Multifunctional MR monitoring of the healing process after myocardial infarction

Rivaroxaban attenuates neutrophil maturation in the bone marrow niche

Pharmacological inhibition of factor Xa by rivaroxaban has been shown to mediate cardioprotection and is frequently used in patients with, e.g., atrial fibrillation. Rivaroxaban's anti-inflammatory actions are well known, but the underlying mechanisms are still incompletely understood. To date, no study has focused on the effects of rivaroxaban on the bone marrow (BM), despite growing evidence that the BM and its activation are of major importance in the development/progression of cardiovascular disease. Thus, we examined the impact of rivaroxaban on BM composition under homeostatic conditions and in response to a major cardiovascular event. Rivaroxaban treatment of mice for 7 days markedly diminished mature leukocytes in the BM. While apoptosis of BM-derived mature myeloid leukocytes was unaffected, lineage-negative BM cells exhibited a differentiation arrest at the level of granulocyte-monocyte progenitors, specifically affecting neutrophil maturation via downregulation of the transcription factors Spi1 and Csfr1. To assess whether this persists also in situations of increased leukocyte demand, mice were subjected to cardiac ischemia/reperfusion injury (I/R): 7 d pretreatment with rivaroxaban led to reduced cardiac inflammation 72 h after I/R and lowered circulating leukocyte numbers. However, BM myelopoiesis showed a rescue of the leukocyte differentiation arrest, indicating that rivaroxaban's inhibitory effects are restricted to homeostatic conditions and are mainly abolished during emergency hematopoiesis. In translation, ST-elevation MI patients treated with rivaroxaban also exhibited reduced circulating leukocyte numbers. In conclusion, we demonstrate that rivaroxaban attenuates neutrophil maturation in the BM, which may offer a therapeutic option to limit overshooting of the immune response after I/R.

Imaging Methods: Magnetic Resonance Imaging

Myocardial inflammation occurs following activation of the cardiac immune system, producing characteristic changes in the myocardial tissue. Cardiovascular magnetic resonance is the non-invasive imaging gold standard for myocardial tissue characterization, and is able to detect image signal changes that may occur resulting from inflammation, including edema, hyperemia, capillary leak, necrosis, and fibrosis. Conventional cardiovascular magnetic resonance for the detection of myocardial inflammation and its sequela include T2-weighted imaging, parametric T1- and T2-mapping, and gadolinium-based contrast-enhanced imaging. Emerging techniques seek to image several parameters simultaneously for myocardial tissue characterization, and to depict subtle immune-mediated changes, such as immune cell activity in the myocardium and cardiac cell metabolism. This review article outlines the underlying principles of current and emerging cardiovascular magnetic resonance methods for imaging myocardial inflammation.

Multiparametric MRI identifies subtle adaptations for demarcation of disease transition in murine aortic valve stenosis

Aortic valve stenosis (AS) is the most frequent valve disease with relevant prognostic impact. Experimental model systems for AS are scarce and comprehensive imaging techniques to simultaneously quantify function and morphology in disease progression are lacking. Therefore, we refined an acute murine AS model to closely mimic human disease characteristics and developed a high-resolution magnetic resonance imaging (MRI) approach for simultaneous in-depth analysis of valvular, myocardial as well as aortic morphology/pathophysiology to identify early changes in tissue texture and critical transition points in the adaptive process to AS. AS was induced by wire injury of the aortic valve. Four weeks after surgery, cine loops, velocity, and relaxometry maps were acquired at 9.4 T to monitor structural/functional alterations in valve, aorta, and left ventricle (LV). In vivo MRI data were subsequently validated by histology and compared to echocardiography. AS mice exhibited impaired valve opening accompanied by significant valve thickening due to fibrotic remodelling. While control mice showed bell-shaped flow profiles, AS resulted not only in higher peak flow velocities, but also in fragmented turbulent flow patterns associated with enhanced circumferential strain and an increase in wall thickness of the aortic root. AS mice presented with a mild hypertrophy but unaffected global LV function. Cardiac MR relaxometry revealed reduced values for both T1 and T2 in AS reflecting subtle myocardial tissue remodelling with early alterations in mitochondrial function in response to the enhanced afterload. Concomitantly, incipient impairments of coronary flow reserve and myocardial tissue integrity get apparent accompanied by early troponin release. With this, we identified a premature transition point with still compensated cardiac function but beginning textural changes. This will allow interventional studies to explore early disease pathophysiology and novel therapeutic targets.

Regional analysis of inflammation and contractile function in reperfused acute myocardial infarction by in vivo 19F cardiovascular magnetic resonance in pigs

Inflammatory cell infiltration is central to healing after acute myocardial infarction (AMI). The relation of regional inflammation to edema, infarct size (IS), microvascular obstruction (MVO), intramyocardial hemorrhage (IMH), and regional and global LV function is not clear. Here we noninvasively characterized regional inflammation and contractile function in reperfused AMI in pigs using fluorine (¹⁹F) cardiovascular magnetic resonance (CMR). Adult anesthetized pigs underwent left anterior descending coronary artery instrumentation with either 90 min occlusion (n = 17) or without occlusion (sham, n = 5). After 3 days, in surviving animals a perfluorooctyl bromide nanoemulsion was infused intravenously to label monocytes/macrophages. At day 6, in vivo ¹H-CMR was performed with cine, T2 and T2* weighted imaging, T2 and T1 mapping, perfusion and late gadolinium enhancement followed by ¹⁹F-CMR. Pigs were sacrificed for subsequent ex vivo scans and histology. Edema extent was 35 ± 8% and IS was 22 ± 6% of LV mass. Six of ten surviving AMI animals displayed both MVO and IMH (3.3 ± 1.6% and 1.9 ± 0.8% of LV mass). The ¹⁹F signal, reflecting the presence and density of monocytes/macrophages, was consistently smaller than edema volume or IS and not apparent in remote areas. The ¹⁹F signal-to-noise ratio (SNR) > 8 in the infarct border zone was associated with impaired remote systolic wall thickening. A whole heart value of ¹⁹F integral (¹⁹F SNR × milliliter) > 200 was related to initial LV remodeling independently of edema, IS, MVO, and IMH. Thus, ¹⁹F-CMR quantitatively characterizes regional inflammation after AMI and its relation to edema, IS, MVO, IMH and regional and global LV function and remodeling.

Beyond Vessel Diameters: Non-invasive Monitoring of Flow Patterns and Immune Cell Recruitment in Murine Abdominal Aortic Disorders by Multiparametric MRI

The pathophysiology of the initiation and progression of abdominal aortic aneurysms (AAAs) and aortic dissections (AADs) is still unclear. However, there is strong evidence that monocytes and macrophages are of crucial importance in these processes. Here, we utilized a molecular imaging approach based on background-free ¹⁹F MRI and employed perfluorocarbon nanoemulsions (PFCs) for in situ ¹⁹F labeling of monocytes/macrophages to monitor vascular inflammation and AAA/AAD formation in angiotensin II (angII)-treated apolipoproteinE-deficient (apoE^-/-) mice. In parallel, we used conventional ¹H MRI for the characterization of aortic flow patterns and morphology. AngII (1 μg/kg/min) was infused into apoE^-/- mice via osmotic minipumps for 10 days and mice were monitored by multiparametric ¹H/¹⁹F MRI. PFCs were intravenously injected directly after pump implantation followed by additional applications on day 2 and 4 to allow an efficient ¹⁹F loading of circulating monocytes. The combination of angiographic, hemodynamic, and anatomical measurements allowed an unequivocal classification of mice in groups with developing AAAs, AADs or without any obvious aortic vessel alterations despite the exposure to angII. Maximal luminal and external diameters of the aorta were enlarged in AAAs, whereas AADs showed either a slight decrease of the luminal diameter or no alteration. ¹H/¹⁹F MRI after intravenous PFC application demonstrated significantly higher ¹⁹F signals in aortae of mice that developed AAAs or AADs as compared to mice in which no aortic disorders were detected. High resolution ¹H/¹⁹F MRI of excised aortae revealed a patchy pattern of the ¹⁹F signals predominantly in the adventitia of the aorta. Histological analysis confirmed the presence of macrophages in this area and flow cytometry revealed higher numbers of immune cells in aortae of mice that have developed AAA/AAD. Importantly, there was a linear correlation of the ¹⁹F signal with the total number of infiltrated macrophages. In conclusion, our approach enables a precise differentiation between AAA and AAD as well as visualization and quantitative assessment of inflammatory active vascular lesions, and therefore may help to unravel the complex interplay between macrophage accumulation, vascular inflammation, and the development and progression of AAAs and AADs.

Endothelial β1 Integrin-Mediated Adaptation to Myocardial Ischemia

BACKGROUND

 Short episodes of myocardial ischemia can protect from myocardial infarction. However, the role of endothelial β1 integrin in these cardioprotective ischemic events is largely unknown.

OBJECTIVE

 In this study we investigated whether endothelial β1 integrin is required for cardiac adaptation to ischemia and protection from myocardial infarction.

METHODS

 Here we introduced transient and permanent left anterior descending artery (LAD) occlusions in mice. We inhibited β1 integrin by intravenous injection of function-blocking antibodies and tamoxifen-induced endothelial cell (EC)-specific deletion of Itgb1. Furthermore, human ITGB1 was silenced in primary human coronary artery ECs using small interfering RNA. We analyzed the numbers of proliferating ECs and arterioles by immunohistochemistry, determined infarct size by magnetic resonance imaging (MRI) and triphenyl tetrazolium chloride staining, and analyzed cardiac function by MRI and echocardiography.

RESULTS

 Transient LAD occlusions were found to increase EC proliferation and arteriole formation in the entire myocardium. These effects required β1 integrin on ECs, except for arteriole formation in the ischemic part of the myocardium. Furthermore, this integrin subunit was also relevant for basal and mechanically induced proliferation of human coronary artery ECs. Notably, β1 integrin was needed for cardioprotection induced by transient LAD occlusions, and the absence of endothelial β1 integrin resulted in impaired growth of blood vessels into the infarcted myocardium and reduced cardiac function after permanent LAD occlusion.

CONCLUSION

 We showed that endothelial β1 integrin is required for adaptation of the heart to cardiac ischemia and protection from myocardial infarction.

4-hydroxytamoxifen does not deteriorate cardiac function in cardiomyocyte-specific MerCreMer transgenic mice

Conditional, cell-type-specific transgenic mouse lines are of high value in cardiovascular research. A standard tool for cardiomyocyte-restricted DNA editing is the αMHC-MerCreMer/loxP system. However, there is an ongoing debate on the occurrence of cardiac side effects caused by unspecific Cre activity or related to tamoxifen/oil overload. Here, we investigated potential adverse effects of DNA editing by the αMHC-MerCreMer/loxP system in combination with a low-dose treatment protocol with the tamoxifen metabolite 4-hydroxytamoxifen (OH-Txf). αMHC-MerCreMer mice received intraperitoneally OH-Txf (20 mg/kg) for 5 or 10 days. These treatment protocols were highly efficient to induce DNA editing in adult mouse hearts. Multi-parametric magnetic resonance imaging revealed neither transient nor permanent effects on cardiac function during or up to 19 days after 5 day OH-Txf treatment. Furthermore, OH-Txf did not affect cardiac phosphocreatine/ATP ratios assessed by in vivo 31P MR spectroscopy, indicating no Cre-mediated side effects on cardiac energy status. No MRI-based indication for the development of cardiac fibrosis was found as mean T1 relaxation time was unchanged. Histological analysis of myocardial collagen III content after OH-Txf confirmed this result. Last, mean T2 relaxation time was not altered after Txf treatment suggesting no pronounced cardiac lipid accumulation or tissue oedema. In additional experiments, cardiac function was assessed for up to 42 days to investigate potential delayed side effects of OH-Txf treatment. Neither 5- nor 10-day treatment resulted in a depression of cardiac function. Efficient cardiomyocyte-restricted DNA editing that is free of unwanted side effects on cardiac function, energetics or fibrosis can be achieved in adult mice when the αMHC-MerCreMer/loxP system is activated by the tamoxifen metabolite OH-Txf.

Transcranial activation and imaging of low boiling point phase-change contrast agents through the temporal bone using an ultrafast interframe activation ultrasound sequence

PURPOSE

As a cavitation enhancer, low boiling point phase-change contrast agents (PCCA) offer potential for ultrasound-mediated drug delivery in applications including intracerebral hemorrhage or brain tumors. In addition to introducing cavitation, ultrasound imaging also has the ability to provide guidance and monitoring of the therapeutic process by localizing delivery events. However, the highly attenuating skull poses a challenge for achieving an image with useful contrast. In this study, the feasibility of transcranial activation and imaging of low boiling point PCCAs through the human temporal bone was investigated by using a low frequency ultrafast interframe activation ultrasound (UIAU) imaging sequence with singular value decomposition-based denoising.

METHODS

Lipid-shelled PCCAs filled with decafluorobutane were activated and imaged at 37°C in tissue-mimicking phantoms both without and with an ex vivo human skull using the new UIAU sequence and a low frequency diagnostic transducer array at frequencies from 1.5 to 3.5 MHz. A singular value decomposition-based denoising filter was developed and used to further enhance transcranial image contrast. The contrast-to-tissue ratio (CTR) and contrast enhancement (CE) of UIAU was quantitatively evaluated and compared with the amplitude modulation pulse inversion (AMPI) and vaporization detection imaging (VDI) approaches.

RESULTS

Image results demonstrate enhanced contrast in the phantom channel with suppressed background when the low boiling point PCCA was activated both without and with an ex vivo human skull using the UIAU sequence. Quantitative results show that without the skull, low frequency UIAU imaging provided significantly higher image contrast (CTR ≥ 18.56 dB and CE ≥ 18.66 dB) than that of AMPI and VDI (P < 0.05). Transcranial imaging results indicated that the CE of UIAU (≥18.80 dB) was significantly higher than AMPI for free-field activation pressures of 5 and 6 MPa. The CE of UIAU is also significantly higher than that of VDI when PCCAs were activated at 2.5 MHz and 3 MHz (P < 0.05). The CTR (23.30 [20.07-25.56] dB) of denoised UIAU increased by 12.58 dB relative to the non-denoised case and was significantly higher than that of AMPI at an activation pressure of 4 MPa (P < 0.05).

CONCLUSIONS

Results indicate that low boiling point PCCAs can be activated and imaged at low frequencies including imaging through the temporal bone using the UIAU sequence. The UIAU imaging approach provides higher contrast than AMPI and VDI, especially at lower activation pressures with additional removal of electronic noise.

In vivo clearance of 19F MRI imaging nanocarriers is strongly influenced by nanoparticle ultrastructure

Perfluorocarbons hold great promise both as imaging agents, particularly for ¹⁹F MRI, and in therapy, such as oxygen delivery. ¹⁹F MRI is unique in its ability to unambiguously track and quantify a tracer while maintaining anatomic context, and without the use of ionizing radiation. This is particularly well-suited for inflammation imaging and quantitative cell tracking. However, perfluorocarbons, which are best suited for imaging - like perfluoro-15-crown-5 ether (PFCE) - tend to have extremely long biological retention. Here, we showed that the use of a multi-core PLGA nanoparticle entrapping PFCE allows for a 15-fold reduction of half-life in vivo compared to what is reported in literature. This unexpected rapid decrease in ¹⁹F signal was observed in liver, spleen and within the infarcted region after myocardial infarction and was confirmed by whole body NMR spectroscopy. We demonstrate that the fast clearance is due to disassembly of the ~200 nm nanoparticle into ~30 nm domains that remain soluble and are cleared quickly. We show here that the nanoparticle ultrastructure has a direct impact on in vivo clearance of its cargo i.e. allowing fast release of PFCE, and therefore also bringing the possibility of multifunctional nanoparticle-based imaging to translational imaging, therapy and diagnostics.

Anaemia is associated with severe RBC dysfunction and a reduced circulating NO pool: vascular and cardiac eNOS are crucial for the adaptation to anaemia

Anaemia is frequently present in patients with acute myocardial infarction (AMI) and contributes to an adverse prognosis. We hypothesised that, besides reduced oxygen carrying capacity, anaemia is associated with (1) red blood cell (RBC) dysfunction and a reduced circulating nitric oxide (NO) pool, (2) compensatory enhancement of vascular and cardiac endothelial nitric oxide synthase (eNOS) activity, and (3) contribution of both, RBC dysfunction and reduced circulatory NO pool to left ventricular (LV) dysfunction and fatal outcome in AMI. In mouse models of subacute and chronic anaemia from repeated mild blood loss the circulating NO pool, RBC, cardiac and vascular function were analysed at baseline and in reperfused AMI. In anaemia, RBC function resulted in profound changes in membrane properties, enhanced turnover, haemolysis, dysregulation of intra-erythrocytotic redox state, and RBC-eNOS. RBC from anaemic mice and from anaemic patients with acute coronary syndrome impaired the recovery of contractile function of isolated mouse hearts following ischaemia/reperfusion. In anaemia, the circulating NO pool was reduced. The cardiac and vascular adaptation to anaemia was characterised by increased arterial eNOS expression and activity and an eNOS-dependent increase of end-diastolic left ventricular volume. Endothelial dysfunction induced through genetic or pharmacologic reduction of eNOS-activity abrogated the anaemia-induced cardio-circulatory compensation. Superimposed AMI was associated with decreased survival. In summary, moderate blood loss anaemia is associated with severe RBC dysfunction and reduced circulating NO pool. Vascular and cardiac eNOS are crucial for the cardio-circulatory adaptation to anaemia. RBC dysfunction together with eNOS dysfunction may contribute to adverse outcomes in AMI.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Dissociation of 19F and fluorescence signal upon cellular uptake of dual-contrast perfluorocarbon nanoemulsions

OBJECTIVE

Perfluorocarbon nanoemulsions (PFCs) tagged with fluorescence dyes have been intensively used to confirm the in vivo ¹⁹F magnetic resonance imaging (MRI) localization of PFCs by post mortem histology or flow cytometry. However, only limited data are available on tagged PFCs and the potential dissociation of fluorescence and ¹⁹F label after cellular uptake over time.

MATERIALS AND METHODS

PFCs were coupled to rhodamine (Rho) or carboxyfluorescein (Cfl) and their fate was analyzed after in vitro uptake by J774, RAW and CHO cells by flow cytometry and ¹⁹F MRI. In separate in vivo experiments, the dual-labelled emulsions were intravenously applied into mice and their distribution was monitored in spleen and liver over 24 h. In a final step, time course of fluorescence and ¹⁹F signals from injected emulsions were tracked in a local inflammation model making use of a subcutaneous matrigel depot doped with LPS (lipopolysaccharide).

RESULTS

Internalization of fluorescence-labelled PFCs was associated with a substantial whitening over 24 h in all macrophage cell lines while the ¹⁹F signal remained stable over time. In all experiments, ^CflPFCs were more susceptible to bleaching than ^RhoPFCs. After intravenous injection of ^RhoPFCs, the fluorescence signal in spleen and liver peaked after 30 min and 2 h, respectively, followed by a successive decrease over 24 h, whereas the ¹⁹F signal continuously increased during this observation period. Similar results were found in the matrigel/LPS model, where we observed increasing ¹⁹F signals in the inflammatory hot spot over time while the fluorescence signal of immune cells isolated from the matrigel depot 24 h after its implantation was only marginally elevated over background levels. This resulted in a massive underestimation of the true PFC deposition in the reticuloendothelial system and at inflammatory hot spots.

CONCLUSION

Cellular uptake of fluorescently tagged PFCs leads to a dissociation of the fluorescence and the ¹⁹F label signal over time, which critically impacts on interpretation of long-term experiments validated by histology or flow cytometry.

Graves' orbitopathy occurs sex-independently in an autoimmune hyperthyroid mouse model

Graves’ orbitopathy (GO) is the most common extra thyroidal complication of Graves’ disease (GD) and occurs predominantly in women but more severe in men. The reason for this effect of gender on GO is unknown. Herein we studied the manifestation of GO in both sexes of an induced mouse model in absence of additional risk factors present in patients like advanced age, genetic variabilities or smoking. Male and female mice were immunized with human TSHR A-subunit encoding plasmid. Both sexes comparably developed autoimmune hyperthyroidism characterized by TSHR stimulating autoantibodies, elevated T4 values, hyperplastic thyroids and hearts. Autoimmune mice developed inflammatory eye symptoms and proptosis, although males earlier than females. Serial in vivo¹H/¹⁹F-magnetic resonance imaging revealed elevated inflammatory infiltration, increased fat volume and glycosaminoglycan deposition in orbits of both sexes but most significantly in female mice. Histologically, infiltration of T-cells, extension of brown fat and overall collagen deposition were characteristics of GO in male mice. In contrast, female mice developed predominately macrophage infiltration in muscle and connective tissue, and muscle hypertrophy. Apart from sex-dependent variabilities in pathogenesis, disease classification revealed minor sex-differences in incidence and total outcome. In conclusion, sex does not predispose for autoimmune hyperthyroidism and associated GO.

Quantification of myocardial infarct area based on TRAFFn relaxation time maps - comparison with cardiovascular magnetic resonance late gadolinium enhancement, T1ρ and T2 in vivo

BACKGROUND

Two days after myocardial infarction (MI), the infarct consists mostly on necrotic tissue, and the myocardium is transformed through granulation tissue to scar in two weeks after the onset of ischemia in mice. In the current work, we determined and optimized cardiovascular magnetic resonance (CMR) methods for the detection of MI size during the scar formation without contrast agents in mice.

METHODS

We characterized MI and remote areas with rotating frame relaxation time mapping including relaxation along fictitious field in nth rotating frame (RAFFn), T1ρ and T2 relaxation time mappings at 1, 3, 7, and 21 days after MI. These results were compared to late gadolinium enhancement (LGE) and Sirius Red-stained histology sections, which were obtained at day 21 after MI.

RESULTS

All relaxation time maps showed significant differences in relaxation time between the MI and remote area. Areas of increased signal intensities after gadolinium injection and areas with increased TRAFF2 relaxation time were highly correlated with the MI area determined from Sirius Red-stained histology sections (LGE: R2 = 0.92, P < 0.01, TRAFF2: R2 = 0.95, P < 0.001). Infarct area determined based on T1ρ relaxation time correlated highly with Sirius Red histology sections (R2 = 0.97, P < 0.01). The smallest overestimation of the LGE-defined MI area was obtained for TRAFF2 (5.6 ± 4.2%) while for T1ρ overestimation percentage was > 9% depending on T1ρ pulse power.

CONCLUSION

T1ρ and TRAFF2 relaxation time maps can be used to determine accurately MI area at various time points in the mouse heart. Determination of MI size based on TRAFF2 relaxation time maps could be performed without contrast agents, unlike LGE, and with lower specific absorption rate compared to on-resonance T1ρ relaxation time mapping.

Assessment of liver ischemia reperfusion injury in mice using hepatic T2 mapping: Comparison with histopathology

BACKGROUND

Liver ischemia reperfusion injury (IRI) occurs during liver surgery or transplantation resulting in an inflammatory response, tissue damage, and functional impairment of the organ.

PURPOSE

To assess the feasibility of T₂ mapping for noninvasive quantification of liver edema after partial liver IRI in mice.

STUDY TYPE

Prospective, experimental study.

ANIMAL MODEL

Partial liver IRI was induced in C57BL/6-mice by transient clamping of the left lateral and median liver lobes for 35 (n = 8), 45 (n = 6), 60 (n = 17), or 90 minutes (n = 5). For comparison, healthy C57BL/6-mice were examined as controls (n = 9).

FIELD STRENGTH/SEQUENCE

Functional liver MRI was performed on a 7T scanner using a respiratory-triggered multiecho spin-echo sequence.

ASSESSMENT

Healthy control mice and mice with partial liver IRI on day 1 after surgery, and additionally on day 7 in a subgroup with 60 minutes IRI (n = 8) were examined. Maps of T₂ relaxation time of liver tissue were used to assess distribution, severity of tissue edema (mean T₂ time), and the percentage of edematous liver tissue.

STATISTICAL TEST

One-way analysis of variance (ANOVA) with Tukey's honest significant difference (HSD), paired t-tests, Pearson's test for correlation of MRI parameters with levels of liver enzymes, and histopathology, receiver operating characteristic (ROC) analysis.

RESULTS

Significant tissue edema induced by liver IRI as compared to the control group was detected by increased mean T₂ times in groups with 60 minutes (P < 0.001) and 90 minutes IRI (P < 0.001). The percentage of edematous liver tissue significantly increased with longer ischemia times (controls 3.4 ± 0.4%, 35 minutes 5.3 ± 0.6%, 45 minutes 23.3 ± 7.6%, 60 minutes 39.7 ± 3.6%, 90 minutes 51.3 ± 4.5%). Mean T₂ times and the percentage of edematous liver tissue significantly correlated with elevation of liver enzymes (P < 0.001), histological evidence of liver injury (r = 0.80 and r = 0.82, P < 0.001), and neutrophil infiltration (r = 0.70 and r = 0.74, P < 0.001). In the subgroup with follow-up, the severity (P < 0.01) and extent of liver edema decreased significantly over time (P < 0.01).

DATA CONCLUSION

T₂ mapping allows quantification and follow-up of liver injury in mice.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;48:1586-1594.

Nrf2 Deficiency Unmasks the Significance of Nitric Oxide Synthase Activity for Cardioprotection

The transcription factor nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is a key master switch that controls the expression of antioxidant and cytoprotective enzymes, including enzymes catalyzing glutathione de novo synthesis. In this study, we aimed to analyze whether Nrf2 deficiency influences antioxidative capacity, redox state, NO metabolites, and outcome of myocardial ischemia reperfusion (I/R) injury. In Nrf2 knockout (Nrf2 KO) mice, we found elevated eNOS expression and preserved NO metabolite concentrations in the aorta and heart as compared to wild types (WT). Unexpectedly, Nrf2 KO mice have a smaller infarct size following myocardial ischemia/reperfusion injury than WT mice and show fully preserved left ventricular systolic function. Inhibition of NO synthesis at onset of ischemia and during early reperfusion increased myocardial damage and systolic dysfunction in Nrf2 KO mice, but not in WT mice. Consistent with this, infarct size and diastolic function were unaffected in eNOS knockout (eNOS KO) mice after ischemia/reperfusion. Taken together, these data suggest that eNOS upregulation under conditions of decreased antioxidant capacity might play an important role in cardioprotection against I/R. Due to the redundancy in cytoprotective mechanisms, this fundamental antioxidant property of eNOS is not evident upon acute NOS inhibition in WT mice or in eNOS KO mice until Nrf2-related signaling is abrogated.

Multimodal assessment of orbital immune cell infiltration and tissue remodeling during development of graves disease by 1 H19 F MRI

PURPOSE

To evaluate key molecular and cellular features of Graves orbitopathy (GO) by simultaneous monitoring of alterations in morphology, inflammatory patterns, and tissue remodeling.

METHODS

To this end, we utilized a murine model of GO induced by immunization with a human thyroid-stimulating hormone receptor A-subunit plasmid. Altogether, 52 mice were used: 27 GOs and 25 controls (Ctrl) immunized with β-galactasidose plasmid. From these, 17 GO and 12 Ctrl mice were subjected to multimodal MRI at 9.4T, whereas 23 mice only underwent histology. Beyond anatomical hydrogen-1 (¹ H) MRI, we employed transverse relaxation time (T₂ ) mapping for visualization of edema, chemical exchange saturation transfer (CEST) for detection of hyaluronan, and fluorine-19 (¹⁹ F) MRI for tracking of in situ-labeled immune cells after intravenous injection of perfluorcarbons (PFCs).

RESULTS

¹ H/¹⁹ F MRI demonstrated substantial infiltration of PFC-loaded immune cells in peri and retro-orbital regions of GO mice, whereas healthy Ctrls showed only minor ¹⁹ F signals. In parallel, T₂ mapping indicated onset of edema in periorbital tissue and adjacent ocular glands (P = 0.038/0.017), which were associated with enhanced orbital CEST signals in GO mice (P = 0.031). Concomitantly, a moderate expansion of retrobulbar fat (P = 0.029) was apparent; however, no signs for extraocular myopathy were detectable. ¹⁹ F MRI-based visualization of orbital inflammation exhibited the highest significance level to discriminate between GO and Ctrl mice (P = 0.006) and showed the best correlation with the clinical score (P = 0.0007).

CONCLUSION

The present approach permits the comprehensive characterization of orbital tissue and holds the potential for accurate GO diagnosis in the clinical setting. Magn Reson Med 80:711-718, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

MR Assessment of Acute Pathologic Process after Myocardial Infarction in a Permanent Ligation Mouse Model: Role of Magnetic Nanoparticle-Contrasted MRI

We evaluated the relationship between myocardial infarct size and inflammatory response using cardiac magnetic resonance imaging (CMR) in an acute myocardial infarction (AMI) mouse model. Myocardial infarction (MI) was induced in 14 mice by permanent ligation of the left anterior descending artery. Late gadolinium enhancement (LGE), manganese-enhanced MRI (MEMRI), and magnetofluorescent nanoparticle MRI (MNP-MRI) were performed 1, 2, and 3 days after MI, respectively. The size of the enhanced lesion was quantitatively determined using Otsu's thresholding method in area-based and sector-based approaches and was compared statistically. Linear correlation between the enhanced lesion sizes was evaluated by Pearson's correlation coefficients. Differences were compared using Bland-Altman analysis. The size of the inflammatory area determined by MNP-MRI (57.1 ± 10.1%) was significantly larger than that of the infarct area measured by LGE (40.8 ± 11.7%, P < 0.0001) and MEMRI (44.1 ± 14.9%, P < 0.0001). There were significant correlations between the sizes of the infarct and inflammatory lesions (MNP-MRI versus LGE: r = 0.3418, P = 0.0099; MNP-MRI versus MEMRI: r = 0.4764, P = 0.0002). MNP-MRI provides information about inflammatory responses in a mouse model of AMI. Thus, MNP-MRI associated with LGE and MEMRI may play an important role in monitoring the disease progression in MI.

Cardiovascular Magnetic Resonance Relaxometry Predicts Regional Functional Outcome After Experimental Myocardial Infarction

BACKGROUND

Cardiovascular magnetic resonance with gadolinium-based contrast agents has established as gold standard for tissue characterization after myocardial infarction (MI). Beyond accurate diagnosis, the value of cardiovascular magnetic resonance to predict the outcome after MI has yet to be substantiated.

METHODS AND RESULTS

Recent cardiovascular magnetic resonance approaches were systematically compared for quantification of tissue injury and functional impairment after MI using murine models with permanent left anterior descending coronary artery ligation (n=14) or 50 minutes ischemia/reperfusion (n=13). Cardiovascular magnetic resonance included native/postcontrast T1 maps, T2 maps, and late gadolinium enhancement at days 1 and 21 post-MI. For regional correlation of parametric and functional measures, the left ventricle was analyzed over 200 sectors. For T1 mapping, we used retrospective triggering with variable flip angle analysis. Sectoral analysis of native T1 maps already revealed in the acute phase after MI substantial discrepancies in myocardial tissue texture between the 2 MI models (native T1 day 1: permanent ligation, 1280.0±162.6 ms; ischemia/reperfusion, 1115.0±140.5 ms; P<0.001; n=14/13), which were later associated with differential functional outcome (left ventricular ejection fraction day 21: permanent ligation, 24.5±7.0%; ischemia/reperfusion, 33.7±11.6%; P<0.05; n=14/13). At this early time, any other parameter was indicative for the subsequent worsening of left ventricular ejection fraction in permanent ligation mice. Linear regression of acute individual measures with contractile function in corresponding areas at day 21 demonstrated for early native T1 values the best correlation with the later functional impairment (R² =0.94).

CONCLUSIONS

The present T1 mapping approach permits accurate characterization of local tissue injury and holds the potential for sensitive and graduated prognosis of the functional outcome after MI without gadolinium-based contrast agents.

LRP5/canonical Wnt signalling and healing of ischemic myocardium

LRP5 (low-density lipoprotein receptor-related protein 5) activates canonical Wnt signalling. LRP5 plays multiple roles including regulation of lipoprotein and cholesterol homeostasis as well as innate immunity cell function. However, it is not known whether LRP5 has a role in the myocardium. The aim of this study was to investigate LRP5 and Wnt signalling in myocardial remodelling after acute myocardial infarction (MI). Wnt protein levels were determined in a hypercholesterolemic porcine model of MI, in Lrp5 ^-/- C57Bl6 mice, in cultured cardiomyocytes and in human explanted hearts with previous MI episodes. 21 days post-MI, there was upregulation of LRP5 in the ischemic myocardium of hypercholesterolemic pigs as well as an upregulated expression of proteins of the Wnt pathway. We demonstrate via overexpression and silencing experiments that LRP5 induces Wnt pathway activation in isolated cardiomyocytes. Hypoxia and lipid-loading induced the expression of Wnt proteins, whereas this effect is blocked in LRP5-silenced cardiomyocytes. To characterize the function of the LRP5-Wnt axis upregulation in the heart, we induced MI in wild-type and Lrp5 ^-/- mice. Lrp5 ^-/- mice had significantly larger infarcts than Wt mice, indicating a protective role of LRP5 in injured myocardium. The LRP5 upregulation in post-MI hearts seen in pigs and mice was also evident in human hearts as dyslipidemic patients with previous episodes of ischemia have higher expression of LRP5 and Wnt-signalling genes than non-ischemic dilated hearts. We demonstrate an upregulation of LRP5 and the Wnt signalling pathway that it is a prosurvival healing response of cardiomyocytes upon injury.

High field magnetic resonance imaging of rodents in cardiovascular research

Transgenic and gene knockout rodent models are primordial to study pathophysiological processes in cardiovascular research. Over time, cardiac MRI has become a gold standard for in vivo evaluation of such models. Technical advances have led to the development of magnets with increasingly high field strength, allowing specific investigation of cardiac anatomy, global and regional function, viability, perfusion or vascular parameters. The aim of this report is to provide a review of the various sequences and techniques available to image mice on 7-11.7 T magnets and relevant to the clinical setting in humans. Specific technical aspects due to the rise of the magnetic field are also discussed.

Myocardial T2 mapping reveals age- and sex-related differences in volunteers

BACKGROUND

T2 mapping indicates to be a sensitive method for detection of tissue oedema hidden beyond the detection limits of T2-weighted Cardiovascular Magnetic Resonance (CMR). However, due to variability of baseline T2 values in volunteers, reference values need to be defined. Therefore, the aim of the study was to investigate the effects of age and sex on quantitative T2 mapping with a turbo gradient-spin-echo (GRASE) sequence at 1.5 T. For that reason, we studied sensitivity issues as well as technical and biological effects on GRASE-derived myocardial T2 maps. Furthermore, intra- and interobserver variability were calculated using data from a large volunteer group.

METHODS

GRASE-derived multiecho images were analysed using dedicated software. After sequence optimization, validation and sensitivity measurements were performed in muscle phantoms ex vivo and in vivo. The optimized parameters were used to analyse CMR images of 74 volunteers of mixed sex and a wide range of age with typical prevalence of hypertension and diabetes. Myocardial T2 values were analysed globally and according to the 17 segment model. Strain-encoded (SENC) imaging was additionally performed to investigate possible effects of myocardial strain on global or segmental T2 values.

RESULTS

Ex vivo studies in muscle phantoms showed, that GRASE-derived T2 values were comparable to those acquired by a standard multiecho spinecho sequence but faster by a factor of 6. Besides that, T2 values reflected tissue water content. The in vivo measurements in volunteers revealed intra- and interobserver correlations with R2=0.91 and R2=0.94 as well as a coefficients of variation of 2.4% and 2.2%, respectively. While global T2 time significantly decreased towards the heart basis, female volunteers had significant higher T2 time irrespective of myocardial region. We found no correlation of segmental T2 values with maximal systolic, diastolic strain or heart rate. Interestingly, volunteers´ age was significantly correlated to T2 time while that was not the case for other coincident cardiovascular risk factors.

CONCLUSION

GRASE-derived T2 maps are highly reproducible. However, female sex and aging with typical prevalence of hypertension and diabetes were accompanied by increased myocardial T2 values. Thus, sex and age must be considered as influence factors when using GRASE in a diagnostic manner.