The advancing clinical impact of molecular imaging in CVD

Coronary Atherosclerosis: Pathophysiologic Basis for Diagnosis and Management

Coronary atherosclerosis is a long lasting and continuously evolving disease with multiple clinical manifestations ranging from asymptomatic to stable angina, acute coronary syndrome (ACS), heart failure (HF) and sudden cardiac death (SCD). Genetic and environmental factors contribute to the development and progression of coronary atherosclerosis. In this review, current knowledge related to the diagnosis and management of coronary atherosclerosis based on pathophysiologic mechanisms will be discussed. In addition to providing state-of-the-art concepts related to coronary atherosclerosis, special consideration will be given on how to apply data from epidemiologic studies and randomized clinical trials to the individual patient. The greatest challenge for the clinician in the twenty-first century is not in absorbing the fast accumulating new knowledge, but rather in applying this knowledge to the individual patient.

18F-FDG uptake in main arterial branches of patients with large vessel vasculitis: visual and semiquantitative analysis

OBJECTIVE

Over the last decade, the contribution of (18)F-FDG (FDG) PET/CT imaging to the diagnosis of large vessel vasculitis has been widely investigated. The aim of this study was to evaluate a more extensive role for PET/CT in grading vascular inflammation in patients with different clinical stages of disease.

METHODS

The images of 66 PET/CT studies of 34 patients, performed at diagnosis and/or during follow-up were reviewed. FDG uptake in different regions of aorta and in its major branches was visually (regional Score: rS) and semiquantitatively (regional SUVmean: rSUV) assessed. The global vascular uptake was also evaluated for each study by summing all rSs (summed Score; sS) and averaging rSUVs (averaged SUV; aSUV). FDG uptake in 15 PET/CT studies of control age-matched subjects without signs or symptoms of vasculitis was also analyzed.

RESULTS

Higher levels of regional and global FDG uptake were found at diagnosis in comparison with follow-up studies of 12 patients with complete longitudinal observation (p value range 0.0552-0.0026). In the latter group high values were generally observed when disease relapse or incomplete response to therapy (active disease) occurred, whereas lower uptake was found in studies of remitted patients (p = <0.01), whose FDG levels were similar to those of control subjects. At ROC analysis performed on all image dataset, optimal cut-off levels of regional and global FDG vascular uptake provided a good discrimination between 25 patients at diagnosis and 15 control subjects (aSUV greater than 0.697; PPV = 92.3; NPV = 92.9). Major overlap was observed among FDG levels of 21 patients with active disease and in remission (aSUV greater than 0.653; PPV = 58.3; NPV = 94.1). Similar performances of visual and semiquantitative analyses were found when areas under curves (AUCs) were compared.

CONCLUSIONS

(18)F-FDG PET/CT has a promising role in grading inflammation in patients with large arteries vasculitis. Nevertheless, a cut-off based analysis of FDG vascular uptake is not sufficient to separate patients with active and inactive disease during follow-up.

Intravascular NIRF Molecular Imaging Approaches in Coronary Artery Disease

Progression of vulnerable coronary atherosclerotic plaques underlies the majority of acute myocardial infarction and sudden cardiac death episodes. Recent advances in biological/molecular imaging technology are now enabling the accurate identification of high-risk plaques and stents in living subjects. Due to their smaller caliber and susceptibility to cardiorespiratory motion, noninvasive molecular imaging of human coronary arteries remains challenging. Therefore, intravascular high-resolution molecular imaging approaches appear necessary to resolve molecular features of human coronary arteries and stents. Here we present recent progress in intravascular near-infrared fluorescence (NIRF) molecular imaging, including the evolution from standalone NIRF systems to those integrated with structural imaging methods including optical coherence tomography and intravascular ultrasound. Preclinical demonstrations of imaging inflammation, fibrin, and endothelial impairment are highlighted. We then close with a discussion of translation of NIRF imaging to the cardiac catheterization laboratory and showcase first-in-human intracoronary imaging results of NIR autofluorescence in CAD.

In vivo MR and Fluorescence Dual-modality Imaging of Atherosclerosis Characteristics in Mice Using Profilin-1 Targeted Magnetic Nanoparticles

Aims: This study aims to explore non-invasive imaging of atherosclerotic plaque through magnetic resonance imaging (MRI) and near-infrared fluorescence (NIRF) by using profilin-1 targeted magnetic iron oxide nanoparticles (PF₁-Cy5.5-DMSA-Fe₃O₄-NPs, denoted as PC-NPs) as multimodality molecular imaging probe in murine model of atherosclerosis. Methods and Results: PC-NPs were constructed by conjugating polyclonal profilin-1 antibody and NHS-Cy5.5 fluorescent dye to the surface of DMSA-Fe₃O₄-nanoparticles via condensation reaction. Murine atherosclerosis model was induced in apoE^-/- mice by high fat and cholesterol diet (HFD) for 16 weeks. The plaque areas in aortic artery were detected with Oil Red O staining. Immunofluorescent staining and Western blot analysis were applied respectively to investigate profilin-1 expression. CCK-8 assay and transwell migration experiment were performed to detect vascular smooth muscle cells (VSMCs) proliferation. In vivo MRI and NIRF imaging of atherosclerotic plaque were carried out before and 36 h after intravenous injection of PC-NPs. Oil Red O staining showed that the plaque area was significantly increased in HFD group (p<0.05). Immunofluorescence staining revealed that profilin-1 protein was highly abundant within plaque in HFD group and co-localized with α-smooth muscle actin. Profilin-1 siRNA intervention could inhibit VSMCs proliferation and migration elicited by ox-LDL (p<0.05). In vivo MRI and NIRF imaging revealed that PC-NPs accumulated in atherosclerotic plaque of carotid artery. There was a good correlation between the signals of MRI and ex vivo fluorescence intensities of NIRF imaging in animals with PC-NPs injection. Conclusion: PC-NPs is a promising dual modality imaging probe, which may improve molecular diagnosis of plaque characteristics and evaluation of pharmaceutical interventions for atherosclerosis.

Combined PET/MRI: Multi-modality Multi-parametric Imaging Is Here: Summary Report of the 4th International Workshop on PET/MR Imaging; February 23-27, 2015, Tübingen, Germany

This paper summarises key themes and discussions from the 4th international workshop dedicated to the advancement of the technical, scientific and clinical applications of combined positron emission tomography (PET)/magnetic resonance imaging (MRI) systems that was held in Tübingen, Germany, from February 23 to 27, 2015. Specifically, we summarise the three days of invited presentations from active researchers in this and associated fields augmented by round table discussions and dialogue boards with specific topics. These include the use of PET/MRI in cardiovascular disease, paediatrics, oncology, neurology and multi-parametric imaging, the latter of which was suggested as a key promoting factor for the wider adoption of integrated PET/MRI. Discussions throughout the workshop and a poll taken on the final day demonstrated that attendees felt more strongly that PET/MRI has further advanced in both technical versatility and acceptance by clinical and research-driven users from the status quo of last year. Still, with only minimal evidence of progress made in exploiting the true complementary nature of the PET and MRI-based information, PET/MRI is still yet to achieve its potential. In that regard, the conclusion of last year's meeting "the real work has just started" still holds true.

SPECT/CT Imaging of High-Risk Atherosclerotic Plaques using Integrin-Binding RGD Dimer Peptides

Vulnerable atherosclerotic plaques with unique biological signatures are responsible for most major cardiovascular events including acute myocardial infarction and stroke. However, current clinical diagnostic approaches for atherosclerosis focus on anatomical measurements such as the degree of luminal stenosis and wall thickness. An abundance of neovessels with elevated expression of integrin αvβ3 is closely associated with an increased risk of plaque rupture. Herein we evaluated the potential of an αvβ3 integrin-targeting radiotracer, (99m)Tc-IDA-D-[c(RGDfK)]2, for SPECT/CT imaging of high-risk plaque in murine atherosclerosis models. In vivo uptake of (99m)Tc-IDA-D-[c(RGDfK)]2 was significantly higher in atherosclerotic aortas than in relatively normal aortas. Comparison with the negative-control peptide, (99m)Tc-IDA-D-[c(RADfK)]2, proved specific binding of (99m)Tc-IDA-D-[c(RGDfK)]2 for plaque lesions in in vivo SPECT/CT and ex vivo autoradiographic imaging. Histopathological characterization revealed that a prominent SPECT signal of (99m)Tc-IDA-D-[c(RGDfK)]2 corresponded to the presence of high-risk plaques with a large necrotic core, a thin fibrous cap, and vibrant neoangiogenic events. Notably, the RGD dimer based (99m)Tc-IDA-D-[c(RGDfK)]2 showed better imaging performance in comparison with the common monomeric RGD peptide probe (123)I-c(RGDyV) and fluorescence tissue assay corroborated this. Our preclinical data demonstrated that (99m)Tc-IDA-D-[c(RGDfK)]2 SPECT/CT is a sensitive tool to noninvasively gauge atherosclerosis beyond vascular anatomy by assessing culprit plaque neovascularization.

Macrophages in vascular inflammation--From atherosclerosis to vasculitis

The spectrum of vascular inflammatory disease ranges from atherosclerosis and hypertension, widespread conditions affecting large proportions of the population, to the vasculitides, rare syndromes leading to fast and irreversible organ failure. Atherosclerosis progresses over decades, inevitably proceeding through multiple phases of disease and causes its major complications when the vessel wall lesion ruptures, giving rise to lumen-occlusive atherothrombosis. Vasculitides of medium and large arteries progress rapidly, causing tissue ischemia through lumen-occlusive intimal hyperplasia. In both disease entities, macrophages play a decisive role in pathogenesis, but function in the context of other immune cells that direct their differentiation and their functional commitments. In atherosclerosis, macrophages are involved in the removal of lipids and tissue debris and make a critical contribution to tissue damage and wall remodeling. In several of the vasculitides, macrophages contribute to granuloma formation, a microstructural platform optimizing macrophage-T-cell interactions, antigen containment and inflammatory amplification. By virtue of their versatility and plasticity, macrophages are able to promote a series of pathogenic functions, ranging from the release of cytokines and enzymes, the production of reactive oxygen species, presentation of antigen and secretion of tissue remodeling factors. However, as short-lived cells that lack memory, macrophages are also amendable to reprogramming, making them promising targets for anti-inflammatory interventions.

Inflammation, Atherosclerosis, and Coronary Artery Disease: PET/CT for the Evaluation of Atherosclerosis and Inflammation

Atherosclerosis is a prevalent cardiovascular disease marked by inflammation and the formation of plaque within arterial walls. As the disease progresses, there is an increased risk of major cardiovascular events. Owing to the nature of atherosclerosis, it is imperative to develop methods to further understand the physiological implications and progression of the disease. The combination of positron emission tomography (PET)/computed tomography (CT) has proven to be promising for the evaluation of atherosclerotic plaques and inflammation within the vessel walls. The utilization of the radiopharmaceutical tracer, 18F-fluorodeoxyglucose (¹⁸F-FDG), with PET/CT is invaluable in understanding the pathophysiological state involved in atherosclerosis. In this review, we will discuss the use of ¹⁸F-FDG-PET/CT imaging for the evaluation of atherosclerosis and inflammation both in preclinical and clinical studies. The potential of more specific novel tracers will be discussed. Finally, we will touch on the potential benefits of using the newly introduced combined PET/magnetic resonance imaging (MRI) for non-invasive imaging of atherosclerosis.

PEG modification on (111)In-labeled phosphatidyl serine liposomes for imaging of atherosclerotic plaques

INTRODUCTION

Previously, we reported a probe for imaging of atherosclerotic plaques: (111)In-labeled liposomes. Liposomes were modified with phosphatidylserine (PS) because macrophages recognize PS and phagocytize apoptotic cells in plaques. PS modification was successful and we could visualize atherosclerotic plaques by single-photon emission computed tomography (SPECT). However, too-rapid blood clearance reduced accumulation of PS-liposomes in plaques in vivo. Therefore, in the present study, PS-liposomes were modified with polyethylene glycol (PEG) to retard the rate of blood clearance.

METHODS

PS-liposomes (size, 100 nm or 200 nm) were PEGylated with PEG2000 or PEG5000 at 1 or 5 mol%, and radiolabeled with (111)In. For the study of uptake in vitro, liposomes were incubated with mouse peritoneal macrophages. Biodistribution studies in vivo were carried out in ddY mice. En face autoradiograms were obtained with apoE(-/-) mice upon intravenous injection of (111)In-liposomes.

RESULTS

Uptake was decreased significantly at 5 mol% PEGylation in 100-nm PS-liposomes (*P<0.05 vs. 0 mol%). All the PEGylated liposomes tested showed significantly lower uptake than the non-PEGylated control in 200-nm liposomes. In vivo results showed slower blood clearance in PEGylated liposomes. Autoradiograms in apoE(-/-) mice were well matched with Oil Red O staining. Additionally, 200-nm PS-liposomes modified with 5%PEG2000 ([(111)In]5%PEG2000PS200) showed the highest uptake to the region in vivo.

CONCLUSIONS

As expected, PEGylation retarded the rate of blood clearance. In addition, it affected liposome uptake by macrophages in vitro. These results suggest that the balance between the rate of blood clearance and macrophage recognition is important, and [(111)In]5%PEG2000PS200 showed the best results in our investigation.

Dual modality intravascular optical coherence tomography (OCT) and near-infrared fluorescence (NIRF) imaging: a fully automated algorithm for the distance-calibration of NIRF signal intensity for quantitative molecular imaging

Intravascular optical coherence tomography (IVOCT) is a well-established method for the high-resolution investigation of atherosclerosis in vivo. Intravascular near-infrared fluorescence (NIRF) imaging is a novel technique for the assessment of molecular processes associated with coronary artery disease. Integration of NIRF and IVOCT technology in a single catheter provides the capability to simultaneously obtain co-localized anatomical and molecular information from the artery wall. Since NIRF signal intensity attenuates as a function of imaging catheter distance to the vessel wall, the generation of quantitative NIRF data requires an accurate measurement of the vessel wall in IVOCT images. Given that dual modality, intravascular OCT-NIRF systems acquire data at a very high frame-rate (>100 frames/s), a high number of images per pullback need to be analyzed, making manual processing of OCT-NIRF data extremely time consuming. To overcome this limitation, we developed an algorithm for the automatic distance-correction of dual-modality OCT-NIRF images. We validated this method by comparing automatic to manual segmentation results in 180 in vivo images from six New Zealand White rabbit atherosclerotic after indocyanine-green injection. A high Dice similarity coefficient was found (0.97 ± 0.03) together with an average individual A-line error of 22 µm (i.e., approximately twice the axial resolution of IVOCT) and a processing time of 44 ms per image. In a similar manner, the algorithm was validated using 120 IVOCT clinical images from eight different in vivo pullbacks in human coronary arteries. The results suggest that the proposed algorithm enables fully automatic visualization of dual modality OCT-NIRF pullbacks, and provides an accurate and efficient calibration of NIRF data for quantification of the molecular agent in the atherosclerotic vessel wall.

Imaging of oxidation-specific epitopes with targeted nanoparticles to detect high-risk atherosclerotic lesions: progress and future directions

Oxidation-specific epitopes (OSE) within developing atherosclerotic lesions are key antigens that drive innate and adaptive immune responses in atherosclerosis, leading to chronic inflammation. Oxidized phospholipids and malondialdehyde-lysine epitopes are well-characterized OSE present in human atherosclerotic lesions, particularly in pathologically defined vulnerable plaques. Using murine and human OSE-specific antibodies as targeting agents, we have developed radionuclide and magnetic resonance based nanoparticles, containing gadolinium, manganese or lipid-coated ultrasmall superparamagnetic iron oxide, to non-invasively image OSE within experimental atherosclerotic lesions. These methods quantitate plaque burden, allow detection of lesion progression and regression, plaque stabilization, and accumulation of OSE within macrophage-rich areas of the artery wall, suggesting they detect the most active lesions. Future studies will focus on using "natural" antibodies, lipopeptides, and mimotopes for imaging applications. These approaches should enhance the clinical translation of this technique to image, monitor, evaluate efficacy of novel therapeutic agents, and guide optimal therapy of high-risk atherosclerotic lesions.

Molecular intravascular imaging approaches for atherosclerosis

Coronary artery disease (CAD) is an inflammatory process that results in buildup of atherosclerosis, typically lipid-rich plaque in the arterial wall. Progressive narrowing of the vessel wall and subsequent plaque rupture can lead to myocardial infarction and death. Recent advances in intravascular fluorescence imaging techniques have provided exciting coronary artery-targeted platforms to further characterize the molecular changes that occur within the vascular wall as a result of atherosclerosis and following coronary stent-induced vascular injury. This review will summarize exciting recent developments in catheter-based imaging of coronary arterial-sized vessels; focusing on two-dimensional near-infrared fluorescence imaging (NIRF) molecular imaging technology as an approach to specifically identify inflammation and fibrin directly within coronary artery-sized vessels. Intravascular NIRF is anticipated to provide new insights into the in vivo biology underlying high-risk plaques, as well as high-risks stents prone to stent restenosis or stent thrombosis.