18F-sodium fluoride positron emission tomography for molecular imaging of coronary atherosclerosis based on computed tomography analysis

18F-sodium fluoride positron emission tomography following coronary computed tomography angiography in predicting long-term coronary events: a 5-year follow-up study

PURPOSE

The predictive value of 18F-sodium fluoride (18F-NaF) positron emission tomography (PET) in combination with coronary computed tomography (CT) angiography (CCTA) for future coronary events has attracted interest. We evaluated the potential of 18F-NaF PET/CT following CCTA to predict major coronary events (MACE) during a 5-year follow-up period.

METHODS

Forty patients with coronary atherosclerotic lesions detected on CCTA underwent 18F-NaF PET/CT examination. Each lesion was evaluated for luminal stenosis and high-risk plaque (HRP) with < 30 Hounsfield units and a > 1.1 remodeling index on CCTA. Focal 18F-NaF uptake in each lesion was quantified using the maximum tissue-to-background ratio (TBRmax), and the maximum TBRmax per patient (M-TBRmax) was determined. We followed MACE (cardiac death, acute coronary syndrome, and/or coronary revascularization > 6 months after 18F-NaF PET/CT) for 5 years.

RESULTS

In total, 142 coronary lesions were analyzed. Eleven patients experienced any MACE. Patients with MACE showed a higher M-TBRmax than those without (1.40 ± .19 vs. 1.18 ± .18, P = .0011), and the optimal M-TBRmax cutoff to predict MACE was 1.29. Patients with M-TBRmax of ≥ 1.29 had a higher risk of MACE than those with lower values (P = .012, log-rank test), whereas patients with obstructive stenosis and those with HRP did not. Multivariate Cox proportional analysis adjusted for age, sex, coronary risk factors, and CCTA findings showed that M-TBRmax of ≥ 1.29 remained an independent predictor of 5-year MACE (hazard ratio, 5.4; 95% confidence interval, 1.1-25.4; P = .034).

CONCLUSION

18F-NaF PET/CT following CCTA provides useful strategies to predict 5-year MACE.

The reliability and utility of on-site CT-derived fractional flow reserve (FFR) based on fluid structure interactions: comparison with FFRCT based on computational fluid dynamics, invasive FFR, and resting full-cycle ratio

Fractional flow reserve (FFR) derived off-site by coronary computed tomography angiography (CCTA) (FFRCT) is obtained by applying the principles of computational fluid dynamics. This study aimed to validate the overall reliability of on-site CCTA-derived FFR based on fluid structure interactions (CT-FFR) and assess its clinical utility compared with FFRCT, invasive FFR, and resting full-cycle ratio (RFR). We calculated the CT-FFR for 924 coronary vessels in 308 patients who underwent CCTA for clinically suspected coronary artery disease. Of these patients, 35 patients with at least one obstructive stenosis (> 50%) detected on CCTA underwent both CT-FFR and FFRCT for further investigation. Furthermore, 24 and 20 patients underwent invasive FFR and RFR in addition to CT-FFR, respectively. The inter-observer correlation (r) of CT-FFR was 0.93 (95% confidence interval [CI] 0.85-0.97, P < 0.0001) with a mean absolute difference of - 0.0042 (limits of agreement - 0.073, 0.064); 97.3% of coronary arteries without obstructive lesions on CCTA had negative results for ischemia on CT-FFR (> 0.80). The correlation coefficient between CT-FFR and FFRCT for 105 coronary vessels was 0.87 (95% CI 0.82-0.91, P < 0.0001) with a mean absolute difference of - 0.012 (limits of agreement - 0.12, 0.10). CT-FFR correlated well with both invasive FFR (r = 0.66, 95% CI 0.36-0.84, P = 0.0003) and RFR (r = 0.78, 95% CI 0.51-0.91, P < 0.0001). These data suggest that CT-FFR can potentially substitute for FFRCT and correlates closely with invasive FFR and RFR with high reproducibility. Our findings should be proven by further clinical investigation in a larger cohort.

A longitudinal pilot study to assess temporal changes in coronary arterial 18F-sodium fluoride uptake

PURPOSE

How coronary arterial 18F-sodium fluoride (18F-NaF) uptake on positron emission tomography changes over the long term and what clinical factors impact the changes remain unclear. We sought to investigate the topics in this study.

METHODS

We retrospectively studied 15 patients with ≥1 coronary atherosclerotic lesion/s detected on cardiac computed tomography who underwent baseline and follow-up (interval of >3 years) 18F-NaF positron emission tomography/computed tomography scans. Focal 18F-NaF uptake in each lesion was quantified using maximum tissue-to-background ratio (TBRmax). The temporal change in TBRmax was assessed using a ratio of follow-up to baseline TBRmax (R-TBRmax).

RESULTS

A total of 51 lesions were analyzed. Mean R-TBRmax was 0.96 ± 0.21. CT-based lesion features (location, obstructive stenosis, plaque types, features of high-risk plaque) did not correlate with an increase in R-TBRmax. In multivariate analysis, baseline TBRmax significantly correlated with higher follow-up TBRmax (β = 0.57, P < 0.0001), and the presence of diabetes mellitus significantly correlated with both higher follow-up TBRmax (β = 0.34, P = 0.001) and elevated R-TBRmax (β = 0.40, P = 0.003).

CONCLUSION

Higher coronary arterial 18F-NaF uptake is likely to remain continuously high. Diabetes mellitus affects the long-term increase in coronary arterial 18F-NaF uptake.

Non-calcified active atherosclerosis plaque detection with 18F-NaF and 18F-FDG PET/CT dynamic imaging

Arterial inflammation is an indicator of atheromatous plaque vulnerability to detach and to obstruct blood vessels in the heart or in the brain thus causing heart attack or stroke. To date, it is difficult to predict the plaque vulnerability. This study was aimed to assess the behavior of 18F-sodium fluoride (18F-NaF) and 18F-fluorodeoxyglucose (18F-FDG) uptake in the aorta and iliac arteries as a function of plaque density on CT images. We report metabolically active artery plaques associated to inflammation in the absence of calcification. 18 elderly volunteers were recruited and imaged with computed tomography (CT) and positron emission tomography (PET) with 18F-NaF and 18F-FDG. A total of 1338 arterial segments were analyzed, 766 were non-calcified and 572 had calcifications. For both 18F-NaF and 18F-FDG, the mean SUV values were found statistically significantly different between non-calcified and calcified artery segments. Clustering CT non-calcified segments, excluding blood, resulted in two clusters C1 and C2 with a mean density of 30.63 ± 5.06 HU in C1 and 43.06 ± 4.71 HU in C2 (P < 0.05), and their respective SUV were found statistically different in 18F-NaF and 18F-FDG. The 18F-NaF images showed plaques not detected on CT images, where the 18F-FDG SUV values were high in comparison to artery walls without plaques. The density on CT images alone corresponding to these plaques could be further investigated to see whether it can be an indicator of the active plaques.

Molecular imaging research in atherosclerosis: A 23-year scientometric and visual analysis

Background: Cardiovascular and cerebrovascular diseases are major global health problems, and the main cause is atherosclerosis. Recently, molecular imaging has been widely employed in the diagnosis and therapeutic applications of a variety of diseases, including atherosclerosis. Substantive facts have announced that molecular imaging has broad prospects in the early diagnosis and targeted treatment of atherosclerosis. Objective: We conducted a scientometric analysis of the scientific publications over the past 23 years on molecular imaging research in atherosclerosis, so as to identify the key progress, hotspots, and emerging trends. Methods: Original research and reviews regarding molecular imaging in atherosclerosis were retrieved from the Web of Science Core Collection database. Microsoft Excel 2021 was used to analyze the main findings. CiteSpace, VOSviewer, and a scientometric online platform were used to perform visualization analysis of the co-citation of journals and references, co-occurrence of keywords, and collaboration between countries/regions, institutions, and authors. Results: A total of 1755 publications were finally included, which were published by 795 authors in 443 institutions from 59 countries/regions. The United States was the top country in terms of the number and centrality of publications in this domain, with 810 papers and a centrality of 0.38, and Harvard University published the largest number of articles (182). Fayad, ZA was the most productive author, with 73 papers, while LIBBY P had the most co-citations (493). CIRCULATION was the top co-cited journal with a frequency of 1,411, followed by ARTERIOSCL THROM VAS (1,128). The co-citation references analysis identified eight clusters with a well-structured network (Q = 0.6439) and highly convincing clustering (S = 0.8865). All the studies calculated by keyword co-occurrence were divided into five clusters: "nanoparticle," "magnetic resonance imaging," "inflammation," "positron emission tomography," and "ultrasonography". Hot topics mainly focused on cardiovascular disease, contrast media, macrophage, vulnerable plaque, and microbubbles. Sodium fluoride ⁃PET, targeted drug delivery, OCT, photoacoustic imaging, ROS, and oxidative stress were identified as the potential trends. Conclusion: Molecular imaging research in atherosclerosis has attracted extensive attention in academia, while the challenges of clinical transformation faced in this field have been described in this review. The findings of the present research can inform funding agencies and researchers toward future directions.

Assessment of atherosclerosis in multiple myeloma and smoldering myeloma patients using 18F- sodium fluoride PET/CT

BACKGROUND

To compare the NaF uptake in the thoracic aorta and whole heart, as an early indicator of atherosclerosis, in multiple myeloma (MM) and smoldering multiple myeloma (SMM) patients with a healthy control (HC) group.

METHODS

Forty-four untreated myeloma patients (35 MM and nine SMM) and twenty-six age and gender-matched HC subjects were collected. Each individual's NaF uptake in three parts of the aorta (AA: ascending aorta, AR: aortic arch, DA: descending aorta) and the whole heart was segmented. Average global standardized uptake value means were derived by sum of the product of each slice area divided by the sum of those slice areas. Results were reported as target to background ratio (TBR).

RESULTS

There was a significant difference between the NaF uptake in the thoracic aorta of myeloma and HC groups [AA (myeloma = 1.82 ± 0.21, HC = 1.24 ± 0.02), AR (myeloma = 1.71 ± 0.19, HC = 1.28 ± 0.03) and DA (myeloma = 1.96 ± 0.28, HC = 1.38 ± 0.03); P-values < 0.001]. The difference in the whole heart NaF uptake between two groups was also significant (P < 0.001).

CONCLUSIONS

We observed a higher uptake of NaF in the thoracic aorta and whole heart of myeloma patients in comparison to the matched control group.

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.

Iterative reconstruction incorporating background correction improves quantification of [18F]-NaF PET/CT images of patients with abdominal aortic aneurysm

BACKGROUND

A confounding issue in [18F]-NaF PET/CT imaging of abdominal aortic aneurysms (AAA) is the spill in contamination from the bone into the aneurysm. This study investigates and corrects for this spill in contamination using the background correction (BC) technique without the need to manually exclude the part of the AAA region close to the bone.

METHODS

Seventy-two (72) datasets of patients with AAA were reconstructed with the standard ordered subset expectation maximization (OSEM) algorithm incorporating point spread function (PSF) modelling. The spill in effect in the aneurysm was investigated using two target regions of interest (ROIs): one covering the entire aneurysm (AAA), and the other covering the aneurysm but excluding the part close to the bone (AAAexc). ROI analysis was performed by comparing the maximum SUV in the target ROI (SUVmax(T)), the corrected cSUVmax (SUVmax(T) - SUVmean(B)) and the target-to-blood ratio (TBR = SUVmax(T)/SUVmean(B)) with respect to the mean SUV in the right atrium region.

RESULTS

There is a statistically significant higher [18F]-NaF uptake in the aneurysm than normal aorta and this is not correlated with the aneurysm size. There is also a significant difference in aneurysm uptake for OSEM and OSEM + PSF (but not OSEM + PSF + BC) when quantifying with AAA and AAAexc due to the spill in from the bone. This spill in effect depends on proximity of the aneurysms to the bone as close aneurysms suffer more from spill in than farther ones.

CONCLUSION

The background correction (OSEM + PSF + BC) technique provided more robust AAA quantitative assessments regardless of the AAA ROI delineation method, and thus it can be considered as an effective spill in correction method for [18F]-NaF AAA studies.

Imaging Inflammation in Patients and Animals: Focus on PET Imaging the Vulnerable Plaque

Acute coronary syndrome (ACS) describes a range of conditions associated with the rupture of high-risk or vulnerable plaque. Vulnerable atherosclerotic plaque is associated with many changes in its microenvironment which could potentially cause rapid plaque progression. Present-day PET imaging presents a plethora of radiopharmaceuticals designed to image different characteristics throughout plaque progression. Improved knowledge of atherosclerotic disease pathways has facilitated a growing number of pathophysiological targets for more innovative radiotracer design aimed at identifying at-risk vulnerable plaque and earlier intervention opportunity. This paper reviews the efficacy of PET imaging radiotracers ¹⁸F-FDG, ¹⁸F-NaF, ⁶⁸Ga-DOTATATE, ⁶⁴Cu-DOTATATE and ⁶⁸Ga-pentixafor in plaque characterisation and risk assessment, as well as the translational potential of novel radiotracers in animal studies. Finally, we discuss our murine PET imaging experience and the challenges encountered.

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.

Clinical implications of 18F-sodium fluoride uptake in subclinical aortic valve calcification: Its relation to coronary atherosclerosis and its predictive value

BACKGROUND

Uptake of 18F-sodium fluoride (18F-NaF) on positron emission tomography (PET) reflects active calcification. Application of this technique in the early phase of aortic valve calcification (AVC) is of clinical interest. We investigated clinical implications of 18F-NaF uptake in subclinical AVC evaluated simultaneously with coronary atherosclerosis, and the utility of 18F-NaF uptake in predicting AVC progression.

METHODS

We studied 25 patients with subclinical AVC and coronary plaques detected on computed tomography (CT) who underwent 18F-NaF PET/CT. AVC score, volume, mean density, and the presence of high-risk coronary plaque were evaluated on CT in each patient. Focal 18F-NaF uptake in AVC and in coronary plaques was quantified with the maximum tissue-to-background ratio (TBRmax).

RESULTS

There were positive correlations between AVC TBRmax (A-TBRmax) and AVC parameters on CT. The 14 patients with high-risk coronary plaque had significantly higher A-TBRmax than those without such plaque (1.60 ± 0.18 vs 1.42 ± 0.13, respectively; P = 0.012). A-TBRmax positively correlated with maximum TBRmax of coronary plaque per patient (r = 0.55, P = 0.0043). In the 11 patients who underwent follow-up CT scan, A-TBRmax positively correlated with subsequent increase in AVC score (r = 0.74, P = 0.0091).

CONCLUSION

Our 18F-NaF PET- and CT-based data indicate relationships between calcification activity in subclinical AVC and characteristics of coronary atherosclerosis. 18F-NaF PET may provide new information regarding molecular conditions and future progression of subclinical AVC.

Imaging coronary plaques using 3D motion-compensated [18F]NaF PET/MR

Background

Cardiac PET has recently found novel applications in coronary atherosclerosis imaging using [¹⁸F]NaF as a radiotracer, highlighting vulnerable plaques. However, the resulting uptakes are relatively small, and cardiac motion and respiration-induced movement of the heart can impair the reconstructed images due to motion blurring and attenuation correction mismatches. This study aimed to apply an MR-based motion compensation framework to [¹⁸F]NaF data yielding high-resolution motion-compensated PET and MR images.

Methods

Free-breathing 3-dimensional Dixon MR data were acquired, retrospectively binned into multiple respiratory and cardiac motion states, and split into fat and water fraction using a model-based reconstruction framework. From the dynamic MR reconstructions, both a non-rigid cardiorespiratory motion model and a motion-resolved attenuation map were generated and applied to the PET data to improve image quality. The approach was tested in 10 patients and focal tracer hotspots were evaluated concerning their target-to-background ratio, contrast-to-background ratio, and their diameter.

Results

MR-based motion models were successfully applied to compensate for physiological motion in both PET and MR. Target-to-background ratios of identified plaques improved by 7 ± 7%, contrast-to-background ratios by 26 ± 38%, and the plaque diameter decreased by −22 ± 18%. MR-based dynamic attenuation correction strongly reduced attenuation correction artefacts and was not affected by stent-related signal voids in the underlying MR reconstructions.

Conclusions

The MR-based motion correction framework presented here can improve the target-to-background, contrast-to-background, and width of focal tracer hotspots in the coronary system. The dynamic attenuation correction could effectively mitigate the risk of attenuation correction artefacts in the coronaries at the lung-soft tissue boundary. In combination, this could enable a more reproducible and reliable plaque localisation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-020-05180-4.

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.

Analytical quantification of aortic valve 18F-sodium fluoride PET uptake

BACKGROUND

Challenges to cardiac PET-CT include patient motion, prolonged image acquisition and a reduction of counts due to gating. We compared two analytical tools, FusionQuant and OsiriX, for quantification of gated cardiac 18F-sodium fluoride (18F-fluoride) PET-CT imaging.

METHODS

Twenty-seven patients with aortic stenosis were included, 15 of whom underwent repeated imaging 4 weeks apart. Agreement between analytical tools and scan-rescan reproducibility was determined using the Bland-Altman method and Lin's concordance correlation coefficients (CCC).

RESULTS

Image analysis was faster with FusionQuant [median time (IQR) 7:10 (6:40-8:20) minutes] compared with OsiriX [8:30 (8:00-10:10) minutes, p = .002]. Agreement of uptake measurements between programs was excellent, CCC = 0.972 (95% CI 0.949-0.995) for mean tissue-to-background ratio (TBRmean) and 0.981 (95% CI 0.965-0.997) for maximum tissue-to-background ratio (TBRmax). Mean noise decreased from 11.7% in the diastolic gate to 6.7% in motion-corrected images (p = .002); SNR increased from 25.41 to 41.13 (p = .0001). Aortic valve scan-rescan reproducibility for TBRmax was improved with FusionQuant using motion correction compared to OsiriX (error ± 36% vs ± 13%, p < .001) while reproducibility for TBRmean was similar (± 10% vs ± 8% p = .252).

CONCLUSION

18F-fluoride PET quantification with FusionQuant and OsiriX is comparable. FusionQuant with motion correction offers advantages with respect to analysis time and reproducibility of TBRmax values.

Integrated cardiovascular assessment of atherosclerosis using PET/MRI

Atherosclerosis is a systemic inflammatory disease typified by the development of lipid-rich atheroma (plaques), the rupture of which are a major cause of myocardial infarction and stroke. Anatomical evaluation of the plaque considering only the degree of luminal stenosis overlooks features associated with vulnerable plaques, such as high-risk morphological features or pathophysiology, and hence risks missing vulnerable or ruptured non-stenotic plaques. Consequently, there has been interest in identifying these markers of vulnerability using either MRI for morphology, or positron emission tomography (PET) for physiological processes involved in atherogenesis. The advent of hybrid PET/MRI scanners offers the potential to combine the strengths of PET and MRI to allow comprehensive assessment of the atherosclerotic plaque. This review will discuss the principles and technical aspects of hybrid PET/MRI assessment of atherosclerosis, and consider how combining the complementary modalities of PET and MRI has already furthered our understanding of atherogenesis, advanced drug development, and how it may hold potential for clinical application.

Optimization of reconstruction and quantification of motion-corrected coronary PET-CT

BACKGROUND

Coronary PET shows promise in the detection of high-risk atherosclerosis, but there remains a need to optimize imaging and reconstruction techniques. We investigated the impact of reconstruction parameters and cardiac motion-correction in 18F Sodium Fluoride (18F-NaF) PET.

METHODS

Twenty-two patients underwent 18F-NaF PET within 22 days of an acute coronary syndrome. Optimal reconstruction parameters were determined in a subgroup of six patients. Motion-correction was performed on ECG-gated data of all patients with optimal reconstruction. Tracer uptake was quantified in culprit and reference lesions by computing signal-to-noise ratio (SNR) in diastolic, summed, and motion-corrected images.

RESULTS

Reconstruction using 24 subsets, 4 iterations, point-spread-function modelling, time of flight, and 5-mm post-filtering provided the highest median SNR (31.5) compared to 4 iterations 0-mm (22.5), 8 iterations 0-mm (21.1), and 8 iterations 5-mm (25.6; all P < .05). Motion-correction improved SNR of culprit lesions (n = 33) (24.5[19.9-31.5]) compared to diastolic (15.7[12.4-18.1]; P < .001) and summed data (22.1[18.9-29.2]; P < .001). Motion-correction increased the SNR difference between culprit and reference lesions (10.9[6.3-12.6]) compared to diastolic (6.2[3.6-10.3]; P = .001) and summed data (7.1 [4.8-11.6]; P = .001).

CONCLUSIONS

The number of iterations and extent of post-filtering has marked effects on coronary 18F-NaF PET quantification. Cardiac motion-correction improves discrimination between culprit and reference lesions.

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.

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.

Clinical imaging of cardiovascular inflammation

Cardiovascular disease due to atherosclerosis is the number one cause of morbidity and mortality worldwide. In the past twenty years, compelling preclinical and clinical data have indicated that a maladaptive inflammatory response plays a crucial role in the development of atherosclerosis initiation and progression in the vasculature, all the way to the onset of life-threatening cardiovascular events. Furthermore, inflammation is key to heart and brain damage and healing after myocardial infarction or stroke. Recent evidence indicates that this interplay between the vasculature, organs target of ischemia and the immune system is mediated by the activation of hematopoietic organs (bone marrow and spleen). In this evolving landscape, non-invasive imaging is becoming more and more essential to support either mechanistic preclinical studies to investigate the role of inflammation in cardiovascular disease (CVD), or as a translational tool to quantify inflammation in the cardiovascular system and hematopoietic organs in patients. In this review paper, we will describe the clinical applications of non-invasive imaging to quantify inflammation in the vasculature, infarcted heart and brain, and hematopoietic organs in patients with cardiovascular disease, with specific focus on [18F]FDG PET and other novel inflammation-specific radiotracers. Furthermore, we will briefly describe the most recent clinical applications of other imaging techniques such as MRI, SPECT, CT, CEUS and OCT in this arena.

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.

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.

Relationship between coronary arterial 18F-sodium fluoride uptake and epicardial adipose tissue analyzed using computed tomography

PURPOSE

¹⁸F-Sodium fluoride (¹⁸F-NaF) positron emission tomography (PET) has the potential to detect high-risk coronary plaques. Epicardial adipose tissue (EAT) reportedly correlates with coronary atherosclerosis progression. We evaluated the relationship between coronary arterial ¹⁸F-NaF uptake and EAT findings using computed tomography (CT).

METHODS

We studied 40 patients with ≥ 1 coronary plaque detected on cardiac CT who underwent ¹⁸F-NaF PET/CT. EAT volume was measured using CT and indexed to body surface area in each patient. Each plaque was evaluated for CT-based luminal stenosis and high-risk features. The mean EAT density surrounding each plaque was calculated as perilesional EAT density (PLED) using non-contrast CT images. Focal ¹⁸F-NaF uptake in each plaque was quantified using the maximum tissue-to-background ratio (TBR_max).

RESULTS

EAT volume index was similar between patients with TBR_max ≥ 1.28 (previously reported optimal cutoff to predict coronary events) and those with lower TBR_max, but patients with TBR_max ≥ 1.28 showed higher maximum PLED per patient (- 86 ± 12 Hounsfield units (HU) versus - 98 ± 11 HU, P = 0.0044). In the lesion-based analysis (n = 92), PLED was positively correlated with TBR_max, and the optimal PLED cutoff to identify TBR_max ≥ 1.28 was - 97 HU. On multivariate analysis adjusted for lesion location, obstructive stenosis, and high-risk plaque on CT, PLED ≥ - 97 HU remained a significant predictor of TBR_max ≥ 1.28.

CONCLUSIONS

Increased PLED was associated with significant coronary arterial ¹⁸F-NaF uptake. Step-by-step analyses of EAT density on CT and coronary arterial ¹⁸F-NaF uptake on PET may offer novel strategies for risk prediction in coronary artery disease.

Basic Science of PET Imaging for Inflammatory Diseases

FDG-PET/CT has recently emerged as a useful tool for the evaluation of inflammatory diseases too, in addition to that of malignant diseases. The imaging is based on active glucose utilization by inflammatory tissue. Autoradiography studies have demonstrated high FDG uptake in macrophages, granulocytes, fibroblasts, and granulation tissue. Especially, activated macrophages are responsible for the elevated FDG uptake in some types of inflammation. According to one study, after activation by lipopolysaccharide of cultured macrophages, the [¹⁴C]2DG uptake by the cells doubled, reaching the level seen in glioblastoma cells. In activated macrophages, increase in the expression of total GLUT1 and redistributions from the intracellular compartments toward the cell surface have been reported. In one rheumatoid arthritis model, following stimulation by hypoxia or TNF-α, the highest elevation of the [³H]FDG uptake was observed in the fibroblasts, followed by that in macrophages and neutrophils. As the fundamental mechanism of elevated glucose uptake in both cancer cells and inflammatory cells, activation of glucose metabolism as an adaptive response to a hypoxic environment has been reported, with transcription factor HIF-1α playing a key role. Inflammatory cells and cancer cells seem to share the same molecular mechanism of elevated glucose metabolism, lending support to the notion of usefulness of FDGPET/CT for the evaluation of inflammatory diseases, besides cancer.

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.

Feasibility of Coronary 18F-Sodium Fluoride Positron-Emission Tomography Assessment With the Utilization of Previously Acquired Computed Tomography Angiography

BACKGROUND

We assessed the feasibility of utilizing previously acquired computed tomography angiography (CTA) with subsequent positron-emission tomography (PET)-only scan for the quantitative evaluation of ¹⁸F-NaF PET coronary uptake.

METHODS AND RESULTS

Forty-five patients (age 67.1±6.9 years; 76% males) underwent CTA (CTA1) and combined ¹⁸F-NaF PET/CTA (CTA2) imaging within 14 [10, 21] days. We fused CTA1 from visit 1 with ¹⁸F-NaF PET (PET) from visit 2 and compared visual pattern of activity, maximal standard uptake (SUVmax) values, and target to background ratio (TBR) measurements on (PET/CTA1) fused versus hybrid (PET/CTA2). On PET/CTA2, 226 coronary plaques were identified. Fifty-eight coronary segments from 28 (62%) patients had high ¹⁸F-NaF uptake (TBR >1.25), whereas 168 segments had lesions with ¹⁸F-NaF TBR ≤1.25. Uptake in all lesions was categorized identically on coregistered PET/CTA1. There was no significant difference in ¹⁸F-NaF uptake values between PET/CTA1 and PET/CTA2 (SUVmax, 1.16±0.40 versus 1.15±0.39; P=0.53; TBR, 1.10±0.45 versus 1.09±0.46; P=0.55). The intraclass correlation coefficient for SUVmax and TBR was 0.987 (95% CI, 0.983-0.991) and 0.986 (95% CI, 0.981-0.992). There was no fixed or proportional bias between PET/CTA1 and PET/CTA2 for SUVmax and TBR. Cardiac motion correction of PET scans improved reproducibility with tighter 95% limits of agreement (±0.14 for SUVmax and ±0.15 for TBR versus ±0.20 and ±0.20 on diastolic imaging; P<0.001).

CONCLUSIONS

Coronary CTA/PET protocol with CTA first followed by PET-only allows for reliable and reproducible quantification of ¹⁸F-NaF coronary uptake. This approach may facilitate selection of high-risk patients for PET-only imaging based on results from prior CTA, providing a practical workflow for clinical application.

Current and Emerging Preclinical Approaches for Imaging-Based Characterization of Atherosclerosis

Atherosclerotic plaques can remain quiescent for years, but become life threatening upon rupture or disruption, initiating clot formation in the vessel lumen and causing acute myocardial infarction and ischemic stroke. Whether and how a plaque ruptures is determined by its macroscopic structure and microscopic composition. Rupture-prone plaques usually consist of a thin fibrous cap with few smooth muscle cells, a large lipid core, a dense infiltrate of inflammatory cells, and neovessels. Such lesions, termed high-risk plaques, can remain asymptomatic until the thrombotic event. Various imaging technologies currently allow visualization of morphological and biological characteristics of high-risk atherosclerotic plaques. Conventional protocols are often complex and lack specificity for high-risk plaque. Conversely, new imaging approaches are emerging which may overcome these limitations. Validation of these novel imaging techniques in preclinical models of atherosclerosis is essential for effective translational to clinical practice. Imaging the vessel wall, as well as its biological milieu in small animal models, is challenging because the vessel wall is a small structure that undergoes continuous movements imposed by the cardiac cycle as it is adjacent to circulating blood. The focus of this paper is to provide a state-of-the-art review on techniques currently available for preclinical imaging of atherosclerosis in small animal models and to discuss the advantages and limitations of each approach.

Radionuclide Imaging of Atherothrombotic Diseases

Purpose of Review

A variety of approaches and molecular targets have emerged in recent years for radionuclide-based imaging of atherosclerosis and vulnerable plaque using single photon emission computed tomography (SPECT) and positron emission tomography (PET), with numerous methods focused on characterizing the mechanisms underlying plaque progression and rupture. This review highlights the ongoing developments in both the preclinical and clinical environment for radionuclide imaging of atherosclerosis and atherothrombosis.

Recent Findings

Numerous physiological processes responsible for the evolution of high-risk atherosclerotic plaque, such as inflammation, thrombosis, angiogenesis, and microcalcification, have been shown to be feasible targets for SPECT and PET imaging. For each physiological process, specific molecular markers have been identified that allow for sensitive non-invasive detection and characterization of atherosclerotic plaque.

Summary

The capabilities of SPECT and PET imaging continue to evolve for physiological evaluation of atherosclerosis. This review summarizes the latest developments related to radionuclide imaging of atherothrombotic diseases.

Molecular Imaging of Vascular Calcification with 18F-Sodium-Fluoride in Patients Infected with Human Immunodeficiency Virus

¹⁸F-Sodium Fluoride (NaF) accumulates in areas of active hydroxyapatite deposition and potentially unstable atherosclerotic plaques. We assessed the presence of atherosclerotic plaques in 50 adult patients with HIV (HIV+) who had undergone two cardiac computed tomography scans to measure coronary artery calcium (CAC) progression. CAC and its progression are predictive of an unfavorable prognosis. Tracer uptake was quantified in six arterial territories: aortic arch, innominate carotid artery, right and left internal carotid arteries, left coronary (anterior descending and circumflex) and right coronary artery. Thirty-one patients showed CAC progression and 19 did not. At least one territory with high NaF uptake was observed in 150 (50%) of 300 arterial territories. High NaF uptake was detected more often in non-calcified than calcified areas (68% vs. 32%), and in patients without than in those with prior CAC progression (68% vs. 32%). There was no correlation between clinical and demographic variables and NaF uptake. In clinically stable HIV+ patients, half of the arterial territories showed a high NaF uptake, often in the absence of macroscopic calcification. NaF uptake at one time point did not correlate with prior progression of CAC. Prospective studies will demonstrate the prognostic significance of high NaF uptake in HIV+ patients.

Assessment of artery calcification in atherosclerosis with dynamic 18F-FDG-PET/CT imaging in elderly subjects

Glucose metabolism in atherosclerotic arteries has been shown to be an indicator of inflammation, which might be a precursor of plaque rupture. In this prospective study, we assessed the correlation between artery calcification and glucose metabolism by means of 18F-FDG PET/CT imaging in elderly subjects. Nineteen elderly subjects, with age ranging from 65 to 85 years, underwent CT and dynamic 18F-FDG-PET imaging. The artery calcification was determined with a threshold of 130 Hounsfield units. Intensity of calcification and ratio of calcification area to total artery area were classified in four sequential classes from CT images. The CT artery images were also classified as having single or multi-spot calcifications. Their respective glucose metabolism was assessed with fractional uptake rate (FUR). Factor analysis was used in this study to separate blood images from tissue to extract the blood time activity curves for FUR calculations. The artery images in PET data were corrected for partial volume effect. The total arterial segments analyzed were 1332, with 1085 without calcification (81%), 247 (19%) with calcification, and 94 segments were having multi-spot of calcifications. There was a statistically significant difference in FUR values between non-calcified to calcified segments and between subjects under medication to non-medication when comparing the subjects based on calcification area. No statistically significant differences of FUR were found between single spot as a function of intensity, while in the multi-spots, there was a statistically significant difference for all artery segments. Metabolism activity varies for non-calcified to calcified segments. Based on the metabolic activity represented by FUR, calcifications in multi-spots have different effects than in single spots.

18 F-Sodium Fluoride Imaging of Coronary Atherosclerosis in Ambulatory Patients With Diabetes Mellitus

Objective—

Although patients with diabetes mellitus (DM) are considered at high risk of cardiovascular events, there is growing evidence that this notion is incorrect. Atherosclerosis imaging may identify patients at risk.

Approach and Results—

We performed coronary atherosclerosis with 18 F-sodium fluoride (NaF) positron emission tomography/computed tomography and gated chest computed tomography for coronary artery calcium in 88 consecutive ambulatory patients with DM on a stable medical regimen. NaF has been shown to localize avidly in culprit lesions of patients with acute coronary syndromes and may identify unstable plaques. NaF activity was measured as target (coronary arteries)-to-background (left ventricular pool) ratio (TBR). High TBR was defined as ≥1.5. The mean age of the cohort was 54±14 years, 55% had type 2 DM, 65% were men, the median HgbA1c (hemoglobin A1c) and LDL (low-density lipoprotein) cholesterol were 7.5% (interquartile range, 7.1–8.5) and 1.9 mmol/L (interquartile range, 1.5–2.6), respectively. Mean coronary artery calcium score was 374±773, and median TBR was 1.2. Coronary artery TBR ≥1.5 was detected in 13 (15%) patients. In univariable analyses, male sex ( P =0.0002), estimated glomerular filtration rate ( P =0.02), and total coronary artery calcium score ( P =0.04) were associated with TBR. In multivariable analyses, TBR >median was associated with male sex ( P =0.0001) and statin use ( P =0.042).

Conclusions—

In ambulatory patients with DM asymptomatic for cardiovascular disease, the prevalence of potentially vulnerable plaques detected with NaF was low, but in the absence of follow-up data at this stage, we cannot assess the import of this information. Future research will establish whether NaF imaging helps risk stratify patients with DM.

Clinical Trial Registration—

URL: http://www.clinicaltrials.gov . Unique identifier: NCT03530176.

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.

Fluorine-18: an untapped resource in inorganic chemistry

Advances in the field of fluorine chemistry have been applied extensively to the syntheses of 18F-labelled organic compounds and radiopharmaceuticals. However, 18F has sparely been used as a tool to explore inorganic chemistry and can be viewed as a research area worthy of further development. This review highlights the application of 18F in development of inorganic fluorinating agents, mechanistic studies and imaging tools.

Predictive Value of 18F-Sodium Fluoride Positron Emission Tomography in Detecting High-Risk Coronary Artery Disease in Combination With Computed Tomography

Background Application of 18F-sodium fluoride (18F-NaF) positron emission tomography ( PET ) to coronary artery disease has attracted interest. We investigated the utility of 18F-NaF uptake for predicting coronary events and evaluated the combined use of coronary computed tomography (CT) angiography ( CCTA ) and 18F-NaF PET /CT in coronary artery disease risk assessment. Methods and Results This study included patients with ≥1 coronary atherosclerotic lesion detected on CCTA who underwent 18F-NaF PET / CT . High-risk plaque on CCTA was defined as plaque with low density (<30 Hounsfield units) and high remodeling index (>1.1). Focal 18F-NaF uptake in each lesion was quantified using the maximum tissue:background ratio ( TBR max), and maximum TBR max per patient (M- TBR max) was determined. Thirty-two patients having a total of 112 analyzed lesions were followed for 2 years after 18F-NaF PET / CT scan, and 11 experienced coronary events (acute coronary syndrome and/or late coronary revascularization [after 3 months]). Patients with coronary events had higher M- TBR max than those without (1.39±0.18 versus 1.19±0.17, respectively; P=0.0034). The optimal M- TBR max cutoff to predict coronary events was 1.28 (area under curve: 0.79). Patients with M- TBR max ≥1.28 had a higher risk of earlier coronary events than those with lower M- TBR max ( P=0.0062 by log-rank test). In patient-based (n=41) and lesion-based (n=143) analyses of CCTA findings that predicted higher coronary 18F-NaF uptake, the presence of high-risk plaque was a significant predictor of both M- TBR max ≥1.28 and TBR max ≥1.28. Conclusions 18F-NaF PET / CT has the potential to detect high-risk coronary artery disease and individual coronary lesions and to predict future coronary events when combined with CCTA . Clinical Trial Registration URL : www.umin.ac.jp . Unique identifier: UMIN 000013735.

Three-Hour Delayed Imaging Improves Assessment of Coronary 18F-Sodium Fluoride PET

Coronary ¹⁸F-sodium fluoride (¹⁸F-NaF) PET identifies ruptured plaques in patients with recent myocardial infarction and localizes to atherosclerotic lesions with active calcification. Most studies to date have performed the PET acquisition 1 h after injection. Although qualitative and semiquantitative analysis is feasible with 1-h images, residual blood-pool activity often makes it difficult to discriminate plaques with ¹⁸F-NaF uptake from noise. We aimed to assess whether delayed PET performed 3 h after injection improves image quality and uptake measurements. Methods: Twenty patients (67 ± 7 y old, 55% male) with stable coronary artery disease underwent coronary CT angiography (CTA) and PET/CT both 1 h and 3 h after the injection of 266.2 ± 13.3 MBq of ¹⁸F-NaF. We compared the visual pattern of coronary uptake, maximal background (blood pool) activity, noise, SUV_max, corrected SUV_max (cSUV_max), and target-to-background (TBR) ratio in lesions defined by CTA on 1-h versus 3-h ¹⁸F-NaF PET. Results: On 1-h PET, 26 CTA lesions with ¹⁸F-NaF PET uptake were identified in 12 (60%) patients. On 3-h PET, we detected ¹⁸F-NaF PET uptake in 7 lesions that were not identified on 1-h PET. The median cSUV_max and TBRs of these lesions were 0.48 (interquartile range [IQR], 0.44-0.51) and 1.45 (IQR, 1.39-1.52), respectively, compared with -0.01 (IQR, -0.03-0.001) and 0.95 (IQR, 0.90-0.98), respectively, on 1-h PET (both P < 0.001). Across the entire cohort, 3-h PET SUV_max was similar to 1-h PET measurements (1.63 [IQR, 1.37-1.98] vs. 1.55 [IQR, 1.43-1.89], P = 0.30), and the background activity was lower (0.71 [IQR, 0.65-0.81] vs. 1.24 [IQR, 1.05-1.31], P < 0.001). On 3-h PET, TBR, cSUV_max, and noise were significantly higher (respectively: 2.30 [IQR, 1.70-2.68] vs. 1.28 [IQR, 0.98-1.56], P < 0.001; 0.38 [IQR, 0.27-0.70] vs. 0.90 [IQR, 0.64-1.17], P < 0.001; and 0.10 [IQR, 0.09-0.12] vs. 0.07 [IQR, 0.06-0.09], P = 0.02). Median cSUV_max and TBR increased by 92% (range, 33%-225%) and 80% (range, 20%-177%), respectively. Conclusion: Blood-pool activity decreases on delayed imaging, facilitating the assessment of ¹⁸F-NaF uptake in coronary plaques. Median TBR increases by 80%, leading to the detection of more plaques with significant uptake than are detected using the standard 1-h protocol. A greater than 1-h delay may improve the detection of ¹⁸F-NaF uptake in coronary artery plaques.

Diagnosis and Monitoring of Osteoporosis With 18F-Sodium Fluoride PET: An Unavoidable Path for the Foreseeable Future

The prevalence of metabolic bone diseases particularly osteoporosis and its precursor, osteopenia, continue to grow as serious global health issues today. On a worldwide perspective, 200million people suffer from osteoporosis and in 2005, over 2million fracture incidents were estimated due to osteoporosis in the United States. Currently, osteoporosis and other metabolic bone diseases are evaluated primarily through dual energy X-ray absorptiometry, and rarely by bone biopsy with tetracycline labeling or Technetium-99m (^99mTc) based bone scintigraphy. Deficiencies in these methods have prompted the use of more precise methods of assessment. This review highlights the use of ¹⁸F-sodium fluoride (NaF) with PET (NaF-PET), NaF-PET/CT, or NaF-PET/MRI in the evaluation of osteoporosis and osteopenia in the lumbar spine and hip. This imaging modality provides a molecular perspective with respect to the underlying metabolic alterations that lead to osseous disorders by measuring bone turnover through standardized uptake values. Its sensitivity and ability to examine the entire skeletal system make it a more superior imaging modality compared to standard structural imaging techniques. Further research is needed to determine its accuracy in reflecting the efficacy of therapeutic interventions in metabolic bone diseases.

Role of new imaging modalities in pursuit of the vulnerable plaque and the vulnerable patient

Numerous biomarkers and imaging modalities were investigated during the past few decades to identify patients harboring plaques at high risk of rupturing and causing catastrophic events. The classical description of a vulnerable plaque included a large lipid core, covered by a thin fibrous cap and evidence of inflammation especially around the hinge points of the plaque. Unfortunately, the search has resulted to a large extent in a failure to accurately identify the site of a future event. In time the search focus switched to the vulnerable patient rather than the individual vulnerable plaques, but the debate continues as to the more appropriate approach to risk assessment. This review discusses the most recent developments in molecular, anatomical and functional imaging directed at identifying a patient at high-risk of coronary artery disease events.

Data on analysis of coronary atherosclerosis on computed tomography and 18F-sodium fluoride positron emission tomography

This article contains the data showing illustrative examples of plaque classification on coronary computed tomography angiography (CCTA) and measurement of ¹⁸F-sodium fluoride (¹⁸F-NaF) uptake in coronary atherosclerotic lesions on positron emission tomography (PET). We divided the lesions into one of three plaque types on CCTA (calcified plaque, non-calcified plaque, partially calcified plaque). Focal ¹⁸F-NaF uptake of each lesion was quantified using maximum tissue-to-background ratio. This article also provides a representative case with a non-calcified coronary plaque detected on CCTA and identified on ¹⁸F-NaF PET/non-contrast computed tomography based on a location of a vessel branch as a landmark. These complement the data reported by Kitagawa et al. (2017) [1].