MRI of atherosclerose

This thesis is aimed at visualization of atherosclerotic plaques with MRI. Noninvasive screening for subclinical atherosclerosis as well as detection of high-risk atherosclerotic plaque in an established population of cardiovascular patients is important for patient management. Anatomical MRI utilizing intrinsic contrast based on different MR relaxation properties of plaque components, contrast-enhanced MRI using non-targeted ultra-small particles of iron oxide (USPIOs) and contrast enhanced MRI using micelles targeting novel targets of advanced atherosclerosis disease in plaque, were investigated. An overview of literature on the subject of MRI of atherosclerosis is provided (chapter 2 and 3). We succeeded in ex vivo discrimination between lipid core and foam cells and better discrimination between lipid core and stable plaque components in human carotid plaque using FLASH with fat suppression (FS) when compared to FLASH without FS (Chapter 4). Addition of Inversion Recovery Spin Echo (IRSE) to a panel of standard techniques (T1w/T2w/PDw) allowed for better identification of intra-plaque hemorrhage. Some research groups have targeted the macrophages in atherosclerotic plaques with USPIOs. The tremendous T2* contrast observed in these studies was also seen in our mouse study, however we were only able to relate the negative contrast caused by USPIOs to uptake in peri-aortic lymph nodes (Chapter 5). Blooming, the effect of spreading of negative contrast beyond the location of uptake of iron oxides, may account for mistaking uptake of USPIOs by peri-aortic lymph node macrophages for uptake by plaque macrophages. Molecular MRI of atherosclerotic plaque has been performed with contrast agents directed against a variety of targets. Some of these targets are macrophage membrane-bound receptors, like the CB2 receptor (Chapter 6 and 8). Other targets are intracellular proteins stored in macrophage granules and released from macrophages to the extracellular matrix, like NGAL (Chapter 6 and 7). We showed increased enhancement at 72 hours after injection of NGAL/24p3 targeted micelles when compared to isotype antibody conjugated micelles (Chapter 7). Just from a practical point of view, in most studies post-injection MRI was planned at 24, 48 and 72 hours after injection. However, accurate definition of the timing window for visualization of specific uptake would be more appropriate in order to increase sensitivity and specificity of the test outcome. Therefore, we performed continuous T1 weighted imaging of the aorta after injection of both CB2-R targeted and control micelles (Chapter 8). Moreover the relation between aortic normalized enhancement ratios (NER) and gadolinium content was calculated from standard T1 weighted images and from inversion-recovery prepared T1 weighted images. The relation was stronger when the NER was calculated from the inversion-recovery prepared T1 weighted image, however we advise averaging of several NER values calculated from sequential images, because of a substantial inter-scan variability. As a separate subject, performance of a self-gated cardiac MRI method has been shown comparable to a prospectively gated cardiac MRI method for evaluation of cardiac function parameters in mice after permanent occlusion of the left anterior descending artery (Chapter 9)

Does physical training increase insulin sensitivity in chronic heart failure patients?

To determine the effect of training on insulin sensitivity (IS) and how this relates to peak 02 (peak oxygen uptake) in CHF (chronic heart failure), 77 CHF patients (New York Heart Association class, II/III; men/women, 59/18; age, 60 +/- 9 years; body mass index, 26.7 +/- 3.9 kg/m(2); left ventricular ejection fraction, 26.9 +/- 8.1%; expressed as means +/- S.D.) participated in the study. Patients were randomly assigned to a training or control group (TrG or CG respectively). Sixty-one patients completed the study. Patients participated in training (combined strength and endurance exercises) four times per week, two times supervised and two times at home. Before and after intervention, anthropometry, IS (euglycaemic hyperinsulinaemic clamp) and peak 02 (incremental cycle ergometry) were assessed. Intervention did not affect IS significantly, even though IS increased by 20% in TrG and 11% in CG (not significant). Peak (V)over dot O-2 increased as a result of training (6% increase in TrG; 2% decrease in CG; P <0.05). In both groups (TrG and CG), the change in IS correlated positively with the change in peak (V)over dot O-2 (r = 0.30, P <0.05). Training resulted in an increase in peak (V)over dot O-2, but not in IS. Whether physical training actually increases IS in CHF patients remains unclear

Characterization and in vitro and in vivo testing of CB2-receptor- and NGAL-targeted paramagnetic micelles for molecular MRI of vulnerable atherosclerotic plaque.

Atherosclerotic plaque macrophages express the peripheral cannabinoid receptor (CB2-R) and promote fibrous cap degradation by secretion of neutrophil gelatinase-associated lipocalin 2 (NGAL). In this study, we report the preparation, characterization, and in vitro and in vivo testing of double-labeled (MR and fluorescent) CB2-R- and NGAL-targeted micelles. Procedures/Results Specific CB2-R agonists or antibodies directed to 24p3 (mouse homolog of NGAL) were incorporated into di-oleoyl-polyethylene glycol-phosphatidylethanolamine 1000 (DOPE-PEG1000) micelles or di-stearoyl-polyethylene glycol-phosphatidylethanolamine 2000 (DSPE-PEG2000) micelles. The hydrodynamic diameter, determined by dynamic light scattering, was 16.5 and 19.0 nm for CB2-R-targeted DOPE-PEG1000 and DSPE-PEG2000 micelles, respectively, and 23.0 nm for Ab-conjugated DSPE-PEG2000 micelles. In vitro and in vivo MRI and fluorescence microscopy showed specific binding of CB2-R-targeted and 24p3-targeted micelles to in vitro systems and to aortic plaque in apoE−/−/eNOS−/− mice, respectively. Conclusions CB2-R- and NGAL-targeted micelles show promise as tools for in vivo characterization of vulnerable plaqu

Molecular MRI of murine atherosclerotic plaque targeting NGAL : a protein associated with unstable human plaque characteristics

Aims Neutrophil gelatinase-associated lipocalin (NGAL) is an effector molecule of the innate immune system. One of its actions is the prolongation of matrix metalloproteinase-9 (MMP-9) activity by the formation of a degradation-resistant NGAL/MMP-9 complex. We studied NGAL in human atherosclerotic lesions and we examined whether NGAL could act as a target for molecular imaging of atherosclerotic plaques. Methods and results Increased levels of NGAL and the NGAL/MMP-9 complex were associated with high lipid content, high number of macrophages, high interleukin-6 (IL-6) and IL-8 levels, and low smooth muscle cell content in human atherosclerotic lesions obtained during carotid endarterectomy (n = 122). Moreover, plaque levels of NGAL tended to be higher when intra-plaque haemorrhage (IPH) or luminal thrombus was present (n 77) than without the presence of IPH or thrombus (n 30). MMP-9 and -8 activities were strongly related to NGAL levels. The enhancement on magnetic resonance (MR) images of the abdominal aorta of ApoE(-/-)/eNOS(-/-) mice was observed at 72 h after injection of NGAL/24p3-targeted micelles. The specificity of these results was validated by histology, and co-localization of micelles, macrophages, and NGAL/24p3 was observed. Conclusion NGAL is highly expressed in atheromatous human plaques and associated with increased MMP-9 activity. NGAL can be detected in murine atherosclerotic arteries using targeted high-resolution MR imaging. Therefore, we conclude that NGAL might serve as a novel imaging target for the detection of high-risk plaques