Black blood gradient echo cine magnetic resonance imaging of the mouse heart

Feasibility of Free-Breathing, Non-ECG-Gated, Black-Blood Cine Magnetic Resonance Images With Multitasking in Measuring Left Ventricular Function Indices

OBJECTIVE

To clinically validate the feasibility and accuracy of cine images acquired through the multitasking method, with no electrocardiogram gating and free-breathing, in measuring left ventricular (LV) function indices by comparing them with those acquired through the balanced steady-state free precession (bSSFP) method, with multiple breath-holds and electrocardiogram gating.

MATERIALS AND METHODS

Forty-three healthy volunteers (female:male, 30:13; mean age, 23.1 ± 2.3 years) and 36 patients requiring an assessment of LV function for various clinical indications (female:male, 22:14; 57.8 ± 11.3 years) were enrolled in this prospective study. Each participant underwent cardiac magnetic resonance imaging (MRI) using the multiple breath-hold bSSFP method and free-breathing multitasking method. LV function parameters were measured for both MRI methods. Image quality was assessed through subjective image quality scores (1 to 5) and calculation of the contrast-to-noise ratio (CNR) between the myocardium and blood pool. Differences between the two MRI methods were analyzed using the Bland-Altman plot, paired t-test, or Wilcoxon signed-rank test, as appropriate.

RESULTS

LV ejection fraction (LVEF) was not significantly different between the two MRI methods (P = 0.222 in healthy volunteers and P = 0.343 in patients). LV end-diastolic mass was slightly overestimated with multitasking in both healthy volunteers (multitasking vs. bSSFP, 60.5 ± 10.7 g vs. 58.0 ± 10.4 g, respectively; P < 0.001) and patients (69.4 ± 18.1 g vs. 66.8 ± 18.0 g, respectively; P = 0.003). Acceptable and comparable image quality was achieved for both MRI methods (multitasking vs. bSSFP, 4.5 ± 0.7 vs. 4.6 ± 0.6, respectively; P = 0.203). The CNR between the myocardium and blood pool showed no significant differences between the two MRI methods (18.89 ± 6.65 vs. 18.19 ± 5.83, respectively; P = 0.480).

CONCLUSION

Multitasking-derived cine images obtained without electrocardiogram gating and breath-holding achieved similar image quality and accurate quantification of LVEF in healthy volunteers and patients.

Progressive Cardiac Metabolic Defects Accompany Diastolic and Severe Systolic Dysfunction in Spontaneously Hypertensive Rat Hearts

Background Cardiac metabolic abnormalities are present in heart failure. Few studies have followed metabolic changes accompanying diastolic and systolic heart failure in the same model. We examined metabolic changes during the development of diastolic and severe systolic dysfunction in spontaneously hypertensive rats (SHR). Methods and Results We serially measured myocardial glucose uptake rates with dynamic 2-[18F] fluoro-2-deoxy-d-glucose positron emission tomography in vivo in 9-, 12-, and 18-month-old SHR and Wistar Kyoto rats. Cardiac magnetic resonance imaging determined systolic function (ejection fraction) and diastolic function (isovolumetric relaxation time) and left ventricular mass in the same rats. Cardiac metabolomics was performed at 12 and 18 months in separate rats. At 12 months, SHR hearts, compared with Wistar Kyoto hearts, demonstrated increased isovolumetric relaxation time and slightly reduced ejection fraction indicating diastolic and mild systolic dysfunction, respectively, and higher (versus 9-month-old SHR decreasing) 2-[18F] fluoro-2-deoxy-d-glucose uptake rates (Ki). At 18 months, only few SHR hearts maintained similar abnormalities as 12-month-old SHR, while most exhibited severe systolic dysfunction, worsening diastolic function, and markedly reduced 2-[18F] fluoro-2-deoxy-d-glucose uptake rates. Left ventricular mass normalized to body weight was elevated in SHR, more pronounced with severe systolic dysfunction. Cardiac metabolite changes differed between SHR hearts at 12 and 18 months, indicating progressive defects in fatty acid, glucose, branched chain amino acid, and ketone body metabolism. Conclusions Diastolic and severe systolic dysfunction in SHR are associated with decreasing cardiac glucose uptake, and progressive abnormalities in metabolite profiles. Whether and which metabolic changes trigger progressive heart failure needs to be established.

Obesity-Induced Coronary Microvascular Disease Is Prevented by iNOS Deletion and Reversed by iNOS Inhibition

Coronary microvascular disease (CMD) caused by obesity and diabetes is major contributor to heart failure with preserved ejection fraction; however, the mechanisms underlying CMD are not well understood. Using cardiac magnetic resonance applied to mice fed a high-fat, high-sucrose diet as a model of CMD, we elucidated the role of inducible nitric oxide synthase (iNOS) and 1400W, an iNOS antagonist, in CMD. Global iNOS deletion prevented CMD along with the associated oxidative stress and diastolic and subclinical systolic dysfunction. The 1400W treatment reversed established CMD and oxidative stress and preserved systolic/diastolic function in mice fed a high-fat, high-sucrose diet. Thus, iNOS may represent a therapeutic target for CMD.

Reproducibility of Systolic Strain in Mice Using Cardiac Magnetic Resonance Feature Tracking of Black-Blood Cine Images

PURPOSE

Mouse models are widely utilized to enhance our understanding of cardiac disease. The goal of this study is to investigate the reproducibility of strain parameters that were measured in mice using cardiac magnetic resonance (CMR) feature-tracking (CMR42, Canada).

METHODS

We retrospectively analyzed black-blood CMR datasets from thirteen C57BL/6 B6.SJL-CD45.1 mice (N = 10 female, N = 3 male) that were imaged previously. The circumferential, longitudinal, and radial (Ecc, Ell, and Err, respectively) parameters of strain were measured in the mid-ventricular region of the left ventricle. Intraobserver and interobserver reproducibility were assessed for both the end-systolic (ES) and peak strain.

RESULTS

The ES strain had larger intraclass correlation coefficient (ICC) values when compared to peak strain, for both the intraobserver and interobserver reproducibility studies. Specifically, the intraobserver study showed excellent reproducibility for all three ES strain parameters, namely, Ecc (ICC 0.95, 95% CI 0.83-0.98), Ell (ICC 0.90, 95% CI 0.59-0.97), and Err (ICC 0.92, 95% CI 0.73-0.97). This was also the case for the interobserver study, namely, Ecc (ICC 0.92, 95% CI 0.60-0.98), Ell (ICC 0.76, 95% CI 0.33-0.93), and Err (ICC 0.93, 95% CI 0.68-0.98). Additionally, the coefficient of variation values were all < 10%.

CONCLUSION

The results of this preliminary study showed excellent reproducibility for all ES strain parameters, with good to excellent reproducibility for the peak strain parameters. Moreover, all ES strain parameters had larger ICC values than the peak strain. In general, these results imply that feature-tracking with CMR42 software and black-blood cine images can be reliably used to assess strain patterns in mice.

Case report: Use of unenhanced cardiac MR to evaluate low flow states for thrombus

Left atrial spontaneous echo contrast is an echocardiographic finding associated with hypercoagulable states and stasis and can be clinically correlated with a thromboembolism in patients with atrial fibrillation, mitral stenosis, or mitral valve repair. Black blood double inversion recovery cardiac magnetic resonance is used to map out cardiac anatomy and demonstrates higher signal in blood flowing at a slower rate, such as at the sites of aortic aneurysms, aortic dissections and vessel occlusions. Here, we present the case of a 66-year-old woman with persistent atrial fibrillation and left atrial findings of spontaneous echo contrast and high signal on black blood double inversion recovery cardiac magnetic resonance imaging, raising concern for a thrombus. However, this was ruled out with subsequent bright blood cardiac magnetic resonance imaging demonstrating high signal with transitory central swirling low signal in the left atrium. Thus, cardiac magnetic resonance imaging can be used to demonstrate non-thrombotic low flow states in cases where contrast is contraindicated.

Accelerated fatty acid composition MRI of epicardial adipose tissue: Development and application to eplerenone treatment in a mouse model of obesity-induced coronary microvascular disease

Purpose

To develop an accelerated MRI method to quantify the epicardial adipose tissue (EAT) fatty acid composition (FAC) and test the hypothesis that eplerenone (EPL) shifts the EAT FAC toward unsaturation in obese mice.

Methods

Undersampled multi‐echo gradient echo imaging employing a dictionary‐based compressed‐sensing reconstruction and iterative decomposition with echo asymmetry and least‐squares–based mapping (IDEAL) was developed, validated, and used to study EAT in obese mice scanned at 7T. Fully sampled and rate 2, 2.5, 3, and 3.5 undersampled image data were acquired, reconstructed, and assessed using RMSE and structural similarity (SSIM). Two groups of mice were studied: untreated (control, n = 10) and EPL‐treated (n = 10) mice fed a high‐fat high‐sucrose diet. MRI included imaging of EAT FAC, EAT volume, and myocardial perfusion reserve.

Results

Rate 3 acceleration provided RMSE <5% and structural similarity >0.85 for FAC MRI. After 6 weeks of diet, EPL‐treated compared to untreated mice had a reduced EAT saturated fatty acid fraction (0.27 ± 0.09 vs. 0.39 ± 0.07, P < 0.05) and increased EAT unsaturation degree (4.37 ± 0.32 vs. 3.69 ± 0.58, P < 0.05). Also, EAT volume in EPL‐treated compared to untreated mice was reduced (8.1 ± 0.6 mg vs. 11.4 ± 0.7 mg, P < 0.01), and myocardial perfusion reserve was improved (1.83 ± 0.15 vs. 1.61 ± 0.17, P < 0.05).

Conclusion

Rate 3 accelerated FAC MRI enabled accurate quantification of EAT FAC in mice. EPL treatment shifted the EAT FAC toward increased unsaturation and was associated with improvement of coronary microvascular function.

Click here for author‐reader discussions

Cardiac MRI Assessment of Mouse Myocardial Infarction and Regeneration

Small animal models are indispensable for cardiac regeneration research. Studies in mouse and rat models have provided important insights into the etiology and mechanisms of cardiovascular diseases and accelerated the development of therapeutic strategies. It is vitally important to be able to evaluate the therapeutic efficacy and have reliable surrogate markers for therapeutic development for cardiac regeneration research. Magnetic resonance imaging (MRI), a versatile and noninvasive imaging modality with excellent penetration depth, tissue coverage, and soft-tissue contrast, is becoming a more important tool in both clinical settings and research arenas. Cardiac MRI (CMR) is versatile, noninvasive, and capable of measuring many different aspects of cardiac functions, and, thus, is ideally suited to evaluate therapeutic efficacy for cardiac regeneration. CMR applications include assessment of cardiac anatomy, regional wall motion, myocardial perfusion, myocardial viability, cardiac function assessment, assessment of myocardial infarction, and myocardial injury. Myocardial infarction models in mice are commonly used model systems for cardiac regeneration research. In this chapter, we discuss various CMR applications to evaluate cardiac functions and inflammation after myocardial infarction.

Metabolic Changes in Spontaneously Hypertensive Rat Hearts Precede Cardiac Dysfunction and Left Ventricular Hypertrophy

Background

Sustained pressure overload leads to changes in cardiac metabolism, function, and structure. Both time course and causal relationships between these changes are not fully understood. Therefore, we studied spontaneously hypertensive rats (SHR) during early hypertension development and compared them to control Wistar Kyoto rats.

Methods and Results

We serially evaluated myocardial glucose uptake rates (Ki) with dynamic 2‐[¹⁸F] fluoro‐2‐deoxy‐D‐glucose positron emission tomography, and ejection fraction and left ventricular mass to body weight ratios with cardiac magnetic resonance imaging in vivo, determined glucose uptake and oxidation rates in isolated perfused hearts, and analyzed metabolites, mammalian target of rapamycin activity and endoplasmic reticulum stress in dissected hearts. When compared with Wistar Kyoto rats, SHR demonstrated increased glucose uptake rates (Ki) in vivo, and reduced ejection fraction as early as 2 months of age when hypertension was established. Isolated perfused SHR hearts showed increased glucose uptake and oxidation rates starting at 1 month. Cardiac metabolite analysis at 2 months of age revealed elevated pyruvate, fatty acyl‐ and branched chain amino acid‐derived carnitines, oxidative stress, and inflammation. Mammalian target of rapamycin activity increased in SHR beginning at 2 months. Left ventricular mass to body weight ratios and endoplasmic reticulum stress were elevated in 5 month‐old SHR.

Conclusions

Thus, in a genetic hypertension model, chronic cardiac pressure overload promptly leads to increased myocardial glucose uptake and oxidation, and to metabolite abnormalities. These coincide with, or precede, cardiac dysfunction while left ventricular hypertrophy develops only later. Myocardial metabolic changes may thus serve as early diagnostic markers for hypertension‐induced left ventricular hypertrophy.

4D cardiac imaging at clinical 3.0T provides accurate assessment of murine myocardial function and viability

OBJECTIVES

We validate a 4D strategy tailored for 3T clinical systems to simultaneously quantify function and infarct size in wild type mice after ischemia/reperfusion, with improved spatial and temporal resolution by comparison to previous published protocols using clinical field MRI systems.

METHODS

C57BL/6J mice underwent 60min ischemia/reperfusion (n=14) or were controls without surgery (n=6). Twenty-four hours after surgery mice were imaged with gadolinium injection and sacrificed for post-mortem MRI and histology with serum also taken for Troponin I levels. The double ECG- and respiratory-triggered 3D FLASH (Fast Low Angle Shot) gradient echo (GRE) cine sequence had an acquired isotropic resolution of 344μm, TR/TE of 7.8/2.9ms and acquisition time 25-35min. The conventional 2D FLASH cine sequence had the same in-plane resolution of 344μm, 1mm slice thickness and TR/TE 11/5.4ms for an acquisition time of 20-25min plus 5min for planning. Left ventricle (LV) and right ventricle (RV) volumes were measured and functional parameters compared 2D to 3D, left to right and for inter and intra observer reproducibility. MRI infarct volume was compared to histology.

RESULTS

For the function evaluation, the 3D cine outperformed 2D cine for spatial and temporal resolution. Protocol time for the two methods was equivalent (25-35min). Flow artifacts were reduced (p=0.008) and epi/endo-cardial delineation showed good intra and interobserver reproducibility. Paired t-test comparing ejection volume left to right showed no significant difference for 3D (p=0.37), nor 2D (p=0.30) and correlation slopes of left to right EV were 1.17 (R²=0.75) for 2D and 1.05 (R²=0.50) for 3D. Quantifiable 'late gadolinium enhancement' infarct volume was seen only with the 3D cine and correlated to histology (R²=0.89). Left ejection fraction and MRI-measured infarct volume correlated (R²>0.3).

CONCLUSIONS

The 4D strategy, with contrast injection, was validated in mice for function and infarct quantification from a single scan with minimal slice planning.

Cardiovascular magnetic resonance detects the progression of impaired myocardial perfusion reserve and increased left-ventricular mass in mice fed a high-fat diet

BACKGROUND

Impaired myocardial perfusion reserve (MPR) is prevalent in obesity and diabetes, even in the absence of obstructive coronary artery disease (CAD), and is prognostic of adverse events. We sought to establish the time course of reduced MPR and to investigate associated vascular and tissue properties in mice fed a high-fat diet (HFD), as they are an emerging model of human obesity, diabetes, and reduced MPR without obstructive CAD.

METHODS

C57Bl/6 mice fed a HFD or a low-fat diet (control) were imaged at 6, 12, 18 and 24 weeks post-diet. The cardiovascular magnetic resonance (CMR) protocol included multi-slice cine imaging to assess ejection fraction (EF), left-ventricular (LV) mass, LV wall thickness (LVWT), and LV volumes, and first-pass perfusion CMR to quantify MPR. Coronary vascular reactivity, aortic atherosclerosis, myocardial capillary density and tissue fibrosis were also assessed.

RESULTS

Body weight was increased in HFD mice at 6-24 weeks post-diet (p < 0.05 vs. control). MPR in HFD mice was reduced and LV mass and LVWT were increased in HFD mice at 18 and 24 weeks post-diet (p < 0.05 vs. control). Coronary arteriolar vascular reactivity to adenosine and acetylcholine were reduced in HFD mice (p < 0.05 vs. control). There were no significant differences in cardiac volumes, EF, or capillary density measurements between the two groups. Histology showed interstitial fibrosis in HFD and no aortic atherosclerosis in either group.

CONCLUSIONS

C57Bl/6 mice fed a HFD for 18-24 weeks have progressively increased LV mass and impaired MPR with fibrosis, normal capillary density and no aortic plaque. These results establish C57Bl/6 mice fed a HFD for 18-24 weeks as a model of impaired MPR without obstructive CAD due to obesity and diabetes.

Development of Real-Time Magnetic Resonance Imaging of Mouse Hearts at 9.4 Tesla--Simulations and First Application

A novel method for real-time magnetic resonance imaging for the assessment of cardiac function in mice at 9.4 T is proposed. The technique combines a highly undersampled radial gradient echo acquisition with an image reconstruction utilizing both parallel imaging and compressed sensing. Simulations on an in silico phantom were performed to determine the achievable acceleration factor and to optimize regularization parameters. Several parameters characterizing the quality of the reconstructed images (such as spatial and temporal image sharpness or compartment areas) were calculated for this purpose. Subsequently, double-gated segmented cine data as well as non-gated undersampled real-time data using only six projections per timeframe (temporal resolution  ∼ 10 ms) were acquired in a mid-ventricular slice of four normal mouse hearts in vivo. The highly accelerated data sets were then subjected to the introduced reconstruction technique and results were validated against the fully sampled references. Functional parameters obtained from real-time and fully sampled data agreed well with a comparable accuracy for left-ventricular volumes and a slightly larger scatter for mass. This study introduces and validates a real-time cine-MRI technique, which significantly reduces scan time in preclinical cardiac functional imaging and has the potential to investigate mouse models with abnormal heart rhythm.

Analysis of left ventricular function of the mouse heart during experimentally induced hyperthyroidism and recovery

Many of the clinical manifestations of hyperthyroidism are due to the ability of thyroid hormones to alter myocardial contractility and cardiovascular hemodynamics, leading to cardiovascular impairment. In contrast, recent studies highlight also the potential beneficial effects of thyroid hormone administration for clinical or preclinical treatment of different diseases such as atherosclerosis, obesity and diabetes or as a new therapeutic approach in demyelinating disorders. In these contexts and in the view of developing thyroid hormone-based therapeutic strategies, it is, however, important to analyze undesirable secondary effects on the heart. Animal models of experimentally induced hyperthyroidism therefore represent important tools for investigating and monitoring changes of cardiac function. In our present study we use high-field cardiac MRI to monitor and follow-up longitudinally the effects of prolonged thyroid hormone (triiodothyronine) administration focusing on murine left ventricular function. Using a 9.4 T small horizontal bore animal scanner, cinematographic MRI was used to analyze changes in ejection fraction, wall thickening, systolic index and fractional shortening. Cardiac MRI investigations were performed after sustained cycles of triiodothyronine administration and treatment arrest in adolescent (8 week old) and adult (24 week old) female C57Bl/6 N mice. Triiodothyronine supplementation of 3 weeks led to an impairment of cardiac performance with a decline in ejection fraction, wall thickening, systolic index and fractional shortening in both age groups but with a higher extent in the group of adolescent mice. However, after a hormonal treatment cessation of 3 weeks, only young mice are able to partly restore cardiac performance in contrast to adult mice lacking this recovery potential and therefore indicating a presence of chronically developed heart pathology.

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.

Extracellular superoxide dismutase ameliorates skeletal muscle abnormalities, cachexia, and exercise intolerance in mice with congestive heart failure

BACKGROUND

Congestive heart failure (CHF) is a leading cause of morbidity and mortality, and oxidative stress has been implicated in the pathogenesis of cachexia (muscle wasting) and the hallmark symptom, exercise intolerance. We have previously shown that a nitric oxide-dependent antioxidant defense renders oxidative skeletal muscle resistant to catabolic wasting. Here, we aimed to identify and determine the functional role of nitric oxide-inducible antioxidant enzyme(s) in protection against cardiac cachexia and exercise intolerance in CHF.

METHODS AND RESULTS

We demonstrated that systemic administration of endogenous nitric oxide donor S-nitrosoglutathione in mice blocked the reduction of extracellular superoxide dismutase (EcSOD) protein expression, as well as the induction of MAFbx/Atrogin-1 mRNA expression and muscle atrophy induced by glucocorticoid. We further showed that endogenous EcSOD, expressed primarily by type IId/x and IIa myofibers and enriched at endothelial cells, is induced by exercise training. Muscle-specific overexpression of EcSOD by somatic gene transfer or transgenesis (muscle creatine kinase [MCK]-EcSOD) in mice significantly attenuated muscle atrophy. Importantly, when crossbred into a mouse genetic model of CHF (α-myosin heavy chain-calsequestrin), MCK-EcSOD transgenic mice had significant attenuation of cachexia with preserved whole body muscle strength and endurance capacity in the absence of reduced HF. Enhanced EcSOD expression significantly ameliorated CHF-induced oxidative stress, MAFbx/Atrogin-1 mRNA expression, loss of mitochondria, and vascular rarefaction in skeletal muscle.

CONCLUSIONS

EcSOD plays an important antioxidant defense function in skeletal muscle against cardiac cachexia and exercise intolerance in CHF.

Quantification of left ventricular volumes, mass, and ejection fraction using cine displacement encoding with stimulated echoes (DENSE) MRI

PURPOSE

To test the hypothesis that magnitude images from cine displacement encoding with stimulated echoes (DENSE) magnetic resonance imaging (MRI) can accurately quantify left ventricular (LV) volumes, mass, and ejection fraction (EF).

MATERIALS AND METHODS

Thirteen mice (C57BL/6J) were imaged using a 7T ClinScan MRI. A short-axis stack of cine T2-weighted black blood (BB) images was acquired for calculation of LV volumes, mass, and EF using the gold standard sum-of-slices methodology. DENSE images were acquired during the same imaging session in three short-axis (basal, mid, apical) and two long-axis orientations. A custom surface fitting algorithm was applied to epicardial and endocardial borders from the DENSE magnitude images to calculate volumes, mass, and EF. Agreement between the DENSE-derived measures and BB-derived measures was assessed via coefficient of variation (CoV).

RESULTS

3D surface reconstruction was completed on the order of seconds from segmented images, and required fewer slices to be segmented. Volumes, mass, and EF from DENSE-derived surfaces matched well with BB data (CoVs ≤11%).

CONCLUSION

LV mass, volumes, and EF in mice can be quantified through sparse (five slices) sampling with DENSE. This consolidation significantly reduces the time required to assess both mass/volume-based measures of cardiac function and advanced cardiac mechanics.

Glucose regulation of load-induced mTOR signaling and ER stress in mammalian heart

BACKGROUND

Changes in energy substrate metabolism are first responders to hemodynamic stress in the heart. We have previously shown that hexose-6-phosphate levels regulate mammalian target of rapamycin (mTOR) activation in response to insulin. We now tested the hypothesis that inotropic stimulation and increased afterload also regulate mTOR activation via glucose 6-phosphate (G6P) accumulation.

METHODS AND RESULTS

We subjected the working rat heart ex vivo to a high workload in the presence of different energy-providing substrates including glucose, glucose analogues, and noncarbohydrate substrates. We observed an association between G6P accumulation, mTOR activation, endoplasmic reticulum (ER) stress, and impaired contractile function, all of which were prevented by pretreating animals with rapamycin (mTOR inhibition) or metformin (AMPK activation). The histone deacetylase inhibitor 4-phenylbutyrate, which relieves ER stress, also improved contractile function. In contrast, adding the glucose analogue 2-deoxy-d-glucose, which is phosphorylated but not further metabolized, to the perfusate resulted in mTOR activation and contractile dysfunction. Next we tested our hypothesis in vivo by transverse aortic constriction in mice. Using a micro-PET system, we observed enhanced glucose tracer analog uptake and contractile dysfunction preceding dilatation of the left ventricle. In contrast, in hearts overexpressing SERCA2a, ER stress was reduced and contractile function was preserved with hypertrophy. Finally, we examined failing human hearts and found that mechanical unloading decreased G6P levels and ER stress markers.

CONCLUSIONS

We propose that glucose metabolic changes precede and regulate functional (and possibly also structural) remodeling of the heart. We implicate a critical role for G6P in load-induced mTOR activation and ER stress.

A nonpeptide angiotensin II type 2 receptor agonist does not attenuate postmyocardial infarction left ventricular remodeling in mice

Cardiac overexpression of the angiotensin II type 2 receptor (AT2 R) attenuates left ventricular (LV) remodeling after myocardial infarction (MI) in transgenic mice. We hypothesized that a novel nonpeptide AT2 R agonist, compound 21 (C21), would attenuate post-MI LV remodeling. Fifty-nine mice were studied for 28 days after 1-hour surgical occlusion-reperfusion of the left anterior descending coronary artery. Immediately thereafter, 23 mice received 0.3 mg·kg·d of C21 via Alzet osmotic minipump, 16 received 10 mg·kg·d of the AT1 R antagonist candesartan in drinking water, and 20 were untreated controls. Cardiac magnetic resonance imaging measured ejection fraction (EF), LV end-systolic, and end-diastolic volumes (ESVI and EDVI) indexed to weight serially post MI. Infarct size was measured on day 1 by late gadolinium-enhanced cardiac magnetic resonance imaging. At baseline, heart rate, blood pressure, EDVI, ESVI, and EF were similar between groups. Mean infarct size (42%-45% of LV mass) was similar between groups. C21-treated animals demonstrated adverse LV remodeling (increased EDVI and ESVI at all post-MI time points) compared with control. Candesartan therapy preserved left ventricular EF at day 28 compared with the C21-treated group. Thus, direct stimulation of the AT2 R by C21 at 0.3 mg·kg·d does not attenuate post-MI LV remodeling in reperfused MI in mice.

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.

Improved method for quantification of regional cardiac function in mice using phase-contrast MRI

Phase-contrast magnetic resonance imaging is a technique that allows for characterization of regional cardiac function and for measuring transmural myocardial velocities in human hearts with high temporal and spatial resolution. The application of this technique (also known as tissue phase mapping) to murine hearts has been very limited so far. The aim of our study was to implement and to optimize tissue phase mapping for a comprehensive assessment of murine transmural wall motion. Baseline values for regional motion patterns in mouse hearts, based on the clinically used American Heart Association's 17-segment model, were established, and a detailed motion analysis of mouse heart for the entire cardiac cycle (including epicardial and endocardial motion patterns) is provided. Black-blood contrast was found to be essential to obtain reproducible velocity encoding. Tissue phase mapping of the mouse heart permits the detailed assessment of regional myocardial velocities. While a proof-of-principle application in a murine ischemia-reperfusion model was performed, future studies are warranted to assess its potential for the investigation of systolic and diastolic functions in genetically and surgically manipulated mouse models of human heart disease.

Partial restoration of cardio-vascular defects in a rescued severe model of spinal muscular atrophy

Spinal muscular atrophy (SMA) is a leading genetic cause of infantile death. Loss of a gene called Survival Motor Neuron 1 (SMN1) and, as a result, reduced levels of the Survival Motor Neuron (SMN) protein leads to SMA development. SMA is characterized by the loss of functional motor neurons in the spinal cord. However, accumulating evidence suggests the contribution of other organs to the composite SMA phenotype and disease progression. A growing number of congenital heart defects have been identified in severe SMA patients. Consistent with the clinical cases, we have recently identified developmental and functional heart defects in two SMA mouse models, occurring at embryonic stage in a severe SMA model and shortly after birth in a less severe model (SMN∆7). Our goal was to examine the late stage cardiac abnormalities in untreated SMN∆7 mice and to determine whether gene replacement therapy restores cardiac structure/function in rescued SMN∆7 model. To reveal the extent of the cardiac structural/functional repair in the rescued mice, we analyzed the heart of untreated and treated SMN∆7 model using self-complementary Adeno-associated virus (serotype 9) expressing the full-length SMN cDNA. We examined the characteristics of the heart failure such as remodeling, fibrosis, oxidative stress, and vascular integrity in both groups. Our results clearly indicate that fibrosis, oxidative stress activation, vascular remodeling, and a significant decrease in the number of capillaries exist in the SMA heart. The cardiac structural defects were improved drastically in the rescued animals, however, the level of impairment was still significant compared to the age-matched wildtype littermates. Furthermore, functional analysis by in vivo cardiac magnetic resonance imaging (MRI) revealed that the heart of the treated SMA mice still exhibits functional defects. In conclusion, cardiac abnormalities are only partially rescued in post-birth treated SMA animals and these abnormalities may contribute to the premature death of vector-treated SMA animals with seemingly rescued motor function but an average life span of less than 70 days as reported in several studies.

Assessment of global cardiac function

High-resolution magnetic resonance cine imaging (cine-MRI) allows for a non-invasive assessment of ventricular function and mass in normal mice and in genetically and surgically modified mouse models of cardiac disease. The assessment of myocardial mass and function by cine-MRI does not rely on geometric assumptions, as the hearts are covered from the base to the apex, typically by a stack of two-dimensional images. The MR data acquisition is then followed by image segmentation of specific cine frames in each slice to obtain geometric and functional parameters, such as end-diastolic volume (EDV), end-systolic volume (ESV) or ejection fraction (EF). This technique has been well established in clinical routine application and it is now also becoming the reference method in experimental cardiovascular MRI. The cine images are typically acquired in short- and long-axis orientations of the heart to facilitate an accurate assessment of cardiac functional parameters. These views can be difficult to identify, particularly in animals with diseased hearts. Furthermore, data analysis can be the source of a systematic error, mainly for myocardial mass measurement. We have established protocols that allow for a quick and reproducible way of obtaining the relevant cardiac views for cine-MRI, and for accurate image analysis.

Cardiovascular magnetic resonance imaging in small animals

Noninvasive imaging studies involving small animals are becoming increasingly important in preclinical pharmacological, genetic, and biomedical cardiovascular research. Especially small animal magnetic resonance imaging (MRI) using high field and clinical MRI systems has gained significant importance in recent years. Compared to other imaging modalities, like computer tomography, MRI can provide an excellent soft tissue contrast, which enables the characterization of different kinds of tissues without the use of contrast agents. In addition, imaging can be performed with high spatial and temporal resolution. Small animal MRI cannot only provide anatomical information about the beating murine heart; it can also provide functional and molecular information, which makes it a unique imaging modality. Compared to clinical MRI examinations in humans, small animal MRI is associated with additional challenges. These included a smaller size of all cardiovascular structures and a up to ten times higher heart rate. Dedicated small animal monitoring devices make a reliable cardiac triggering and respiratory gating feasible. MRI in combination with molecular probes enables the noninvasive imaging of biological processes at a molecular level. Different kinds of iron oxide or gadolinium-based contrast agents can be used for this purpose. Compared to other molecular imaging modalities, like single photon emission computed tomography (SPECT) and positron emission tomography (PET), MRI can also provide imaging with high spatial resolution, which is of high importance for the assessment of the cardiovascular system. The sensitivity for detection of MRI contrast agents is however lower compared to sensitivity of radiation associated techniques like PET and SPECT. This chapter is divided into the following sections: (1) "Introduction," (2) "Principals of Magnetic Resonance Imaging," (3) "MRI Systems for Preclinical Imaging and Experimental Setup," and (4) "Cardiovascular Magnetic Resonance Imaging."

Displacement-encoded and manganese-enhanced cardiac MRI reveal that nNOS, not eNOS, plays a dominant role in modulating contraction and calcium influx in the mammalian heart

Within cardiomyocytes, endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) are thought to modulate L-type calcium channel (LTCC) function and sarcoplasmic reticulum calcium cycling, respectively. However, divergent results from mostly invasive prior studies suggest more complex roles. To elucidate the roles of nNOS and eNOS in vivo, we applied noninvasive cardiac MRI to study wild-type (WT), eNOS(-/-), and nNOS(-/-) mice. An in vivo index of LTCC flux (LTCCI) was measured at baseline (Bsl), dobutamine (Dob), and dobutamine + carbacholamine (Dob + CCh) using manganese-enhanced MRI. Displacement-encoded MRI assessed contractile function by measuring circumferential strain (E(cc)) and systolic (dE(cc)/dt) and diastolic (dE(cc)/dt(diastolic)) strain rates at Bsl, Dob, and Dob + CCh. Bsl LTCCI was highest in nNOS(-/-) mice (P < 0.05 vs. WT and eNOS(-/-)) and increased only in WT and eNOS(-/-) mice with Dob (P < 0.05 vs. Bsl). LTCCI decreased significantly from Dob levels with Dob + CCh in all mice. Contractile function, as assessed by E(cc), was similar in all mice at Bsl. With Dob, E(cc) increased significantly in WT and eNOS(-/-) but not nNOS(-/-) mice (P < 0.05 vs. WT and eNOS(-/-)). With Dob + CCh, E(cc) returned to baseline levels in all mice. Systolic blood pressure, measured via tail plethysmography, was highest in eNOS(-/-) mice (P < 0.05 vs. WT and nNOS(-/-)). Mice deficient in nNOS demonstrate increased Bsl LTCC function and an attenuated contractile reserve to Dob, whereas eNOS(-/-) mice demonstrate normal LTCC and contractile function under all conditions. These results suggest that nNOS, not eNOS, plays the dominant role in modulating Ca(2+) cycling in the heart.

Mouse phenotyping with MRI

The field of mouse phenotyping with magnetic resonance imaging (MRI) is rapidly growing, motivated by the need for improved tools for characterizing and evaluating mouse models of human disease. Image results can provide important comparisons of human conditions with mouse disease models, evaluations of treatment, development or disease progression, as well as direction for histological or other investigations. Effective mouse MRI studies require attention to many aspects of experiment design. In this chapter, we provide details and discussion of important practical considerations: hardware requirements, mouse handling for in vivo imaging, specimen preparation for ex vivo imaging, sequence and contrast agent selection, study size, and quantitative image analysis. We focus particularly on anatomical phenotyping, an important and accessible application that has shown a high potential for impact in many mouse models at our imaging center.

T₂ -weighted MRI of post-infarct myocardial edema in mice

T(2) -weighted, cardiac magnetic resonance imaging (T(2) w CMR) can be used to noninvasively detect and quantify the edematous region that corresponds to the area at risk (AAR) following myocardial infarction (MI). Previously, CMR has been used to examine structure and function in mice, expediting the study of genetic manipulations. To date, CMR has not been applied to imaging of post-MI AAR in mice. We developed a whole-heart, T(2) w CMR sequence to quantify the AAR in mouse models of ischemia and infarction. The ΔB(0) and ΔB(1) environment around the mouse heart at 7 T were measured, and a T(2) -preparation sequence suitable for these conditions was developed. Both in vivo T(2) w and late gadolinium enhanced CMR were performed in mice after 20-min coronary occlusions, resulting in measurements of AAR size of 32.5 ± 3.1 (mean ± SEM)% left ventricular mass, and MI size of 50.1 ± 6.4% AAR size. Excellent interobserver agreement and agreement with histology were also found. This T(2) w imaging method for mice may allow for future investigations of genetic manipulations and novel therapies affecting the AAR and salvaged myocardium following reperfused MI.

A fast black-blood sequence for four-dimensional cardiac manganese-enhanced MRI in mouse

The increasing number of mouse models of cardiac diseases requires improvements in the current MRI tools. Anatomic and functional cardiac phenotyping by MRI calls for both time and space resolution in three dimensions. Black-blood contrast is often needed for the accurate delineation of myocardium and chambers, and is consistent with manganese contrast enhancement. In this article, we propose a fast, three-dimensional, time-resolved (four-dimensional), black-blood MRI sequence that allows mouse heart imaging at 10 periods of the cardiac cycle within 30  min at an isotropic resolution of 200  µm. Two-dimensional imaging was possible within 80  s. Blood cancellation was achieved by employing bipolar gradients without the use of a double inversion recovery preparation scheme. Saturation slices were added in two-dimensional experiments for better blood nulling. The rapidity of the two-dimensional acquisition protocol allowed the measurement of the time course of contrast enhancement on manganese infusion. Owing to the very high contrast-to-noise ratio, manganese-enhanced MRI in four dimensions made possible the accurate assessment of regional cardiac volumes in healthy animals. In experimentally infarcted mice, the size of the ischemic zone could be measured easily with this method. The technique might be valuable in evaluating mouse heart diseases and their follow-up in longitudinal studies.

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.

Three-dimensional T1 mapping of the mouse heart using variable flip angle steady-state MR imaging

Cardiac MR T(1) mapping is a promising quantitative imaging tool for the diagnosis and evaluation of cardiomyopathy. Here, we present a new preclinical cardiac MRI method enabling three-dimensional T(1) mapping of the mouse heart. The method is based on a variable flip angle analysis of steady-state MR imaging data. A retrospectively triggered three-dimensional FLASH (fast low-angle shot) sequence (3D IntraGate) enables a constant repetition time and maintains steady-state conditions. 3D T(1) mapping of the complete mouse heart could be achieved in 20 min. High-quality, bright-blood T(1) maps were obtained with homogeneous T(1) values (1764 ± 172 ms) throughout the myocardium. The repeatability coefficient of R(1) (1/T(1) ) in a specific region of the mouse heart was between 0.14 and 0.20 s(-1) , depending on the number of flip angles. The feasibility for detecting regional differences in ΔR(1) was shown with pre- and post-contrast T(1) mapping in mice with surgically induced myocardial infarction, for which ΔR(1) values up to 0.83 s(-1) were found in the infarct zone. The sequence was also investigated in black-blood mode, which, interestingly, showed a strong decrease in the apparent mean T(1) of healthy myocardium (905 ± 110 ms). This study shows that 3D T(1) mapping in the mouse heart is feasible and can be used to monitor regional changes in myocardial T(1), particularly in relation to pathology and in contrast-enhanced experiments to estimate local concentrations of (targeted) contrast agent.

Cardiovascular phenotyping of the mouse heart using a 4D radial acquisition and liposomal Gd-DTPA-BMA

MR microscopy has enormous potential for small-animal cardiac imaging because it is capable of producing volumetric images at multiple time points to accurately measure cardiac function. MR has not been used as frequently as ultrasound to measure cardiac function in the small animal because the MR methods required relatively long scan times, limiting throughput. Here, we demonstrate four-dimensional radial acquisition in conjunction with a liposomal blood pool agent to explore functional differences in three populations of mice: six C57BL/6J mice, six DBA/2J mice, and six DBA/2J CSQ+ mice, all with the same gestational age and approximately the same weight. Cardiovascular function was determined by measuring both left ventricular and right ventricular end diastolic volume, end systolic volume, stroke volume, and ejection fraction. Statistical significance was observed in end diastolic volume, end systolic volume, and ejection fraction for left ventricular measurements between all three populations of mice. No statistically significant difference was observed in stroke volume in either the left or right ventricle for any of the three populations of mice. This study shows that MRI is capable of efficient, high-throughput, four-dimensional cardiovascular phenotyping of the mouse.

Improved arterial spin labeling after myocardial infarction in mice using cardiac and respiratory gated look-locker imaging with fuzzy C-means clustering

Experimental myocardial infarction (MI) in mice is an important disease model, in part due to the ability to study genetic manipulations. MRI has been used to assess cardiac structural and functional changes after MI in mice, but changes in myocardial perfusion after acute MI have not previously been examined. Arterial spin labeling noninvasively measures perfusion but is sensitive to respiratory motion and heart rate variability and is difficult to apply after acute MI in mice. To account for these factors, a cardiorespiratory-gated arterial spin labeling sequence using a fuzzy C-means algorithm to retrospectively reconstruct images was developed. Using this method, myocardial perfusion was measured in remote and infarcted regions at 1, 7, 14, and 28 days post-MI. Baseline perfusion was 4.9 +/- 0.5 mL/g min and 1 day post-MI decreased to 0.9 +/- 0.8 mL/g min in infarcted myocardium (P < 0.05 versus baseline) while remaining at 5.2 +/- 0.8 mL/g min in remote myocardium. During the subsequent 28 days, perfusion in the remote zone remained unchanged, while a partial recovery of perfusion in the infarct zone was seen. This technique, when applied to genetically engineered mice, will allow for the investigation of the roles of specific genes in myocardial perfusion during infarct healing.

Small-animal molecular imaging methods

The ability to trace or identify specific molecules within a specific anatomic location provides insight into metabolic pathways, tissue components, and tracing of solute transport mechanisms. With the increasing use of small animals for research, such imaging must have sufficiently high spatial resolution to allow anatomic localization as well as sufficient specificity and sensitivity to provide an accurate description of the molecular distribution and concentration.

METHODS

Imaging methods based on electromagnetic radiation, such as PET, SPECT, MRI, and CT, are increasingly applicable because of recent advances in novel scanner hardware and image reconstruction software and the availability of novel molecules that have enhanced sensitivity in these methodologies.

RESULTS

Small-animal PET has been advanced by the development of detector arrays that provide higher resolution and positron-emitting elements that allow new molecular tracers to be labeled. Micro-MRI has been improved in terms of spatial resolution and sensitivity through increased magnet field strength and the development of special-purpose coils and associated scan protocols. Of particular interest is the associated ability to image local mechanical function and solute transport processes, which can be directly related to the molecular information. This ability is further strengthened by the synergistic integration of PET with MRI. Micro-SPECT has been improved through the use of coded aperture imaging approaches as well as image reconstruction algorithms that can better deal with the photon-limited scan data. The limited spatial resolution can be partially overcome by integrating SPECT with CT. Micro-CT by itself provides exquisite spatial resolution of anatomy, but recent developments in high-spatial-resolution photon counting and spectrally sensitive imaging arrays, combined with x-ray optical devices, hold promise for actual molecular identification by virtue of the chemical bond lengths of molecules, especially biopolymers.

CONCLUSION

Given the increasing use of small animals for evaluating new clinical imaging techniques and providing more insight into pathophysiologic phenomena as well as the availability of improved detection systems, scanning protocols, and associated software, the sensitivity and specificity of molecular imaging are increasing.

Cardiovascular MRI in small animals

Imaging studies of cardiovascular disease in small rodents have become a prerequisite in preclinical cardiovascular research. Transgenic and gene-knockout models of cardiovascular diseases enables the investigation of the influence of single genes or groups of genes on disease pathogenesis. In addition, experimental and genetically altered models provide valuable in vivo platforms to investigate the efficacy of novel drugs and contrast agents. Owing to the excellent soft tissue contrast, high spatial and temporal resolution, as well as the tomographic nature of MRI, anatomy and function can be assessed with unique accuracy and reproducibility. Furthermore, using novel targeted MRI contrast agents, molecular changes associated with cardiovascular disease can be investigated in the same imaging session. This review focuses on recent advances in hardware, imaging sequences and probe design.

4D retrospective black blood trueFISP imaging of mouse heart

The purpose of this study was to demonstrate the feasibility of steady-state True fast imaging with steady precession (TrueFISP) four-dimensional imaging of mouse heart at high resolution and its efficiency for cardiac volumetry. Three-dimensional cine-imaging of control and hypoxic mice was carried out at 4.7 T without magnetization preparation or ECG-triggering. The k-space lines were acquired with the TrueFISP sequence (pulse repetition time/echo time = 4/2 ms) in a repeated sequential manner. Retrospective reordering of raw data allowed the reconstruction of 10 three-dimensional images per cardiac cycle. The acquisition scheme used an alternating radiofrequency phase and sum-of-square reconstruction method. Black-blood three-dimensional images at around 200 mum resolution were produced without banding artifact throughout the cardiac cycle. High contrast to noise made it possible to estimate cavity volumes during diastole and systole. Right and left ventricular stroke volume was significantly higher in hypoxic mice vs controls (20.2 +/- 2 vs 15.1 +/- 2; P < 0.05, 24.9 +/- 2 vs 20.4 +/- 2; P < 0.05, respectively). In conclusion, four-dimensional black-blood TrueFISP imaging in living mice is a method of choice to investigate cardiac abnormalities in mouse models.

Recent advances in small-animal cardiovascular imaging

Because of the development of gene knockout and transgenic technologies, small animals, such as mice and rats, have become the most widely used animals for cardiovascular imaging studies. Imaging can provide a method to serially evaluate the effect of a particular genetic mutation or pharmacologic therapy (1). In addition, imaging can be used as a noninvasive screening tool for particular cardiovascular phenotypes. Outcome measures of therapeutic efficacy, such as ejection fraction, left ventricular mass, and ventricular volume, can be determined noninvasively as well. Furthermore, small-animal imaging can be used to develop and test new molecular imaging probes (2,3). However, the small size of the heart and rapid heart rate of murine models create special challenges for cardiovascular imaging.

Molecular Imaging of Myocardial Injury: A Magnetofluorescent Approach

The role of molecular imaging in enhancing the understanding of myocardial injury and repair is rapidly expanding. Moreover, in recent years magnetic resonance and fluorescence-based approaches have been added to the molecular imaging armamentarium and have been used to image selected molecular and cellular targets in the myocardium. Apoptosis, necrosis, macrophage infiltration, myeloperoxidase activity, cathepsin activity, and type 1 collagen have all been imaged in vivo with a magnetofluorescent (MRI and/or fluorescence) approach. This review highlights the potential of these and other magnetofluorescent agents, with particular focus on their role in ischemic heart disease.

Four-dimensional MR microscopy of the mouse heart using radial acquisition and liposomal gadolinium contrast agent

Magnetic resonance microscopy (MRM) has become an important tool for small animal cardiac imaging. In relation to competing technologies (microCT and ultrasound), MR is limited by spatial resolution, temporal resolution, and acquisition time. All three of these limitations have been addressed by developing a four-dimensional (4D) (3D plus time) radial acquisition (RA) sequence. The signal-to-noise ratio (SNR) has been optimized by minimizing the echo time (TE) (300 us). The temporal resolution and throughput have been improved by center-out trajectories resulting in repetition time (TR) <2.5 ms. The contrast has been enhanced through the use of a liposomal blood pool agent that reduces the T(1) of the blood to <400 ms. We have developed protocols for three specific applications: 1) high-throughput with spatial resolution of 87 x 87 x 352 um(3) (voxel volume = 2.7 nL) and acquisition time of 16 min; 2) high-temporal resolution with spatial resolution of 87 x 87 x 352 um(3) (voxel volume = 2.7 nL) and temporal resolution at 4.8 ms and acquisition time of 32 minutes; and 3) high-resolution isotropic imaging at 87 x 87 x 87 um(3) (voxel volume = 0.68 nL) and acquisition time of 31 min. The 4D image arrays allow direct measure of cardiac functional parameters dependent on chamber volumes, e.g., ejection fraction (EF), end diastolic volume (EDV), and end systolic volume (ESV).

Postinfarction myocardial scarring in mice: molecular MR imaging with use of a collagen-targeting contrast agent

PURPOSE

To prospectively evaluate a gadolinium-based collagen-targeting contrast agent, EP-3533, for in vivo magnetic resonance (MR) imaging of myocardial fibrosis in a mouse model of healed myocardial infarction (MI).

MATERIALS AND METHODS

All procedures were performed in accordance with protocols approved by the animal care and use committee. MI was induced in eight mice by means of occlusion of the left anterior descending coronary artery followed by reperfusion. Four MR examinations were performed in each animal: one examination before, one examination 1 day after, and two examinations 6 weeks after the MI. For the latter two examinations, electrocardiographically gated inversion-recovery gradient-echo MR images were acquired before and serially (every 5 minutes) after the intravenous injection of either gadopentetate dimeglumine or EP-3533. The image enhancement kinetic properties of the postinfarction scar, normal myocardium, and blood were compared.

RESULTS

Dynamic T1-weighted MR imaging revealed the washout time constants for EP-3533 to be significantly longer than those for gadopentetate dimeglumine in regions of postinfarction scarring (mean, 194.8 minutes +/-116.8 [standard deviation] vs 25.5 minutes +/- 4.2; P < .05) and in normal myocardium (mean, 45.4 minutes +/- 16.7 vs 25.1 minutes +/- 9.7; P < .05). Findings on postmortem histologic sections stained for collagen correlated well with EP-3533-enhanced areas seen on inversion-recovery MR images. Fifty minutes after EP-3533 injection, the postinfarction scar tissue samples, as compared with the normal myocardium, had a twofold higher concentration of gadolinium.

CONCLUSION

Use of the gadolinium-based collagen-targeting contrast agent, EP-3533, enabled in vivo molecular MR imaging of fibrosis in a mouse model of healed postinfarction myocardial scarring.

Cardiac structure and function during ageing in energetically compromised Guanidinoacetate N-methyltransferase (GAMT)-knockout mice - a one year longitudinal MRI study

BACKGROUND

High-resolution magnetic resonance imaging (cine-MRI) is well suited for determining global cardiac function longitudinally in genetically or surgically manipulated mice, but in practice it is seldom used to its full potential. In this study, male and female guanidinoacetate N-methyltransferase (GAMT) knockout, and wild type littermate mice were subjected to a longitudinal cine-MRI study at four time points over the course of one year. GAMT is an essential enzyme in creatine biosynthesis, such that GAMT deficient mice are entirely creatine-free. Since creatine plays an important role in the buffering and transfer of high-energy phosphate bonds in the heart, it was hypothesized that lack of creatine would be detrimental for resting cardiac performance during ageing.

METHODS

Measurements of cardiac structure (left ventricular mass and volumes) and function (ejection fraction, stroke volume, cardiac output) were obtained using high-resolution cine-MRI at 9.4 T under isoflurane anaesthesia.

RESULTS

There were no physiologically significant differences in cardiac function between wild type and GAMT knockout mice at any time point for male or female groups, or for both combined (for example ejection fraction: 6 weeks (KO vs. WT): 70 +/- 6% vs. 65 +/- 7%; 4 months: 70 +/- 6% vs. 62 +/- 8%; 8 months: 62 +/- 11% vs. 62 +/- 6%; 12 months: 61 +/- 7% vs. 59 +/- 11%, respectively).

CONCLUSION

These findings suggest the presence of comprehensive adaptations in the knockout mice that can compensate for a lack of creatine. Furthermore, this study clearly demonstrates the power of cine-MRI for accurate non-invasive, serial cardiac measurements. Cardiac growth curves could easily be defined for each group, in the same set of animals for all time points, providing improved statistical power, and substantially reducing the number of mice required to conduct such a study. This technique should be eminently useful for following changes of cardiac structure and function during ageing.

Cardiac magnetic resonance imaging in small rodents using clinical 1.5 T and 3.0 T scanners

Cardiac magnetic resonance (CMR) imaging can provide noninvasive, high resolution images of heart anatomy, viability, perfusion, and function. However, the adoption of clinical CMR imaging protocols for small rodents has been limited due to the small heart size and rapid heart rates. Therefore, most CMR studies in small rodents have been performed on non-clinical, high-field MR magnets. Because such high-field systems are not readily available at most institutions, the technical aspects that are needed to perform CMR on clinical 1.5 T and 3.0 T MR scanners are presented in this paper. Equipment requirements are presented, and a comprehensive description of the methods needed to complete a CMR exam including the animal preparation, imaging, and image analysis are discussed. In addition, the advanced applications of myocardial tagging and delayed-contrast-enhanced imaging are reviewed for the assessment of regional contractile function and myocardial viability, respectively.

Multi-parameter in vivo cardiac magnetic resonance imaging demonstrates normal perfusion reserve despite severely attenuated beta-adrenergic functional response in neuronal nitric oxide synthase knockout mice

AIMS

The role of neuronal nitric oxide synthase (nNOS) in regulating contractile function remains controversial, and in regulating myocardial perfusion is uninvestigated. We used magnetic resonance imaging (MRI) to phenotype nNOS(-/-) and wild-type (WT) mice regarding left ventricular (LV) structure, baseline function, beta-adrenergic responsiveness, and perfusion reserve.

METHODS AND RESULTS

Cine MRI showed higher LV mass to end-diastolic volume ratio (2.3 +/- 0.2 mg/microL nNOS(-/-) vs. 1.7 +/- 0.1 mg/microL WT; P=0.032) and LV ejection fraction (64.9 +/- 2.1% nNOS(-/-) vs. 55.8 +/- 1.1% WT; P = 0.003) in nNOS(-/-). Myocardial tagging demonstrated similar baseline systolic circumferential strain (Ecc) in nNOS(-/-) and WT. With dobutamine, the normal change in Ecc was nearly absent in nNOS(-/-) (-0.5 +/- 0.3% nNOS(-/-) vs. -2.2 +/- 0.3% WT; P = 0.001), and the systolic strain rate (dEcc/dt) response to dobutamine seen in WT was reduced in nNOS(-/-) (-29 +/- 13%/s nNOS(-/-) vs. -106+/-16%/s WT; P = 0.001). Diastolic strain rate increased significantly with dobutamine only in WT. Arterial spin labelling showed that baseline perfusion and perfusion reserve with either dobutamine or an adenosine receptor agonist are normal in nNOS(-/-).

CONCLUSION

MRI provides non-invasive in vivo evidence that nNOS does not play a role in basal contractile function or myocardial perfusion, but is required for increasing cardiac inotropy and lusitropy upon beta-adrenergic stimulation.

Comparison between prospective and retrospective triggering for mouse cardiac MRI

High-resolution magnetic resonance imaging (MRI) has evolved into one of the major non-invasive tools to study the healthy and diseased mouse heart. This study presents a Cartesian CINE MRI protocol based on a fast low-angle shot sequence with a navigator echo to generate cardiac triggering and respiratory gating signals retrospectively, making the use of ECG leads and respiratory motion sensors obsolete. MRI of the in vivo mouse heart using this sequence resulted in CINE images with no detectable cardiac and respiratory motion artefacts. The retrospective method allows for steady-state imaging of the mouse heart, which is essential for quantitative contrast-enhanced MRI studies. A comparison was made between prospective and retrospective methods in terms of the signal-to-noise ratio and the contrast-to-noise ratio between blood and myocardial wall, as well as global cardiac functional indices: end-diastolic volume, end-systolic volume, stroke volume and ejection fraction. The retrospective method resulted in almost constant left-ventricle wall signal intensity throughout the cardiac cycle, at the expense of a decrease in the signal-to-noise ratio and the contrast-to-noise ratio between blood and myocardial wall as compared with the prospective method. Prospective and retrospective sequences yielded comparable global cardiac functional indices. The largest mean relative difference found was 8% for the end-systolic volume.

4D cardiac MRI in the mouse

With the introduction of mouse models for the study of cardiac morphogenesis, there arises a need for new imaging protocols that can capture both morphological and functional information. High-resolution 2D cardiac cine MRI has often been used to quantify left and right ventricular function. In this study we propose a 3D isotropic cardiac cine MRI protocol with a voxel size of 200 microm(3) as a means of studying cardiac multi-chamber morphology and function. A black blood sequence was used to enhance blood myocardium contrast. Manual segmentation of the ventricles was used to measure ventricular volumes at end diastole and end systole. This method is demonstrated on an Irx4-deficient mouse model. We have been able to identify the volumes of both ventricles dynamically and to show differences in ejection fraction in the mutant. We have also identified an abnormality of the papillary muscle in the mutant that had been missed in previous phenotyping with ultrasound and histology.

MR in mouse models of cardiac disease

Transgenic and knockout mice can be used to study the genes and basic mechanisms involved in heart disease, and have therefore assumed a central role in modern cardiac research. MRI and MRS techniques have recently been developed for mice that enable the quantitative or semi-quantitative in vivo assessment of cardiac anatomy, function, perfusion, infarction, Ca(2+) influx, and metabolism. With these techniques, the normal mouse heart has been shown to be well suited as a model of human cardiac disease. The roles of individual genes in normal cardiac physiology have recently been studied by MR, including the role of neuronal nitric oxide synthase in beta-adrenergic stimulation, the roles of the inducible nitric oxide synthase and myoglobin in function, dilation, and energetics, and the role of cardiac troponin I in contractility. Furthermore, with a mouse model of myocardial infarction, the roles of the angiotensin II type 2 receptor, xanthine oxidase inhibitors, blood coagulation factor XIII, and inducible nitric oxide synthase in post-infarct function and remodeling have been further elucidated. Non-invasive in vivo MRI and MRS in mice provide a unique and powerful means for phenotyping genetically engineered mice and can improve our understanding of the roles of specific genes and proteins in cardiac physiology and pathophysiology.

Gender and strain variations in left ventricular cardiac function and mass determined with magnetic resonance imaging at 7 tesla in adult mice

OBJECTIVES

We sought to assess with magnetic resonance imaging (MRI) the influence of strain type and gender on left ventricular (LV) global function and mass in 3 inbred mouse strains with a normal cardiac phenotype.

MATERIALS AND METHODS

A total of 30 mice from 3 inbred strains (C57BL/6, 29S2/SvPasCrl, and C3HFeJ) were studied on a 7.05-T MR scanner using ECG-triggered cine sequences. LV mass and volumes were calculated with inclusion and exclusion of papillary muscles (PMs) in the LV wall.

RESULTS

Significant differences were found with strain and gender (P < 0.001), with strain-effect but no gender-effect for ejection fraction (EF), end-diastolic volume (EDV), and end-systolic volume (ESV). There were no differences in LV mass between strains but lower values in female mice except in the C3H strain. The exclusion of PMs led to the relative underestimation of EF (-6.1%) and of LV mass (-6.4 mg) and the relative overestimation of EDV (6.3 microL) and ESV (5.3 microL). Inter- and intraobserver reproducibility was better when PMs were included.

CONCLUSION

The use of MRI demonstrates cardiac interstrain and gender-related phenotypic diversities that are essential factors to consider when building genomic databases and designing studies.

Cardiac function estimation from MRI using a heart model and data assimilation: advances and difficulties

In this paper, we present a framework to estimate local ventricular myocardium contractility using clinical MRI, a heart model and data assimilation. First, we build a generic anatomical model of the ventricles including muscle fibre orientations and anatomical subdivisions. Then, this model is deformed to fit a clinical MRI, using a semi-automatic fuzzy segmentation, an affine registration method and a local deformable biomechanical model. An electromechanical model of the heart is then presented and simulated. Finally, a data assimilation procedure is described, and applied to this model. Data assimilation makes it possible to estimate local contractility from given displacements. Presented results on fitting to patient-specific anatomy and assimilation with simulated data are very promising. Current work on model calibration and estimation of patient parameters opens up possibilities to apply this framework in a clinical environment.