Quantitative assessment of the atherosclerotic burden of the aorta by combined FDG-PET and CT image analysis: a new concept

The value of FDG-PET/CT imaging in the assessment, monitoring, and management of COVID-19

The pathogenesis of Coronavirus Disease 2019 (COVID-19) involves cytokine-driven recruitment and accumulation of inflammatory cells at sites of infection. These activated neutrophils, monocytes, and effector T cells are highly glycolytic and thus appear as [18]F-labeled fluorodeoxyglucose (FDG) avid sites on positron emission tomography (PET) imaging. FDG-PET-computed tomography (FDG-PET/CT) is a highly sensitive modality for the detection, monitoring, and assessing response related to COVID-19 disease activity that holds significant clinical relevance. To date, concerns over cost, access, and undue radiation exposure have limited the use of FDG-PET/CT in COVID-19 to a small number of individuals where PET-based interventions were already indicated. In this review, we summarize the existing literature on the use of FDG-PET in the detection and monitoring of COVID-19 with particular focus on several areas of clinical relevance that warrant future research: (1) incidental early detection of subclinical COVID-19 in patients who have undergone FDG-PET for other underlying diseases, (2) standardized quantitative assessment of COVID-19 disease burden at specific points in time, and (3) analysis of FDG-PET/CT data leading to better characterization of COVID-19 pathogenesis. Employing FDG-PET/CT for these purposes may allow for the earliest detection of COVID-19-associated venous thromboembolism (VTE), standardized monitoring of disease progression and response to treatment, and better characterization of the acute and chronic complications of this disease.

Association of Apremilast With Vascular Inflammation and Cardiometabolic Function in Patients With Psoriasis: The VIP-A Phase 4, Open-label, Nonrandomized Clinical Trial

Importance

Psoriasis is an inflammatory condition associated with metabolic and cardiovascular disease. Apremilast, a phosphodiesterase 4 inhibitor, is commonly used for psoriasis and can cause weight loss.

Objective

To determine the association between apremilast and aortic vascular inflammation as assessed by 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT), cardiometabolic markers (primary outcomes at week 16), and abdominal fat composition.

Design, Setting, and Participants

A single-arm, open-label, interventional, nonrandomized clinical trial in which the imaging and laboratory outcomes were measured by an investigator who was blinded to time was conducted between April 11, 2017, and August 17, 2021, at 7 dermatology sites in the United States. A total of 101 patients with moderate to severe psoriasis were screened, 70 enrolled, 60 completed week 16, and 39 completed week 52.

Intervention

Apremilast, 30 mg, twice daily.

Main Outcomes and Measures

Aortic vascular inflammation (measured by FDG-PET/CT), 68 cardiometabolic biomarkers, and abdominal fat composition (measured by CT) at week 16 and week 52 compared with baseline.

Results

The mean (SD) age of the 70 patients was 47.5 (14.6) years, 54 were male (77.1%), 4 were Black (5.7%), and 58 were White (82.9%). There was no change in aortic vascular inflammation at week 16 (target to background ratio, -0.02; 95% CI, -0.08 to 0.05; P = .61) or week 52 (target to background ratio, -0.07; 95% CI, -0.15 to 0.01; P = .09) compared with baseline. At week 16, potentially beneficial decreases in interleukin 1b, valine, leucine, isoleucine, fetuin A, and branched-chain amino acids were observed. At week 52 compared with baseline, potentially beneficial decreases in ferritin, β-hydroxybutyrate, acetone, and ketone bodies, with an increase in apolipoprotein A-1, were observed, but there was a reduction in cholesterol efflux. There was an approximately 5% to 6% reduction in subcutaneous and visceral adiposity at week 16 that was maintained at week 52.

Conclusions and Relevance

The findings of this nonrandomized clinical trial suggest that apremilast has a neutral association with aortic vascular inflammation, variable but generally beneficial associations with a subset of cardiometabolic biomarkers, and associations with reductions in visceral and subcutaneous fat, indicating that the drug may have an overall benefit for patients with cardiometabolic disease and psoriasis.

Trial Registration

ClinicalTrials.gov Identifier: NCT03082729.

PET-CTBased Quantitative Parameters for Assessment of Treatment Response and Disease Activity in Cancer and Noncancerous Disorders

The various semiquantitative and quantitative PET-CT parameters provide measurement of disease activity and assessment of treatment response in the PET-CT studies. These include standardized uptake value (SUV), metabolic tumor volume (MTV) and total lesion glycolysis (TLG), and total metabolic tumor volume (TMTV). Thresholding and adaptive thresholding methods are commonly used algorithms for the evaluation of global disease activity. Readily available commercial software frequently in-built with the current generation PET-CT scanners for providing easy, less time consuming, highly reproducible, and more accurate measurement of global disease activity on PET-CT imaging in evaluation of malignant as well as benign disorders.

Potential and Most Relevant Applications of Total Body PET/CT Imaging

ABSTRACT

The introduction of total body (TB) PET/CT instruments over the past 2 years has initiated a new and exciting era in medical imaging. These instruments have substantially higher sensitivity (up to 68 times) than conventional modalities and therefore allow imaging the entire body over a short period. However, we need to further refine the imaging protocols of this instrument for different indications. Total body PET will allow accurate assessment of the extent of disease, particularly, including the entire axial and appendicular skeleton. Furthermore, delayed imaging with this instrument may enhance the sensitivity of PET for some types of cancer. Also, this modality may improve the detection of venous thrombosis, a common complication of cancer and chemotherapy, in the extremities and help prevent pulmonary embolism. Total body PET allows assessment of atherosclerotic plaques throughout the body as a systematic disease. Similarly, patients with widespread musculoskeletal disorders including both oncologic and nononcologic entities, such as degenerative joint disease, rheumatoid arthritis, and osteoporosis, may benefit from the use of TB-PET. Finally, quantitative global disease assessment provided by this approach will be superior to conventional measurements, which do not reflect overall disease activity. In conclusion, TB-PET imaging may have a revolutionary impact on day-to-day practice of medicine and may become the leading imaging modality in the future.

Imaging Atherosclerosis by PET, With Emphasis on the Role of FDG and NaF as Potential Biomarkers for This Disorder

Molecular imaging has emerged in the past few decades as a novel means to investigate atherosclerosis. From a pathophysiological perspective, atherosclerosis is characterized by microscopic inflammation and microcalcification that precede the characteristic plaque buildup in arterial walls detected by traditional assessment methods, including anatomic imaging modalities. These processes of inflammation and microcalcification are, therefore, prime targets for molecular detection of atherosclerotic disease burden. Imaging with positron emission tomography/computed tomography (PET/CT) using 18F-fluorodeoxyglucose (FDG) and 18F-sodium fluoride (NaF) can non-invasively assess arterial inflammation and microcalcification, respectively. FDG uptake reflects glucose metabolism, which is particularly increased in atherosclerotic plaques retaining macrophages and undergoing hypoxic stress. By contrast, NaF uptake reflects the exchange of hydroxyl groups of hydroxyapatite crystals for fluoride producing fluorapatite, a key biochemical step in calcification of atherosclerotic plaque. Here we review the existing literature on FDG and NaF imaging and their respective values in investigating the progression of atherosclerotic disease. Based on the large volume of data that have been introduced to the literature and discussed in this review, it is clear that PET imaging will have a major role to play in assessing atherosclerosis in the major and coronary arteries. However, it is difficult to draw definitive conclusions on the potential role of FDG in investigating atherosclerosis given the vast number of studies with different designs, image acquisition methods, analyses, and interpretations. Our experience in this domain of research has suggested that NaF may be the tool of choice over FDG in assessing atherosclerosis, especially in the setting of coronary artery disease (CAD). Specifically, global NaF assessment appears to be superior in detecting plaques in tissues with high background FDG activity, such as the coronary arteries.

A Randomized Placebo-Controlled Trial of Secukinumab on Aortic Vascular Inflammation in Moderate-to-Severe Plaque Psoriasis (VIP-S)

Psoriasis, a chronic immune-mediated disease, is associated with an increased risk of cardiovascular events and mortality. Secukinumab selectively neutralizes IL-17A and has reported high efficacy with a favorable safety profile in various psoriatic disease manifestations. Subsequent to the 12-week randomized, placebo-controlled, double-blind treatment period, patients with moderate-to-severe psoriasis received secukinumab for 40 weeks. Vascular inflammation using 18F-2-fluorodeoxyglucose-positron emission tomography/computed tomography imaging and blood-based cardiometabolic was assessed at week 0, 12, and 52. The difference in change in aortic inflammation from baseline to week 12 for secukinumab (n = 46) versus placebo (n = 45) was -0.053 (95% confidence interval = -0.169 to 0.064; P= 0.37). Small increases in total cholesterol, low-density lipoprotein, and low-density lipoprotein particles, but no changes in markers of inflammation, adiposity, insulin resistance, or predictors of diabetes, were observed with secukinumab treatment compared with placebo. At week 52, reductions in TNF-α (P= 0.0063) and ferritin (P= 0.0354), and an increase in fetuin-A (P= 0.0024), were observed with secukinumab treatment compared with baseline. No significant changes in aortic inflammation or markers of advanced lipoprotein characterization, adiposity, or insulin resistance were observed with secukinumab treatment compared with baseline. Secukinumab exhibited a neutral impact on aortic vascular inflammation and biomarkers of cardiometabolic disease after 52 weeks of treatment.

Potential Applications of PET-Based Novel Quantitative Techniques in Pediatric Diseases and Disorders

The progress made in hybrid PET imaging during the past decades has significantly expanded the role of this modality in both clinical and research applications. Semi-quantitative PET/CT has been the workhorse of clinical PET/CT due to its simplicity and availability. In addition to semi-quantitative PET/CT, volumetric PET and global metabolic activity have recently shown promise in a more accurate assessment of various diseases. PET/CT has been widely used in pediatric oncologic and non-oncologic diseases. Here we have highlighted few of the pitfalls in the quantitative PET/CT and their potential remedies which have potential in PET/CT evaluation of pediatric diseases.

PET Scan with Fludeoxyglucose/Computed Tomography in Low-Grade Vascular Inflammation

Fluorodeoxyglucose-PET/computed tomography combines the high sensitivity of PET with the excellent spatial resolution provided by computed tomography, making it a potentially powerful tool for capturing and quantifying early vascular diseases. Patients with chronic inflammatory states have an increased risk of cardiovascular events; there is also increased vascular fluorodeoxyglucose uptake seen compared with healthy controls. This review examines the use of fluorodeoxyglucose-PET/computed tomography in assessing low-grade vascular inflammation in chronic inflammation and then reviews fluorodeoxyglucose-PET/computed tomography as a tool in monitoring the efficacy of various treatments known to modulate cardiovascular disease.

A Phase IV, Randomized, Double-Blind, Placebo-Controlled Crossover Study of the Effects of Ustekinumab on Vascular Inflammation in Psoriasis (the VIP-U Trial)

Psoriasis is a T helper type 17 autoimmune disease associated with an increased risk cardiovascular events and mortality. Ustekinumab, an antibody to p40, blocks cytokines IL-12 and IL-23, and is a highly effective and safe treatment for psoriasis. We conducted a randomized double-blinded placebo-controlled trial to determine the effect of ustekinumab on aortic vascular inflammation (AVI) measured by imaging, and key biomarkers of inflammation, lipid, and glucose metabolism in the blood of patients with moderate-to-severe psoriasis. A total of 43 patients were randomized, and at week 12, ustekinumab-treated patients had a -18.65% (95% confidence interval = -29.45% to -7.85%) reduction in AVI, a reduction in inflammatory biomarkers, and an increase in apolipoprotein B lipoproteins compared with placebo. At week 12, placebo patients were crossed over such that all patients received ustekinumab for 52 weeks. At the end of 52 weeks of ustekinumab treatment, there was no change in AVI compared with baseline, inflammatory markers were reduced, and there were increases in selected measures of lipids and leptin. These results show that blockade of IL-12 and/or IL-23 may transiently reduce AVI, with more durable reduction in inflammatory cytokines associated with cardiovascular disease.

18F-FDG PET/CT for the quantification of inflammation in large carotid artery plaques

BACKGROUND

There is currently no consensus on the methodology for quantification of 18F-FDG uptake in inflammation in atherosclerosis. In this study, we explore different methods for quantification of 18F-FDG uptake in carotid atherosclerotic plaques and correlate the uptake values to histological assessments of inflammation.

METHODS AND RESULTS

Forty-four patients with atherosclerotic stenosis ≥70% of the internal carotid artery underwent 18F-FDG PET/CT. Maximum standardized uptake values (SUVmax) from all plaque-containing slices were collected. SUVmax for the single highest and the mean of multiple slices with and without blood background correction (by subtraction (cSUV) or by division (target-to-background ratio (TBR)) were calculated. Following endarterectomy 30 plaques were assessed histologically. The length of the plaques at CT was 6-32 mm. The 18F-FDG uptake in the plaques was 1.15-2.66 for uncorrected SUVs, 1.16-3.19 for TBRs, and 0.20-1.79 for cSUVs. There were significant correlations between the different uptake values (r = 0.57-0.99, P < 0.001). Methods with and without blood background correction showed similar, moderate correlations to the amount of inflammation assessed at histology (r = 0.44-0.59, P < 0.02).

CONCLUSIONS

In large stenotic carotid plaques, 18F-FDG uptake reflects the inflammatory status as assessed at histology. Increasing number of PET slices or background correction did not change the correlation.

Novel Quantitative PET Techniques for Clinical Decision Support in Oncology

Quantitative image analysis has deep roots in the usage of positron emission tomography (PET) in clinical and research settings to address a wide variety of diseases. It has been extensively employed to assess molecular and physiological biomarkers in vivo in healthy and disease states, in oncology, cardiology, neurology, and psychiatry. Quantitative PET allows relating the time-varying activity concentration in tissues/organs of interest and the basic functional parameters governing the biological processes being studied. Yet, quantitative PET is challenged by a number of degrading physical factors related to the physics of PET imaging, the limitations of the instrumentation used, and the physiological status of the patient. Moreover, there is no consensus on the most reliable and robust image-derived PET metric(s) that can be used with confidence in clinical oncology owing to the discrepancies between the conclusions reported in the literature. There is also increasing interest in the use of artificial intelligence based techniques, particularly machine learning and deep learning techniques in a variety of applications to extract quantitative features (radiomics) from PET including image segmentation and outcome prediction in clinical oncology. These novel techniques are revolutionizing clinical practice and are now offering unique capabilities to the clinical molecular imaging community and biomedical researchers at large. In this report, we summarize recent developments and future tendencies in quantitative PET imaging and present example applications in clinical decision support to illustrate its potential in the context of clinical oncology.

A novel method to assess subchondral bone formation using [18F]NaF-PET in the evaluation of knee degeneration

PURPOSE

Fluorine-18-sodium fluoride-PET ([F]NaF-PET) facilitates direct assessment of subchondral bone formation to evaluate degeneration in articulating joints. No standards exist for the quantification of joint activity using [F]NaF-PET, and many techniques rely on focal uptake to characterize an entire region of interest. This study proposes a novel method of quantitative global knee analysis to assess regions of expected bone remodeling in the evaluation of knee degeneration.

PATIENTS AND METHODS

The study population consisted of 18 patients with rheumatoid arthritis who underwent [F]NaF-PET/computed tomography imaging. The maximum standardized uptake value (knee SUVmax) in addition to a target-to-background ratio (TBR) that represents global knee activity adjusted for systemic bone formation measured at the lateral femoral neck (global knee TBR) were calculated. A radiologist scored standard radiographs of the knee in nine patients using the Kellgren-Lawrence grading system.

RESULTS

Patients with greater [F]NaF uptake demonstrated greater knee deterioration, which was corroborated by the radiograph findings. Average Kellgren-Lawrence grading was strongly associated with both global knee TBR (Spearman ρ=0.69, P=0.04) and knee SUVmax scores (Spearman ρ=0.93, P=0.0003).

CONCLUSION

Assessment of global activity within the joint is a feasible and clinically useful technique for characterizing disease activity with a single value. Furthermore, a ratio based on systemic bone turnover in a nonarticulating, weight-bearing site adjusts for differences in bone formation related to bodyweight or metabolic bone diseases. We hypothesize that a global knee TBR score may be more sensitive at detecting changes in disease progression, as new spatially distinct lesions with a lower SUV that develop within an region of interest would not be detected by the SUVmax methodology. Longitudinal studies assessing sensitivity with larger patient cohorts are needed to further validate this methodology.

18F-NaF and 18F-FDG as molecular probes in the evaluation of atherosclerosis

The early detection of atherosclerotic disease is vital to the effective prevention and management of life-threatening cardiovascular events such as myocardial infarctions and cerebrovascular accidents. Given the potential for positron emission tomography (PET) to visualize atherosclerosis earlier in the disease process than anatomic imaging modalities such as computed tomography (CT), this application of PET imaging has been the focus of intense scientific inquiry. Although ¹⁸F-FDG has historically been the most widely studied PET radiotracer in this domain, there is a growing body of evidence that ¹⁸F-NaF holds significant diagnostic and prognostic value as well. In this article, we review the existing literature on the application of ¹⁸F-FDG and ¹⁸F-NaF as PET probes in atherosclerosis and present the findings of original animal and human studies that have examined how well ¹⁸F-NaF uptake correlates with vascular calcification and cardiovascular risk.

Psoriasis as a human model of disease to study inflammatory atherogenesis

Inflammation is known to play a significant role in the process of atherogenesis and cardiovascular disease (CVD). Indeed, patients with chronic inflammatory diseases are at increased risk for cardiovascular events. However, the mechanisms linking chronic inflammation and CVD remain poorly understood. Psoriasis, a chronic inflammatory skin disease associated with a greater risk of early cardiovascular events, provides a suitable human model to study the pathophysiology of inflammatory atherogenesis in humans. Additionally, cytokines such as TNF-α, IL-17A, and other immune pathways are the common links between the pathogenesis of psoriasis and atherosclerosis, and hence the approved treatments for psoriasis, which include selective cytokine inhibition (e.g., anti-TNF, anti-IL-17A, and anti-IL-12/23) and immune modulation (e.g., methotrexate or cyclosporine), provide an opportunity to examine the effect of modulating these pathways on atherogenesis. We have been using this human model in a large, prospective cohort study, and this review summarizes our approach and results of using this human model to study inflammatory atherogenesis. Specifically, we review simultaneous multimodal imaging of several vascular beds using ¹⁸fludeoxyglucose positron emission tomography/computed tomography, ¹⁸fludeoxyglucose positron emission tomography/MRI, and coronary computed tomography angiography as well as cardiovascular biomarkers to better understand how modulation of inflammation may impact vascular diseases.

Quantification of aging effects upon global knee inflammation by 18F-FDG-PET

OBJECTIVE

The goal of this study was to quantify aging effects upon the global knee joint and surrounding capsule and soft tissue inflammation using fluorine-18 fluorodeoxyglucose (18F-FDG) PET imaging.

METHODS

This reanalysis of a prospective study included 64 patients who had undergone 18F-FDG-PET for evaluation of hip joint prostheses, and whose scans included the knee joints in the field of view. Mean patient age was 53 years (range: 33-84 years). A fixed-sized three-dimensional region of interest was placed around each knee joint, paying close attention to exclude the popliteal vessels. 18F-FDG-avid regions in each knee joint were then segmented using an adaptive contrast-oriented thresholding method, and metabolically active volume (MAV), mean standardized uptake value (SUV mean), partial volume-corrected SUV mean (cSUV mean), and partial volume-corrected mean metabolic volumetric product (cMVP mean = cSUV mean × MAV) of the segmented regions were calculated. Finally, global knee inflammation (GKI) for each knee joint was calculated as the sum of cMVP mean in all segmented regions. Association of GKI with age was assessed with Pearson's correlation and linear regression methods, and GKI was compared between patients at different ages - between patients younger than 55 years and those older than 55 years - using the unpaired t-test.

RESULTS

The correlation coefficient of GKI with advancing age was 0.57 (P = 0.02). In the linear regression model, considering GKI as the dependent variable and age and sex as independent covariates, the β coefficient of age was 2.1 (95% confidence interval: 1.1-3.2). For patients aged younger than 55 years versus those aged older than 55 years, the mean GKI was 157 and 190 cm3, respectively (P = 0.01).

CONCLUSION

Through the use of novel quantitative techniques, we were able to calculate GKI and demonstrate a significant increase in the entity of joint inflammation with advancing age. As degenerative disease is age-related and inflammation is implicated in its pathogenesis, our findings further support this association. These preliminary data suggest that this approach can potentially provide a means to objectively quantify the degree of inflammation in various joint disorders, and possibly in other knee degenerative/inflammatory diseases.

Assessment of atherosclerosis in large vessel walls: A comprehensive review of FDG-PET/CT image acquisition protocols and methods for uptake quantification

There is growing evidence showing the importance of fluorodeoxyglucose positron emission tomography (FDG-PET) in the evaluation of vessel wall inflammation and atherosclerosis. Although this imaging modality has been increasingly used, there are various methods for image acquisition and evaluating FDG uptake activity in the vessel walls and atherosclerotic lesions, including qualitative visual scaling, semi-quantitative, and quantitative evaluations. Using each of these image acquisition protocols and measurement methods may result in different findings. In this review, we are going to describe the various image acquisition methods and common measurement strategies reflected in the literature and discuss their advantages and flaws.

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.

Variability and uncertainty of 18F-FDG PET imaging protocols for assessing inflammation in atherosclerosis: suggestions for improvement

PET with (18)F-FDG shows promise for the evaluation of metabolic activities in atherosclerotic plaques. Although recommendations regarding the acquisition and measurement protocols to be used for (18)F-FDG PET imaging of atherosclerosis inflammation have been published, there is no consensus regarding the most appropriate protocols, and the image reconstruction approach has been especially overlooked. Given the small size of the targeted lesions, the reconstruction and measurement methods might strongly affect the results. We determined the differences in results due to the protocol variability and identified means of increasing the measurement reliability.

METHODS

An extensive literature search was performed to characterize the variability in atherosclerosis imaging and quantification protocols. Highly realistic simulations of atherosclerotic carotid lesions based on real patient data were designed to determine how the acquisition and processing protocol parameters affected the measured values.

RESULTS

In 49 articles, we identified 53 different acquisition protocols, 51 reconstruction protocols, and 46 quantification methods to characterize atherosclerotic lesions from (18)F-FDG PET images. The most important parameters affecting the measurement accuracy were the number of iterations used for reconstruction and the postfiltering applied to the reconstructed images, which could together make the measured standardized uptake values (SUVs) vary by a factor greater than 3. Image sampling, acquisition duration, and metrics used for the measurements also affected the results to a lesser extent (SUV varying by a factor of 1.3 at most). For an acceptable SUV variability, the lowest bias in SUV was observed using an 8-min acquisition per bed position; ordered-subset expectation maximization reconstruction with at least 120 maximum likelihood expectation maximization equivalent iterations, including a point spread function model using a 1 mm(3) voxel size; and no postfiltering. Because of the partial-volume effect, measurement bias remained greater than 60%. The use and limitations of the target-to-blood activity ratio metrics are also presented and discussed.

CONCLUSION

(18)F-FDG PET protocol harmonization is needed in atherosclerosis imaging. Optimized protocols can significantly reduce the measurement errors in wall activity estimates, but PET systems with higher spatial resolution and advanced partial-volume corrections will be required to accurately assess plaque inflammation from (18)F-FDG PET.

Assessment of global cardiac uptake of radiolabeled iron oxide nanoparticles in apolipoprotein-E-deficient mice: implications for imaging cardiovascular inflammation

PURPOSE

Atherosclerosis is a leading cause of death in industrialized countries and is characterized by the accumulation of lipids and inflammatory cells, including macrophages, in blood vessel walls. Therefore, the ability to image macrophages could help identify plaques that are precursors of acute thrombotic events. Previous research has shown that long-circulating nanoparticles could be used to detect macrophages within atherosclerotic plaques of the aorta. By conducting this study, we investigated whether global cardiac uptake of radiolabeled nanoparticles could allow assessment of total macrophage burden in the coronary arteries.

PROCEDURES

Dextran-coated iron oxide nanoparticles (IONPs) were labeled with iodine-125 via Bolton-Hunter (sulfosuccinimidyl-3-[4-hydroxyphenyl]propionate) method. IONPs were characterized by means of dynamic light scattering and transmission electronic microscopy. Biodistribution studies were performed in healthy and atherosclerotic mice. Additionally, digital autoradiography of hearts from both healthy and atherosclerotic mice was performed to assess regional and global atherosclerotic burden.

RESULTS

The [(125)I]IONPs exhibited high radiolabel stability and long blood circulation, which eventually led to high heart uptake in apoE -/- mice when compared with healthy controls. Furthermore, digital autoradiography showed substantially enhanced emission of signals from the hearts of atherosclerotic mice, while no or minimal cardiac signals were detected in healthy mice.

CONCLUSIONS

This preparation showed adequate physical-chemical properties for in vivo studies, such as small size (∼30 nm), good radiolabel stability, and long circulation time. There was also significant accumulation in the heart of apoE-/- mice compared with that of healthy control animals. These findings suggest that radiolabeled dextran-coated iron oxide nanoparticles may have potential to become a useful tool to detect macrophages in the atherosclerosis plaques of coronary arteries; however, these preliminary findings should be confirmed by further studies in a larger scale in various atherosclerosis models.

An update on novel quantitative techniques in the context of evolving whole-body PET imaging

Since its foundation PET has established itself as one of the standard imaging modalities enabling the quantitative assessment of molecular targets in vivo. In the past two decades, quantitative PET has become a necessity in clinical oncology. Despite introduction of various measures for quantification and correction of PET parameters, there is debate on the selection of the appropriate methodology in specific diseases and conditions. In this review, we have focused on these techniques with special attention to topics such as static and dynamic whole body PET imaging, tracer kinetic modeling, global disease burden, texture analysis and radiomics, dual time point imaging and partial volume correction.

Validation of FDG uptake in the arterial wall as an imaging biomarker of atherosclerotic plaques with 18F-fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET/CT)

BACKGROUND AND PURPOSE

From the literature, the prevalence of fluorodeoxyglucose (FDG) uptake in large artery atherosclerotic plaques shows great heterogeneity. We retrospectively reviewed 100 consecutive patients who underwent FDG-positron emission tomography-computed tomography (PET/CT) imaging of their whole body, to evaluate FDG uptake in the arterial wall.

MATERIALS AND METHODS

We retrospectively evaluated 100 whole-body PET-CT scans. The PET images coregistered with CT were reviewed for abnormal 18F-FDG uptake. The mean standard uptake value (SUV) was measured in regions of interest (ROIs). The prevalence of PET+ plaques was determined based on the qualitative PET review, used as the gold standard in a receiver-operating characteristic (ROC) curve analysis to determine an optimal threshold for the quantitative PET analysis.

RESULTS

The qualitative, visual assessment demonstrated FDG uptake in the arterial walls of 26 patients. A total of 85 slices exhibited FDG uptake within the arterial wall of 37 artery locations. 11, 17, and 2 patients exhibited FDG uptake within the wall of carotid arteries, of the aorta, and of the iliac arteries, respectively. Only 4 of the 26 patients had positive FDG uptake in more than one artery location. In terms of quantitative analysis, a threshold of 2.8 SUV was associated with a negative predictive value of 99.4% and a positive predictive value of 100% to predict qualitative PET+ plaques. A threshold of 1.8 SUV was associated with a negative predictive value of 100% and a positive predictive value of 99.4%. Area under the ROC curve was .839.

CONCLUSION

The prevalence of PET uptake in arterial walls in a consecutive population of asymptomatic patients is low and usually confined to one type of artery, and its clinical relevance in terms of vulnerability to ischemic events remains to be determined.

Correlation between Aortic Wall Thickness and Coronary Artery Disease by 64 Slice Multidetector Computed Tomography

INTRODUCTION

Atherosclerotic cardiovascular disease is a dispersed pathology involving the coronary arteries, carotid arteries, aorta and peripheral arteries. It has been previously suggested that coronary and aortic atherosclerosis may be associated. Imaging of the aorta and the aortic wall can be performed by various imaging modalities including state-of-the-art multidetector computer tomography (MDCT). This study aimed to investigate a possible association between the MDCT-measured thickness of the thoracic aorta and the presence of coronary artery disease (CAD) as well as its severity.

METHODS

Three hundred and fifty candidates of coronary computer tomography angiography (CTA) with signs and symptoms suggestive of CAD were recruited in Tabriz Parsian and Iran CTA Centers. Contrast-enhanced MDCT examinations were performed using a 64 detector scanner. Maximum aortic wall thickness in the mid-portion of descending thoracic aorta (region of pulmonary trunk to diaphragm) was measured perpendicular to the center of the vessel.

RESULTS

CAD was confirmed in 189 cases (54%) and the remaining 161 cases served as controls. The mean age of the cases, as well as the percentage of male subjects was significantly higher in the CAD group. The mean aortic wall thickness was also significantly higher in the patient group (2.21±0.63 mm vs. 1.88±0.58 mm; P<0.001). In multivariate analysis, however, the two groups turned up comparable as to the aortic wall thickness (P=0.31). The optimal cut-off point of aortic wall thickness was ≥2 mm in discriminating between CAD+ and CAD- groups, with a corresponding sensitivity and specificity of 65% and 57%, respectively. There was no significant association between aortic wall thickness and the severity of CAD (the number of significantly occluded coronary arteries).

CONCLUSION

Aortic wall thickness is apparently neither an independent predictor of CAD nor is it associated with the severity of CAD in candidates of CTA.

Overview of positron emission tomography, hybrid positron emission tomography instrumentation, and positron emission tomography quantification

Positron emission tomography (PET) is a powerful quantitative molecular imaging technique that is complementary to structural imaging techniques for purposes of disease detection and characterization. This review article provides a brief overview of PET, hybrid PET instrumentation, and PET quantification.

PET/MR imaging: technical aspects and potential clinical applications

Instruments that combine positron emission tomography (PET) and magnetic resonance (MR) imaging have recently been assembled for use in humans, and may have diagnostic performance superior to that of PET/computed tomography (CT) for particular clinical and research applications. MR imaging has major strengths compared with CT, including superior soft-tissue contrast resolution, multiplanar image acquisition, and functional imaging capability through specialized techniques such as diffusion-tensor imaging, diffusion-weighted (DW) imaging, functional MR imaging, MR elastography, MR spectroscopy, perfusion-weighted imaging, MR imaging with very short echo times, and the availability of some targeted MR imaging contrast agents. Furthermore, the lack of ionizing radiation from MR imaging is highly appealing, particularly when pediatric, young adult, or pregnant patients are to be imaged, and the safety profile of MR imaging contrast agents compares very favorably with iodinated CT contrast agents. MR imaging also can be used to guide PET image reconstruction, partial volume correction, and motion compensation for more accurate disease quantification and can improve anatomic localization of sites of radiotracer uptake, improve diagnostic performance, and provide for comprehensive regional and global structural, functional, and molecular assessment of various clinical disorders. In this review, we discuss the historical development, software-based registration, instrumentation and design, quantification issues, potential clinical applications, potential clinical roles of image segmentation and global disease assessment, and challenges related to PET/MR imaging.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.13121038/-/DC1.

Imaging atherosclerosis with hybrid [18F]fluorodeoxyglucose positron emission tomography/computed tomography imaging: what Leonardo da Vinci could not see

Prodigious efforts and landmark discoveries have led toward significant advances in our understanding of atherosclerosis. Despite significant efforts, atherosclerosis continues globally to be a leading cause of mortality and reduced quality of life. With surges in the prevalence of obesity and diabetes, atherosclerosis is expected to have an even more pronounced impact upon the global burden of disease. It is imperative to develop strategies for the early detection of disease. Positron emission tomography (PET) imaging utilizing [(18)F]fluorodeoxyglucose (FDG) may provide a non-invasive means of characterizing inflammatory activity within atherosclerotic plaque, thus serving as a surrogate biomarker for detecting vulnerable plaque. The aim of this review is to explore the rationale for performing FDG imaging, provide an overview into the mechanism of action, and summarize findings from the early application of FDG PET imaging in the clinical setting to evaluate vascular disease. Alternative imaging biomarkers and approaches are briefly discussed.

Relation between popliteal-tibial artery atherosclerosis and global glycolytic metabolism in the affected diabetic foot: a pilot study using quantitative FDG-PET

BACKGROUND

We evaluated the extent and the degree of active atherosclerosis in the popliteal-tibial arteries by quantitative techniques using [(18)F]-2-fluoro-2-deoxy-d-glucose-positron emission tomography (FDG-PET). An effort was made to determine whether there was a positive correlation between the level of ongoing atherosclerosis and subsequent chronic ischemia in the diabetic foot.

METHODS

A total of 115 diabetic patients were enrolled in this prospective research study and underwent lower-extremity FDG-PET and magnetic resonance imaging. We selected 36 diabetic patients with complicated diabetic foot for this analysis (11 men, 25 women; mean age, 59.5 years; age range, 36-85 years). Ten nondiabetic control participants (6 men, 4 women; mean age, 59.7 years; age range, 27-88 years) were also included for comparison. Only patients with blood glucose levels less than 200 mg/100 mL were enrolled in this study. The metabolic volumetric product (MVP), for each patient was calculated multiplying foot average maximum standard uptake value (SUV(max)) by foot volume. Pearson correlation analysis between foot mean SUV(max) alone and the degree of atherosclerosis and between the foot MVP and the degree of atherosclerosis was performed as measured by FDG-PET. A t test was used to assess for significant differences in foot SUV(max) and foot MVP among varying degrees of atherosclerosis, using P < .05 as the criterion for statistical significance.

RESULTS

Foot SUV(max) significantly correlated (P < .05) with the ratio of popliteal-tibial artery SUV(max) to background SUV (Pearson correlation coefficient = 0.49). Foot MVP was also significantly correlated (P < .05) with the ratio of popliteal-tibial artery SUV(max) to background SUV (Pearson correlation coefficient = 0.69).

CONCLUSIONS

Using FDG-PET, one may be able to detect and quantify the degree of increased metabolic activity of early active atherosclerosis and the associated chronic tissue ischemic effects at the regional level and globally throughout the whole body during the asymptomatic phase of disease.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Quantification of atherosclerotic plaque activity and vascular inflammation using [18-F] fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT)

Conventional non-invasive imaging modalities of atherosclerosis such as coronary artery calcium (CAC) and carotid intimal medial thickness (C-IMT) provide information about the burden of disease. However, despite multiple validation studies of CAC, and C-IMT, these modalities do not accurately assess plaque characteristics, and the composition and inflammatory state of the plaque determine its stability and, therefore, the risk of clinical events. [(18)F]-2-fluoro-2-deoxy-D-glucose (FDG) imaging using positron-emission tomography (PET)/computed tomography (CT) has been extensively studied in oncologic metabolism. Studies using animal models and immunohistochemistry in humans show that FDG-PET/CT is exquisitely sensitive for detecting macrophage activity, an important source of cellular inflammation in vessel walls. More recently, we and others have shown that FDG-PET/CT enables highly precise, novel measurements of inflammatory activity of activity of atherosclerotic plaques in large and medium-sized arteries. FDG-PET/CT studies have many advantages over other imaging modalities: 1) high contrast resolution; 2) quantification of plaque volume and metabolic activity allowing for multi-modal atherosclerotic plaque quantification; 3) dynamic, real-time, in vivo imaging; 4) minimal operator dependence. Finally, vascular inflammation detected by FDG-PET/CT has been shown to predict cardiovascular (CV) events independent of traditional risk factors and is also highly associated with overall burden of atherosclerosis. Plaque activity by FDG-PET/CT is modulated by known beneficial CV interventions such as short term (12 week) statin therapy as well as longer term therapeutic lifestyle changes (16 months). The current methodology for quantification of FDG uptake in atherosclerotic plaque involves measurement of the standardized uptake value (SUV) of an artery of interest and of the venous blood pool in order to calculate a target to background ratio (TBR), which is calculated by dividing the arterial SUV by the venous blood pool SUV. This method has shown to represent a stable, reproducible phenotype over time, has a high sensitivity for detection of vascular inflammation, and also has high inter-and intra-reader reliability. Here we present our methodology for patient preparation, image acquisition, and quantification of atherosclerotic plaque activity and vascular inflammation using SUV, TBR, and a global parameter called the metabolic volumetric product (MVP). These approaches may be applied to assess vascular inflammation in various study samples of interest in a consistent fashion as we have shown in several prior publications.

Clinical utility of FDG-PET and PET/CT in non-malignant thoracic disorders

There have been several endeavors made to investigate the potential role of 2-deoxy-2-[(18)F]fluoro-D-glucose positron emission tomography (FDG-PET) (and tracers) and PET-computed tomography imaging in various benign disorders, particularly those related to thoracic structures. These various conditions can be broadly categorized into three groups: (a) infectious diseases (mycobacterial, fungal, bacterial infection), (b) active granulomatous disease such as sarcoidosis, and (c) other non-infectious/inflammatory conditions or proliferative disorders (e.g., radiation pneumonitis, post-lung transplant lymphoproliferative disorders, occupational pleuropulmonary complications, and post-surgical conditions), all of which can demonstrate varying degrees of FDG uptake on PET scans based upon the degree of inflammatory activity. This article reviews the current state of this very important application of FDG-PET imaging.

Pediatric lymphoma: metabolic tumor burden as a quantitative index for treatment response evaluation

OBJECTIVE

Metabolic tumor burden (MTB) incorporates the advantages of the existing indices: the metabolic volume of the lesion calculated by size-dependent thresholding on positron emission tomography-computed tomography (PET-CT) along with its aggressiveness as determined by standardized uptake value (SUV). This study was conducted to investigate whether MTB can be used as an objective index for monitoring therapy response in pediatric lymphoma.

METHODS

Forty-two pediatric patients (35 male and 7 female) with histologically proven lymphomas (26 Hodgkin's and 16 non-Hodgkin's) were evaluated. MTB was assessed in baseline, early interim (after 2 cycles) and post-therapy PET-CT studies using RT_Image software. Size-dependent thresholding based on a phantom study conducted at our institute was used for the calculation of metabolic tumor volume (MTV). MTB was given as the product of MTV and the SUVmean. Summation of MTB from all lesions gave the whole body MTB. Baseline, early interim and post-therapy SUVmax and whole body MTB of the partial and complete response group were compared.

RESULTS

Of 42 patients, 37 had complete response and 5 had partial response at the end of therapy based on clinical, CECT and bone marrow biopsy findings. SUVmax showed an overall reduction of 87.4% while MTB showed a reduction of 96.4% between baseline and early interim PET-CT scan. Similarly, SUVmax showed an overall reduction of 95.2% while MTB showed a reduction of 99.6% between baseline and post-therapy scan. There was significant difference between MTB of partial response and complete response group at baseline and early interim PET-CT (p 0.031 and 0.012, respectively). No such significant difference was found for SUVmax.

CONCLUSION

Whole body MTB appears to be useful quantitative parameter for the assessment of treatment response using PET-CT in pediatric lymphoma patients.

Detection and Quantification of Molecular Calcification by PET/Computed Tomography: A New Paradigm in Assessing Atherosclerosis

Atherosclerotic plaque rupture is the leading cause of acute coronary syndrome. Molecular calcification represent as one of the early stages of plaque evolution has been hypothesized to play a potential role in atherosclerotic plaque instability and subsequent rupture. Several invasive and non-invasive structural imaging techniques have been utilized to diagnose atherosclerosis, but none of these methods are capable of detecting and quantifying molecular calcification. Fluorine-18-Sodium Fluoride (18F-NaF) positron emission tomography/computed tomography (PET/CT) imaging allows detection and quantification of arterial molecular calcification in heart and across multiple vessels. In this review the authors discuss the feasibility, application and potential future of 18F-NaF-PET/CT in detecting molecular calcification and in defining the future risk of atherosclerotic plaque rupture in the affected vessels.

Role of Global Disease Assessment by Combined PET-CT-MR Imaging in Examining Cardiovascular Disease

Atherosclerosis is considered a chronic inflammatory disease, and thereafter the degree of this pathologic process is considered to be a major determinant in plaque stability and in forecasting future events. Over the past decade, (18)F-fluorodeoxyglucose PET/computed tomography has become a well-established imaging modality in evaluating various inflammatory disorders, and has been shown to be very useful in evaluating plaque activity in major arteries. This emerging noninvasive imaging modality has great potential in evaluating plaque vulnerability and in predicting the risk of future rupture and consequent thrombosis.

Hypoxia but not inflammation augments glucose uptake in human macrophages: Implications for imaging atherosclerosis with 18fluorine-labeled 2-deoxy-D-glucose positron emission tomography

OBJECTIVES

This study investigated the regulation of glucose uptake in cells that participate in atherogenesis by stimuli relevant to this process, to gain mechanistic insight into the origin of the (18)fluorine-labeled 2-deoxy-D-glucose (FdG) uptake signals observed clinically.

BACKGROUND

Patient studies suggest that positron emission tomography (PET) using FdG can detect "active" atherosclerotic plaques, yet the mechanism giving rise to FdG signals remains unknown.

METHODS

We exposed cells to conditions thought to operate in atheroma and determined rates of glucose uptake.

RESULTS

Hypoxia, but not pro-inflammatory cytokines, potently stimulated glucose uptake in human macrophages and foam cells. Statins attenuated this process in vitro, suggesting that these agents have a direct effect on human macrophages. Immunohistochemical study of human plaques revealed abundant expression of proteins regulating glucose utilization, predominantly in macrophage-rich regions of the plaques-regions previously proved hypoxic. Smooth-muscle cells and endothelial cells markedly increased rates of glucose uptake when exposed to pro-inflammatory cytokines.

CONCLUSIONS

Glucose uptake and, probably, FdG uptake signals in atheroma may reflect hypoxia-stimulated macrophages rather than mere inflammatory burden. Cytokine-activated smooth-muscle cells also may contribute to the FdG signal.

Systemic and vascular inflammation in patients with moderate to severe psoriasis as measured by [18F]-fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET/CT): a pilot study

OBJECTIVE

To evaluate the feasibility of using [18F]-fluorodeoxyglucose positron emission tomography-computed tomography (FDG-PET/CT) to detect and quantify systemic inflammation in patients with psoriasis.

DESIGN

Case series with a nested case-control study.

SETTING

Referral dermatology and preventive cardiology practices.

PARTICIPANTS

Six patients with psoriasis affecting more than 10% of their body surface area and 4 controls age and sex matched to 4 of the patients with psoriasis for a nested case-control study.

MAIN OUTCOME MEASURES

The FDG uptake in the liver, musculoskeletal structures, and aorta measured by mean standardized uptake value, a measure of FDG tracer uptake by macrophages and other inflammatory cells.

RESULTS

FDG-PET/CT identified numerous foci of inflammation in 6 patients with psoriasis within the skin, liver, joints, tendons, and aorta. Inflammation in the joints was observed in a patient with psoriatic arthritis as well as in 1 patient with no history of joint disease or joint symptoms. In a nested case-control study, FDG-PET/CT imaging demonstrated increased vascular inflammation in multiple segments of the aorta compared with controls. These findings persisted after adjustment for traditional cardiovascular risk factors in multivariate analysis (mean β = 0.33; P < .001). Patients with psoriasis further demonstrated increased hepatic inflammation after adjusting for cardiovascular risk factors (β = 0.18; P < .001), but the association was no longer significant when adjusted for alcohol intake (β = -0.25; P = .07).

CONCLUSION

FDG-PET/CT is a sensitive tool for identifying inflammation and can be used to identify clinically observed inflammation in the skin and subclinical inflammation in the blood vessels, joints, and liver of patients with psoriasis.

When to image carotid plaque inflammation with FDG PET/CT

OBJECTIVE

Quantification of 18-fluorodeoxyglucose (FDG) uptake in inflamed high-risk carotid atherosclerotic plaques is challenged by the spatial resolution of positron emission tomography (PET) and luminal blood activity. Late acquisition protocols have been used to overcome these challenges to enhance the contrast between the plaque and blood-pool FDG activity. However, for prospective studies the late acquisition is inconvenient for the patient and staff, and most retrospective studies of plaque uptake use data from early acquisition protocols. The objective was to evaluate changes in the quantification methods of FDG uptake in carotid artery plaques between early and late PET scans.

METHODS

FDG uptake 1 and 3 h after tracer injection was compared in 19 carotid artery plaques. The average plaque maximum standardized uptake value (SUVmax) and a target to background ratio (TBR), using venous blood-pool activity as background, were evaluated at the two time points. These methods have been shown earlier to quantitate the degree of inflammation in late hour scans.

RESULTS

A good individual plaque FDG uptake consistency was found between the two time points for SUVmax, r²=0.86. In contrast, the ratio method did not conserve the results between the two time points: TBR r²=0.34. For both methods, absolute values changed over time. TBR values generally increased as blood pool activity decreased, whereas the individual plaque SUVmax values showed both increases and decreases over time.

CONCLUSION

Identification of carotid plaque inflammation with PET can be performed 1 h after FDG injection using SUVmax for plaque FDG uptake quantification.

Aortic atheroma and adverse cerebral outcome: risk, diagnosis, and management options

Aortic atheromatous disease is a common finding in the patient presenting for cardiac surgery. Adverse neurologic outcome has been closely linked to the extent of aortic atherosclerosis. In order to optimize perioperative outcomes, the location and severity of disease needs accurate characterization using multimodal techniques. Although various preoperative radiographic techniques have variably identified patients with significant atheroma, intraoperative echocardiographic imaging has proven most useful in localizing and characterizing the degree of aortic atheroma. Epiaortic assessment of the ascending aorta has been utilized in guiding surgical modifications and interventions aimed at reducing the risk of neurologic injury. Although no particular technique has been definitely studied, avoidance of the identifiable atheromatous aortic region has been a main feature of the various modifications employed to optimize neurologic outcome after cardiac surgery.