FDG–PET imaging of atherosclerosis: Do we know what we see?

Novel Imaging-Based Biomarkers for Identifying Carotid Plaque Vulnerability

Carotid artery disease has traditionally been assessed based on the degree of luminal narrowing. However, this approach, which solely relies on carotid stenosis, is currently being questioned with regard to modern risk stratification approaches. Recent guidelines have introduced the concept of the "vulnerable plaque," emphasizing specific features such as thin fibrous caps, large lipid cores, intraplaque hemorrhage, plaque rupture, macrophage infiltration, and neovascularization. In this context, imaging-based biomarkers have emerged as valuable tools for identifying higher-risk patients. Non-invasive imaging modalities and intravascular techniques, including ultrasound, computed tomography, magnetic resonance imaging, intravascular ultrasound, optical coherence tomography, and near-infrared spectroscopy, have played pivotal roles in characterizing and detecting unstable carotid plaques. The aim of this review is to provide an overview of the evolving understanding of carotid artery disease and highlight the significance of imaging techniques in assessing plaque vulnerability and informing clinical decision-making.

Various Aspects of Fasting on the Biodistribution of Radiopharmaceuticals

It is demonstrated that fasting can alter the biodistribution of radiopharmaceuticals in nuclear medicine. Various studies have highlighted that fasting is interpreted to be easy for physicians during PET study, fasting is one of the most important factors determining the usefulness of this protocol. It is well documented that fasting can suppress normal ¹⁸F-FDG PET uptake during nuclear cardiology. However, there is no consensus about the usefulness of fasting on radiopharmaceuticals, especially on ¹⁸F-FDG in PET imaging, but special attention should be paid to the setting of the fasting duration. Nevertheless, it does seem we still need extensive clinical studies in the future. The present study aims to review the various aspects of fasting, especially metabolic alteration on radiopharmaceutical biodistribution. In this study, we focused more on the effect of fasting on ¹⁸F-FDG biodistribution, which alters its imaging contrast in cardiology and cancer imaging. Therefore, shifting substrate metabolism from glucose to free fatty acids during fasting can be an alternative approach to suppress physiological myocardial uptake.

PET-Based Imaging with 18F-FDG and 18F-NaF to Assess Inflammation and Microcalcification in Atherosclerosis and Other Vascular and Thrombotic Disorders

Positron emission tomography (PET) imaging with ¹⁸F-fluorodeoxyglucose (FDG) represents a method of detecting and characterizing arterial wall inflammation, with potential applications in the early assessment of vascular disorders such as atherosclerosis. By portraying early-stage molecular changes, FDG-PET findings have previously been shown to correlate with atherosclerosis progression. In addition, recent studies have suggested that microcalcification revealed by ¹⁸F-sodium fluoride (NaF) may be more sensitive at detecting atherogenic changes compared to FDG-PET. In this review, we summarize the roles of FDG and NaF in the assessment of atherosclerosis and discuss the role of global assessment in quantification of the vascular disease burden. Furthermore, we will review the emerging applications of FDG-PET in various vascular disorders, including pulmonary embolism, as well as inflammatory and infectious vascular diseases.

Carotid Plaque Inflammation Assessed by 18F-FDG PET/CT and Lp-PLA2 Is Higher in Symptomatic Patients

Carotid plaque inflammation assessed by 2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) and lipoprotein-associated phospholipase A2 (Lp-PLA₂) levels are higher in symptomatic patients. The aim of this study was to assess correlations between ¹⁸F-FDG uptake on PET scan of carotid artery plaques, plasma levels of Lp-PLA₂, and cerebrovascular symptoms. The study included 45 consecutive patients (22 symptomatic, 23 asymptomatic) with >70% carotid stenosis. Patients were examined by hybrid PET/CT, and maximum standardized uptake values (SUV_max) were recorded. Blood samples were obtained, and plasma was stored at -80 °C for subsequent Lp-PLA₂ analysis. Symptomatic and asymptomatic patients showed no significant difference in classical cardiovascular risk factors. Asymptomatic carotid stenosis patients more frequently had a history of coronary artery disease (P = .025) and peripheral artery disease (P = .012). The symptomatic group had higher ¹⁸F-FDG uptake in carotid plaques (P < .001), higher plasma Lp-PLA₂ (P < .01), and higher high-sensitive C-reactive protein (P = .022). 2-Deoxy-2-[¹⁸F]fluoro-D-glucose uptake on PET/CT and plasma Lp-PLA₂ show a statistically significant association with the symptomatic status of carotid plaques.

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.

Assessing the feasibility of NaF-PET/CT versus FDG-PET/CT to detect abdominal aortic calcification or inflammation in rheumatoid arthritis patients

OBJECTIVE

We aimed to determine whether NaF-PET/CT or FDG-PET/CT can detect abdominal aortic molecular calcification and inflammation in patients with rheumatoid arthritis (RA).

METHODS

In this study, 18 RA patients (4 women, 14 men; mean age 56.0 ± 11.7) and 18 healthy controls (4 women, 14 men; mean age 55.8 ± 11.9) were included. The controls were matched to patients by sex and age (± 4 years). All subjects of this study underwent NaF-PET/CT scanning 90 min following the administration of NaF. FDG-PET/CT imaging was performed 180 min following intravenous FDG injection. Using OsiriX software, the global mean standardized uptake value (global SUVmean) in abdominal aorta was calculated for both FDG and NaF. The NaF SUVmean and FDG SUVmean were divided by the blood pool activity providing target-to-background ratios (TBR) namely, NaF-TBRmean and FDG-TBRmean. The CT calcium volume score was obtained using a growing region algorithm based on Hounsfield units.

RESULTS

The average NaF-TBRmean score among RA patients was significantly greater than that of healthy controls (median 1.61; IQR 1.49-1.88 and median 1.40; IQR 1.23-1.52, P = 0.002). The average CT calcium volume score among RA patients was also significantly greater than that of healthy controls (median 1.96 cm³; IQR 0.57-5.48 and median 0.004 cm³; IQR 0.04-0.05, P < 0.001). There was no significant difference between the average FDG-TBRmean scores in the RA patients when compared to healthy controls (median 1.29; IQR 1.13-1.52 and median 1.29; IQR 1.13-1.52, respectively, P = 0.98).

CONCLUSION

Quantitative assessment with NaF-PET/CT identifies increased molecular calcification in the wall of the abdominal aorta among patients with RA as compared with healthy controls, while quantitative assessment with FDG-PET/CT did not identify a difference in aortic vessel wall FDG uptake between the RA and healthy control groups.

Network Medicine: A Clinical Approach for Precision Medicine and Personalized Therapy in Coronary Heart Disease

Early identification of coronary atherosclerotic pathogenic mechanisms is useful for predicting the risk of coronary heart disease (CHD) and future cardiac events. Epigenome changes may clarify a significant fraction of this "missing hereditability", thus offering novel potential biomarkers for prevention and care of CHD. The rapidly growing disciplines of systems biology and network science are now poised to meet the fields of precision medicine and personalized therapy. Network medicine integrates standard clinical recording and non-invasive, advanced cardiac imaging tools with epigenetics into deep learning for in-depth CHD molecular phenotyping. This approach could potentially explore developing novel drugs from natural compounds (i.e. polyphenols, folic acid) and repurposing current drugs, such as statins and metformin. Several clinical trials have exploited epigenetic tags and epigenetic sensitive drugs both in primary and secondary prevention. Due to their stability in plasma and easiness of detection, many ongoing clinical trials are focused on the evaluation of circulating miRNAs (e.g. miR-8059 and miR-320a) in blood, in association with imaging parameters such as coronary calcifications and stenosis degree detected by coronary computed tomography angiography (CCTA), or functional parameters provided by FFR/CT and PET/CT. Although epigenetic modifications have also been prioritized through network based approaches, the whole set of molecular interactions (interactome) in CHD is still under investigation for primary prevention strategies.

Assessment of atherosclerotic plaque calcification using F18-NaF PET-CT

BACKGROUND

The aim of the present study was to evaluate the uptake of F18-NaF by the arterial wall in patients with high cardiovascular (CV) risk profile. The tracer uptake was assessed in relation to gender and the number of CV risk factors.

METHODS AND RESULTS

25 patients without known CV disease were included and evaluated by PET-CT with F18-NaF: 14 (56%) men and 11 (44%) women. The mean target-to-background ratio (TBR: max SUV/mean blood-pool SUV) but not the corrected uptake per lesion (CUL: max SUV - mean blood-pool SUV) was higher in men than women (TBR: 1.8 ± 0.6 vs 1.7 ± 0.2; P = 0.04; CUL: 0.7 ± 0.3 vs W 0.6 ± 0.1; P = 0.4). Patients with >3 CV risk factors had higher CUL (0.8 ± 0.1 vs 0.6 ± 0.2; P = 0.01) but not TBR (1.8 ± 0.2 vs 1.7 ± 0.6; P = 0.7) than patients with <3 risk factors.

CONCLUSIONS

The TBR but not CUL is higher in men than women while the CUL but not TBR is related to the number of CV risk factors. These results are hypothesis-generating and require validation in larger studies.

Reference values for fluorine-18-fluorodeoxyglucose and fluorine-18-sodium fluoride uptake in human arteries: a prospective evaluation of 89 healthy adults

OBJECTIVE

Reference values of fluorine-18-fluorodeoxyglucose (F-FDG) and fluorine-18-sodium fluoride (F-NaF) uptake in human arteries are unknown. The aim of this study was to determine age-specific and sex-specific reference values of arterial F-FDG and F-NaF uptake.

PARTICIPANTS AND METHODS

Uptake of F-FDG and F-NaF was determined in the ascending aorta, aortic arch, and descending thoracic aorta. In addition, F-FDG uptake was determined in the carotid arteries and F-NaF uptake was determined in the coronary arteries. Arterial F-FDG and F-NaF uptake were quantified as the blood pool subtracted maximum activity concentration in kBq/ml (BS F-FDGmax and BS F-NaFmax, respectively). In addition to determining reference values, we evaluated the influence of age and sex on BS F-FDGmax and BS F-NaFmax.

RESULTS

Arterial F-FDG and F-NaF uptake was assessed in 89 healthy adults aged 21-75 years (mean age: 44±14 years, 53% men). Both BS F-FDGmax and BS F-NaFmax increased with age. BS F-FDGmax increased with age in the descending aorta (β=0.28; P=0.003), whereas BS F-NaFmax increased with age in the ascending aorta (β=0.18; P<0.001), aortic arch (β=0.19; P=0.006), descending aorta (β=0.33; P<0.001), and coronary arteries (β=0.20; P=0.009), respectively. BS F-FDGmax and BS F-NaFmax were not influenced by sex, except for BS F-FDGmax in the ascending aorta.

CONCLUSION

Prospective evaluation of 89 healthy adults generated age-specific and sex-specific reference values of arterial F-FDG and F-NaF uptake. Our findings indicate that arterial F-FDG and F-NaF uptake tend to increase with age.

18 F-Fluorodeoxyglucose-Positron Emission Tomography As an Imaging Biomarker in a Prospective, Longitudinal Cohort of Patients With Large Vessel Vasculitis

OBJECTIVE

To assess the clinical value of ¹⁸ F-fluorodeoxyglucose (FDG) positron emission tomography (PET) in a prospective cohort of patients with large vessel vasculitis (LVV) and comparator subjects.

METHODS

Patients with Takayasu arteritis and giant cell arteritis were studied, along with a comparator group consisting of patients with hyperlipidemia, patients with diseases that mimic LVV, and healthy controls. Participants underwent clinical evaluation and FDG-PET imaging, and patients with LVV underwent serial imaging at 6-month intervals. We calculated sensitivity and specificity of FDG-PET interpretation for distinguishing patients with clinically active LVV from comparator subjects and from patients with disease in clinical remission. A qualitative summary score based on global arterial FDG uptake, the PET Vascular Activity Score (PETVAS), was used to study associations between activity on PET scan and clinical characteristics and to predict relapse.

RESULTS

A total of 170 FDG-PET scans were performed in 115 participants (56 patients with LVV and 59 comparator subjects). FDG-PET distinguished patients with clinically active LVV from comparator subjects with a sensitivity of 85% (95% confidence interval [95% CI] 69, 94) and a specificity of 83% (95% CI 71, 91). FDG-PET scans were interpreted as active vasculitis in most patients with LVV in clinical remission (41 of 71 [58%]). Clinical disease activity status, disease duration, body mass index, and glucocorticoid use were independently associated with activity on PET scan. Among patients who underwent PET during clinical remission, future clinical relapse was more common in patients with a high PETVAS than in those with a low PETVAS (55% versus 11%; P = 0.03) over a median follow-up period of 15 months.

CONCLUSION

FDG-PET provides information about vascular inflammation that is complementary to, and distinct from, clinical assessment in LVV. FDG-PET scan activity during clinical remission was associated with future clinical relapse.

Visualization of Synthetic Vascular Smooth Muscle Cells in Atherosclerotic Carotid Rat Arteries by F-18 FDG PET

Synthetic vascular smooth muscle cells (VSMCs) play important roles in atherosclerosis, in-stent restenosis, and transplant vasculopathy. We investigated the synthetic activity of VSMCs in the atherosclerotic carotid artery using ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography (PET). Atherosclerosis was induced in rats by partial ligation of the right carotid artery coupled with an atherogenic diet and vitamin D injections (2 consecutive days, 600,000 IU/day). One month later, rats were imaged by F-18 FDG PET. The atherosclerotic right carotid arteries showed prominent luminal narrowing with neointimal hyperplasia. The regions with neointimal hyperplasia were composed of α-smooth muscle actin-positive cells with decreased expression of smooth muscle myosin heavy chain. Surrogate markers of synthetic VSMCs such as collagen type III, cyclophilin A, and matrix metallopeptidase-9 were increased in neointima region. However, neither macrophages nor neutrophils were observed in regions with neointimal hyperplasia. F-18 FDG PET imaging and autoradiography showed elevated FDG uptake into the atherosclerotic carotid artery. The inner vessel layer showed higher tracer uptake than the outer layer. Consistently, the expression of glucose transporter 1 was highly increased in neointima. The present results indicate that F-18 FDG PET may be a useful tool for evaluating synthetic activities of VSMCs in vascular remodeling disorders.

Hyaluronan Nanoparticles Selectively Target Plaque-Associated Macrophages and Improve Plaque Stability in Atherosclerosis

Hyaluronan is a biologically active polymer, which can be formulated into nanoparticles. In our study, we aimed to probe atherosclerosis-associated inflammation by using hyaluronan nanoparticles and to determine whether they can ameliorate atherosclerosis. Hyaluronan nanoparticles (HA-NPs) were prepared by reacting amine-functionalized oligomeric hyaluronan (HA) with cholanic ester and labeled with a fluorescent or radioactive label. HA-NPs were characterized in vitro by several advanced microscopy methods. The targeting properties and biodistribution of HA-NPs were studied in apoe^-/- mice, which received either fluorescent or radiolabeled HA-NPs and were examined ex vivo by flow cytometry or nuclear techniques. Furthermore, three atherosclerotic rabbits received ⁸⁹Zr-HA-NPs and were imaged by PET/MRI. The therapeutic effects of HA-NPs were studied in apoe^-/- mice, which received weekly doses of 50 mg/kg HA-NPs during a 12-week high-fat diet feeding period. Hydrated HA-NPs were ca. 90 nm in diameter and displayed very stable morphology under hydrolysis conditions. Flow cytometry revealed a 6- to 40-fold higher uptake of Cy7-HA-NPs by aortic macrophages compared to normal tissue macrophages. Interestingly, both local and systemic HA-NP-immune cell interactions significantly decreased over the disease progression. ⁸⁹Zr-HA-NPs-induced radioactivity in atherosclerotic aortas was 30% higher than in wild-type controls. PET imaging of rabbits revealed 6-fold higher standardized uptake values compared to the muscle. The plaques of HA-NP-treated mice contained 30% fewer macrophages compared to control and free HA-treated group. In conclusion, we show favorable targeting properties of HA-NPs, which can be exploited for PET imaging of atherosclerosis-associated inflammation. Furthermore, we demonstrate the anti-inflammatory effects of HA-NPs in atherosclerosis.

Adaptive Response of T and B Cells in Atherosclerosis

Atherosclerosis is a chronic inflammatory disease that is initiated by the retention and accumulation of cholesterol-containing lipoproteins, particularly low-density lipoprotein, in the artery wall. In the arterial intima, lipoprotein components that are generated through oxidative, lipolytic, and proteolytic activities lead to the formation of several danger-associated molecular patterns, which can activate innate immune cells as well as vascular cells. Moreover, self- and non-self-antigens, such as apolipoprotein B-100 and heat shock proteins, can contribute to vascular inflammation by triggering the response of T and B cells locally. This process can influence the initiation, progression, and stability of plaques. Substantial clinical and experimental data support that the modulation of adaptive immune system may be used for treating and preventing atherosclerosis. This may lead to the development of more selective and less harmful interventions, while keeping host defense mechanisms against infections and tumors intact. Approaches such as vaccination might become a realistic option for cardiovascular disease, especially if they can elicit regulatory T and B cells and the secretion of atheroprotective antibodies. Nevertheless, difficulties in translating certain experimental data into new clinical therapies remain a challenge. In this review, we discuss important studies on the function of T- and B-cell immunity in atherosclerosis and their manipulation to develop novel therapeutic strategies against cardiovascular disease.

Vessel Wall Imaging of the Intracranial and Cervical Carotid Arteries

Vessel wall imaging can depict the morphologies of atherosclerotic plaques, arterial walls, and surrounding structures in the intracranial and cervical carotid arteries beyond the simple luminal changes that can be observed with traditional luminal evaluation. Differentiating vulnerable from stable plaques and characterizing atherosclerotic plaques are vital parts of the early diagnosis, prevention, and treatment of stroke and the neurological adverse effects of atherosclerosis. Various techniques for vessel wall imaging have been developed and introduced to differentiate and analyze atherosclerotic plaques in the cervical carotid artery. High-resolution magnetic resonance imaging (HR-MRI) is the most important and popular vessel wall imaging technique for directly evaluating the vascular wall and intracranial artery disease. Intracranial artery atherosclerosis, dissection, moyamoya disease, vasculitis, and reversible cerebral vasoconstriction syndrome can also be diagnosed and differentiated by using HR-MRI. Here, we review the radiologic features of intracranial artery disease and cervical carotid artery atherosclerosis on HR-MRI and various other vessel wall imaging techniques (e.g., ultrasound, computed tomography, magnetic resonance, and positron emission tomography-computed tomography).

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

This study sought to discover which atherosclerotic plaque components co-localize with enhanced [(18)F]-fluorodeoxyglucose (FDG) uptake in carotid positron emission tomography (PET) images. Although in vivo PET currently lacks the resolution, high-resolution ex vivo FDG-microPET with histology validation of excised carotid plaque might accomplish this goal. Thirteen patients were injected with FDG before carotid endarterectomy. After excision, the plaque specimens were scanned by microPET and magnetic resonance imaging, and then serially sectioned for histological analysis. Two analyses were performed using generalized linear mixed models: (1) a PET-driven analysis which sampled high and low FDG uptake areas from PET images to identify their components in matched histology specimens; and (2) a histology-driven analysis where specific plaque components were selected and matched to corresponding PET images. In the PET-driven analysis, regions of high FDG uptake were more likely to contain inflammatory cells (p < 0.001) and neovasculature (p = 0.008) than regions of low FDG uptake. In the histology-driven analysis, regions with inflammatory cells (p = 0.001) and regions with loose extracellular matrix (p = 0.001) were associated with enhanced FDG uptake. Furthermore, areas of complex inflammatory cell infiltrate (co-localized macrophages, lymphocytes and foam cells) had the highest FDG uptake among inflammatory subgroups (p < 0.001). In conclusion, in carotid plaque, regions of inflammatory cell infiltrate, particularly complex one, co-localized with enhanced FDG uptake in high-resolution FDG-microPET images. Loose extracellular matrix and areas containing neovasculature also produced FDG signal. This study points to the potential ability of FDG-PET to detect the cellular components of the vulnerable plaque.

Aortic ¹⁸F-FDG uptake in patients suffering from granulomatosis with polyangiitis

PURPOSE

The objective of the study was to systematically assess aortic inflammation in patients with granulomatosis with polyangiitis (GPA) using (18)F-2-deoxy-2-[(18)F]fluoro-D-glucose (FDG) positron emission tomography (PET)/CT.

METHODS

Aortic inflammation was studied in PET/CT scans obtained from 21 patients with GPA; 14 patients with sarcoidosis were included as disease controls, 7 patients with stage I or II head and neck carcinoma ascertained during routine clinical practice were used as healthy controls (HC) and 5 patients with large vessel vasculitis (LVV) were used as positive controls. Aortic (18)F-FDG uptake was expressed as the blood-normalized maximum standardized uptake value (SUVmax), known as the target to background ratio (mean TBRmax).

RESULTS

The mean TBRmax (interquartile range) of the aorta in patients with GPA, sarcoidosis, HC and LVV were 1.75 (1.32-2.05), 1.62 (1.54-1.74), 1.29 (1.22-1.52) and 2.03 (1.67-2.45), respectively. The mean TBRmax was significantly higher in patients suffering from GPA or LVV compared to HC (p < 0.05 and p < 0.005, respectively) and tended to be higher in patients suffering from sarcoidosis, but this did not reach statistical significance (p = 0.098). The mean TBRmax of the most diseased segment was significantly higher compared to HC [1.57 (1.39-1.81)] in LVV patients [2.55 (2.22-2.82), p < 0.005], GPA patients [2.17 (1.89-2.83), p < 0.005] and patients suffering from sarcoidosis [2.04 (1.88-2.20), p < 0.05]. In GPA patients, the mean TBRmax of the aorta was significantly higher in patients with previous renal involvement [2.01 (1.69-2.53)] compared to patients without renal involvement in the past [1.60 (1.51-1.80), p < 0.05]. Interrater reproducibility with a second reader was high (all intraclass correlation coefficients >0.9).

CONCLUSION

Patients suffering from GPA show marked aortic FDG uptake.

Contemporary carotid imaging: from degree of stenosis to plaque vulnerability

Carotid artery stenosis is a well-established risk factor of ischemic stroke, contributing to up to 10%-20% of strokes or transient ischemic attacks. Many clinical trials over the last 20 years have used measurements of carotid artery stenosis as a means to risk stratify patients. However, with improvements in vascular imaging techniques such as CT angiography and MR angiography, ultrasonography, and PET/CT, it is now possible to risk stratify patients, not just on the degree of carotid artery stenosis but also on how vulnerable the plaque is to rupture, resulting in ischemic stroke. These imaging techniques are ushering in an emerging paradigm shift that allows for risk stratifications based on the presence of imaging features such as intraplaque hemorrhage (IPH), plaque ulceration, plaque neovascularity, fibrous cap thickness, and presence of a lipid-rich necrotic core (LRNC). It is important for the neurosurgeon to be aware of these new imaging techniques that allow for improved patient risk stratification and outcomes. For example, a patient with a low-grade stenosis but an ulcerated plaque may benefit more from a revascularization procedure than a patient with a stable 70% asymptomatic stenosis with a thick fibrous cap. This review summarizes the current state-of-the-art advances in carotid plaque imaging. Currently, MRI is the gold standard in carotid plaque imaging, with its high resolution and high sensitivity for identifying IPH, ulceration, LRNC, and inflammation. However, MRI is limited due to time constraints. CT also allows for high-resolution imaging and can accurately detect ulceration and calcification, but cannot reliably differentiate LRNC from IPH. PET/CT is an effective technique to identify active inflammation within the plaque, but it does not allow for assessment of anatomy, ulceration, IPH, or LRNC. Ultrasonography, with the aid of contrast enhancement, is a cost-effective technique to assess plaque morphology and characteristics, but it is limited in sensitivity and specificity for detecting LRNC, plaque hemorrhage, and ulceration compared with MRI. Also summarized is how these advanced imaging techniques are being used in clinical practice to risk stratify patients with low- and high-grade carotid artery stenosis. For example, identification of IPH on MRI in patients with low-grade carotid artery stenosis is a risk factor for failure of medical therapy, and studies have shown that such patients may fair better with carotid endarterectomy (CEA). MR plaque imaging has also been found to be useful in identifying revascularization candidates who would be better candidates for CEA than carotid artery stenting (CAS), as high intraplaque signal on time of flight imaging is associated with vulnerable plaque and increased rates of adverse events in patients undergoing CAS but not CEA.

Imaging of small animal peripheral artery disease models: recent advancements and translational potential

Peripheral artery disease (PAD) is a broad disorder encompassing multiple forms of arterial disease outside of the heart. As such, PAD development is a multifactorial process with a variety of manifestations. For example, aneurysms are pathological expansions of an artery that can lead to rupture, while ischemic atherosclerosis reduces blood flow, increasing the risk of claudication, poor wound healing, limb amputation, and stroke. Current PAD treatment is often ineffective or associated with serious risks, largely because these disorders are commonly undiagnosed or misdiagnosed. Active areas of research are focused on detecting and characterizing deleterious arterial changes at early stages using non-invasive imaging strategies, such as ultrasound, as well as emerging technologies like photoacoustic imaging. Earlier disease detection and characterization could improve interventional strategies, leading to better prognosis in PAD patients. While rodents are being used to investigate PAD pathophysiology, imaging of these animal models has been underutilized. This review focuses on structural and molecular information and disease progression revealed by recent imaging efforts of aortic, cerebral, and peripheral vascular disease models in mice, rats, and rabbits. Effective translation to humans involves better understanding of underlying PAD pathophysiology to develop novel therapeutics and apply non-invasive imaging techniques in the clinic.

A pilot trial to examine the effect of high-dose niacin on arterial wall inflammation using fluorodeoxyglucose positron emission tomography

RATIONALE AND OBJECTIVES

Although studies have reported direct inhibition of inflammatory pathways with niacin, the effect of niacin on arterial wall inflammation remains unknown. We examined the effect of niacin on arterial (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT).

MATERIALS AND METHODS

Nine statin-treated patients with coronary disease were randomized to niacin 6000 mg/day or placebo. FDG-PET/CT and lipids were assessed at baseline and at 12 weeks. FDG was quantified in the aorta, right carotid artery, and left carotid artery as the target-to-background ratio (TBR) and target-to-background difference (TBD).

RESULTS

Eight patients completed the study. No significant changes in FDG measured by aortic, left carotid, or right carotid TBR or TBD were seen in either group. Compared to baseline, niacin-treated subjects exhibited a significant 29% reduction in low-density lipoprotein cholesterol (LDL-C; 95% confidence interval [CI], -50% to 8%; P = .01) and a nonsignificant 29% reduction in LDL particle number (LDL-P; 95% CI, -58% to 0.2%; P = .07). A nonsignificant 11% increase in HDL-C (95% CI, -15% to 37%; P = .30) and 8% decrease in HDL-P (95% CI, -44% to 28%; P = .51) were observed with niacin treatment. In a pooled analysis, changes in LDL-P were positively correlated with FDG uptake in the aorta (TBR r = 0.66, P = .08; TBD r = 0.75, P = .03), left carotid (TBR r = 0.65, P = .08; TBD r = 0.74, P = .03), and right carotid (TBR r = 0.54, P = .17; TBD r = 0.61, P = .11).

CONCLUSIONS

In this pilot study, adding niacin to statin therapy did not affect arterial wall inflammation measured by FDG-PET/CT. However, an association between changes in arterial FDG uptake and LDL-P was observed. Larger studies are needed to definitively examine the effect of niacin on arterial wall inflammation.

[¹⁸F]-fluorodeoxyglucose PET imaging of atherosclerosis

[(18)F]-fluorodeoxyglucose PET ((18)FDG PET) imaging has emerged as a promising tool for assessment of atherosclerosis. By targeting atherosclerotic plaque glycolysis, a marker for plaque inflammation and hypoxia, (18)FDG PET can assess plaque vulnerability and potentially predict risk of atherosclerosis-related disease, such as stroke and myocardial infarction. With excellent reproducibility, (18)FDG PET can be a surrogate end point in clinical drug trials, improving trial efficiency. This article summarizes key findings in the literature, discusses limitations of (18)FDG PET imaging of atherosclerosis, and reports recommendations to optimize imaging protocols.

Imaging subclinical atherosclerosis: is it ready for prime time? A review

Imaging subclinical atherosclerosis holds the promise of individualized cardiovascular (CV) risk assessment. The large arsenal of noninvasive imaging techniques available today is playing an increasingly important role in the diagnosis and monitoring of subclinical atherosclerosis. However, there is a debate about the advisability of clinical screens for subclinical atherosclerosis and which modality is the most appropriate for monitoring risk and atherosclerosis progression. This article offers an overview of the traditional and emerging noninvasive imaging modalities used to detect early atherosclerosis, surveys population studies addressing the value of subclinical atherosclerosis detection, and also examines guideline recommendations for their clinical implementation. The clinical relevance of this manuscript lies in the potential of current imaging technology to improve CV risk prediction based on traditional risk factors and the present recommendations for subclinical atherosclerosis assessment. Noninvasive imaging will also help to identify individuals at high CV who would benefit from intensive prevention or therapeutic interventions.

Periaortic brown adipose tissue as a major determinant of [¹⁸F]-fluorodeoxyglucose vascular uptake in atherosclerosis-prone, apoE-/- mice

BACKGROUND

[18F]-fluorodeoxyglucose (FDG) has been suggested for the clinical and experimental imaging of inflammatory atherosclerotic lesions. Significant FDG uptake in brown adipose tissue (BAT) has been observed both in humans and mice. The objective of the present study was to investigate the influence of periaortic BAT on apolipoprotein E-deficient (apoE-/-) mouse atherosclerotic lesion imaging with FDG.

METHODS

ApoE-/- mice (36 ± 2 weeks-old) were injected with FDG (12 ± 2 MBq). Control animals (Group A, n = 7) were injected conscious and kept awake at room temperature (24°C) throughout the accumulation period. In order to minimize tracer activity in periaortic BAT, Group B (n = 7) and C (n = 6) animals were injected under anaesthesia at 37°C and Group C animals were additionally pre-treated with propranolol. PET/CT acquisitions were performed prior to animal euthanasia and ex vivo analysis of FDG biodistribution.

RESULTS

Autoradiographic imaging indicated higher FDG uptake in atherosclerotic lesions than in the normal aortic wall (all groups, P<0.05) and the blood (all groups, P<0.01) which correlated with macrophage infiltration (R = 0.47; P<0.001). However, periaortic BAT uptake was either significantly higher (Group A, P<0.05) or similar (Group B and C, P = NS) to that observed in atherosclerotic lesions and was shown to correlate with in vivo quantified aortic FDG activity.

CONCLUSION

Periaortic BAT FDG uptake was identified as a confounding factor while using FDG for the non-invasive imaging of mouse atherosclerotic lesions.

Noninvasive assessment of hypoxia in rabbit advanced atherosclerosis using ¹⁸F-fluoromisonidazole positron emission tomographic imaging

BACKGROUND

Hypoxia is an important microenvironmental factor influencing atherosclerosis progression by inducing foam-cell formation, metabolic adaptation of infiltrated macrophages, and plaque neovascularization. Therefore, imaging plaque hypoxia could serve as a marker of lesions at risk.

METHODS AND RESULTS

Advanced aortic atherosclerosis was induced in 18 rabbits by atherogenic diet and double balloon endothelial denudation. Animals underwent (18)F-fluoromisonidazole positron emission tomographic and (18)F-fluorodeoxyglucose positron emission tomographic imaging after 6 to 8 months (atherosclerosis induction) and 12 to 16 months (progression) of diet initiation. Four rabbits fed standard chow served as controls. Radiotracer uptake of the abdominal aorta was measured using standardized uptake values. After imaging, plaque hypoxia (pimonidazole), macrophages (RAM-11), neovessels (CD31), and hypoxia-inducible factor-1α were assessed by immunohistochemistry.(18)F-fluoromisonidazole uptake increased with time on diet (standardized uptake value mean, 0.10±0.01 in nonatherosclerotic animals versus 0.20±0.03 [P=0.002] at induction and 0.25±0.03 [P<0.001] at progression). Ex vivo positron emission tomographic imaging corroborated the (18)F-fluoromisonidazole uptake by the aorta of atherosclerotic rabbits. (18)F-fluorodeoxyglucose uptake also augmented in atherosclerotic animals, with an standardized uptake value mean of 0.43±0.02 at induction versus 0.35±0.02 in nonatherosclerotic animals (P=0.031) and no further increase at progression. By immunohistochemistry, hypoxia was mainly located in the macrophage-rich areas within the atheromatous core, whereas the macrophages close to the lumen were hypoxia-negative. Intraplaque neovessels were found predominantly in macrophage-rich hypoxic regions (pimonidazole(+)/hypoxia-inducible factor-1α(+)/RAM-11(+)).

CONCLUSIONS

Plaque hypoxia increases with disease progression and is present in macrophage-rich areas associated with neovascularization. (18)F-fluoromisonidazole positron emission tomographic imaging emerges as a new tool for the detection of atherosclerotic lesions.

Coronary and carotid atherosclerosis: how useful is the imaging?

The recent advancement of imaging modalities has made possible visualization of atherosclerosis disease in all phases of its development. Markers of subclinical atherosclerosis or even the most advanced plaque features are acquired by invasive (IVUS, OCT) and non-invasive imaging modalities (US, MRI, CTA). Determining plaques prone to rupture (vulnerable plaques) might help to identify patients at risk for myocardial infarction or stroke. The most accepted features of plaque vulnerability include: thin cap fibroatheroma, large lipid core, intimal spotty calcification, positive remodeling and intraplaque neovascularizations. Today, research is focusing on finding imaging techniques that are less invasive, less radiation and can detect most of the vulnerable plaque features. While, carotid atherosclerosis can be visualized using noninvasive imaging, such as US, MRI and CT, imaging plaque feature in coronary arteries needs invasive imaging modalities. However, atherosclerosis is a systemic disease with plaque development simultaneously in different arteries and data acquisition in carotid arteries can add useful information for prediction of coronary events.

Optimizing 18F-FDG PET/CT imaging of vessel wall inflammation: the impact of 18F-FDG circulation time, injected dose, uptake parameters, and fasting blood glucose levels

PURPOSE

(18)F-FDG PET is increasingly used for imaging of vessel wall inflammation. However, limited data are available on the impact of methodological variables, i.e. prescan fasting glucose, FDG circulation time and injected FDG dose, and of different FDG uptake parameters, in vascular FDG PET imaging.

METHODS

Included in the study were 195 patients who underwent vascular FDG PET/CT of the aorta and the carotids. Arterial standardized uptake values (meanSUVmax), target-to-background ratios (meanTBRmax) and FDG blood-pool activity in the superior vena cava (SVC) and the jugular veins (JV) were quantified. Vascular FDG uptake values classified according to the tertiles of prescan fasting glucose levels, the FDG circulation time, and the injected FDG dose were compared using ANOVA. Multivariate regression analyses were performed to identify the potential impact of all variables described on the arterial and blood-pool FDG uptake.

RESULTS

Tertile analyses revealed FDG circulation times of about 2.5 h and prescan glucose levels of less than 7.0 mmol/l, showing a favorable relationship between arterial and blood-pool FDG uptake. FDG circulation times showed negative associations with aortic meanSUVmax values as well as SVC and JV FDG blood-pool activity, but positive correlations with aortic and carotid meanTBRmax values. Prescan glucose levels were negatively associated with aortic and carotid meanTBRmax and carotid meanSUVmax values, but were positively correlated with SVC blood-pool uptake. The injected FDG dose failed to show any significant association with vascular FDG uptake.

CONCLUSION

FDG circulation times and prescan blood glucose levels significantly affect FDG uptake in the aortic and carotid walls and may bias the results of image interpretation in patients undergoing vascular FDG PET/CT. The injected FDG dose was less critical. Therefore, circulation times of about 2.5 h and prescan glucose levels less than 7.0 mmol/l should be preferred in this setting.

Neuroimaging of the vulnerable plaque

Plaque vulnerability due to inflammation has been shown to be a participating factor in the degenerative process in the arterial wall that contributes to stenosis and embolism. This is believed to have an important role to play also in the genesis of stroke or cerebrovascular diseases. In order to appropriately screen patients for treatment, there is an absolute need to directly or indirectly visualize both the normal carotid and the suspected plaque. This can be done with a variety of techniques ranging from ultrasound to computed tomography (CT) and magnetic resonance imaging (MRI). In addition to angiographic techniques, direct imaging of the plaque can be done either by ultrasound or by the so-called molecular imaging techniques, i.e. positron emission tomography (PET). These findings, together with other clinical and paraclinical parameters should finally guide the therapeutic choice.

Assessment of inflammation in large arteries with 18F-FDG-PET in elderly

This paper presents repeated measurements of atherosclerosis using bimodality positron emission tomography and computed tomography (PET/CT) with 18F-fluorodeoxyglucose (18F-FDG) to assess its uptake in aorta, iliac and femoral arteries in three groups of elderly subjects classified as normals (N), hypercholesterolemics (H) and with stable angina (A) in a 12 months follow-up (T0 to T12). The subjects in group H were taking rosuvastatin (20mg/d) for 12 months before the second scan. The calcifications in the arteries were determined by CT imaging and the artery PET images were analyzed slice by slice. The standard uptake values (SUVs) for 18F-FDG uptake were classified in two main groups: calcified and non-calcified arteries and each main group comprises six sub-groups for the three subject groups N, H and A, and for the two measurements 12 months apart. Although the calcifications were present at some portions of the arteries in all subjects (23%, 36% and 44% of calcified sites to total sites analyzed, respectively, in groups N, H and A), the results show the most noticeable SUV changes after 12 months was in group N of non-calcified arteries. In the three groups, the calcified arteries showed no significant differences between T0 and T12 while significant differences were observed for the non-calcified arteries. However, there were no significant changes at T12 between groups N and H following rosuvastatin intake in group H. In conclusion, the quantitative analysis with 18F-FDG-PET/CT could be efficient in the localization of the inflammation and evaluation of its progression in atherosclerosis instead of global evaluations with systemic inflammation biomarkers.

PET imaging of chemokine receptors in vascular injury-accelerated atherosclerosis

Atherosclerosis is the pathophysiologic process behind lethal cardiovascular diseases. It is a chronic inflammatory progression. Chemokines can strongly affect the initiation and progression of atherosclerosis by controlling the trafficking of inflammatory cells in vivo through interaction with their receptors. Some chemokine receptors have been reported to play an important role in plaque development and stability. However, the diagnostic potential of chemokine receptors has not yet been explored. The purpose of this study was to develop a positron emitter-radiolabeled probe to image the upregulation of chemokine receptor in a wire-injury-accelerated apolipoprotein E knockout (ApoE(-/-)) mouse model of atherosclerosis.

METHODS

A viral macrophage inflammatory protein II (vMIP-II) was used to image the upregulation of multiple chemokine receptors through conjugation with DOTA for (64)Cu radiolabeling and PET. Imaging studies were performed at 2 and 4 wk after injury in both wire-injured ApoE(-/-) and wild-type C57BL/6 mice. Competitive PET blocking studies with nonradiolabeled vMIP-II were performed to confirm the imaging specificity. Specific PET blocking with individual chemokine receptor antagonists was also performed to verify the upregulation of a particular chemokine receptor. In contrast, (18)F-FDG PET imaging was performed in both models to evaluate tracer uptake. Immunohistochemistry on the injury and sham tissues was performed to assess the upregulation of chemokine receptors.

RESULTS

(15)O-CO PET showed decreased blood volume in the femoral artery after the injury. (64)Cu-DOTA-vMIP-II exhibited fast in vivo pharmacokinetics with major renal clearance. PET images showed specific accumulation around the injury site, with consistent expression during the study period. Quantitative analysis of tracer uptake at the injury lesion in the ApoE(-/-) model showed a 3-fold increase over the sham-operated site and the sites in the injured wild-type mouse. (18)F-FDG PET showed significantly less tracer accumulation than (64)Cu-DOTA-vMIP-II, with no difference observed between injury and sham sites. PET blocking studies identified chemokine receptor-mediated (64)Cu-DOTA-vMIP-II uptake and verified the presence of 8 chemokine receptors, and this finding was confirmed by immunohistochemistry.

CONCLUSION

(64)Cu-DOTA-vMIP-II was proven a sensitive and useful PET imaging probe for the detection of 8 up-regulated chemokine receptors in a model of injury-accelerated atherosclerosis.

Complementarity between (18)F-FDG PET/CT and Ultrasonography or Angiography in Carotid Plaque Characterization

Background and Purpose

To estimate clinical roles of 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) versus angiography and ultrasonography in carotid plaque characterization.

Methods

We characterized two groups of patients with recently (<1 month) symptomatic (n=14; age=71.8±8.6 years, mean±SD) or chronic (n=13, age=68.9±9.0 years) carotid stenosis using a battery of imaging tests: diffusion magnetic resonance (MR) imaging, MR or transfemoral angiography, duplex ultrasonography (DUS), and carotid FDG-PET/computed tomography.

Results

The degree of angiographic stenosis was greater in patients with recently symptomatic carotid plaques (67.5±21.5%) than in patients with chronic carotid plaques (32.4±26.8%, p=0.001). Despite the significant difference in the degree of stenosis, lesional maximum standardized uptake values (maxSUVs) on the carotid FDG-PET did not differ between the recently symptomatic (1.56±0.53) and chronic (1.56±0.34, p=0.65) stenosis groups. However, lesional-to-contralesional maxSUV ratios were higher in the recently symptomatic stenosis group (113±17%) than in the chronic stenosis group (98±10%, p=0.017). The grayscale median value of the lesional DUS echodensities was lower in the recently symptomatic stenosis group (28.2±10.0, n=9) than in the chronic stenosis group (53.9±14.0, n=8; p=0.001). Overall, there were no significant correlations between angiographic stenosis, DUS echodensity, and FDG-PET maxSUV. Case/subgroup analyses suggested complementarity between imaging modalities.

Conclusions

There were both correspondences and discrepancies between the carotid FDG-PET images and DUS or angiography data. Further studies are required to determine whether FDG-PET could improve the clinical management of carotid stenosis.

PET imaging of inflammation biomarkers

Inflammation plays a significant role in many disease processes. Development in molecular imaging in recent years provides new insight into the diagnosis and treatment evaluation of various inflammatory diseases and diseases involving inflammatory process. Positron emission tomography using (18)F-FDG has been successfully applied in clinical oncology and neurology and in the inflammation realm. In addition to glucose metabolism, a variety of targets for inflammation imaging are being discovered and utilized, some of which are considered superior to FDG for imaging inflammation. This review summarizes the potential inflammation imaging targets and corresponding PET tracers, and the applications of PET in major inflammatory diseases and tumor associated inflammation. Also, the current attempt in differentiating inflammation from tumor using PET is also discussed.

Small animal positron emission tomography imaging and in vivo studies of atherosclerosis

Atherosclerosis is a growing health challenge globally, and despite our knowledge of the disease has increased over the last couple of decades, many unanswered questions remain. As molecular imaging can be used to visualize, characterize and measure biological processes at the molecular and cellular levels in living systems, this technology represents an opportunity to investigate some of these questions in vivo. In addition, molecular imaging may be translated into clinical use and eventually pave the way for more personalized treatment regimes in patients. Here, we review the current knowledge obtained from in vivo positron emission tomography studies of atherosclerosis performed in small animals.

(18)F-FDG PET imaging of murine atherosclerosis: association with gene expression of key molecular markers

Aim

To study whether ¹⁸F-FDG can be used for in vivo imaging of atherogenesis by examining the correlation between ¹⁸F-FDG uptake and gene expression of key molecular markers of atherosclerosis in apoE^−/− mice.

Methods

Nine groups of apoE^−/− mice were given normal chow or high-fat diet. At different time-points, ¹⁸F-FDG PET/contrast-enhanced CT scans were performed on dedicated animal scanners. After scans, animals were euthanized, aortas removed, gamma counted, RNA extracted from the tissue, and gene expression of chemo (C-X-C motif) ligand 1 (CXCL-1), monocyte chemoattractant protein (MCP)-1, vascular cell adhesion molecule (VCAM)-1, cluster of differentiation molecule (CD)-68, osteopontin (OPN), lectin-like oxidized LDL-receptor (LOX)-1, hypoxia-inducible factor (HIF)-1α, HIF-2α, vascular endothelial growth factor A (VEGF), and tissue factor (TF) was measured by means of qPCR.

Results

The uptake of ¹⁸F-FDG increased over time in the groups of mice receiving high-fat diet measured by PET and ex vivo gamma counting. The gene expression of all examined markers of atherosclerosis correlated significantly with ¹⁸F-FDG uptake. The strongest correlation was seen with TF and CD68 (p<0.001). A multivariate analysis showed CD68, OPN, TF, and VCAM-1 to be the most important contributors to the uptake of ¹⁸F-FDG. Together they could explain 60% of the ¹⁸F-FDG uptake.

Conclusion

We have demonstrated that ¹⁸F-FDG can be used to follow the progression of atherosclerosis in apoE^−/− mice. The gene expression of ten molecular markers representing different molecular processes important for atherosclerosis was shown to correlate with the uptake of ¹⁸F-FDG. Especially, the gene expressions of CD68, OPN, TF, and VCAM-1 were strong predictors for the uptake.

The use and importance of liposomes in positron emission tomography

Among different imaging modalities, Positron Emission Tomography (PET) gained importance in routine hospital practice depending on ability to diagnose diseases in early stages and tracing of therapy by obtaining metabolic information. The combination of PET with Computed Tomography (CT) forms hybrid imaging modality that gives chance to obtain better images having higher resolution by fusing both functional and anatomical images in the same imaging modality at the same time. Therefore, better contrast agents are essentially needed. The advance in research about developing drug delivery systems as specific nanosized targeted systems gained an additional importance for obtaining better diagnosis and therapy of different diseases. Liposomes appear to be more attractive drug delivery systems in delivering either drugs or imaging ligands to target tissue or organ of diseases with higher accumulation by producing in nano-scale, long circulating by stealth effect and specific targeting by modifying with specific ligands or markers. The combination of positron emitting radionuclides with liposomes are commonly in research level nowadays and there is no commercially available liposome formulation for PET imaging. However by conjugating positron emitter radionuclide with liposomes can form promising diagnostic agents for improved diagnosis and following up treatments by increasing image signal/contrast in the target tissue in lower concentrations by specific targeting as the most important advantage of liposomes. More accurate and earlier diagnosis of several diseases can be obtained even in molecular level with the use of stable and effectively radiolabeled molecular target specific nano sized liposomes with longer half-lived positron emitting radionuclides.

Effects of p38 mitogen-activated protein kinase inhibition on vascular and systemic inflammation in patients with atherosclerosis

OBJECTIVES

This study sought to determine the effects of a p38 mitogen-activated protein kinase inhibitor, losmapimod, on vascular inflammation, by (18)F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography imaging.

BACKGROUND

The p38 mitogen-activated protein kinase cascade plays an important role in the initiation and progression of inflammatory diseases, including atherosclerosis.

METHODS

Patients with atherosclerosis on stable statin therapy (n = 99) were randomized to receive losmapimod 7.5 mg once daily (lower dose [LD]), twice daily (higher dose [HD]) or placebo for 84 days. Vascular inflammation was assessed by FDG positron emission tomography/computed tomography imaging of the carotid arteries and aorta; analyses focused on the index vessel (the artery with the highest average maximum tissue-to-background ratio [TBR] at baseline). Serum inflammatory biomarkers and FDG uptake in visceral and subcutaneous fat were also measured.

RESULTS

The primary endpoint, change from baseline in average TBR across all segments in the index vessel, was not significantly different between HD and placebo (ΔTBR: -0.04 [95% confidence interval [CI]: -0.14 to +0.06], p = 0.452) or LD and placebo (ΔTBR: -0.02 [95% CI: -0.11 to +0.06], p = 0.579). However, there was a statistically significant reduction in average TBR in active segments (TBR ≥1.6) (HD vs. placebo: ΔTBR: -0.10 [95% CI: -0.19 to -0.02], p = 0.0125; LD vs. placebo: ΔTBR: -0.10 [95% CI: -0.18 to -0.02], p = 0.0194). The probability of a segment being active was also significantly reduced for HD when compared with placebo (OR: 0.57 [95% CI: 0.41 to 0.81], p = 0.002). Within the HD group, reductions were observed in placebo-corrected inflammatory biomarkers including high-sensitivity C-reactive protein (% reduction: -28% [95% CI: -46 to -5], p = 0.023) as well as FDG uptake in visceral fat (ΔSUV: -0.05 [95% CI: -0.09 to -0.01], p = 0.018), but not subcutaneous fat.

CONCLUSIONS

Despite nonsignificant changes for the primary endpoint of average vessel TBR, HD losmapimod reduced vascular inflammation in the most inflamed regions, concurrent with a reduction in inflammatory biomarkers and FDG uptake in visceral fat. These results suggest a systemic anti-inflammatory effect. (A Study to Evaluate the Effects of 3 Months Dosing With GW856553, as Assessed FDG-PET/CT Imaging; NCT00633022).

Comparison of 18F-fluoro-deoxy-glucose, 18F-fluoro-methyl-choline, and 18F-DPA714 for positron-emission tomography imaging of leukocyte accumulation in the aortic wall of experimental abdominal aneurysms

OBJECTIVE

Abdominal aortic aneurysm (AAA) is a frequent form of atherothrombotic disease, whose natural history is to enlarge and rupture. Indicators other than AAA diameter would be useful for preventive surgery decision-making, including positron-emission tomography (PET) methods permitting visualization of aortic wall leukocyte activation relevant to prognostic AAA evaluation. In this study, we compare three PET tracers of activated leukocytes, 18F-fluoro-deoxy-glucose (FDG), 18F-fluoro-methyl-choline (FCH), and 18F-DPA714 (a peripheral benzodiazepine receptor antagonist) for in vivo PET quantification of aortic wall inflammation in rat experimental AAAs, in correlation with histopathological studies of lesions.

METHODS

AAAs were induced by orthotopic implantation of decellularized guinea pig abdominal aorta in 46 Lewis rats. FDG-PET (n = 20), FCH-PET (n = 8), or both (n = 12) were performed 2 weeks to 4 months after the graft, 1 hour after tracer injection (30 MBq). Six rats (one of which had FDG-PET) underwent 18F-DPA714-PET. Rats were sacrificed after imaging; AAAs and normal thoracic aortas were cut into axial sections for quantitative autoradiography and histologic studies, including ED1 (macrophages) and CD8 T lymphocyte immunostaining. Ex vivo staining of AAAs and thoracic aortas with 18F-DPA714 and unlabeled competitors was performed.

RESULTS

AAAs developed in 35 out of 46 cases. FCH uptake in AAAs was lower than that of FDG in all cases on imaging, with lower AAA-to-background maximal standardized uptake value (SUV(max)) ratios (1.78 ± 0.40 vs 2.71 ± 0.54; P < .01 for SUV(max) ratios), and lower AAA-to-normal aorta activity ratios on autoradiography (3.52 ± 1.26 vs 8.55 ± 4.23; P < .005). FDG AAA-to-background SUV(max) ratios correlated with the intensity of CD8 + ED1 staining (r = .76; P < .03). FCH AAA-to-background SUV(max) ratios correlated with the intensity of ED1 staining (r = .80; P < .03). 18F-DPA714 uptake was similar in AAAs and in normal aortas, both in vivo and ex vivo.

CONCLUSIONS

In rat experimental AAA, characterized by an important aortic wall leukocytes activity, FDG-PET showed higher sensitivity than FCH-PET and 18F-DPA714-PET to detect activated leukocytes. This enhances potential interest of this tracer for prognostic evaluation of AAA in patients.

Effects of age, diet, and type 2 diabetes on the development and FDG uptake of atherosclerotic plaques

OBJECTIVES

This study investigated the effects of age, duration of a high-fat diet, and type 2 diabetes on atherosclerotic plaque development and uptake of (18)F-fluorodeoxyglucose ((18)F-FDG) in 2 mouse models.

BACKGROUND

The animal's age and start time and duration of a high-fat diet have effects on plaque composition in atherosclerotic mice.

METHODS

The aortas of atherosclerotic low-density lipoprotein receptor deficient mice expressing only apolipoprotein B100 (LDLR(-/-)ApoB(100/100)) and atherosclerotic and diabetic mice overexpressing insulin-like growth factor II (IGF-II/LDLR(-/-)ApoB(100/100)) were investigated at 4, 6, and 12 months of age and older after varying durations of high-fat diet. C57BL/6N mice on normal chow served as controls. Plaque size (intima-to-media ratio), macrophage density (Mac-3 staining), and plaque uptake of (18)F-FDG were studied by means of in vivo positron emission tomography/computed tomography by ex vivo autoradiography and by histological and immunohistochemical methods.

RESULTS

From the ages of 4 to 6 months and 12 months and older, the plaque size increased and the macrophage density decreased. Compared with the controls, the in vivo imaging showed increased aortic (18)F-FDG uptake at 4 and 6 months, but not at 12 months and older. Autoradiography showed focal (18)F-FDG uptake in plaques at all time points (average plaque-to-normal vessel wall ratio: 2.4 ± 0.4, p < 0.001) with the highest uptake in plaques with high macrophage density. There were no differences in the plaque size, macrophage density, or uptake of (18)F-FDG between LDLR(-/-)ApoB(100/100) and IGF-II/LDLR(-/-)ApoB(100/100) mice at any time point.

CONCLUSIONS

The 6-month-old LDLR(-/-)ApoB(100/100) and IGF-II/LDLR(-/-)ApoB(100/100) mice demonstrated highly inflamed, large, and extensive atherosclerotic plaques after 4 months of a high-fat diet, presenting a suitable model for studying the imaging of atherosclerotic plaque inflammation with (18)F-FDG. The presence of type 2 diabetes did not confound evaluation of plaque inflammation with (18)F-FDG.

High-resolution imaging of human atherosclerotic carotid plaques with micro 18F-FDG PET scanning exploring plaque vulnerability

AIMS

FDG-PET can be used to identify vulnerable plaques in atherosclerotic disease. Clinical FDG-PET camera systems are restricted in terms of resolution for the visualization of detailed inflammation patterns in smaller vascular structures. The aim of the study is to evaluate the possible added value of a high-resolution microPET system in excised carotid plaques using FDG.

METHODS AND RESULTS

In this study, 17 patients with planned carotid endarterectomy were included. Excised plaques were incubated in FDG and subsequently imaged with microPET. Macrophage presence in plaques was evaluated semi-quantitatively by immunohistochemistry. Plaque calcification was assessed additionally with CT and correlated to FDG uptake. Finally, FDG uptake and macrophage infiltration were compared with patient symptomatology. Heterogeneous distributions and variable intensities of FDG uptake were found within the plaques. A positive correlation between the distribution of macrophages and the FDG uptake (r = 0.68, P < .01) was found. A negative correlation was found between areas of calcifications and FDG uptake (r = -0.84, P < .001). Ratio FDG(max) values as well as degree of CD68 accumulation were significantly higher in CVA patients compared with TIA or amaurosis fugax patients (P < .05) and CVA patients compared with asymptomatic patients (P < .05).

CONCLUSION

This ex vivo study demonstrates that excised carotid plaques can be visualized in detail using FDG microPET. Enhancement of clinical PET/CT resolution for similar imaging results in patients is needed.

What can be seen by 18F-FDG PET in atherosclerosis imaging? The effect of foam cell formation on 18F-FDG uptake to macrophages in vitro

(18)F-FDG PET is a promising tool for detecting vulnerable plaques, depending on the extent of macrophage infiltration; however, it is still not clear which stage of the lesion can be detected by (18)F-FDG PET.

METHODS

In this study, we investigated the effect of foam cell formation on (18)F-FDG uptake using cultured mouse peritoneal macrophages.

RESULTS

(18)F-FDG accumulation was increased by foam cell formation, but the uptake was decreased to the control level after complete differentiation to foam cells. Changes in hexokinase activity tended to accompany changes in (18)F-FDG uptake. In contrast, changes in glucose-6-phosphatase activity and glucose transporter 1 expression did not parallel (18)F-FDG uptake.

CONCLUSION

Our results suggest that (18)F-FDG PET detects the early stage of foam cell formation in atherosclerosis.

PET imaging of aortic atherosclerosis: Is combined imaging of plaque anatomy and function an amaranthine quest or conceivable reality?

Traditionally, blood vessels have been studied using contrast luminography to determine the site, extent and severity of luminal compromise by atherosclerotic deposits. Similar anatomical data can now be acquired non-invasively using ultrasound, computed tomography or magnetic resonance imaging. Plaque stability is an important determinant of subsequent vascular events and currently functional data on the stability of plaque is less well provided by these methods. The search for non-invasive techniques to image combined plaque anatomy and function has been pursued with visionary anticipation. This expectation may soon be realised as imaging with radionuclide-labelled atheroma-targeted contrast agents has demonstrated that plaque functional characteristics can now be shown. Increasingly positron emission tomography/computed tomography (PET/CT) imaging with (18)F fluorodexoyglucose (FDG) and other radionuclides is being used to determine culprit plaques in complex clinically scenarios. Clinically, this information may prove extremely valuable in the assessment of stable and unstable patients and its use in prime time medical practice is eagerly awaited. We will discuss the current clinical applications of functional atheroma imaging in the aorta and highlight the promising preclinical data on novel image biomarkers of plaque instability. If clinical science is able to successfully translate these advances in vascular imaging from the bench to the bedside, a new paradigm will be achieved in cardiovascular diagnostics.

Noninvasive Positron Emission Tomography Imaging of Coronary Arterial Inflammation

The importance of inflammation to atherothrombosis has led to the pursuit of noninvasive imaging methods to measure inflammation within the arterial wall. There is substantial evidence supporting the use of (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) imaging for evaluation of atherosclerotic plaque inflammation. However, coronary imaging with this technique has been limited, due to several technical hurdles. Nonetheless, early experiences in coronary FDG-PET imaging have been encouraging. This review outlines the development of vascular PET imaging and its potential use for evaluation of coronary artery disease.

Excessive aortic inflammation in chronic obstructive pulmonary disease: an 18F-FDG PET pilot study

Chronic obstructive pulmonary disease (COPD) patients exhibit increased cardiovascular risk, even after controlling for smoking. Inflammation may underlie this observation.

METHODS

We measured vascular inflammation in both COPD patients and controls using (18)F-FDG PET/CT. Aortic inflammation was expressed as the target-to-background ratio (TBR) of the standardized uptake value in 7 COPD patients, 5 metabolic syndrome patients, and 7 ex-smokers.

RESULTS

Abdominal aortic mean TBR (+/-SD) was greater in COPD patients than in ex-smoker controls (1.60 +/- 0.13 vs. 1.34 +/- 0.15, P = 0.0001). Aortic arch and abdominal aorta mean TBRs were higher in metabolic syndrome patients than in COPD patients (aortic arch, 1.80 +/- 0.18 vs. 1.53 +/- 0.18, P = 0.001, and abdominal aorta, 1.71 +/- 0.14 vs. 1.60 +/- 0.13, P = 0.001).

CONCLUSION

COPD patients exhibited aortic inflammation that fell between the aortic inflammation exhibited by ex-smokers and that by metabolic syndrome patients. This may in part explain the increased risk of cardiovascular disease in COPD patients.

Imaging atherosclerotic plaque inflammation by fluorodeoxyglucose with positron emission tomography: ready for prime time?

Inflammation is a determinant of atherosclerotic plaque rupture, the event leading to most myocardial infarctions and strokes. Although conventional imaging techniques identify the site and severity of luminal stenosis, the inflammatory status of the plaque is not addressed. Positron emission tomography imaging of atherosclerosis using the metabolic marker fluorodeoxyglucose allows quantification of arterial inflammation across multiple vessels. This review sets out the background and current and potential future applications of this emerging biomarker of cardiovascular risk, along with its limitations.