18F-NaF PET is less affected and can be utilized to analyze the coronary arteries in addition to the peripheral vasculature.

Conclusion

Molecular imaging with PET/CT has become a useful tool in the early detection of atherosclerosis. 18F-NaF PET/CT shows promise in the early global assessment of atherosclerosis, but further prospective studies are needed to confirm its role in this area.

18F-Sodium fluoride uptake is associated with severity of atherosclerotic stenosis in stable ischemic heart disease

BACKGROUND

Increased uptake of 18F-Sodium fluoride (18F-NaF) PET has potential to identify atherosclerotic plaques that are vulnerable to rupture. Whether 18F-NaF PET can evaluate the significance of atherosclerotic plaque in patients with stable coronary artery disease is less clear. We evaluated 18F-NaF PET uptake in coronary arteries in patients without acute coronary artery syndrome to determine the association of 18F-NaF signal uptake with severity of coronary stenosis.

METHODS AND RESULTS

We retrospectively identified 114 patients who received both regadenoson stress 82Rb myocardial perfusion PET and 18F-NaF PET study with an average interval of 5 months. Out of this cohort, forty-one patients underwent invasive coronary angiography. In a patient-based analysis, patients with ischemic regadenoson stress 82Rb PET had significantly higher coronary 18F-NaF uptake than patients with normal myocardial perfusion (P < .01). Among the 41 patients who underwent coronary angiography, per-vessel 18F-NaF uptake in both obstructive and nonobstructive coronary arteries was significantly higher than in normal coronary arteries (P < .05) regardless of the severity of coronary calcification. There was poor correlation between calcification and 18F-NaF uptake in coronary arteries (r = 0.41) CONCLUSION: Coronary arterial 18F-NaF uptake is associated with coronary stenosis severity in patients with stable coronary artery disease. 18F-NaF PET studies may be useful for characterizing coronary atherosclerotic plaques.

Choosing between anatomy and function is not always evident for the heart of end-stage renal disease patients. How low can we go?

Patients with chronic kidney disease (CKD) are at a very high risk of adverse cardiovascular events. In CKD patients, vascular calcification is more prevalent, appears at an earlier age, and is more severe than in the general population. CKD physiology rather than the effects of dialysis is the primary driver of microvascular disease in these patients. Considering the significant morbidity and mortality attributable to cardiovascular disease in the CKD population, risk stratification remains an important challenge. Topics such as function vs anatomy to properly risk stratify these patients, as well as future perspectives on non-invasive techniques, will be addressed.

Temporal relationship between 18F-sodium fluoride uptake in the abdominal aorta and evolution of CT-verified vascular calcification

BACKGROUND

Fluoride-18 sodium fluoride (18F-NaF) localizes in microcalcifications in atheroma. The microcalcifications may aggregate, passing the resolution threshold to visualize on computed tomography (CT). We evaluated serial NaF positron emission tomography (PET)-CT scans to determine the temporal relationship between vascular NaF uptake and CT evident calcification in the abdominal aorta.

METHODS

Prostate cancer patients who had at least 3 NaF PET-CT scans over at least 1.5 years were retrospectively enrolled. Regions of interest were traced in the abdominal aorta on both PET and CT images, excluding skeletal NaF activity. The maximum standardized uptake value (SUVmax) of NaF and the density and volume of calcium (exceeding 130 HU) were summed and divided by the number of slices to produce the SUVmax/slice and the mm3·slice-1 of calcium.

RESULTS

Of 437 patients, 45 patients met criteria. NaF uptake waxed and waned between scans, while the calcium volume plateaued or increased over time. NaF uptake correlated with calcium volume on the baseline scan (P = .60, < .0001†) and calcium volume increment, especially from 1.0 to 1.5 years (r = .79, P < .0001†). Patients with persistently high NaF uptake showed a higher calcium volume increment (0-1.5 years) than patients with low or transiently high NaF uptake.

CONCLUSIONS

Abdominal aortic NaF uptake varied over time. NaF uptake on the baseline scans and high NaF uptake on the serial scans preceded an increase in calcium volume, especially by 1.0-1.5 years. Persistently high NaF uptake was associated with a greater increment in calcium volume than patients with transiently elevated or persistently low fluoride uptake.

HAP-Multitag, a PET and Positive MRI Contrast Nanotracer for the Longitudinal Characterization of Vascular Calcifications in Atherosclerosis

Vascular microcalcifications are associated with atherosclerosis plaque instability and, therefore, to increased mortality. Because of this key role, several imaging probes have been developed for their in vivo identification. Among them, [¹⁸F]FNa is the gold standard, showing a large uptake in the whole skeleton by positron emission tomography. Here, we push the field toward the combined anatomical and functional early characterization of atherosclerosis. For this, we have developed hydroxyapatite (HAP)-multitag, a bisphosphonate-functionalized ⁶⁸Ga core-doped magnetic nanoparticle showing high affinity toward most common calcium salts present in microcalcifications, particularly HAP. We characterized this interaction in vitro and in vivo, showing a massive uptake in the atherosclerotic lesion identified by positron emission tomography (PET) and positive contrast magnetic resonance imaging (MRI). In addition, this accumulation was found to be dependent on the calcification progression, with a maximum uptake in the microcalcification stage. These results confirmed the ability of HAP-multitag to identify vascular calcifications by PET/(T₁)MRI during the vulnerable stages of the plaque progression.

Sodium fluoride in cardiovascular disorders: A systematic review

BACKGROUND

18-Fluorine sodium fluoride is a well-known radiotracer used for bone metastasis diagnosis. Its uptake correlation with cardiovascular (CV) risk was primarily suggested in oncological patients. Moreover, as a specific marker of microcalcification, it seems to correlate with CV disease progression and plaque instability.

METHODS AND RESULTS

Our purpose was to systematically review clinical studies that characterized the use of this marker in CV conditions. In atherosclerosis, most studies report a positive correlation with the burden of CV risk factors and vascular calcification. A higher uptake was found in culprit plaques/rupture sites in coronary and carotid arteries and it was also linked to high-risk features in histology and intravascular imaging analysis of the plaques. In aortic stenosis, this tracer displayed an increasing uptake with disease severity.

CONCLUSIONS

Sodium fluoride positron emission tomography is a promising non-invasive technique to identify high-risk plaques, which sets ground to a potential use of this tracer in evaluating atherosclerotic disease progression and degenerative changes in aortic valve stenosis. Nevertheless, there is a need for further prospective evidence that demonstrates this technique's value in predicting clinical events, adjusting treatment strategies, and improving patient outcomes.

Two-year change in 18F-sodium fluoride uptake in major arteries of healthy subjects and angina pectoris patients

To examine 2-year changes in carotid and aortic ¹⁸F-sodium fluoride (NaF) uptake in both healthy controls and angina pectoris patients. Twenty-nine healthy subjects and 20 angina pectoris patients underwent 90-min NaF-PET/CT twice 2 years apart. The carotids and three sections of the aorta (arch, thoracic, abdominal) were manually segmented. NaF uptake was expressed as the mean and total standardized uptake values without and with partial volume correction (SUVmean, SUVtotal and pvcSUVmean, pvcSUVtotal). Insignificant tendencies were higher NaF uptake in angina patients at both time points with less uptake in healthy subjects and higher uptake in angina patients after 2 years. Thus, aortic pvcSUVmean of angina patients was 1.14 ± 0.35 and 1.29 ± 0.71 at baseline and after 2 years vs. 0.99 ± 0.31 and 0.95 ± 0.28 in healthy subjects. A similar pattern was observed for the carotid pvcSUVmean. NaF uptake at baseline could not predict a change in CT-calcification after 2 years. NaF uptake in all parts of the aorta correlated positively with age. There was an insignificant, but consistent, tendency for slightly higher arterial NaF uptake in the angina group indicating more ongoing microcalcification at both time points in patients than healthy subjects. The 2-year changes were in both groups very small suggesting that the atherosclerotic process is slow, albeit with a tendency of slight decreases among healthy controls and slight increases in angina patients despite statin therapy in half of these.

Effects of administration route on uptake kinetics of 18F-sodium fluoride positron emission tomography in mice

¹⁸F-sodium fluoride (¹⁸F-NaF) is a positron emission tomography (PET) radiotracer widely used in skeletal imaging and has also been proposed as a biomarker of active calcification in atherosclerosis. Like most PET radiotracers, ¹⁸F-NaF is typically administered intravenously. However in small animal research intravenous administrations can be challenging, because partial paravenous injection is common due to the small calibre of the superficial tail veins and repeat administrations via tail veins can lead to tissue injury therefore limiting the total number of longitudinal scanning points. In this paper, the feasibility of using intra-peritoneal route of injection of ⁸F-NaF to study calcification in mice was studied by looking at the kinetic and uptake profiles of normal soft tissues and bones versus intra-vascular injections. Dynamic PET was performed for 60 min on nineteen isoflurane-anesthetized male Swiss mice after femoral artery (n = 7), femoral vein (n = 6) or intraperitoneal (n = 6) injection of ⁸F-NaF. PET data were reconstructed and the standardised uptake value (SUV) and standardised uptake value ratio (SUVr) were estimated from the last three frames between 45- and 60-min and ⁸F-NaF uptake constant (K_i) was derived by Patlak graphical analysis. In soft tissue, the ¹⁸F-NaF perfusion phase changes depending on the type on injection route, whereas the uptake phase is similar regardless of the administration route. In bone tissue SUV, SUVr and K_i measures were not significantly different between the three administration routes. Comparison between PET and CT measures showed that bones that had the highest CT density displayed the lowest PET activity and conversely, bones where CT units were low had high ⁸F-NaF uptake. Intraperitoneal injection is a valid and practical alternative to the intra-vascular injections in small-animal ¹⁸F-NaF PET imaging providing equivalent pharmacokinetic data. CT outcome measures report on sites of stablished calcification whereas PET measures sites of higher complexity and active calcification.

Dynamic monitoring of active calcification in atherosclerosis by 18F-NaF PET imaging

The objective was to dynamically monitor the progression of atherosclerotic plaques in ApoE-/- mice with 18F-NaF PET imaging. The ApoE-/- mice were used to develop atherosclerosis models, and the C57BL/6 J mice were used as control. 18F-NaF PET was performed when the mice were 12, 20, and 30 weeks of age. Serum lipids and lipoproteins profiles, inflammatory cytokines, and calcification factors were tested by ELISA. The lipid distribution, morphology, and calcification of plaque were evaluated by Oil Red O, HE, and alizarin red staining. The correlation between imaging and the extent of calcification was analyzed by Pearson correlation analysis. The uptake of 18F-NaF in the aorta was gradually increased with each weekly extension. Compared with the ApoE-/- mice at the age of 12 weeks and 20 weeks, the levels of lipoprotein, inflammatory cytokines, and calcification factors were higher at 30 weeks. In Oil Red O, HE, and alizarin red staining, the extent of the lipid area and calcification increased with time. The correlation analysis showed that the uptake of 18F-NaF in the aorta was related to the extent of calcification. 18F-NaF may dynamically monitor the progression of atherosclerotic plaques and ongoing microcalcification formation.

Advances in Quantitative Analysis of 18F-Sodium Fluoride Coronary Imaging

¹⁸F-sodium fluoride (¹⁸F-NaF) positron emission tomography (PET) has emerged as a promising noninvasive imaging tool for the assessment of active calcification processes in coronary artery disease. ¹⁸F-NaF uptake colocalizes to high-risk and ruptured atherosclerotic plaques. Most recently, ¹⁸F-NaF coronary uptake was shown to be a robust and independent predictor of myocardial infarction in patients with advanced coronary artery disease. In this review, we provide an overview of the advances in coronary ¹⁸F-NaF imaging. In particular, we discuss the recently developed and validated motion correction techniques which address heart contractions, tidal breathing, and patient repositioning during the prolonged PET acquisitions. Additionally, we discuss a novel quantification approach—the coronary microcalcification activity (which has been inspired by the widely employed method in oncology total active tumor volume measurement). This new method provides a single number encompassing ¹⁸F-NaF activity within the entire coronary vasculature rather than just information regarding a single area of most intense tracer uptake.

The ability of Micropure® ultrasound technique to identify microcalcifications in carotid plaques

OBJECTIVES

To study the ability of Micropure® ultrasound technique to identify microcalcifications in carotid plaques.

METHODS

Forty-four carotids in 22 patients were enrolled in this study and were detected by routine ultrasound examination and Micropure® examination at the same time to identify microcalcifications in plaques. The results were compared with the tissue-background ratio (TBR) in ¹⁸F-NaF PET-CT imaging, which was performed one or two days after the ultrasound examination.

RESULTS

In the 44 carotids, plaques were detected in 37 carotids. Microcalcifications were detected by the Micropure® technique in 32 carotids, which were located surrounded by macrocalcifications in 23 carotids, in the fibre cap in 12 carotids, and in the base of the plaque in 6 carotids. Microcalcifications were not detected in 12 carotids. In ¹⁸F-NaF PET-CT examination, TBR > 1.61 (range 1.62-3.99, mean 2.25 ± 0.58) was detected in 37 carotids, and TBR < 1.61 was detected in 7 carotids. There were no significant differences between the two methods in detecting microcalcifications (p = 0.180). The sensitivity of the Micropure® technique in detecting microcalcifications was 81.08 %, and the specificity was 71.43 %.

CONCLUSIONS

Microcalcifications in the carotid artery detected by the Micropure® technique were well in accordance with ¹⁸F-NaF PET-CT scanning, with better sensitivity and specificity.

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.

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.

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.

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.

Calcification in Atherosclerotic Plaque Vulnerability: Friend or Foe?

Calcification is a clinical marker of atherosclerosis. This review focuses on recent findings on the association between calcification and plaque vulnerability. Calcified plaques have traditionally been regarded as stable atheromas, those causing stenosis may be more stable than non-calcified plaques. With the advances in intravascular imaging technology, the detection of the calcification and its surrounding plaque components have evolved. Microcalcifications and spotty calcifications represent an active stage of vascular calcification correlated with inflammation, whereas the degree of plaque calcification is strongly inversely related to macrophage infiltration. Asymptomatic patients have a higher content of plaque calcification than that in symptomatic patients. The effect of calcification might be biphasic. Plaque rupture has been shown to correlate positively with the number of spotty calcifications, and inversely with the number of large calcifications. There may be certain stages of calcium deposition that may be more atherogenic. Moreover, superficial calcifications are independently associated with plaque rupture and intraplaque hemorrhage, which may be due to the concentrated and asymmetrical distribution of biological stress in plaques. Conclusively, calcification of differential amounts, sizes, shapes, and positions may play differential roles in plaque homeostasis. The surrounding environments around the calcification within plaques also have impacts on plaque homeostasis. The interactive effects of these important factors of calcifications and plaques still await further study.

Coronary Atherosclerosis Imaging

Identifying patients at increased risk of coronary artery disease, before the atherosclerotic complications become clinically evident, is the aim of cardiovascular prevention. Imaging techniques provide direct assessment of coronary atherosclerotic burden and pathological characteristics of atherosclerotic lesions which may predict the progression of disease. Atherosclerosis imaging has been traditionally based on the evaluation of coronary luminal narrowing and stenosis. However, the degree of arterial obstruction is a poor predictor of subsequent acute events. More recent techniques focus on the high-resolution visualization of the arterial wall and the coronary plaques. Most acute coronary events are triggered by plaque rupture or erosion. Hence, atherosclerotic plaque imaging has generally focused on the detection of vulnerable plaque prone to rupture. However, atherosclerosis is a dynamic process and the plaque morphology and composition may change over time. Most vulnerable plaques undergo progressive transformation from high-risk to more stable and heavily calcified lesions, while others undergo subclinical rupture and healing. Although extensive plaque calcification is often associated with stable atherosclerosis, the extent of coronary artery calcification strongly correlates with the degree of atherosclerosis and with the rate of future cardiac events. Inflammation has a central role in atherogenesis, from plaque formation to rupture, hence in the development of acute coronary events. Morphologic plaque assessment, both invasive and non-invasive, gives limited information as to the current activity of the atherosclerotic disease. The addition of nuclear imaging, based on radioactive tracers targeted to the inflammatory components of the plaques, provides a highly sensitive assessment of coronary disease activity, thus distinguishing those patients who have stable disease from those with active plaque inflammation.

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

OBJECTIVE

¹⁸F-sodium fluoride (NaF) position emission tomography (PET) activity correlates with high-risk plaque. We examined the correlation between ¹⁸F-NaF PET activity and extent of calcification (microcalcification and macrocalcification) in coronary arteries. Approach and Results: Eighteen ex vivo human coronary arteries were imaged with ¹⁸F-NaF PET/CT, and target to background ratios were analyzed from 101 plaques. Histopathologic analysis evaluated for microcalcification and macrocalcification, plaque morphology, and inflammation. Plaques with microcalcification demonstrated higher ¹⁸F-NaF PET activity (n=84; mean target to background ratio±SD, 9.0±9.7,) than plaques without microcalcification (n=17, 2.9±3.8; P<0.0001). Higher ¹⁸F-NaF PET activity was associated with advanced plaques characterized by fibroatheroma (n=54, 10.7±10.3) compared with plaques with intimal thickening (n=22, 3.5±3.9) or pathological intimal thickening (n=25, 6.1±8.4; P=0.004). No significant association was found between ¹⁸F-NaF PET activity and inflammation (P=0.08).

CONCLUSIONS

In ex vivo human coronary arteries, higher ¹⁸F-NaF PET activity was associated with microcalcification and advanced plaque morphology. Since microcalcification and fibroatheromas are high-risk plaque features, ¹⁸F-NaF PET/CT may improve risk-stratification.

Vulnerable plaque imaging using 18F-sodium fluoride positron emission tomography

Positron emission tomography (PET) with ¹⁸F-sodium fluoride (¹⁸F-NaF) has emerged as a promising non-invasive imaging modality to identify high-risk and ruptured atherosclerotic plaques. By visualizing microcalcification, ¹⁸F-NaF PET holds clinical promise in refining how we evaluate coronary artery disease, shifting our focus from assessing disease burden to atherosclerosis activity. In this review, we provide an overview of studies that have utilized ¹⁸F-NaF PET for imaging atherosclerosis. We discuss the associations between traditional coronary artery disease measures (risk factors) and ¹⁸F-NaF plaque activity. We also present the data on the histological validation as well as show how ¹⁸F-NaF uptake is associated with plaque morphology on intravascular and CT imaging. Finally, we discuss the technical challenges associated with ¹⁸F-NaF coronary PET highlighting recent advances in this area.

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.

Atherosclerosis imaging with 18F-sodium fluoride PET: state-of-the-art review

Purpose

We examined the literature to elucidate the role of 18F-sodium fluoride (NaF)-PET in atherosclerosis.

Methods

Following a systematic search of PubMed/MEDLINE, Embase, and Cochrane Library included articles underwent subjective quality assessment with categories low, medium, and high. Of 2811 records, 1780 remained after removal of duplicates. Screening by title and abstract left 41 potentially eligible full-text articles, of which 8 (about the aortic valve (n = 1), PET/MRI feasibility (n = 1), aortic aneurysms (n = 1), or quantification methodology (n = 5)) were dismissed, leaving 33 published 2010–2012 (n = 6), 2013–2015 (n = 11), and 2016–2018 (n = 16) for analysis.

Results

They focused on coronary (n = 8), carotid (n = 7), and femoral arteries (n = 1), thoracic aorta (n = 1), and infrarenal aorta (n = 1). The remaining 15 studies examined more than one arterial segment. The literature was heterogeneous: few studies were designed to investigate atherosclerosis, 13 were retrospective, 9 applied both FDG and NaF as tracers, 24 NaF only. Subjective quality was low in one, medium in 13, and high in 19 studies. The literature indicates that NaF is a very specific tracer that mimics active arterial wall microcalcification, which is positively associated with cardiovascular risk. Arterial NaF uptake often presents before CT-calcification, tends to decrease with increasing density of CT-calcification, and appears, rather than FDG-avid foci, to progress to CT-calcification. It is mainly surface localized, increases with age with a wide scatter but without an obvious sex difference. NaF-avid microcalcification can occur in fatty streaks, but the degree of progression to CT-calcification is unknown. It remains unknown whether medical therapy influences microcalcification. The literature held no therapeutic or randomized controlled trials.

Conclusion

The literature was heterogeneous and with few clear cut messages. NaF-PET is a new approach to detect and quantify microcalcification in early-stage atherosclerosis. NaF uptake correlates with cardiovascular risk factors and appears to be a good measure of the body’s atherosclerotic burden, potentially suited also for assessment of anti-atherosclerotic therapy.

Electronic supplementary material

The online version of this article (10.1007/s00259-019-04603-1) contains supplementary material, which is available to authorized users.

Clinical implication of 18F-NaF PET/computed tomography indexes of aortic calcification in coronary artery disease patients: correlations with cardiovascular risk factors

OBJECTIVE

Vascular calcification is known to be associated with cardiovascular risk factors. Recently, F-NaF PET has been reported to be effective for detecting early and active vascular calcification. In this study, correlations between F-NaF PET/computed tomography (CT) findings and cardiovascular risk factors were investigated in patients with suspected coronary artery disease.

PATIENTS AND METHODS

Forty patients with suspected coronary artery disease underwent F-NaF PET/CT. The maximum and overall burden of calcifying activity, and the overall burden of calcium deposition in the descending thoracic aorta (DTA) were measured on F-NaF PET/CT and they were compared with cardiovascular risk factors, particularly, with those related to metabolic syndrome.

RESULTS

The maximum and overall burden of calcifying activity in DTA measured on F-NaF PET were significantly correlated with diabetes mellitus (P = 0.030 and 0.049, respectively) and serum HbA1c level (ρ = 0.433 and 0.344, respectively). In contrast, the overall burden of calcium deposition measured on CT was significantly correlated with hypertension (P < 0.001). The overall burden of calcium deposition was also significantly correlated with metabolic syndrome (P = 0.002) and 10-year cardiovascular disease risk score (P = 0.004) CONCLUSION: F-NaF uptake is closely related to diabetes mellitus, whereas aortic calcification on CT is closely related to hypertension. Although F-NaF uptake in DTA can be a potential prognostic factor, aortic calcification on CT is a more significant prognostic factor for overall cardiovascular risk than F-NaF uptake.

Diagnostic Tests for Vascular Calcification

Vascular calcification (VC) is the heterogeneous endpoint of multiple vascular insults, which varies by arterial bed, the layer of the arterial wall affected, and is propagated by diverse cellular and biochemical mechanisms. A variety of in vivo and ex vivo techniques have been applied to the analysis of VC in preclinical studies, but clinical examination has principally relied on a number of noninvasive and invasive imaging modalities for detection and quantitation. Most imaging methods suffer from suboptimal spatial resolution, leading to the inability to distinguish medial from intimal VC and insufficient sensitivity to detect microcalcifications that are indicative of active mineral deposition and of vulnerable plaques which may be prone to rupture. Serum biomarkers lack specificity for VC and cannot discriminate pathology. Overall, uncertainties surrounding the sensitivity and specificity of different VC testing modalities, the absence of a clear cause-effect relationship, and lack of any evidence-based diagnostic or therapeutic protocols in relation to VC testing in chronic kidney disease has yielded weak or ungraded recommendations for their use in clinical practice. While VC is recognized as a key manifestation of chronic kidney disease-mineral and bone disorder and those with an increasing burden of VC are considered to be at higher cardiovascular risk, routine screening is not currently recommended.

Coronary Artery Microcalcification: Imaging and Clinical Implications

Strategies to prevent acute coronary and cerebrovascular events are based on accurate identification of patients at increased cardiovascular (CV) risk who may benefit from intensive preventive measures. The majority of acute CV events are precipitated by the rupture of the thin cap overlying the necrotic core of an atherosclerotic plaque. Hence, identification of vulnerable coronary lesions is essential for CV prevention. Atherosclerosis is a highly dynamic process involving cell migration, apoptosis, inflammation, osteogenesis, and intimal calcification, progressing from early lesions to advanced plaques. Coronary artery calcification (CAC) is a marker of coronary atherosclerosis, correlates with clinically significant coronary artery disease (CAD), predicts future CV events and improves the risk prediction of conventional risk factors. The relative importance of coronary calcification, whether it has a protective effect as a stabilizing force of high-risk atherosclerotic plaque has been debated until recently. The extent of calcium in coronary arteries has different clinical implications. Extensive plaque calcification is often a feature of advanced and stable atherosclerosis, which only rarely results in rupture. These macroscopic vascular calcifications can be detected by computed tomography (CT). The resulting CAC scoring, although a good marker of overall coronary plaque burden, is not useful to identify vulnerable lesions prone to rupture. Unlike macrocalcifications, spotty microcalcifications assessed by intravascular ultrasound or optical coherence tomography strongly correlate with plaque instability. However, they are below the resolution of CT due to limited spatial resolution. Microcalcifications develop in the earliest stages of coronary intimal calcification and directly contribute to plaque rupture producing local mechanical stress on the plaque surface. They result from a healing response to intense local macrophage inflammatory activity. Most of them show a progressive calcification transforming the early stage high-risk microcalcification into the stable end-stage macroscopic calcification. In recent years, new developments in noninvasive cardiovascular imaging technology have shifted the study of vulnerable plaques from morphology to the assessment of disease activity of the atherosclerotic lesions. Increased disease activity, detected by positron emission tomography (PET) and magnetic resonance (MR), has been shown to be associated with more microcalcification, larger necrotic core and greater rates of events. In this context, the paradox of increased coronary artery calcification observed in statin trials, despite reduced CV events, can be explained by the reduction of coronary inflammation induced by statin which results in more stable macrocalcification.

Molecular imaging of endothelial activation and mineralization in a mouse model of accelerated atherosclerosis

PURPOSE

Preclinical imaging of endothelial activation and mineralization using both positron emission tomography (PET) and magnetic resonance (MR) remains scarce.

PROCEDURES

A group of uremic ApoE-/- (Ur), non-uremic ApoE-/- (NUr), and control C57Bl/6 J mice (Ctl) were investigated. Mineralization process was assessed using sodium fluoride ([18F]NaF) PET, and MR imaging combined with intravenous injection of MPIO-αVCAM-1 was used to evaluate endothelial activation. Micro- and macrocalcifications were evaluated by flame atomic absorption spectroscopy and von Kossa staining, respectively.

RESULTS

Ur mice showed an active and sustained mineralization process compared to Ctl mice (p = 0.002) using [18F]NaF PET imaging. Calcium plasma level was increased in Ur (2.54 ± 0.09 mM, n = 17) compared to NUr and Ctl mice (2.24 ± 0.01, n = 22, and 2.14 ± 0.02, n = 27, respectively; p < 0.0001). Likewise, vascular calcium content was increased in Ur (0.51 ± 0.06 μg Ca2+ per milligram of dry weight aorta, n = 11) compared to NUr (0.27 ± 0.05, n = 9, p = 0.013) and Ctl (0.28 ± 0.05, n = 11, p = 0.014). Ur mice also had a higher inflammatory state using MPIO-αVCAM-1 MR (p global = 0.01, post hoc analysis Ur vs. Ctl p = 0.003) associated with increased VCAM-1 expression (p global = 0.02). Aortic remodeling at the level of the brachiocephalic trunk, brachiocephalic trunk itself, and aortic arch in Ur mice was also demonstrated using MR.

CONCLUSIONS

Preclinical molecular imaging allowed in vivo characterization of the early phase of atherosclerosis. [18F]NaF PET showed early and sustained vascular mineralization in uremic ApoE-/- mice. MPIO-αVCAM-1 MR imaging demonstrated aortic endothelial activation, predominantly in segments with vascular remodeling.

18F-sodium fluoride positron emission tomography assessed microcalcifications in culprit and non-culprit human carotid plaques

BACKGROUND

18F-NaF positron emission tomography (PET) targets microcalcifications. We compared in vitro microPET assessed 18F-NaF uptake between culprit and non-culprit human carotid plaques. Furthermore, we compared 18F-NaF uptake with calcification visualized on microcomputed tomography (microCT).

METHODS

Carotid plaques from stroke patients undergoing surgery were incubated in 18F-NaF and scanned using a microPET and a microCT scan. The average PET assessed 18F-NaF uptake was expressed as percentage of the incubation dose per gram (%Inc/g). 18F-NaF PET volume of interest (VOI) was compared with CT calcification VOI.

RESULTS

23 carotid plaques (17 culprit, 6 non-culprit) were included. The average 18F-NaF uptake in culprit carotid plaques was comparable with the uptake in non-culprit carotid plaques (median 2.32 %Inc/g [IQR 1.98 to 2.81] vs. median 2.35 %Inc/g [IQR 1.77 to 3.00], P = 0.916). Only a median of 10% (IQR 4 to 25) of CT calcification VOI showed increased 18F-NaF uptake, while merely a median of 35% (IQR 6 to 42) of 18F-NaF PET VOI showed calcification on CT.

CONCLUSIONS

18F-NaF PET represents a different stage in the calcification process than CT. We observed a similar PET assessed 18F-NaF uptake and pattern in culprit and non-culprit plaques of high-risk patients, indicating that this method may be of more value in early atherosclerotic stenosis development.

Evaluation of aortic 18F-NaF tracer uptake using PET/CT as a predictor of aortic calcification in postmenopausal women: A longitudinal study

Introduction

Aortic calcification as detected by computed tomography is associated with arterial stiffening and is an important predictor of cardiovascular morbidity and mortality. Uptake of ¹⁸F-sodium fluoride (¹⁸F-NaF) in the aortic wall reflects metabolically active areas of calcification. The aim of this study was to determine if ¹⁸F-NaF uptake in the aorta is associated with calcification and progression of calcification as detected by computed tomography.

Methods

Twenty-one postmenopausal women (mean age 62 ± 6 years) underwent assessment of aortic ¹⁸F-NaF uptake using positron emission tomography/computer tomography at baseline and a repeat computed tomography scan after a mean follow-up of 3.8 ± 1.3 years. Tracer uptake was quantified by calculating the target-to-background (TBR) ratios at baseline and follow-up. Calcification was assessed at baseline and follow-up using computed tomography.

Results

Over the follow-up period, aortic calcium volume increased from 0.46 ± 0.62 to 0.71 ± 0.93 cm³ (P < 0.05). However, the change in calcium volume did not correlate with baseline TBR either unadjusted (r = 0.00, P = 1.00) or adjusted for age and baseline calcium volume (beta coefficient = −0.18, P = 0.42). TBR at baseline did not differ between participants with (n = 16) compared to those without (n = 5) progression in calcium volume (2.43 ± 0.46 vs. 2.31 ± 0.38, P = 0.58). In aortic segments identified to have the highest tracer uptake at baseline, calcium volume did not significantly change over the follow-up period (P = 0.41).

Conclusion

In a cohort of postmenopausal women, ¹⁸F-NaF uptake as measured by TBR in the lumbar aorta did not predict progression of aortic calcification as detected by computed tomography over a four-year follow-up.

Performance Evaluation of a Dedicated Preclinical PET/CT System for the Assessment of Mineralization Process in a Mouse Model of Atherosclerosis

PURPOSE

The purpose of this study was to assess the impact of positron emission tomography/X-ray computed tomography (PET/CT) acquisition and reconstruction parameters on the assessment of mineralization process in a mouse model of atherosclerosis.

PROCEDURES

All experiments were performed on a dedicated preclinical PET/CT system. CT was evaluated using five acquisition configurations using both a tungsten wire phantom for in-plane resolution assessment and a bar pattern phantom for cross-plane resolution. Furthermore, the radiation dose of these acquisition configurations was calculated. The PET system was assessed using longitudinal line sources to determine the optimal reconstruction parameters by measuring central resolution and its coefficient of variation. An in vivo PET study was performed using uremic ApoE^-/-, non-uremic ApoE^-/-, and control mice to evaluate optimal PET reconstruction parameters for the detection of sodium [¹⁸F]fluoride (Na[¹⁸F]F) aortic uptake and for quantitative measurement of Na[¹⁸F]F bone influx (Ki) with a Patlak analysis.

RESULTS

For CT, the use of 1 × 1 and 2 × 2 binning detector mode increased both in-plane and cross-plane resolution. However, resolution improvement (163 to 62 μm for in-plane resolution) was associated with an important radiation dose increase (1.67 to 32.78 Gy). With PET, 3D-ordered subset expectation maximization (3D-OSEM) algorithm increased the central resolution compared to filtered back projection (1.42 ± 0.35 mm vs. 1.91 ± 0.08, p < 0.001). The use of 3D-OSEM with eight iterations and a zoom factor 2 yielded optimal PET resolution for preclinical study (FWHM = 0.98 mm). These PET reconstruction parameters allowed the detection of Na[¹⁸F]F aortic uptake in 3/14 ApoE^-/- mice and demonstrated a decreased Ki in uremic ApoE^-/- compared to non-uremic ApoE^-/- and control mice (p < 0.006).

CONCLUSIONS

Optimizing reconstruction parameters significantly impacted on the assessment of mineralization process in a preclinical model of accelerated atherosclerosis using Na[¹⁸F]F PET. In addition, improving the CT resolution was associated with a dramatic radiation dose increase.

18F-Fluoride Positron Emission Tomographic Imaging of Penile Arteries and Erectile Dysfunction

BACKGROUND

Fluorine-18 sodium fluoride (NaF), a bone-seeking radiopharmaceutical used to detect osseous metastases, localizes in regions of microcalcification in atherosclerosis.

OBJECTIVES

To determine if atherosclerosis of penile arteries plays a role in erectile dysfunction (ED), this study analyzed NaF images in prostate cancer patients.

METHODS

NaF positron emission tomography-computed tomography bone scans were evaluated in 437 prostate cancer patients (age 66.6 ± 8.7 years). Their urologic histories were reviewed for prevalent ED (diagnosed before the scan date) or incident ED (no ED at first scan, but developed during 1-year follow-up); patients with no ED (neither before the scan nor during follow-up) were included as a control group. A semicircular region of interest was set on the dorsal one-half of the penis (to avoid residual excreted activity in the urethra) on 5 contiguous slices at the base of the penis on positron emission tomography-computed tomography coronal reconstructions, and the average standardized uptake value (SUVmax) was described as NaF uptake.

RESULTS

Of 437 patients, 336 (76.9%) had prevalent ED, 60 incident ED (13.7%), and 41 had no ED (9.4%). SUVmax in patients with prevalent (median 1.88; interquartile range [IQR]: 1.67 to 2.16) or incident (median 1.86; IQR: 1.72 to 2.08) ED was significantly higher than no ED (median 1.42; IQR: 1.25 to 1.54) patients (p < 0.001). After adjustment for other risk factors, the odds ratio of prevalent or incident ED was 25.2 (95% confidence interval: 9.5 to 67.0) for every 0.5-U increment in SUVmax with receptor operating characteristic area of 0.91 (95% confidence interval: 0.88 to 0.94).

CONCLUSIONS

NaF uptake in penile vessels suggests that atherosclerosis is associated with ED in prostate cancer patients. The importance of NaF uptake needs to be tested in noncancer subjects and cause-effect relationship needs to be established.

Correlation of fluorine 18-labeled sodium fluoride uptake and arterial calcification on whole-body PET/CT in cancer patients

BACKGROUND

Fluorine-18-labeled sodium fluoride (F-NaF) uptake measured with PET in the vessel walls can indicate active microcalcification, a potential biomarker of higher-risk plaques, which are not indicated by macrocalcification measured with computed tomography (CT). The aim of this study was to determine the extent to which F-NaF uptake is correlated with calcification at arterial plaques in cancer patients undergoing whole-body PET/CT imaging.

PATIENTS AND METHODS

Image data from 179 patients who underwent F-NaF PET/CT were evaluated retrospectively. Plaques were categorized into four groups by calcium score (CS) on CT: CS1 (≥1000); CS2 (400-999); CS3 (100-399), and CS4 (<100) and into three groups by F-NaF target-to-background ratio (TBR) on PET: TBRlow (≤1.0), TBRmedium (1.0-1.5), and TBRhigh (>1.5). Correlations between F-NaF uptake and CS were evaluated.

RESULTS

Plaques with F-NaF uptake or arterial calcification were observed in 122 (76%) of the 179 patients. We found a weak but statistically significant positive correlation between CS and F-NaF uptake. The TBR in CS1 plaques was higher than those in CS3 and CS4 plaques, and the TBR in CS2 plaques was higher than that in CS3 plaques (P<0.05). Compared with patients whose plaques were with F-NaF uptake (TBR>1.5) or arterial calcification (CS>0), patients without plaques of F-NaF uptake or calcification were significantly younger (P=0.00) or with significantly more women (P=0.02).

CONCLUSION

Our finding of a weak but significant positive correlation between F-NaF uptake and arterial calcification suggests that F-NaF PET/CT could provide complementary information of active microcalcification for atherosclerosis evaluation in cancer patients.

Molecular imaging of diabetes and diabetic complications: Beyond pancreatic β-cell targeting

Diabetes is a chronic non-communicable disease affecting over 400 million people worldwide. Diabetic patients are at a high risk of various complications, such as cardiovascular, renal, and other diseases. The pathogenesis of diabetes (both type 1 and type 2 diabetes) is associated with a functional impairment of pancreatic β-cells. Consequently, most efforts to manage and prevent diabetes have focused on preserving β-cells and their function. Advances in imaging techniques, such as magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography, and single-photon-emission computed tomography, have enabled noninvasive and quantitative detection and characterization of the population and function of β-cells in vivo. These advantages aid in defining and monitoring the progress of diabetes and determining the efficacy of anti-diabetic therapies. Beyond β-cell targeting, molecular imaging of biomarkers associated with the development of diabetes, e.g., lymphocyte infiltration, insulitis, and metabolic changes, may also be a promising strategy for early detection of diabetes, monitoring its progression, and occurrence of complications, as well as facilitating exploration of new therapeutic interventions. Moreover, molecular imaging of glucose uptake, production and excretion in specified tissues is critical for understanding the pathogenesis of diabetes. In the current review, we summarize and discuss recent advances in noninvasive imaging technologies for imaging of biomarkers beyond β-cells for early diagnosis of diabetes, investigation of glucose metabolism, and precise diagnosis and monitoring of diabetic complications for better management of diabetic patients.

Early angiogenesis detected by PET imaging with 64Cu-NODAGA-RGD is predictive of bone critical defect repair

Therapies using stem cells may be applicable to all fields of regenerative medicine, including craniomaxillofacial surgery. Dental pulp stem cells (DPSCs) have demonstrated in vitro and in vivo osteogenic and proangiogenic properties. The aim of the study was to evaluate whether early angiogenesis investigated by nuclear imaging can predict bone formation within a mouse critical bone defect. Two symmetrical calvarial critical-sized defects were created. Defects were left empty or filled with i) DPSC-containing dense collagen scaffold, ii) 5% hypoxia-primed DPSC-containing dense collagen scaffold, iii) acellular dense collagen scaffold, or iv) left empty. Early angiogenesis assessed by PET using ⁶⁴Cu-NODAGA-RGD as a tracer was found to be correlated with bone formation determined by micro-CT within the defects from day 30, and to be correlated to the late calcium apposition observed at day 90 using ¹⁸F-Na PET. These results suggest that nuclear imaging of angiogenesis, a technique applicable in clinical practice, is a promising approach for early prediction of bone grafting outcome, thus potentially allowing to anticipate alternative regenerative strategies. STATEMENT OF SIGNIFICANCE: Bone defects are a major concern in medicine. As life expectancy increases, the number of bone lesions grows, and occurring complications lead to a delay or even lack of consolidation. Therefore, to be able to predict healing or the absence of scarring at early times would be very interesting. This would not "waste time" for the patient. We report here that early nuclear imaging of angiogenesis, using ⁶⁴Cu-NODAGA-RGD as a tracer, associated with nuclear imaging of mineralization, using ¹⁸F-Na as a tracer, is correlated to late bone healing objectivized by classical histology and microtomography. This nuclear imaging represents a promising approach for early prediction of bone grafting outcome in clinical practice, thus potentially allowing to anticipate alternative regenerative strategies.

Target identification for the diagnosis and intervention of vulnerable atherosclerotic plaques beyond 18F-fluorodeoxyglucose positron emission tomography imaging: promising tracers on the horizon

Cardiovascular disease is the major cause of morbidity and mortality in developed countries and atherosclerosis is the major cause of cardiovascular disease. Atherosclerotic lesions obstruct blood flow in the arterial vessel wall and can rupture leading to the formation of occlusive thrombi. Conventional diagnostic tools are still of limited value for identifying the vulnerable arterial plaque and for predicting its risk of rupture and of releasing thromboembolic material. Knowledge of the molecular and biological processes implicated in the process of atherosclerosis will advance the development of imaging probes to differentiate the vulnerable plaque. The development of imaging probes with high sensitivity and specificity in identifying high-risk atherosclerotic vessel wall changes and plaques is crucial for improving knowledge-based decisions and tailored individual interventions. Arterial PET imaging with ¹⁸F-FDG has shown promising results in identifying inflammatory vessel wall changes in numerous studies and clinical trials. However, due to its limited specificity in general and its intense physiological uptake in the left ventricular myocardium that impair imaging of the coronary arteries, different PET tracers for the molecular imaging of atherosclerosis have been evaluated. This review describes biological, chemical and medical expertise supporting a translational approach that will enable the development of new or the evaluation of existing PET tracers for the identification of vulnerable atherosclerotic plaques for better risk prediction and benefit to patients.

Partial volume correction for improved PET quantification in 18F-NaF imaging of atherosclerotic plaques

BACKGROUND

Accurate quantification of plaque imaging using 18F-NaF PET requires partial volume correction (PVC).

METHODS

PVC of PET data was implemented by the use of a local projection (LP) method. LP-based PVC was evaluated with an image quality (NEMA) and with a thorax phantom with "plaque-type" lesions of 18-36 mL. The validated PVC method was then applied to a cohort of 17 patients, each with at least one plaque in the carotid or ascending aortic arteries. In total, 51 calcified (HU > 110) and 16 non-calcified plaque lesions (HU < 110) were analyzed. The lesion-to-background ratio (LBR) and the relative change of LBR (ΔLBR) were measured on PET.

RESULTS

Following PVC, LBR of the spheres (NEMA phantom) was within 10% of the original values. LBR of the thoracic lesions increased by 155% to 440% when the LP-PVC method was applied to the PET images. In patients, PVC increased the LBR in both calcified [mean = 78% (-8% to 227%)] and non-calcified plaques [mean = 41%, (-9%-104%)].

CONCLUSIONS

PVC helps to improve LBR of plaque-type lesions in both phantom studies and clinical patients. Better results were obtained when the PVC method was applied to images reconstructed with point spread function modeling.

Assessment of Physiological Intracranial Calcification in Healthy Adults Using 18F-NaF PET/CT

The aim of this research study was to determine the role of 18F-Sodium fluoride (NaF) PET/CT imaging in the assessment of physiologic molecular calcification in the intra-cranial structures. We also examined the association of NaF accumulation with age as well as Hounsfield unit (HU) in certain anatomical sites that are known to calcify with normal aging. Methods: A total of 78 healthy subjects from the Cardiovascular Molecular Calcification Assessed by 18F-NaF PET/CT (CAMONA) clinical trial (38 females and 40 males) were included in this retrospective study. The mean age was 45.28 ±14.15 years (21-75). Mean standardized uptake values (SUVmean) was used to measure NaF accumulation in the choroid plexus and epithalamus (pineal gland and habenula). Maximum HU was also measured for each ROI. Correlation analysis was conducted to assess the association between parameters. Results: Mean SUVmean was 0.42 ± 0.26 in the right choroid plexus, 0.39 ±25 in the left choroid plexus, and 0.23±0.08 in the epithalamus. Significant positive correlations were present between NaF uptake and age in the right choroid plexus (r=0.61, P < 0.0001), left choroid plexus (r=0.63, p<0.0001), and epithalamus (r=0.36, P = 0.001). NaF uptake significantly correlated with HU in the right choroid plexus (r=0.52, P < 0.0001), left choroid plexus (r=0.57, p<0.0001), and epithalamus (r=0.25, P = 0.03). Conclusion: NaF could be used in the assessment of physiological calcification in several intracranial structures. We report significant associations between NaF uptake and aging as well as HU in the calcified choroid plexus and epithalamus. Our findings further support the growing interest to utilize NaF for detecting extra-osseous, molecular calcification, and this powerful probe has potential applications in the evaluation of various age-related, neurodegenerative brain processes.