Mouse myocardial first-pass perfusion MR imaging

Self-gated, dynamic contrast-enhanced magnetic resonance imaging with compressed-sensing reconstruction for evaluating endothelial permeability in the aortic root of atherosclerotic mice

High-risk atherosclerotic plaques are characterized by active inflammation and abundant leaky microvessels. We present a self-gated, dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) acquisition with compressed sensing reconstruction and apply it to assess longitudinal changes in endothelial permeability in the aortic root of Apoe-/- atherosclerotic mice during natural disease progression. Twenty-four, 8-week-old, female Apoe-/- mice were divided into four groups (n = 6 each) and imaged with self-gated DCE-MRI at 4, 8, 12, and 16 weeks after high-fat diet initiation, and then euthanized for CD68 immunohistochemistry for macrophages. Eight additional mice were kept on a high-fat diet and imaged longitudinally at the same time points. Aortic-root pseudo-concentration curves were analyzed using a validated piecewise linear model. Contrast agent wash-in and washout slopes (b1 and b2 ) were measured as surrogates of aortic root endothelial permeability and compared with macrophage density by immunohistochemistry. b2 , indicating contrast agent washout, was significantly higher in mice kept on an high-fat diet for longer periods of time (p = 0.03). Group comparison revealed significant differences between mice on a high-fat diet for 4 versus 16 weeks (p = 0.03). Macrophage density also significantly increased with diet duration (p = 0.009). Spearman correlation between b2 from DCE-MRI and macrophage density indicated a weak relationship between the two parameters (r = 0.28, p = 0.20). Validated piecewise linear modeling of the DCE-MRI data showed that the aortic root contrast agent washout rate is significantly different during disease progression. Further development of this technique from a single-slice to a 3D acquisition may enable better investigation of the relationship between in vivo imaging of endothelial permeability and atherosclerotic plaques' genetic, molecular, and cellular makeup in this important model of disease.

Multi-Scale Imaging of Vascular Pathologies in Cardiovascular Disease

Cardiovascular disease entails systemic changes in the vasculature. The endothelial cells lining the blood vessels are crucial in the pathogenesis of cardiovascular disease. Healthy endothelial cells direct the blood flow to tissues as vasodilators and act as the systemic interface between the blood and tissues, supplying nutrients for vital organs, and regulating the smooth traffic of leukocytes into tissues. In cardiovascular diseases, when inflammation is sensed, endothelial cells adjust to the local or systemic inflammatory state. As the inflamed vasculature adjusts, changes in the endothelial cells lead to endothelial dysfunction, altered blood flow and permeability, expression of adhesion molecules, vessel wall inflammation, thrombosis, angiogenic processes, and extracellular matrix production at the endothelial cell level. Preclinical multi-scale imaging of these endothelial changes using optical, acoustic, nuclear, MRI, and multimodal techniques has progressed, due to technical advances and enhanced biological understanding on the interaction between immune and endothelial cells. While this review highlights biological processes that are related to changes in the cardiac vasculature during cardiovascular diseases, it also summarizes state-of-the-art vascular imaging techniques. The advantages and disadvantages of the different imaging techniques are highlighted, as well as their principles, methodologies, and preclinical and clinical applications with potential future directions. These multi-scale approaches of vascular imaging carry great potential to further expand our understanding of basic vascular biology, to enable early diagnosis of vascular changes and to provide sensitive diagnostic imaging techniques in the management of cardiovascular disease.

Quantitative myocardial first-pass perfusion imaging of CO2 -induced vasodilation in rats

Inducible hypercapnia is an alternative for increasing the coronary blood flow necessary to facilitate the quantification of myocardial blood flow during hyperemia. The current study aimed to quantify the pharmacokinetic effect of a CO₂ gas challenge on myocardial perfusion in rats using high-resolution, first-pass perfusion CMR and compared it with pharmacologically induced hyperemia using regadenoson. A dual-contrast, saturation-recovery, gradient-echo sequence with a Cartesian readout was used on a small-animal 9.4-T scanner; additional cine images during hyperemia/rest were recorded with an ultrashort echo time sequence. The mean myocardial blood flow value at rest was 6.1 ± 1.4 versus 13.9 ± 3.7 and 14.3 ± 4 mL/g/min during vasodilation with hypercapnia and regadenoson, respectively. Accordingly, the myocardial flow reserve value was 2.6 ± 1.1 for the gas challenge and 2.5 ± 1.4 for regadenoson. During hyperemia with both protocols, a significantly increased cardiac output was found. It was concluded that hypercapnia leads to significantly increased coronary flow and yields similar myocardial flow reserves in healthy rats as compared with pharmacological stimulation. Accordingly, inducible hypercapnia can be selected as an alternative stressor in CMR studies of myocardial blood flow in small animals.

Comparison of cine and real-time cardiac MRI in rhesus macaques

Cardiac MRI in rhesus macaques, a species of major relevance for preclinical studies on biological therapies, requires artificial ventilation to realize breath holding. To overcome this limitation of standard cine MRI, the feasibility of Real-Time (RT) cardiac MRI has been tested in a cohort of ten adult rhesus macaques using a clinical MR-system. In spite of lower tissue contrast and sharpness of RT-MRI, cardiac functions were similarly well assessed by RT-MRI compared to cine MRI (similar intra-subject repeatability). However, systematic underestimation of the end-diastolic volume (31 ± 9%), end-systolic volume (20 ± 11%), stroke volume (40 ± 12%) and ejection fraction (13 ± 9%) hamper the comparability of RT-MRI results with those of other cardiac MRI methods. Yet, the underestimations were very consistent (< 5% variability) for repetitive measurements, making RT-MRI an appropriate alternative to cine MRI for longitudinal studies. In addition, RT-MRI enabled the analysis of cardio-respiratory coupling. All functional parameters showed lower values during expiration compared to inspiration, most likely due to the pressure-controlled artificial ventilation. In conclusion, despite systematic underestimation of the functional parameters, RT-MRI allowed the assessment of left ventricular function in macaques with significantly less experimental effort, measurement time, risk and burden for the animals compared to cine MRI.

Feasibility of real-time cardiac MRI in mice using tiny golden angle radial sparse

Cardiovascular magnetic resonance imaging has proven valuable for the assessment of structural and functional cardiac abnormalities. Even although it is an established imaging method in small animals, the long acquisition times of gated or self-gated techniques still limit its widespread application. In this study, the application of tiny golden angle radial sparse MRI (tyGRASP) for real-time cardiac imaging was tested in 12 constitutive nexilin (Nexn) knock-out (KO) mice, both heterozygous (Het, N = 6) and wild-type (WT, N = 6), and the resulting functional parameters were compared with a well-established self-gating approach. Real-time images were reconstructed for different temporal resolutions of between 16.8 and 79.8 ms per image. The suggested approach was additionally tested for dobutamine stress and qualitative first-pass perfusion imaging. Measurements were repeated twice within 2 weeks for reproducibility assessment. In direct comparison with the high-quality, self-gated technique, the real-time approach did not show any significant differences in global function parameters for acquisition times below 50 ms (rest) and 31.5 ms (stress). Compared with WT, the end-diastolic volume (EDV) and end-systolic volume (ESV) were markedly higher (P < 0.05) and the ejection fraction (EF) was significantly lower in the Het Nexn-KO mice at rest (P < 0.001). For the stress investigation, a clear decrease of EDV and ESV, and an increase in EF, but maintained stroke volume, could be observed in both groups. Combined with ECG-triggering, tyGRASP provided first-pass perfusion data with a temporal resolution of one image per heartbeat, allowing the quantitative assessment of upslope curves in the blood-pool and myocardium. Excellent inter-study reproducibility was achieved in all the functional parameters. The tyGRASP is a valuable real-time MRI technique for mice, which significantly reduces the scan time in preclinical cardiac functional imaging, providing sufficient image quality for deriving accurate functional parameters, and has the potential to investigate real-time and beat-to-beat changes.

Extended quantitative dynamic contrast-enhanced cardiac perfusion imaging in mice using accelerated data acquisition and spatially distributed, two-compartment exchange modeling

The objective of the present work was to improve data acquisition and quantification of dynamic contrast-enhanced perfusion imaging in the in vivo murine heart. Four-fold undersampled data were acquired in 14 mice and reconstructed using k-t SPARSE. A two-compartment exchange model was employed to provide additional characterization of myocardial tissue based on compartment volumes and the permeability surface area product. The feasibility of the proposed method was tested using compartment-based analysis of contrast-enhanced perfusion data acquired with intravascular and extracellular contrast agents. A significantly different permeability surface area product was measured for the intravascular versus extracellular contrast agent (0.13-0.15 ml/g/min vs 0.86-0.88 ml/g/min). The reduced extravasation also resulted in significantly smaller interstitial volumes of the intravascular versus extracellular agent (9.8-11% vs 45-47%). No difference was found for myocardial blood flow (6.5-7.2 ml/g/min vs 6.0-7.0 ml/g/min). The results presented here show that two-compartment exchange modeling in the in vivo murine heart is feasible and gives access to tissue parameters beyond myocardial blood flow.

Guidelines for measuring cardiac physiology in mice

Cardiovascular disease is a leading cause of death, and translational research is needed to understand better mechanisms whereby the left ventricle responds to injury. Mouse models of heart disease have provided valuable insights into mechanisms that occur during cardiac aging and in response to a variety of pathologies. The assessment of cardiovascular physiological responses to injury or insult is an important and necessary component of this research. With increasing consideration for rigor and reproducibility, the goal of this guidelines review is to provide best-practice information regarding how to measure accurately cardiac physiology in animal models. In this article, we define guidelines for the measurement of cardiac physiology in mice, as the most commonly used animal model in cardiovascular research.

Listen to this article’s corresponding podcast at http://ajpheart.podbean.com/e/guidelines-for-measuring-cardiac-physiology-in-mice/.

Metformin and sulodexide restore cardiac microvascular perfusion capacity in diet-induced obese rats

Background

Disturbances in coronary microcirculatory function, such as the endothelial glycocalyx, are early hallmarks in the development of obesity and insulin resistance. Accordingly, in the present study myocardial microcirculatory perfusion during rest and stress was assessed following metformin or sulodexide therapy in a rat model of diet-induced obesity. Additionally, the effect of degradation of the glycocalyx on myocardial perfusion was assessed in chow-fed rats.

Methods

Rats were fed a high fat diet (HFD) for 8 weeks and were divided into a group without therapy, and groups that received the anti-diabetic drug metformin or the glycocalyx-stabilizing drug sulodexide in their drinking water during the last 4 weeks of the feeding period. Myocardial microvascular perfusion was determined using first-pass perfusion MRI before and after adenosine infusion. The effect of HFD on microcirculatory properties was also assessed by sidestream darkfield (SDF) imaging of the gastrocnemius muscle. In an acute experimental setting, hyaluronidase was administered to chow-fed control rats to determine the effect of enzymatical degradation of the glycocalyx on myocardial perfusion.

Results

HFD-rats developed central obesity and insulin sensitivity was reduced as evidenced by the marked reduction in insulin-induced phosphorylation of Akt in both cardiac and gastrocnemius muscle. We confirmed our earlier findings that the robust increase in myocardial perfusion in chow-fed rats after an adenosine challenge (+56%, p = 0.002) is blunted in HFD rats (+8%, p = 0.68). In contrast, 4-weeks treatment with metformin or sulodexide partly restored the increase in myocardial perfusion during adenosine infusion in HFD rats (+81%, p = 0.002 and +37%, p = 0.02, respectively). Treating chow-fed rats acutely with hyaluronidase, to enzymatically degrade the glyocalyx, completely blunted the increase in myocardial perfusion during stress.

Conclusions

In early stages of HFD-induced insulin resistance myocardial perfusion becomes compromised, a process that can be countered by treatment with both metformin and sulodexide. The adverse effect of acute glycocalyx degradation and protective effect of long-term sulodexide administration on myocardial perfusion provides indirect evidence, suggesting a role for the glycocalyx in preserving coronary microvascular function in pre-diabetic animals.

Noninvasive mapping of endothelial dysfunction in myocardial ischemia by magnetic resonance imaging using an albumin-based contrast agent

Noninvasive preclinical methods for the characterization of myocardial vascular function are crucial to an understanding of the dynamics of ischemic cardiac disease. Ischemic heart disease is associated with myocardial endothelial dysfunction, resulting in leakage of plasma albumin into the extravascular space. These features can be harnessed in a novel noninvasive three-dimensional magnetic resonance imaging method to measure fractional blood volume (fBV) and vascular permeability (permeability-surface area product, PS) using labeled albumin as a blood pool contrast agent. C57BL/6 mice were imaged before and 3 days after myocardial infarction (MI). Following the quantification of endogenous myocardial R₁ , the dynamics of intravenously injected albumin-based contrast agent, extravasating from permeable myocardial blood vessels, were tracked on short-axis magnetic resonance images of the entire heart. This study successfully discriminated between infarcted and remote regions at 3 days post-infarct, based on a reduced fBV and increased PS in the infarcted region. These findings were confirmed using ex vivo fluorescence imaging and histology. We have demonstrated a novel method to quantify blood volume and permeability in the infarcted myocardium, providing an imaging biomarker for the assessment of endothelial dysfunction. This method has the potential to three-dimensionally visualize subtle changes in myocardial permeability and to track endothelial function for longitudinal cardiac studies determining pathophysiological processes during infarct healing.

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 arterial spin labeling

Arterial spin labeling (ASL) is a cardiovascular magnetic resonance (CMR) technique for mapping regional myocardial blood flow. It does not require any contrast agents, is compatible with stress testing, and can be performed repeatedly or even continuously. ASL-CMR has been performed with great success in small-animals, but sensitivity to date has been poor in large animals and humans and remains an active area of research. This review paper summarizes the development of ASL-CMR techniques, current state-of-the-art imaging methods, the latest findings from pre-clinical and clinical studies, and future directions. We also explain how successful developments in brain ASL and small-animal ASL-CMR have helped to inform developments in large animal and human ASL-CMR.

Early impairment of coronary microvascular perfusion capacity in rats on a high fat diet

Background

It remains to be established if, and to what extent, the coronary microcirculation becomes compromised during the development of obesity and insulin resistance. Recent studies suggest that changes in endothelial glycocalyx properties contribute to microvascular dysfunction under (pre-)diabetic conditions. Accordingly, early effects of diet-induced obesity on myocardial perfusion and function were studied in rats under baseline and hyperaemic conditions.

Methods

Rats were fed a high fat diet (HFD) for 6 weeks and myocardial microvascular perfusion was determined using first-pass perfusion MRI before and after adenosine infusion. The effect of HFD on microcirculatory properties was also assessed by sidestream darkfield (SDF) imaging of the gastrocnemius muscle.

Results

HFD-fed rats developed central obesity and insulin sensitivity was reduced as evidenced by the marked reduction in insulin-induced phosphorylation of Akt in both cardiac and gastrocnemius muscle. Early diet-induced obesity did not lead to hypertension or cardiac hypertrophic remodeling. In chow-fed, control rats a robust increase in cardiac microvascular perfusion was observed upon adenosine infusion (+40 %; p < 0.05). In contrast, the adenosine response was abrogated in rats on a HFD (+8 %; N.S.). HFD neither resulted in rarefaction or loss of glycocalyx integrity in skeletal muscle, nor reduced staining intensity of the glycocalyx of cardiac capillaries.

Conclusions

Alterations in coronary microcirculatory function as assessed by first-pass perfusion MRI represent one of the earliest obesity-related cardiac adaptations that can be assessed non-invasively. In this early stage of insulin resistance, disturbances in glycocalyx barrier properties appeared not to contribute to the observed changes in coronary microvascular function.

Small animal cardiovascular MR imaging and spectroscopy

The use of MR imaging and spectroscopy for studying cardiovascular disease processes in small animals has increased tremendously over the past decade. This is the result of the remarkable advances in MR technologies and the increased availability of genetically modified mice. MR techniques provide a window on the entire timeline of cardiovascular disease development, ranging from subtle early changes in myocardial metabolism that often mark disease onset to severe myocardial dysfunction associated with end-stage heart failure. MR imaging and spectroscopy techniques play an important role in basic cardiovascular research and in cardiovascular disease diagnosis and therapy follow-up. This is due to the broad range of functional, structural and metabolic parameters that can be quantified by MR under in vivo conditions non-invasively. This review describes the spectrum of MR techniques that are employed in small animal cardiovascular disease research and how the technological challenges resulting from the small dimensions of heart and blood vessels as well as high heart and respiratory rates, particularly in mice, are tackled.

Repeatability and variability of myocardial perfusion imaging techniques in mice: Comparison of arterial spin labeling and first-pass contrast-enhanced MRI

PURPOSE

Preclinical imaging of myocardial blood flow (MBF) can elucidate molecular mechanisms underlying cardiovascular disease. We compared the repeatability and variability of two methods, first-pass MRI and arterial spin labeling (ASL), for imaging MBF in mice.

METHODS

Quantitative perfusion MRI in mice was performed using both methods at rest, with a vasodilator, and one day after myocardial infarction. Image quality (score of 1-5; 5 best), between-session coefficient of variability (CVbs ), intra-user coefficient of variability (CVintra-user ), and inter-user coefficient of variability (CVinter-user ) were assessed. Acquisition time was 1-2 min for first-pass MRI and approximately 40 min for ASL.

RESULTS

Image quality was higher for ASL (3.94 ± 0.09 versus 2.88 ± 0.10; P < 0.05). Infarct zone CVbs was lower with first-pass (17 ± 3% versus 46 ± 9%; P < 0.05). The stress perfusion CVintra-user was lower for ASL (3 ± 1% versus 14 ± 3%; P < 0.05). The stress perfusion CVinter-user was lower for ASL (4 ± 1% versus 17 ± 4%; P < 0.05).

CONCLUSION

For low MBF conditions such as infarct, first-pass MRI is preferred due to better repeatability and variability. At high MBF such as at vasodilation, ASL may be more suitable due to superior image quality and lower user variability. First-pass MRI has a substantial speed advantage. Magn Reson Med 75:2394-2405, 2016. © 2015 Wiley Periodicals, Inc.

Assessment of Myocardial Fibrosis in Mice Using a T2*-Weighted 3D Radial Magnetic Resonance Imaging Sequence

BACKGROUND

Myocardial fibrosis is a common hallmark of many diseases of the heart. Late gadolinium enhanced MRI is a powerful tool to image replacement fibrosis after myocardial infarction (MI). Interstitial fibrosis can be assessed indirectly from an extracellular volume fraction measurement using contrast-enhanced T1 mapping. Detection of short T2* species resulting from fibrotic tissue may provide an attractive non-contrast-enhanced alternative to directly visualize the presence of both replacement and interstitial fibrosis.

OBJECTIVE

To goal of this paper was to explore the use of a T2*-weighted radial sequence for the visualization of fibrosis in mouse heart.

METHODS

C57BL/6 mice were studied with MI (n = 20, replacement fibrosis), transverse aortic constriction (TAC) (n = 18, diffuse fibrosis), and as control (n = 10). 3D center-out radial T2*-weighted images with varying TE were acquired in vivo and ex vivo (TE = 21 μs-4 ms). Ex vivo T2*-weighted signal decay with TE was analyzed using a 3-component model. Subtraction of short- and long-TE images was used to highlight fibrotic tissue with short T2*. The presence of fibrosis was validated using histology and correlated to MRI findings.

RESULTS

Detailed ex vivo T2*-weighted signal analysis revealed a fast (T2*fast), slow (T2*slow) and lipid (T2*lipid) pool. T2*fast remained essentially constant. Infarct T2*slow decreased significantly, while a moderate decrease was observed in remote tissue in post-MI hearts and in TAC hearts. T2*slow correlated with the presence of diffuse fibrosis in TAC hearts (r = 0.82, P = 0.01). Ex vivo and in vivo subtraction images depicted a positive contrast in the infarct co-localizing with the scar. Infarct volumes from histology and subtraction images linearly correlated (r = 0.94, P<0.001). Region-of-interest analysis in the in vivo post-MI and TAC hearts revealed significant T2* shortening due to fibrosis, in agreement with the ex vivo results. However, in vivo contrast on subtraction images was rather poor, hampering a straightforward visual assessment of the spatial distribution of the fibrotic tissue.

Cardiac perfusion imaging using hyperpolarized (13)C urea using flow sensitizing gradients

Purpose

To demonstrate the feasibility of imaging the first passage of a bolus of hyperpolarized ¹³C urea through the rodent heart using flow‐sensitizing gradients to reduce signal from the blood pool.

Methods

A flow‐sensitizing bipolar gradient was optimized to reduce the bright signal within the cardiac chambers, enabling improved contrast of the agent within the tissue capillary bed. The gradient was incorporated into a dynamic golden angle spiral ¹³C imaging sequence. Healthy rats were scanned during rest (n = 3) and under adenosine stress‐induced hyperemia (n = 3).

Results

A two‐fold increase in myocardial perfusion relative to rest was detected during adenosine stress‐induced hyperemia, consistent with a myocardial perfusion reserve of two in rodents.

Conclusion

The new pulse sequence was used to obtain dynamic images of the first passage of hyperpolarized ¹³C urea in the rodent heart, without contamination from bright signal within the neighboring cardiac lumen. This probe of myocardial perfusion is expected to enable new hyperpolarized ¹³C studies in which the cardiac metabolism/perfusion mismatch can be identified. Magn Reson Med, 2015. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Magn Reson Med 75:1474–1483, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance.

Hepatic arterial spin labelling MRI: an initial evaluation in mice

The development of strategies to combat hepatic disease and augment tissue regeneration has created a need for methods to assess regional liver function. Liver perfusion imaging has the potential to fulfil this need, across a range of hepatic diseases, alongside the assessment of therapeutic response. In this study, the feasibility of hepatic arterial spin labelling (HASL) was assessed for the first time in mice at 9.4 T, its variability and repeatability were evaluated, and it was applied to a model of colorectal liver metastasis. Data were acquired using flow-sensitive alternating inversion recovery-arterial spin labelling (FAIR-ASL) with a Look-Locker readout, and analysed using retrospective respiratory gating and a T1 -based quantification. This study shows that preclinical HASL is feasible and exhibits good repeatability and reproducibility. Mean estimated liver perfusion was 2.2 ± 0.8 mL/g/min (mean ± standard error, n = 10), which agrees well with previous measurements using invasive approaches. Estimates of the variation gave a within-session coefficient of variation (CVWS) of 7%, a between-session coefficient of variation (CVBS) of 9% and a between-animal coefficient of variation (CVA) of 15%. The within-session Bland-Altman repeatability coefficient (RCWS) was 18% and the between-session repeatability coefficient (RCBS) was 29%. Finally, the HASL method was applied to a mouse model of liver metastasis, in which significantly lower mean perfusion (1.1 ± 0.5 mL/g/min, n = 6) was measured within the tumours, as seen by fluorescence histology. These data indicate that precise and accurate liver perfusion estimates can be achieved using ASL techniques, and provide a platform for future studies investigating hepatic perfusion in mouse models of disease.

Pre-clinical functional Magnetic Resonance Imaging Part II: The heart

One third of all deaths worldwide in 2008 were caused by cardiovascular diseases (CVD), and the incidence of CVD related deaths rises ever more. Thus, improved imaging techniques and modalities are needed for the evaluation of cardiac morphology and function. Cardiac magnetic resonance imaging (CMRI) is a minimally invasive technique that is increasingly important due to its high spatial and temporal resolution, its high soft tissue contrast and its ability of functional and quantitative imaging. It is widely accepted as the gold standard of cardiac functional analysis. In the short period of small animal MRI, remarkable progress has been achieved concerning new, fast imaging schemes as well as purpose-built equipment. Dedicated small animal scanners allow for tapping the full potential of recently developed animal models of cardiac disease. In this paper, we review state-of-the-art cardiac magnetic resonance imaging techniques and applications in small animals at ultra-high fields (UHF).

Accelerated dual-contrast first-pass perfusion MRI of the mouse heart: development and application to diet-induced obese mice

PURPOSE

Gene-modified mice may be used to elucidate molecular mechanisms underlying abnormal myocardial blow flow (MBF). We sought to develop a quantitative myocardial perfusion imaging technique for mice and to test the hypothesis that myocardial perfusion reserve (MPR) is reduced in a mouse model of diet-induced obesity (DIO).

METHODS

A dual-contrast saturation-recovery sequence with ky -t undersampling and a motion-compensated compressed sensing reconstruction algorithm was developed for first-pass MRI on a small-bore 7 Tesla system. Control mice were imaged at rest and with the vasodilators ATL313 and Regadenoson (n = 6 each). In addition, we imaged mice fed a high-fat diet (HFD) for 24 weeks.

RESULTS

In control mice, MBF was 5.7 ± 0.8 mL/g/min at rest and it increased to 11.8 ± 0.6 mL/g/min with ATL313 and to 10.4 ± 0.3 mL/g/min with Regadenoson. In HFD mice, we detected normal resting MBF (5.6 ± 0.4 versus 5.0 ± 0.3 on control diet), low MBF at stress (7.7 ± 0.4 versus 10.4 ± 0.3 on control diet, P < 0.05), and reduced MPR (1.4 ± 0.2 versus 2.0 ± 0.3 on control diet, P < 0.05).

CONCLUSION

Accelerated dual-contrast first-pass MRI with motion-compensated compressed sensing provides spatiotemporal resolution suitable for measuring MBF in free-breathing mice, and detected reduced MPR in DIO mice. These techniques may be used to study molecular mechanisms that underlie abnormal myocardial perfusion.

In vivo characterization of rodent cyclic myocardial perfusion variation at rest and during adenosine-induced stress using cine-ASL cardiovascular magnetic resonance

BACKGROUND

Assessment of cyclic myocardial blood flow (MBF) variations can be an interesting addition to the characterization of microvascular function and its alterations. To date, totally non-invasive in vivo methods with this capability are still lacking. As an original technique, a cine arterial spin labeling (ASL) cardiovascular magnetic resonance approach is demonstrated to be able to produce dynamic MBF maps across the cardiac cycle in rats.

METHOD

High-resolution MBF maps in left ventricular myocardium were computed from steady-state perfusion-dependent gradient-echo cine images produced by the cine-ASL sequence. Cyclic changes of MBF over the entire cardiac cycle in seven normal rats were analyzed quantitatively every 6 ms at rest and during adenosine-induced stress.

RESULTS

The study showed a significant MBF increase from end-systole (ES) to end-diastole (ED) in both physiological states. Mean MBF over the cardiac cycle within the group was 5.5 ± 0.6 mL g(-1) min(-1) at rest (MBFMin = 4.7 ± 0.8 at ES and MBFMax = 6.5 ± 0.6 mL g(-1) min(-1) at ED, P = 0.0007). Mean MBF during adenosine-induced stress was 12.8 ± 0.7mL g(-1) min(-1) (MBFMin = 11.7±1.0 at ES and MBFMax = 14.2 ± 0.7 mL g(-1) min(-1) at ED, P = 0.0007). MBF percentage relative variations were significantly different with 27.2 ± 9.3% at rest and 17.8 ± 7.1% during adenosine stress (P = 0.014). The dynamic analysis also showed a time shift of peak MBF within the cardiac cycle during stress.

CONCLUSION

The cyclic change of myocardial perfusion was examined by mapping MBF with a steady-pulsed ASL approach. Dynamic MBF maps were obtained with high spatial and temporal resolution (6 ms) demonstrating the feasibility of non-invasively mapping cyclic myocardial perfusion variation at rest and during adenosine stress. In a pathological context, detailed assessment of coronary responses to infused vasodilators may give valuable complementary information on microvascular functional defects in disease models.

The retrobulbar sinus is superior to the lateral tail vein for the injection of contrast media in small animal cardiac imaging

Cardiac perfusion studies using computed tomography are a common tool in clinical practice. Recent technical advances and the availability of dedicated small animal scanners allow the transfer of these techniques to the preclinical sector in general and to mouse models of cardiac diseases in particular. This necessitates new requirements for contrast injection techniques as a rapid transport of contrast media from the intravenous access to the animal heart. Clinical contrast agents containing high iodine concentrations are used within small animal studies although they exhibit a high viscosity which might limit their transport within the vasculature. The authors provide a comparison of the transport of contrast media following an injection into the lateral tail vein and an injection into the retrobulbar sinus and discuss the anatomy involved. The temporal evolution of a contrast bolus and its in vivo distribution is visualized. It is demonstrated that injecting contrast agents into the lateral tail vein of mice results in a retrograde blood flow to the liver veins and therefore does not deliver a detectable contrast bolus to the heart, and thus it cannot be used for cardiac perfusion studies. By contrast, boli injected into the retrobulbar sinus are rapidly transported to the heart and provide ventricular contrast enabling perfusion studies similar to those in human patients. The results demonstrate that an injection into the retrobulbar sinus is superior to an injection into the lateral tail vein for the delivery of contrast boli to the animal heart, while all drawbacks of an injection into the lateral tail vein are overcome.

Hyperemic stress myocardial perfusion cardiovascular magnetic resonance in mice at 3 Tesla: initial experience and validation against microspheres

BACKGROUND

Dynamic first pass contrast-enhanced myocardial perfusion is the standard CMR method for the estimation of myocardial blood flow (MBF) and MBF reserve in man, but it is challenging in rodents because of the high temporal and spatial resolution requirements. Hyperemic first pass myocardial perfusion CMR during vasodilator stress in mice has not been reported.

METHODS

Five C57BL/6 J mice were scanned on a clinical 3.0 Tesla Achieva system (Philips Healthcare, Netherlands). Vasodilator stress was induced via a tail vein catheter with an injection of dipyridamole. Dynamic contrast-enhanced perfusion imaging (Gadobutrol 0.1 mmol/kg) was based on a saturation recovery spoiled gradient echo method with 10-fold k-space and time domain undersampling (k-t PCA). One week later the mice underwent repeat anaesthesia and LV injections of fluorescent microspheres at rest and at stress. Microspheres were analysed using confocal microscopy and fluorescence-activated cell sorting.

RESULTS

Mean MBF at rest measured by Fermi-function constrained deconvolution was 4.1 ± 0.5 ml/g/min and increased to 9.6 ± 2.5 ml/g/min during dipyridamole stress (P = 0.005). The myocardial perfusion reserve was 2.4 ± 0.54. The mean count ratio of stress to rest microspheres was 2.4 ± 0.51 using confocal microscopy and 2.6 ± 0.46 using fluorescence. There was good agreement between cardiovascular magnetic resonance CMR and microspheres with no significant difference (P = 0.84).

CONCLUSION

First-pass myocardial stress perfusion CMR in a mouse model is feasible at 3 Tesla. Rest and stress MBF values were consistent with existing literature and perfusion reserve correlated closely to microsphere analysis. Data were acquired on a 3 Tesla scanner using an approach similar to clinical acquisition protocols, potentially facilitating translation of imaging findings between rodent and human studies.

Quantitative T 2* assessment of acute and chronic myocardial ischemia/reperfusion injury in mice

OBJECT

Imaging of myocardial infarct composition is essential to assess efficacy of emerging therapeutics. T (2) (*) mapping has the potential to image myocardial hemorrhage and fibrosis by virtue of its short T (2) (*) . We aimed to quantify T (2) (*) in acute and chronic myocardial ischemia/reperfusion (I/R) injury in mice.

MATERIALS AND METHODS

I/R-injury was induced in C57BL/6 mice (n = 9). Sham-operated mice (n = 8) served as controls. MRI was performed at baseline, and 1, 7 and 28 days after surgery. MRI at 9.4 T consisted of Cine, T (2) (*) mapping and late-gadolinium-enhancement (LGE). Mice (n = 6) were histologically assessed for hemorrhage and collagen in the fibrotic scar.

RESULTS

Baseline T (2) (*) values were 17.1 ± 2.0 ms. At day 1, LGE displayed a homogeneous infarct enhancement. T (2) (*) in infarct (12.0 ± 1.1 ms) and remote myocardium (13.9 ± 0.8 ms) was lower than at baseline. On days 7 and 28, LGE was heterogeneous. T (2) (*) in the infarct decreased to 7.9 ± 0.7 and 6.4 ± 0.7 ms, whereas T (2) (*) values in the remote myocardium were 14.2 ± 1.1 and 15.6 ± 1.0 ms. Histology revealed deposition of iron and collagen in parallel with decreased T (2) (*) .

CONCLUSION

T (2) (*) values are dynamic during infarct development and decrease significantly during scar maturation. In the acute phase, T (2) (*) values in infarcted myocardium differ significantly from those in the chronic phase. T (2) (*) mapping was able to confirm the presence of a chronic infarction in cases where LGE was inconclusive. Hence, T (2) (*) may be used to discriminate between acute and chronic infarctions.

Contrast-enhanced MRI of murine myocardial infarction - part II

Mouse models are increasingly used to study the pathophysiology of myocardial infarction in vivo. In this area, MRI has become the gold standard imaging modality, because it combines high spatial and temporal resolution functional imaging with a large variety of methods to generate soft tissue contrast. In addition, (target-specific) MRI contrast agents can be employed to visualize different processes in the cascade of events following myocardial infarction. Here, the MRI sequence has a decisive role in the detection sensitivity of a contrast agent. However, a straightforward translation of clinically available protocols for human cardiac imaging to mice is not feasible, because of the small size of the mouse heart and its extremely high heart rate. This has stimulated intense research in the development of cardiac MRI protocols specifically tuned to the mouse with regard to timing parameters, acquisition strategies, and ECG- and respiratory-triggering methods to find an optimal trade-off between sensitivity, scan time, and image quality. In this review, a detailed analysis is given of the pros and cons of different mouse cardiac MR imaging methodologies and their application in contrast-enhanced MRI of myocardial infarction.

Distribution of lipid-based nanoparticles to infarcted myocardium with potential application for MRI-monitored drug delivery

Adverse cardiac remodeling after myocardial infarction ultimately causes heart failure. To stimulate reparative processes in the infarct, efficient delivery and retention of therapeutic agents is desired. This might be achieved by encapsulation of drugs in nanoparticles. The goal of this study was to characterize the distribution pattern of differently sized long-circulating lipid-based nanoparticles, namely micelles (~15 nm) and liposomes (~100 nm), in a mouse model of myocardial infarction (MI). MI was induced in mice (n=38) by permanent occlusion of the left coronary artery. Nanoparticle accumulation following intravenous administration was examined one day and one week after surgery, representing the acute and chronic phase of MI, respectively. In vivo magnetic resonance imaging of paramagnetic lipids in the micelles and liposomes was employed to monitor the trafficking of nanoparticles to the infarcted myocardium. Ex vivo high-resolution fluorescence microscopy of fluorescent lipids was used to determine the exact location of the nanoparticles in the myocardium. In both acute and chronic MI, micelles permeated the entire infarct area, which renders them very suited for the local delivery of cardioprotective or anti-remodeling drugs. Liposomes displayed slower and more restricted extravasation from the vasculature and are therefore an attractive vehicle for the delivery of pro-angiogenic drugs. Importantly, the ability to non-invasively visualize both micelles and liposomes with MRI creates a versatile approach for the development of effective cardioprotective therapeutic interventions.

Tropism ablation and stealthing of oncolytic adenovirus enhances systemic delivery to tumors and improves virotherapy of cancer

Intravenous delivery of therapeutic virus particles remains a major goal for virotherapy of metastatic cancer. Avoiding phagocytic capture and unwanted infection of nontarget cells is essential for extended plasma particle kinetics, and simply ablating one or the other does not give extended plasma circulation. Here we show that polymer coating of adenovirus type 5 (Ad5) can combine with predosing strategies or Kupffer cell ablation to achieve systemic kinetics with a half-life >60 min, allowing ready access to peripheral tumors. Accumulation of virus particles within tumor nodules is proportional to the area under the plasma concentration/time curve. Polymer coating wild-type Ad5 in this way is known to decrease hepatic toxicity, increasing the dose of virus particles that can be safely administered. Using polymer-coating technology to deliver a replicating Ad5 systemically, virus replication and transgene expression was almost totally confined to tumor tissues, giving a much improved therapeutic index compared with uncoated virus, and complete control of human HepG2 tumor xenografts.

First-pass perfusion CMR two days after infarction predicts severity of functional impairment six weeks later in the rat heart

BACKGROUND

In humans, dynamic contrast CMR of the first pass of a bolus infusion of Gadolinium-based contrast agent has become a standard technique to identify under-perfused regions of the heart and can accurately demonstrate the severity of myocardial infarction. Despite the clinical importance of this method, it has rarely been applied in small animal models of cardiac disease. In order to identify perfusion delays in the infarcted rat heart, here we present a method in which a T1 weighted MR image has been acquired during each cardiac cycle.

METHODS AND RESULTS

In isolated perfused rat hearts, contrast agent infusion gave uniform signal enhancement throughout the myocardium. Occlusion of the left anterior descending coronary artery significantly reduced the rate of signal enhancement in anterior regions of the heart, demonstrating that the first-pass method was sensitive to perfusion deficits. In vivo measurements of myocardial morphology, function, perfusion and viability were made at 2 and 8 days after infarction. Morphology and function were further assessed using cine-MRI at 42 days. The perfusion delay was larger in rat hearts that went on to develop greater functional impairment, demonstrating that first-pass CMR can be used as an early indicator of infarct severity. First-pass CMR at 2 and 8 days following infarction better predicted outcome than cardiac ejection fraction, end diastolic volume or end systolic volume.

CONCLUSION

First-pass CMR provides a predictive measure of the severity of myocardial impairment caused by infarction in a rodent model of heart failure.

Emerging MRI methods in translational cardiovascular research

Cardiac magnetic resonance imaging (CMR) has become a reference standard modality for imaging of left ventricular (LV) structure and function and, using late gadolinium enhancement, for imaging myocardial infarction. Emerging CMR techniques enable a more comprehensive examination of the heart, making CMR an excellent tool for use in translational cardiovascular research. Specifically, emerging CMR methods have been developed to measure the extent of myocardial edema, changes in ventricular mechanics, changes in tissue composition as a result of fibrosis, and changes in myocardial perfusion as a function of both disease and infarct healing. New CMR techniques also enable the tracking of labeled cells, molecular imaging of biomarkers of disease, and changes in calcium flux in cardiomyocytes. In addition, MRI can quantify blood flow velocity and wall shear stress in large blood vessels. Almost all of these techniques can be applied in both pre-clinical and clinical settings, enabling both the techniques themselves and the knowledge gained using such techniques in pre-clinical research to be translated from the lab bench to the patient bedside.