Advanced methods for quantification of infarct size in mice using three-dimensional high-field late gadolinium enhancement MRI

Comparison of transient and permanent LAD ligation in mice using 18F-FDG PET imaging

Objective

Animal models for myocardial injuries represent important cornerstones in cardiovascular research to monitor the pathological processes and therapeutic approaches. We investigated the association of 18F-FDG derived left ventricular metabolic volume (LVMV), defect area and cardiac function in mice after permanent or transient ligation of the left anterior descending artery (LAD).

Methods

Serial non-invasive ECG-gated 2-deoxy-2-[18F]fluoro-d-glucose positron emission tomography (18F-FDG PET) after permanent or transient LAD ligation enabled a longitudinal in vivo correlation of 18F-FDG derived left ventricular metabolic volume to functional parameters and myocardial defect.

Results

The LVMV shows a more prominent drop after permanent than transient LAD ligation and recovers after 30 days. The loss of LVMV correlates with the defect area assessed by QPS software. Cardiac function parameters (e.g., EDV, ESV, SV) by the QGS software positively correlate with LVMV after permanent and transient LAD ligation.

Conclusions

This study provides novel insight into 18F-FDG derived LVMV after permanent and transient LAD ligation by longitudinal in 18F-FDG PET imaging and underlines the associations of the FDG derived parameter and cardiac function.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12149-022-01734-8.

Guidelines for in vivo mouse models of myocardial infarction

Despite significant improvements in reperfusion strategies, acute coronary syndromes all too often culminate in a myocardial infarction (MI). The consequent MI can, in turn, lead to remodeling of the left ventricle (LV), the development of LV dysfunction, and ultimately progression to heart failure (HF). Accordingly, an improved understanding of the underlying mechanisms of MI remodeling and progression to HF is necessary. One common approach to examine MI pathology is with murine models that recapitulate components of the clinical context of acute coronary syndrome and subsequent MI. We evaluated the different approaches used to produce MI in mouse models and identified opportunities to consolidate methods, recognizing that reperfused and nonreperfused MI yield different responses. The overall goal in compiling this consensus statement is to unify best practices regarding mouse MI models to improve interpretation and allow comparative examination across studies and laboratories. These guidelines will help to establish rigor and reproducibility and provide increased potential for clinical translation.

Cardiac 18F-FDG Positron Emission Tomography: An Accurate Tool to Monitor In vivo Metabolic and Functional Alterations in Murine Myocardial Infarction

Cardiac monitoring after murine myocardial infarction, using serial non-invasive cardiac 18F-FDG positron emissions tomography (PET) represents a suitable and accurate tool for in vivo studies. Cardiac PET imaging enables tracking metabolic alterations, heart function parameters and provides correlations of the infarct size to histology. ECG-gated 18F-FDG PET scans using a dedicated small-animal PET scanner were performed in mice at baseline, 3, 14, and 30 days after myocardial infarct (MI) by permanent ligation of the left anterior descending (LAD) artery. The percentage of the injected dose per gram (%ID/g) in the heart, left ventricular metabolic volume (LVMV), myocardial defect, and left ventricular function parameters: end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), and the ejection fraction (EF%) were estimated. PET assessment of the defect positively correlates with post-infarct histology at 3 and 30 days. Infarcted murine hearts show an immediate decrease in LVMV and an increase in %ID/g early after infarction, diminishing in the remodeling process. This study of serial cardiac PET scans provides insight for murine myocardial infarction models by novel infarct surrogate parameters. It depicts that serial PET imaging is a valid, accurate, and multimodal non-invasive assessment.

Quantification of defect contrast in microSPECT imaging of a myocardial phantom

Ischemic heart disease remains a significant public health concern, accentuating the importance of basic research and therapeutic studies of small animals in which myocardial changes can be reproducibly detected and quantified. Few or no studies have investigated the performance of microSPECT in quantifying myocardial lesions. We utilized three versions of a multi-compartment phantom containing two left ventricular myocardial compartments (one uniform and one with a transmural 'cold' defect), a ventricular blood pool, and a background compartment, where each version had a different myocardial wall thickness (0.75, 1.0 and 1.25 mm). Each compartment was imaged separately while acquiring list-mode data. The separate compartment data were manipulated into a single data set with a known defect contrast, blood-pool and background activity. Data were processed with background-free defect-contrast values of 0 (no defect), -0.25, -0.5, -0.75, and -1.0 (all defect), three ratios of blood-pool to myocardial activity, 0 (no blood pool activity), 0.1, and 0.2 (20% of the activity in the healthy myocardial compartment), and three ratios of uniform background 0 (no background activity), 0.1 and 0.2, relative to the healthy myocardial compartment. For each wall thickness, defect contrast, blood-pool, and background activity combination, 25 list-mode noise realizations were generated and reconstructed. Volumes of interest were drawn and used to determine mean contrast recovery coefficients (CRCs) over the noise ensembles. We developed a slope-analysis procedure to estimate a single CRC over all contrast levels, with resulting CRC values (for no blood-pool and no background) of 0.848, 0.946, and 0.834 for the 0.75, 1.0, and 1.25 mm wall thicknesses, respectively. We also determined and validated a reprocessing method to calculate an ideal CRC. This work demonstrates the quantitative abilities of microSPECT for myocardial-defect imaging utilizing CRC and establishes a framework for evaluating defect-imaging capabilities in other systems.

Hexarelin targets neuroinflammatory pathways to preserve cardiac morphology and function in a mouse model of myocardial ischemia-reperfusion

Acute myocardial ischemia and reperfusion injury (IRI) underly the detrimental effects of coronary heart disease on the myocardium. Despite the ongoing advances in reperfusion therapies, there remains a lack of effective therapeutic strategies for preventing IRI. Growth hormone secretagogues (GHS) have been demonstrated to improve cardiac function, attenuate inflammation and modulate the autonomic nervous system (ANS) in models of cardiovascular disease. Recently, we demonstrated a reduction in infarct size after administration of hexarelin (HEX), in a murine model of myocardial infarction. In the present study we employed a reperfused ischemic (IR) model, to determine whether HEX would continue to have a cardioprotective influence in a model of higher clinical relevance. Myocardial ischemia was induced by transient ligation of the left descending coronary artery (tLAD) in C57BL/6 J mice followed by HEX (0.3 mg/kg/day; n = 20) or vehicle (VEH) (n = 18) administration for 21 days, first administered immediately prior-to reperfusion. IR-injured and sham mice were subjected to high-field magnetic resonance imaging to assess left ventricular (LV) function, with HEX-treated mice demonstrating a significant improvement in LV function compared with VEH-treated mice. A significant decrease in interstitial collagen, TGF-β1 expression and myofibroblast differentiation was also seen in the HEX-treated mice after 21 days. HEX treatment shifted the ANS balance towards a parasympathetic predominance; combined with a significant decrease in cardiac troponin-I and TNF-α levels, these findings were suggestive of an anti-inflammatory action on the myocardium mediated via HEX. In this model of IR, HEX appeared to rebalance the deregulated ANS and activate vagal anti-inflammatory pathways to prevent adverse remodelling and LV dysfunction. There are limited interventions focusing on IRI that have been successful in improving clinical outcome in acute myocardial infarction (AMI) patients, this study provides compelling evidence towards the translational potential of HEX where all others have largely failed.

Prognostic value of cardiovascular magnetic resonance in patients with biopsy-proven systemic sarcoidosis

OBJECTIVES

As prognosis in sarcoidosis is determined by cardiac involvement, the objective was to study the added value of cardiovascular magnetic resonance (CMR) in risk stratification.

METHODS

In 114 patients (48 ± 12 years/52% male) with biopsy-proven sarcoidosis, we studied the value of clinical and CMR-derived parameters to predict future events, using sustained ventricular tachycardia, ventricular fibrillation, aborted cardiac death, implantable cardioverter-defibrillator (ICD) placement with appropriate shocks, hospitalization for heart failure, and death as composite endpoint. Median follow-up after CMR was 3.1 years (1.1-5.7 years).

RESULTS

The ejection fraction (EF) was 58.2 ± 9.1% and 54.7 ± 10.8% for left ventricle (LV) and right ventricle (RV), respectively. LV late gadolinium enhancement (LGE) was present in 40 patients (35%) involving 5.1% of the LV mass (IQR, 3.0-12.0%), with concomitant RV involvement in 12 patients (11%). T2-weighting imaging and/or T2 mapping showed active disease in 14 patients. The composite endpoint was reached in 34 patients, with 7 deaths in the LGE-positive group (17.5%), versus two deaths in the LGE-negative group (2.7%) (p = 0.015). At univariate analysis, RVEF (p = 0.009), pulmonary arterial pressure (p = 0.002), and presence of LGE (p < 0.001) and LGE (% of LV) (p < 0.001) were significant. At multivariate analysis, only presence of LGE and LGE (% of LV) was significant (both p = 0.03). At Kaplan-Meier, presence of LGE and an LGE of 3% predicted event-free survival and patient survival. We found no difference in active versus inactive disease with regard to patient survival.

CONCLUSION

Myocardial enhancement at LGE-CMR adds independent prognostic value in risk stratification sarcoidosis patients. In contrast, clinical as well as functional cardiac parameters lack discriminative power.

KEY POINTS

• Sarcoidosis often affects the heart. • Comprehensive CMR, including T2 imaging and LGE enhancement CMR, allows to depict both active and inactive myocardial damage. • Patient prognosis in sarcoidosis is determined by the presence and severity of myocardial involvement at LGE CMR.

Hexarelin treatment preserves myocardial function and reduces cardiac fibrosis in a mouse model of acute myocardial infarction

Ischemic heart disease (IHD) is a leading cause of morbidity and mortality worldwide. Growth hormone secretagogues (GHS) have been shown to improve cardiac function in models of IHD. This study determined whether hexarelin (HEX), a synthetic GHS, preserves cardiac function and morphology in a mouse model of myocardial infarction (MI). MI was induced by ligation of the left descending coronary artery in C57BL/6J mice followed by vehicle (VEH; n = 10) or HEX (0.3 mg/kg/day; n = 11) administration for 21 days. MI-injured and sham mice (treated with VEH; n = 6 or HEX; n = 5) underwent magnetic resonance imaging for measurement of left ventricular (LV) function, mass and infarct size at 24 h and 14 days post-MI. MI-HEX mice displayed a significant improvement (P < 0.05) in LV function compared with MI-VEH mice after 14 days treatment. A significant decrease in LV mass, interstitial collagen and collagen concentration was demonstrated with chronic HEX treatment (for 21 days), accompanied by a decrease in TGF-β1 expression, myofibroblast differentiation and an increase in collagen-degrading MMP-13 expression levels. Furthermore, heart rate variability analysis demonstrated that HEX treatment shifted the balance of autonomic nervous activity toward a parasympathetic predominance and sympathetic downregulation. This was combined with a HEX-dependent decrease in troponin-I, IL-1β and TNF-α levels suggestive of amelioration of cardiomyocyte injury. These results demonstrate that GHS may preserve ventricular function, reduce inflammation and favorably remodel the process of fibrotic healing in a mouse model of MI and hold the potential for translational application to patients suffering from MI.

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/.

Three-dimensional self-gated cardiac MR imaging for the evaluation of myocardial infarction in mouse model on a 3T clinical MR system

Purpose

To develop and assess a three-dimensional (3D) self-gated technique for the evaluation of myocardial infarction (MI) in mouse model without the use of external electrocardiogram (ECG) trigger and respiratory motion sensor on a 3T clinical MR system.

Methods

A 3D T1-weighted GRE sequence with stack-of-stars sampling trajectories was developed and performed on six mice with MIs that were injected with a gadolinium-based contrast agent at a 3T clinical MR system. Respiratory and cardiac self-gating signals were derived from the Cartesian mapping of the k-space center along the partition encoding direction by bandpass filtering in image domain. The data were then realigned according to the predetermined self-gating signals for the following image reconstruction. In order to accelerate the data acquisition, image reconstruction was based on compressed sensing (CS) theory by exploiting temporal sparsity of the reconstructed images. In addition, images were also reconstructed from the same realigned data by conventional regridding method for demonstrating the advantageous of the proposed reconstruction method. Furthermore, the accuracy of detecting MI by the proposed method was assessed using histological analysis as the standard reference. Linear regression and Bland-Altman analysis were used to assess the agreement between the proposed method and the histological analysis.

Results

Compared to the conventional regridding method, the proposed CS method reconstructed images with much less streaking artifact, as well as a better contrast-to-noise ratio (CNR) between the blood and myocardium (4.1 ± 2.1 vs. 2.9 ± 1.1, p = 0.031). Linear regression and Bland-Altman analysis demonstrated that excellent correlation was obtained between infarct sizes derived from the proposed method and histology analysis.

Conclusion

A 3D T1-weighted self-gating technique for mouse cardiac imaging was developed, which has potential for accurately evaluating MIs in mice at 3T clinical MR system without the use of external ECG trigger and respiratory motion sensor.

Improved segmented modified Look-Locker inversion recovery T1 mapping sequence in mice

Object

To develop and evaluate a 2D modified Look-Locker (MOLLI) for high-resolution T₁ mapping in mice using a 3T MRI scanner.

Materials and methods

To allow high-resolution T₁ mapping in mice at high heart rates a multi-shot ECG-triggered 2D MOLLI sequence was developed. In the proposed T₁ mapping sequence the optimal number of sampling points and pause cardiac cycles following an initial adiabatic inversion pulse was investigated in a phantom. Seven native control and eight mice, 3 days post myocardial infarction (MI) after administration of gadolinium were scanned. Two experienced readers graded the visual T₁ map quality.

Results

In T₁ phantoms, there were no significant differences (<0.4% error) between 12, 15 and 20 pause cardiac cycles (p = 0.1, 0.2 and 0.6 respectively) for 8 acquisition cardiac cycles for 600bpm in comparison to the conventional inversion recovery spin echo T₁ mapping sequence for short T₁’s (<600 ms). Subsequently, all in-vivo scans were performed with 8 data acquisitions and 12 pause cardiac cycles to minimize scan time. The mean native T₁ value of myocardium in control animal was 820.5±52 ms. The post-contrast T₁ measured 3 days after MI in scar was 264±59 ms and in healthy myocardium was 512±62 ms. The Bland-Altman analysis revealed mean difference of only -1.06% of infarct size percentage between T₁ maps and LGE.

Conclusions

A multi-shot 2D MOLLI sequence has been presented that allows reliable measurement of high spatial resolution T₁ maps in mice for heart rates up to 600bpm.

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.

MRI Assessment of Cardiomyopathy Induced by β1-Adrenoreceptor Autoantibodies and Protection Through β3-Adrenoreceptor Overexpression

The cardiopathogenic role of autoantibodies (aabs) directed against β1-adrenoreceptors (β1-AR) is well established. In mouse models, they cause progressive dilated cardiomyopathy (DCM) whose characterization with echocardiography requires prolonged protocols with numerous animals, complicating the evaluation of new treatments. Here, we report on the characterization of β1-aabs-induced DCM in mice using 11.7T MRI. C57BL/6J mice (n = 10 per group) were immunized against the β1-AR and left ventricular (LV) systolic function was assessed at 10, 18 and 27 weeks. Increase in LV mass/tibial length ratio was detected as the first modification at 10 weeks together with dilation of cavities, thereby outperforming echocardiography. Significant impairment in diastolic index was also observed in immunized animals before the onset of systolic dysfunction. Morphometric and histological measurements confirmed these observations. The same protocol performed on β3-AR-overexpressing mice and wild-type littermates (n = 8–12 per group) showed that transgenic animals were protected with reduced LV/TL ratio compared to wild-type animals and maintenance of the diastolic index. This study demonstrates that MRI allows a precocious detection of the subtle myocardial dysfunction induced by β1-aabs and that β3-AR stimulation blunts the development of β1-aabs-induced DCM, thereby paving the way for the use of β3AR-stimulating drugs to treat this autoimmune cardiomyopathy.

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.

Gd-TEMDO: Design, Synthesis, and MRI Application

A simple Ugi tetrazole multicomponent reaction allows the synthesis of a novel macrocyclic cyclen derivative with four appendant tetrazole arms in just two steps in excellent yields. This ligand, called TEMDO, turns out to have a high complexation affinity with lanthanoid metals. Here we describe the design, synthesis, solid-state structure, binding constant, and some MRI applications of the Gd-TEMDO complex as the first example of a congeneric family of oligo-amino tetrazoles.

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.

Optical projection tomography permits efficient assessment of infarct volume in the murine heart postmyocardial infarction

Optical projection tomography permits rapid high-resolution imaging of intact murine heart in vitro and identification of tissue heterogeneity within individual optical slices of postmyocardial infarction hearts. Infarct volume derived from >400 slices correlates with in vivo magnetic resonance imaging and avoids the need for histological staining of multiple physical sections.

The extent of infarct injury is a key determinant of structural and functional remodeling following myocardial infarction (MI). Infarct volume in experimental models of MI can be determined accurately by in vivo magnetic resonance imaging (MRI), but this is costly and not widely available. Experimental studies therefore commonly assess injury by histological analysis of sections sampled from the infarcted heart, an approach that is labor intensive, can be subjective, and does not fully assess the extent of injury. The present study aimed to assess the suitability of optical projection tomography (OPT) for identification of injured myocardium and for accurate and efficient assessment of infarct volume. Intact, perfusion-fixed, optically cleared hearts, collected from mice 7 days after induction of MI by coronary artery occlusion, were scanned by a tomograph for autofluorescence emission after UV excitation, generating >400 transaxial sections for reconstruction. Differential autofluorescence permitted discrimination between viable and injured myocardium and highlighted the heterogeneity within the infarct zone. Two-dimensional infarct areas derived from OPT imaging and Masson's trichrome staining of slices from the same heart were highly correlated (r² = 0.99, P < 0.0001). Infarct volume derived from reconstructed OPT sections correlated with volume derived from in vivo late gadolinium enhancement MRI (r² = 0.7608, P < 0.005). Tissue processing for OPT did not compromise subsequent immunohistochemical detection of endothelial cell and inflammatory cell markers. OPT is thus a nondestructive, efficient, and accurate approach for routine in vitro assessment of murine myocardial infarct volume.

Quantitative T2 mapping of the mouse heart by segmented MLEV phase-cycled T2 preparation

PURPOSE

A high-quality, reproducible, multi-slice T2-mapping protocol for the mouse heart is presented.

METHODS

A T2-prepared sequence with composite 90° and 180° radiofrequency pulses in a segmented MLEV phase cycling scheme was developed. The T2-mapping protocol was optimized using simulations and evaluated with phantoms.

RESULTS

Repeatability for determination of myocardial T2 values was assessed in vivo in n = 5 healthy mice on 2 different days. The average baseline T2 of the left ventricular myocardium was 22.5 ± 1.7 ms. The repeatability coefficient for R2 = 1/T2 for measurements at different days was ΔR2 = 6.3 s(−1). Subsequently, T2 mapping was applied in comparison to late-gadolinium-enhancement (LGE) imaging, to assess 1-day-old ischemia/reperfusion (IR) myocardial injury in n = 8 mice. T2 in the infarcts was significantly higher than in remote tissue, whereas remote tissue was not significantly different from baseline. Infarct sizes based on T2 versus LGE showed strong correlation. To assess the time-course of T2 changes in the infarcts, T2 mapping was performed at day 1, 3, and 7 after IR injury in a separate group of mice (n = 16). T2 was highest at day 3, in agreement with the expected time course of edema formation and resolution after myocardial infarction.

CONCLUSION

T2 prepared imaging provides high quality reproducible T2 maps of healthy and diseased mouse myocardium.

In vivo detection of the effects of preconditioning on LNCaP tumors by a TNF-α nanoparticle construct using MRI

The outcome of systemic and local therapies (e.g. chemotherapy, radiotherapy, surgery, focal ablation) for prostate cancer can be significantly improved by using tumor-specific adjuvants prior to treatment ("preconditioning"). We propose to use dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) to monitor the in vivo response of a mouse model of prostate cancer treated with a vascular disruptive agent, TNF-α, delivered on a gold nanoparticle (NP-TNF). Six male nude mice bearing 4-5 week old LNCaP tumors were scanned at 9.4 T. DCE-MRI was performed two days before and 4-5 h after treatment with NP-TNF. An intraperitoneal (i.p.) bolus of gadolinium-DTPA (Gd) was administered and contrast enhancement was measured for 90 min. Concentration-time curves of Gd were calculated and the area under the Gd curve (AUGC) was determined pre- and post-treatment. NP-TNF treatment caused an increase in contrast uptake in tumors. Interestingly, the early concentration (10 min post Gd bolus i.p.) was similar in both untreated and treated conditions; however, 90 min after injection, [Gd] was 3.4 times higher than before treatment. AUGC doubled from (11 ± 6) [Gd] × min before treatment to (22 ± 9) [Gd] × min after treatment. An increase in signal enhancement was also observed in the muscle but to a lesser degree. We also evaluated the kinetics of intravenous Gd administration in mice bearing a jugular vein catheter to mimic the delivery method used in clinical trials. The overall treatment effects were independent of the delivery pathway of the contrast agent. In conclusion, we show that DCE-MRI is suitable to detect changes associated with a vascular disruptive agent in a mouse model of prostate cancer. The ability to characterize the effects of nanoparticle therapy in vivo with non-destructive methods is important, as such compounds, in combination with treatment strategies, are progressing towards clinical trials.

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).

T₁ mapping detects pharmacological retardation of diffuse cardiac fibrosis in mouse pressure-overload hypertrophy

BACKGROUND

Diffuse interstitial fibrosis is present in diverse cardiomyopathies and associated with poor prognosis. We investigated whether magnetic resonance imaging-based T1 mapping could quantify the induction and pharmacological suppression of diffuse cardiac fibrosis in murine pressure-overload hypertrophy.

METHODS AND RESULTS

Mice were subjected to transverse aortic constriction or sham surgery. The angiotensin receptor blocker losartan was given to half the animals. Cine-magnetic resonance imaging performed at 7 and 28 days showed hypertrophy and remodeling and systolic and diastolic dysfunction in transverse aortic constriction groups as expected. Late gadolinium-enhanced magnetic resonance imaging revealed focal signal enhancement at the inferior right ventricular insertion point of transverse aortic constriction mice concordant with the foci of fibrosis in histology. The extracellular volume fraction, calculated from pre- and postcontrast T1 measurements, was elevated by transverse aortic constriction and showed direct linear correlation with picrosirius red collagen volume fraction, thus confirming the suitability of extracellular volume fraction as an in vivo measure of diffuse fibrosis. Treatment with losartan reduced left ventricular dysfunction and prevented increased extracellular volume fraction, indicating that T1 mapping is sensitive to pharmacological prevention of fibrosis.

CONCLUSIONS

Magnetic resonance imaging can detect diffuse and focal cardiac fibrosis in a clinically relevant animal model of pressure overload and is sensitive to pharmacological reduction of fibrosis by angiotensin receptor blockade. Thus, T1 mapping can be used to assess antifibrotic therapeutic strategies.

MRI and PET in mouse models of myocardial infarction

Myocardial infarction is one of the leading causes of death in the Western world. The similarity of the mouse heart to the human heart has made it an ideal model for testing novel therapeutic strategies. In vivo magnetic resonance imaging (MRI) gives excellent views of the heart noninvasively with clear anatomical detail, which can be used for accurate functional assessment. Contrast agents can provide basic measures of tissue viability but these are nonspecific. Positron emission tomography (PET) is a complementary technique that is highly specific for molecular imaging, but lacks the anatomical detail of MRI. Used together, these techniques offer a sensitive, specific and quantitative tool for the assessment of the heart in disease and recovery following treatment. In this paper we explain how these methods are carried out in mouse models of acute myocardial infarction. The procedures described here were designed for the assessment of putative protective drug treatments. We used MRI to measure systolic function and infarct size with late gadolinium enhancement, and PET with fluorodeoxyglucose (FDG) to assess metabolic function in the infarcted region. The paper focuses on practical aspects such as slice planning, accurate gating, drug delivery, segmentation of images, and multimodal coregistration. The methods presented here achieve good repeatability and accuracy maintaining a high throughput.

Improve myocardial T1 measurement in rats with a new regression model: application to myocardial infarction and beyond

PURPOSE

To improve myocardial and blood T1 measurements with a multi-variable T1 fitting model specifically modified for a segmented multi-shot FLASH sequence.

METHODS

The proposed method was first evaluated in a series of phantoms simulating realistic tissues, and then in healthy rats (n = 8) and rats with acute myocardial infarction (MI) induced by coronary artery ligation (n = 8).

RESULTS

By taking into account the saturation effect caused by sampling α-train pulses, and the longitudinal magnetization recovery between readouts, our model provided more accurate T1 estimate than the conventional three-parameter fit in phantoms under realistic gating procedures (error of -0.42 ± 1.73% versus -3.40 ± 1.46%, respectively, when using the measured inversion efficiency, β). The baseline myocardial T1 values in healthy rats was 1636.3 ± 23.4 ms at 7 Tesla. One day postligation, the T1 values in the remote and proximal myocardial areas were 1637.5 ± 62.6 ms and 1740.3 ± 70.5 ms, respectively. In rats with acute MI, regional differences in myocardial T1 values were observed both before and after the administration of gadolinium.

CONCLUSION

The proposed method has improved T1 estimate as validated in phantoms and could advance applications in rodents using quantitative myocardial T1 mapping.

Cardiac-respiratory self-gated cine ultra-short echo time (UTE) cardiovascular magnetic resonance for assessment of functional cardiac parameters at high magnetic fields

BACKGROUND

To overcome flow and electrocardiogram-trigger artifacts in cardiovascular magnetic resonance (CMR), we have implemented a cardiac and respiratory self-gated cine ultra-short echo time (UTE) sequence. We have assessed its performance in healthy mice by comparing the results with those obtained with a self-gated cine fast low angle shot (FLASH) sequence and with echocardiography.

METHODS

2D self-gated cine UTE (TE/TR = 314 μs/6.2 ms, resolution: 129 × 129 μm, scan time per slice: 5 min 5 sec) and self-gated cine FLASH (TE/TR = 3 ms/6.2 ms, resolution: 129 × 129 μm, scan time per slice: 4 min 49 sec) images were acquired at 9.4 T. Volume of the left and right ventricular (LV, RV) myocardium as well as the end-diastolic and -systolic volume was segmented manually in MR images and myocardial mass, stroke volume (SV), ejection fraction (EF) and cardiac output (CO) were determined. Statistical differences were analyzed by using Student t test and Bland-Altman analyses.

RESULTS

Self-gated cine UTE provided high quality images with high contrast-to-noise ratio (CNR) also for the RV myocardium (CNRblood-myocardium = 25.5 ± 7.8). Compared to cine FLASH, susceptibility, motion, and flow artifacts were considerably reduced due to the short TE of 314 μs. The aortic valve was clearly discernible over the entire cardiac cycle. Myocardial mass, SV, EF and CO determined by self-gated UTE were identical to the values measured with self-gated FLASH and showed good agreement to the results obtained by echocardiography.

CONCLUSIONS

Self-gated UTE allows for robust measurement of cardiac parameters of diagnostic interest. Image quality is superior to self-gated FLASH, rendering the method a powerful alternative for the assessment of cardiac function at high magnetic fields.

Non-invasive assessment of experimental autoimmune myocarditis in rats using a 3 T clinical MRI scanner

AIMS

The aim of this study was to assess the use of a 3 T clinical cardiac magnetic resonance (CMR) scanner to detect injury to the heart in experimental autoimmune myocarditis (EAM).

METHODS AND RESULTS

The use of 3 T CMR for the detection of cardiac injury was assessed in EAM (n = 55) and control (n = 10) male Lewis rats. Animals were evaluated with serial CMR imaging studies, using a 3 T scanner, and with 2D echocardiography before, and at 2 and 5 weeks after EAM induction. By CMR, regional wall motion abnormalities were noted in seven out of eight rats with myocarditis 5 weeks after induction. Subsequently, the rats developed significant left ventricular (LV) dilatation, wall thickening, and pericardial effusion. Average LV systolic and diastolic volumes increased from 131 ± 10 to 257 ± 20 µL (P = 0.0008), and from 309 ± 14 to 412 ± 24 µL (P < 0.0001), and ejection fraction markedly deteriorated (from 58 ± 2 to 37 ± 5%; P = 0.0003). Areas of fibrosis were located by late gadolinium enhancement (LGE) CMR at the subepicardium, mainly within the anterior, lateral, and inferior walls. The extent and location of LGE were highly correlated (r = 0.94; P < 0.0001) with areas of myocardial fibrosis by histopathology, with 85% sensitivity and 86% specificity.

CONCLUSION

A clinical 3 T CMR scanner enables accurate detection, quantification, and monitoring of experimental myocarditis in rats, and could be used for translational research to study the pathophysiology of the disease and evaluate novel therapies.

Magnetic resonance imaging and spectroscopy of the murine cardiovascular system

Magnetic resonance imaging (MRI) has emerged as a powerful and reliable tool to noninvasively study the cardiovascular system in clinical practice. Because transgenic mouse models have assumed a critical role in cardiovascular research, technological advances in MRI have been extended to mice over the last decade. These have provided critical insights into cardiac and vascular morphology, function, and physiology/pathophysiology in many murine models of heart disease. Furthermore, magnetic resonance spectroscopy (MRS) has allowed the nondestructive study of myocardial metabolism in both isolated hearts and in intact mice. This article reviews the current techniques and important pathophysiological insights from the application of MRI/MRS technology to murine models of cardiovascular disease.

Imaging the healing murine myocardial infarct in vivo: ultrasound, magnetic resonance imaging and fluorescence molecular tomography

Improved understanding of the processes involved in infarct healing is required for identification of novel therapeutic targets to limit infarct expansion and consequent long-term ventricular remodelling after myocardial infarction. Infarct healing can be modelled effectively in murine models of coronary artery ligation. While imaging the murine heart is challenging due to its size and high rate of contraction, advances in preclinical imaging now permit accurate assessment of myocardial structure and function in vivo after myocardial infarction. Furthermore, rapid development of a range of molecular probes for use in a number of imaging modalities allows more detailed in vivo analysis of processes, including inflammation, fibrosis and angiogenesis. Here we consider the practical application of in vivo imaging by magnetic resonance imaging, ultrasound and fluorescence molecular tomography for assessment of infarct healing in the mouse.

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.

Evaluation of phase-sensitive versus magnitude reconstructed inversion recovery imaging for the assessment of myocardial infarction in mice with a clinical magnetic resonance scanner

PURPOSE

To evaluate phase-sensitive inversion-recovery (PSIR) imaging at 1.5 T in a mouse model of permanent coronary artery ligation as a potentially rapid and robust alternative for the accurate assessment of myocardial infarction (MI) by cardiac magnetic resonance imaging (MRI).

MATERIALS AND METHODS

PSIR late gadolinium enhancement (LGE) imaging was compared to conventional 2D segmented inversion-recovery imaging for the assessment of murine MI.

RESULTS

PSIR images provided comparable contrast and kinetics of intravenously injected gadopentetate dimeglumine (Gd-DTPA). At the mid-ventricular level there was good agreement between conventional IR and PSIR for infarct size assessment. After intravenous injection a limited time window of ∼6 minutes is available for delayed enhancement imaging in mice. Whole-heart infarct imaging with 1 mm thick slices was only possible in this restricted time frame when the PSIR method is applied, avoiding the need for repetitively adapting the correct inversion time. Infarct size determined by PSIR MRI demonstrated good agreement with postmortem histology. Infarct size determined by PSIR LGE MRI inversely correlates with left-ventricular function on day 7 after MI.

CONCLUSION

The PSIR technique provides stable and consistent contrast between hyperenhanced and remote myocardium independent of the selected inversion time (TI) and proved to be a robust, fast, and accurate tool for the assessment of 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.

Contrast-enhanced MRI of murine myocardial infarction - part I

The use of contrast agents has added considerable value to the existing cardiac MRI toolbox that can be used to study murine myocardial infarction, as it enables detailed in vivo visualization of the molecular and cellular processes that occur in the infarcted and remote tissue. A variety of non-targeted and targeted contrast agents to study myocardial infarction are available and under development. Manganese, which acts as a calcium analogue, can be used to assess cell viability. Traditionally, low-molecular-weight Gd-containing contrast agents are employed to measure infarct size in a late gadolinium enhancement experiment. Gd-based blood-pool agents are used to study the vascular status of the myocardium. The use of targeted contrast agents facilitates more detailed imaging of pathophysiological processes in the acute and chronic infarct. Cell death was visualized by contrast agents functionalized with annexin A5 that binds specifically to phosphatidylserine accessible on dying cells and with an agent that binds to the exposed DNA of dead cells. Inflammation in the myocardium was depicted by contrast agents that target cell adhesion molecules expressed on activated endothelium, by contrast agents that are phagocytosed by inflammatory cells, and by using a probe that targets enzymes excreted by inflammatory cells. Cardiac remodeling processes were visualized with a contrast agent that binds to angiogenic vasculature and with an MR probe that specifically binds to collagen in the fibrotic myocardium. These recent advances in murine contrast-enhanced cardiac MRI have made a substantial contribution to the visualization of the pathophysiology of myocardial infarction, cardiac remodeling processes and the progression to heart failure, which helps to design new treatments. This review discusses the advances and challenges in the development and application of MRI contrast agents to study murine myocardial infarction.

Development and characterization of rodent cardiac phantoms: comparison with in vivo cardiac imaging

The increasing availability of rodent models of human cardiovascular disease has led to a need to translate noninvasive imaging techniques such as magnetic resonance imaging (MRI) from the clinic to the animal laboratory. The aim of this study was to develop phantoms simulating the short-axis view of left ventricular motion of rats and mice, thus reducing the need for live animals in the development of MRI.

Cylindrical phantoms were moulded from polyvinyl alcohol (PVA) Cryogel and attached via stiff water-filled tubing to a gear pump. Pulsatile distension of the phantoms was effected by suitable programming of the pump. Cine MRI scanning was carried out at 7 T and compared with in vivo rodent cardiac imaging.

Suitable pulsatile performance was achieved with phantoms for which the PVA material had been subjected to two freeze–thaw cycles, resulting in T1 and T2 relaxation time constants of 1656±124 ms and 55±10 ms, respectively. For the rat phantom operating at 240 beats per min (bpm), the dynamic range of the outer diameter was from 10.3 to 12.4 mm with the wall thickness varying between 1.9 and 1.2 mm. Corresponding figures for the mouse phantom at 480 bpm were outer diameter range from 5.4 to 6.4 mm and wall thickness from 1.5 to 1.2 mm.

Dynamic cardiac phantoms simulating rodent left ventricular motion in the short-axis view were successfully developed and compared with in vivo imaging. The phantoms can be used for future development work with reduced need of live animals.

Noninvasive estimation of infarct size in a mouse model of myocardial infarction by echocardiographic coronary perfusion

OBJECTIVES

Animal models of myocardial infarction (MI) are widely used not only in analyses of the mechanisms but also in testing the efficacy of therapeutic strategies for the disease. It is therefore critically important but almost impossible to exactly evaluate the validity of coronary artery ligation in a mouse model of MI except by anatomic and histologic analyses. We explored a noninvasive method to estimate MI through analyses of coronary perfusion by transthoracic echocardiography in mice before and 1 day after ligation of the left anterior descending coronary artery.

METHODS

Transthoracic echocardiography-based cardiac function, geometry, and coronary perfusion, electrocardiographic findings, and serum troponin I levels were examined in C57BL6/J mice subjected to left anterior descending artery ligation. The histologic infarct size was confirmed by staining the heart with 2,3,5-triphenyltetrazolium chloride.

RESULTS

Among all parameters, the postoperative hyperemic peak diastolic velocity and coronary flow reserve were most correlated with infarct size (R² = .8028 and .5825, respectively; both P < .0001). With an infarct size of 30% or greater indicating successful ligation and less than 30% indicating unsuccessful ligation, receiver operating characteristic curve analysis showed that the postoperative hyperemic peak diastolic velocity and coronary flow reserve most effectively indicated the infarct size level with optimal cutoff values of 480.16 mm/s and 1.89, respectively. Furthermore, impaired cardiac function, an eccentrically expanded left ventricle, typical pathologic electrocardiographic findings, and elevated troponin I levels were observed most often in the mice with an impaired hyperemic peak diastolic velocity and coronary flow reserve.

CONCLUSIONS

The echocardiographic hyperemic peak diastolic velocity and coronary flow reserve can estimate the histologic infarct size in mice with coronary occlusion.

MRI-based prediction of adverse cardiac remodeling after murine myocardial infarction

Myocardial infarction (MI) results in adverse cardiac remodeling leading to heart failure and increased mortality. Experimental mouse models of MI are extensively used to identify mechanisms underlying adverse remodeling, but the extent of remodeling that occurs may be highly variable and can limit the utility to discover new disease pathways. The ability to predict the development of significant late post-MI remodeling would be invaluable in conducting such studies by increasing throughput and efficiency. This study aimed to identify potential thresholds of cardiac magnetic resonance imaging (MRI) parameters measured early after murine MI that would predict the development of significant adverse remodeling at 4 wk. MI was achieved by permanent coronary ligation and animals (n = 84) were followed up for 4 wk subsequently. MRI was used to assess left ventricular (LV) volumes, mass and ejection fraction, as well as infarct size (IS). Late gadolinium enhancement cine-MRI was performed at 2 days with standard cine-MRI at 30 days post-MI. Utilizing multiple logistic regression, we found that IS >36%, at 2 days post-MI, was the overall best single predictor of adverse remodeling at 30 days (sensitivity 80.7%, specificity 88.9%; C-statistic of 0.939 from receiver-operating curve analysis). LV end-systolic volume (LVESV) >32 μl was also an excellent predictor comparable to IS. The combination of IS >36% and/or LVESV >32 μl provided the highest predictive values for late adverse remodeling among multiple predictors. This study demonstrates that MRI-based estimation of IS and ESV during the acute phase of murine MI are good predictors of subsequent adverse remodeling that may aid experimental design.

Manganese kinetics demonstrated double contrast in acute but not in chronic infarction in a mouse model of myocardial occlusion reperfusion

Manganese (Mn(2+)) is considered as a specific MRI contrast agent that enters viable cardiomyocytes through calcium pathways. Compared to extracellular gadolinium based contrast agents, it has the potential to assess cell viability. To date, only information from the washout phase after recirculation has been used for the detection and characterization of myocardial infarct. This study showed for the first time that in a mouse model of coronary occlusion-reperfusion, Mn(2+) wash-in kinetics are different at 24 h after surgery (acute infarction) than at eight days after surgery (chronic infarction). A fast but transient entry of Mn(2+) into the acute infarct area led to a double contrast between infarct and remote areas, whereas entry of Mn(2+) into the chronic infarct area remained reduced compared to remote regions during both wash-in and washout phases. The main hypothesis is that extracellular space is largely enhanced in acute infarction due to cell membrane rupture and interstitial edema, whereas scar tissue is densely composed of collagen fibers that reduce the distribution volume of free Mn(2+) ions. In addition to its ability to accurately depict the infarct area during the redistribution phase, Mn(2+) is also able to discriminate acute versus chronic injury by the observation of double-contrast kinetics in a mouse model of ischemia reperfusion.

In vivo MRI characterization of progressive cardiac dysfunction in the mdx mouse model of muscular dystrophy

AIMS

The mdx mouse has proven to be useful in understanding the cardiomyopathy that frequently occurs in muscular dystrophy patients. Here we employed a comprehensive array of clinically relevant in vivo MRI techniques to identify early markers of cardiac dysfunction and follow disease progression in the hearts of mdx mice.

METHODS AND RESULTS

Serial measurements of cardiac morphology and function were made in the same group of mdx mice and controls (housed in a non-SPF facility) using MRI at 1, 3, 6, 9 and 12 months after birth. Left ventricular (LV) and right ventricular (RV) systolic and diastolic function, response to dobutamine stress and myocardial fibrosis were assessed. RV dysfunction preceded LV dysfunction, with RV end systolic volumes increased and RV ejection fractions reduced at 3 months of age. LV ejection fractions were reduced at 12 months, compared with controls. An abnormal response to dobutamine stress was identified in the RV of mdx mice as early as 1 month. Late-gadolinium-enhanced MRI identified increased levels of myocardial fibrosis in 6, 9 and 12-month-old mdx mice, the extent of fibrosis correlating with the degree of cardiac remodeling and hypertrophy.

CONCLUSIONS

MRI could identify cardiac abnormalities in the RV of mdx mice as young as 1 month, and detected myocardial fibrosis at 6 months. We believe these to be the earliest MRI measurements of cardiac function reported for any mice, and the first use of late-gadolinium-enhancement in a mouse model of congenital cardiomyopathy. These techniques offer a sensitive and clinically relevant in vivo method for assessment of cardiomyopathy caused by muscular dystrophy and other diseases.

Myocardial infarct size and area at risk assessment in mice

Mouse models of myocardial ischemia and infarction are important in cardiovascular research. Reliable and reproducible assessment of the area at risk (AAR) and infarct size (IS) in mice is vital for deciphering mechanisms behind these common diseases, and for developing and evaluating treatment strategies. The present review will briefly describe and discuss the most common methods for determining the AAR and IS in mouse models of cardiovascular disease. Several methods exist for ex vivo assessment of IS. Conventional histological stains target the fibrous scar and require several days to pass from the time of infarct induction until the animal is euthanized, whereas triphenyltetrazolium-based techniques stain the viable tissue surrounding the infarct and can be performed on tissue harvested within a few hours after infarction. The AAR is usually stained by injecting a dye into the circulation. This dye subsequently distributes to perfused tissue but leaves the AAR unstained. In vivo assessment enables serial measurements of the IS and/or AAR and is sometimes preferable to ex vivo techniques. Echocardiography is usually the method of choice but magnetic resonance imaging-based techniques are also used. The aim of the present review was to provide basic researchers with an introduction to the various techniques used to assess and quantify the IS and AAR in experimental mouse models of myocardial ischemia-reperfusion and infarction.

Regional contrast agent quantification in a mouse model of myocardial infarction using 3D cardiac T1 mapping

BACKGROUND

Quantitative relaxation time measurements by cardiovascular magnetic resonance (CMR) are of paramount importance in contrast-enhanced studies of experimental myocardial infarction. First, compared to qualitative measurements based on signal intensity changes, they are less sensitive to specific parameter choices, thereby allowing for better comparison between different studies or during longitudinal studies. Secondly, T1 measurements may allow for quantification of local contrast agent concentrations. In this study, a recently developed 3D T1 mapping technique was applied in a mouse model of myocardial infarction to measure differences in myocardial T1 before and after injection of a liposomal contrast agent. This was then used to assess the concentration of accumulated contrast agent.

MATERIALS AND METHODS

Myocardial ischemia/reperfusion injury was induced in 8 mice by transient ligation of the LAD coronary artery. Baseline quantitative T1 maps were made at day 1 after surgery, followed by injection of a Gd-based liposomal contrast agent. Five mice served as control group, which followed the same protocol without initial surgery. Twenty-four hours post-injection, a second T1 measurement was performed. Local ΔR1 values were compared with regional wall thickening determined by functional cine CMR and correlated to ex vivo Gd concentrations determined by ICP-MS.

RESULTS

Compared to control values, pre-contrast T1 of infarcted myocardium was slightly elevated, whereas T1 of remote myocardium did not significantly differ. Twenty-four hours post-contrast injection, high ΔR1 values were found in regions with low wall thickening values. However, compared to remote tissue (wall thickening > 45%), ΔR1 was only significantly higher in severe infarcted tissue (wall thickening < 15%). A substantial correlation (r = 0.81) was found between CMR-based ΔR1 values and Gd concentrations from ex vivo ICP-MS measurements. Furthermore, regression analysis revealed that the effective relaxivity of the liposomal contrast agent was only about half the value determined in vitro.

CONCLUSIONS

3D cardiac T1 mapping by CMR can be used to monitor the accumulation of contrast agents in contrast-enhanced studies of murine myocardial infarction. The contrast agent relaxivity was decreased under in vivo conditions compared to in vitro measurements, which needs consideration when quantifying local contrast agent concentrations.

MIQuant--semi-automation of infarct size assessment in models of cardiac ischemic injury

BACKGROUND

The cardiac regenerative potential of newly developed therapies is traditionally evaluated in rodent models of surgically induced myocardial ischemia. A generally accepted key parameter for determining the success of the applied therapy is the infarct size. Although regarded as a gold standard method for infarct size estimation in heart ischemia, histological planimetry is time-consuming and highly variable amongst studies. The purpose of this work is to contribute towards the standardization and simplification of infarct size assessment by providing free access to a novel semi-automated software tool. The acronym MIQuant was attributed to this application.

METHODOLOGY/PRINCIPAL FINDINGS

Mice were subject to permanent coronary artery ligation and the size of chronic infarcts was estimated by area and midline-length methods using manual planimetry and with MIQuant. Repeatability and reproducibility of MIQuant scores were verified. The validation showed high correlation (r(midline length) = 0.981; r(area) = 0.970 ) and agreement (Bland-Altman analysis), free from bias for midline length and negligible bias of 1.21% to 3.72% for area quantification. Further analysis demonstrated that MIQuant reduced by 4.5-fold the time spent on the analysis and, importantly, MIQuant effectiveness is independent of user proficiency. The results indicate that MIQuant can be regarded as a better alternative to manual measurement.

CONCLUSIONS

We conclude that MIQuant is a reliable and an easy-to-use software for infarct size quantification. The widespread use of MIQuant will contribute towards the standardization of infarct size assessment across studies and, therefore, to the systematization of the evaluation of cardiac regenerative potential of emerging therapies.

Accelerating global left-ventricular function assessment in mice using reduced slice acquisition and three-dimensional guide-point modelling

BACKGROUND

To investigate the utility of three-dimensional guide-point modeling (GPM) to reduce the time required for CMR evaluation of global cardiac function in mice, by reducing the number of image slices required for accurate quantification of left-ventricular (LV) mass and volumes.

METHODS

Five female C57Bl/6 mice 8 weeks post myocardial infarction induced by permanent occlusion of the left coronary artery, and six male control (un-operated) C57Bl/6 mice, were subject to CMR examination under isoflurane anaesthesia. Contiguous short axis (SAX) slices (1 mm thick 7-9 slices) were obtained together with two long axis (LAX) slices in two chamber and four chamber orientations. Using a mathematical model of the heart to interpolate information between the available slices, GPM LV mass and volumes were determined using full slice (all SAX and two LAX), six slice (four SAX and two LAX) and four slice (two SAX and two LAX) analysis protocols. All results were compared with standard manual volumetric analysis using all SAX slices.

RESULTS

Infarct size was 39.1±5.1% of LV myocardium. No significant differences were found in left ventricular mass and volumes between the standard and GPM full and six slice protocols in infarcted mice (113±10, 116±11, and 117±11 mg respectively for mass), or between the standard and GPM full, six and four slice protocols in control mice, (105±14, 106±10, 104±12, and 105±7 mg respectively for mass). Significant differences were found in LV mass (135±18 mg) and EF using the GPM four slice protocol in infarcted mice (p<0.05).

CONCLUSION

GPM enables accurate analysis of LV function in mice with relatively large infarcts using a reduced six slice acquisition protocol, and in mice with normal/symmetrical left-ventricular topology using a four slice protocol.

Rapid assessment of myocardial infarct size in rodents using multi-slice inversion recovery late gadolinium enhancement CMR at 9.4T

BACKGROUND

Myocardial infarction (MI) can be readily assessed using late gadolinium enhancement (LGE) cardiovascular magnetic resonance (CMR). Inversion recovery (IR) sequences provide the highest contrast between enhanced infarct areas and healthy myocardium. Applying such methods to small animals is challenging due to rapid respiratory and cardiac rates relative to T1 relaxation.

METHODS

Here we present a fast and robust protocol for assessing LGE in small animals using a multi-slice IR gradient echo sequence for efficient assessment of LGE. An additional Look-Locker sequence was used to assess the optimum inversion point on an individual basis and to determine most appropriate gating points for both rat and mouse. The technique was applied to two preclinical scenarios: i) an acute (2 hour) reperfused model of MI in rats and ii) mice 2 days following non-reperfused MI.

RESULTS

LGE images from all animals revealed clear areas of enhancement allowing for easy volume segmentation. Typical inversion times required to null healthy myocardium in rats were between 300-450 ms equivalent to 2-3 R-waves and ~330 ms in mice, typically 3 R-waves following inversion. Data from rats was also validated against triphenyltetrazolium chloride staining and revealed close agreement for infarct size.

CONCLUSION

The LGE protocol presented provides a reliable method for acquiring images of high contrast and quality without excessive scan times, enabling higher throughput in experimental studies requiring reliable assessment of MI.

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.

Accelerated cardiac magnetic resonance imaging in the mouse using an eight-channel array at 9.4 Tesla

MRI has become an important tool to noninvasively assess global and regional cardiac function, infarct size, or myocardial blood flow in surgically or genetically modified mouse models of human heart disease. Constraints on scan time due to sensitivity to general anesthesia in hemodynamically compromised mice frequently limit the number of parameters available in one imaging session. Parallel imaging techniques to reduce acquisition times require coil arrays, which are technically challenging to design at ultrahigh magnetic field strengths. This work validates the use of an eight-channel volume phased-array coil for cardiac MRI in mice at 9.4 T. Two- and three-dimensional sequences were combined with parallel imaging techniques and used to quantify global cardiac function, T(1)-relaxation times and infarct sizes. Furthermore, the rapid acquisition of functional cine-data allowed for the first time in mice measurement of left-ventricular peak filling and ejection rates under intravenous infusion of dobutamine. The results demonstrate that a threefold accelerated data acquisition is generally feasible without compromising the accuracy of the results. This strategy may eventually pave the way for routine, multiparametric phenotyping of mouse hearts in vivo within one imaging session of tolerable duration.