Lukas Wissmann

Whole-heart dynamic three-dimensional magnetic resonance perfusion imaging for the detection of coronary artery disease defined by fractional flow reserve: determination of volumetric myocardial ischaemic burden and coronary lesion location

AIMS Dynamic three-dimensional-cardiac magnetic resonance (3D-CMR) perfusion proved highly diagnostic for the detection of angiographically defined coronary artery disease (CAD) and has been used to assess the efficacy of coronary stenting procedures. The present study aimed to relate significant coronary lesions as assessed by fractional flow reserve (FFR) to the volume of myocardial hypoenhancement on 3D-CMR adenosine stress perfusion imaging and to define the inter-study reproducibility of stress inducible 3D-CMR hypoperfusion. METHODS AND RESULTS A total of 120 patients with known or suspected CAD were examined in two CMR centres using 1.5 T systems. The protocol included cine imaging, 3D-CMR perfusion during adenosine infusion, and at rest followed by delayed enhancement (DE) imaging. Fractional flow reserve was recorded in epicardial coronary arteries and side branches with ≥2 mm luminal diameter and >40% severity stenosis (pathologic FFR < 0.75). Twenty-five patients underwent an identical repeat CMR examination for the determination of inter-study reproducibility of 3D-CMR perfusion deficits induced by adenosine. Three-dimensional CMR perfusion scans were visually classified as pathologic if one or more segments showed an inducible perfusion deficit in the absence of DE. Myocardial ischaemic burden (MIB) was measured by segmentation of the area of inducible hypoenhancement and normalized to left ventricular myocardial volume (MIB, %). Three-dimensional CMR perfusion resulted in a sensitivity, specificity, and diagnostic accuracy of 90, 82, and 87%, respectively. Substantial concordance was found for inter-study reproducibility [Lins correlation coefficient: 0.98 (95% confidence interval: 0.96-0.99)]. CONCLUSION Three-dimensional CMR stress perfusion provided high diagnostic accuracy for the detection of functionally significant CAD. Myocardial ischaemic burden measurements were highly reproducible and allowed the assessment of CAD severity.

Multicenter Evaluation of Dynamic Three-Dimensional Magnetic Resonance Myocardial Perfusion Imaging for the Detection of Coronary Artery Disease Defined by Fractional Flow Reserve

Background—First-pass myocardial perfusion cardiovascular magnetic resonance (CMR) imaging yields high diagnostic accuracy for the detection of coronary artery disease (CAD). However, standard 2D multislice CMR perfusion techniques provide only limited cardiac coverage, and hence considerable assumptions are required to assess myocardial ischemic burden. The aim of this prospective study was to assess the diagnostic performance of 3D myocardial perfusion CMR to detect functionally relevant CAD with fractional flow reserve (FFR) as a reference standard in a multicenter setting. Methods and Results—A total of 155 patients with suspected CAD listed for coronary angiography with FFR were prospectively enrolled from 5 European centers. 3D perfusion CMR was acquired on 3T MR systems from a single vendor under adenosine stress and at rest. All CMR perfusion analyses were performed in a central laboratory and blinded to all clinical data. One hundred fifty patients were successfully examined (mean age 62.9±10 years, 45 female). The prevalence of CAD defined by FFR (<0.8) was 56.7% (85 of 150 patients). The sensitivity and specificity of 3D perfusion CMR were 84.7% and 90.8% relative to the FFR reference. Comparison to quantitative coronary angiography (≥50%) yielded a prevalence of 65.3%, sensitivity and specificity of 76.5% and 94.2%, respectively. Conclusions—In this multicenter study, 3D myocardial perfusion CMR proved highly diagnostic for the detection of significant CAD as defined by FFR.

Reconstruction of divergence-free velocity fields from cine 3D phase-contrast flow measurements.

Three‐dimensional phase‐contrast velocity vector field mapping shows great potential for clinical applications; however measurement inaccuracies may limit the utility and robustness of the technique. While parts of the error in the measured velocity fields can be minimized by background phase estimation in static tissue and magnetic field monitoring, considerable inaccuracies remain. The present work introduces divergence‐reduction processing of 3D phase‐contrast flow data based on a synergistic combination of normalized convolution and divergence‐free radial basis functions. It is demonstrated that this approach effectively addresses erroneous flow for image reconstructions from both fully sampled and undersampled data. Using computer simulations and in vivo data acquired in the aorta of healthy subjects and a stenotic valve patient it is shown that divergence arising from measurement imperfections can be reduced by up to 87% resulting in improved vector field representations. Based on the results obtained it is concluded that integration of the divergence‐free condition into postprocessing of vector fields presents an efficient approach to addressing flow field inaccuracies. Magn Reson Med, 2013.

MRXCAT: Realistic numerical phantoms for cardiovascular magnetic resonance

BackgroundComputer simulations are important for validating novel image acquisition and reconstruction strategies. In cardiovascular magnetic resonance (CMR), numerical simulations need to combine anatomical information and the effects of cardiac and/or respiratory motion. To this end, a framework for realistic CMR simulations is proposed and its use for image reconstruction from undersampled data is demonstrated.MethodsThe extended Cardiac-Torso (XCAT) anatomical phantom framework with various motion options was used as a basis for the numerical phantoms. Different tissue, dynamic contrast and signal models, multiple receiver coils and noise are simulated. Arbitrary trajectories and undersampled acquisition can be selected. The utility of the framework is demonstrated for accelerated cine and first-pass myocardial perfusion imaging using k-t PCA and k-t SPARSE.ResultsMRXCAT phantoms allow for realistic simulation of CMR including optional cardiac and respiratory motion. Example reconstructions from simulated undersampled k-t parallel imaging demonstrate the feasibility of simulated acquisition and reconstruction using the presented framework. Myocardial blood flow assessment from simulated myocardial perfusion images highlights the suitability of MRXCAT for quantitative post-processing simulation.ConclusionThe proposed MRXCAT phantom framework enables versatile and realistic simulations of CMR including breathhold and free-breathing acquisitions.

Hybrid multiband excitation multiecho acquisition for hyperpolarized 13C spectroscopic imaging

Fast dynamic imaging of hyperpolarized 13C‐labeled pyruvate and its downstream metabolites shows great potential for probing metabolic changes in the heart. Sequences that allow for fast encoding of the spectral and spatial information of the myocardial metabolism and optimal signal excitation are usually limited by gradient performance, especially at high magnetic fields. Here we propose a combination of a spectral‐spatial multiband excitation and multiecho readout to overcome these limitations.

Hyperpolarized Metabolic MR Imaging of Acute Myocardial Changes and Recovery after Ischemia-Reperfusion in a Small-Animal Model.

PURPOSE To implement hyperpolarized magnetic resonance (MR) imaging in an animal model of ischemia-reperfusion and to assess in vivo the regional changes in pyruvate metabolism within the 1st hour and at 1 week after a brief episode of coronary occlusion and reperfusion. MATERIALS AND METHODS All animal experiments were performed with adherence to the Swiss Animal Protection law and were approved by the regional veterinary office. A closed-chest rat model was implemented by using an inflatable balloon secured around the left coronary artery. Animals were placed in an MR system 5-7 days after surgery. [1-(13)C]pyruvate was polarized by using a home-built multisample hyperpolarizer. Hyperpolarized pyruvate was injected at five stages: at baseline; at reperfusion after 15 minutes of coronary occlusion; and at 30 minutes, 60 minutes, and 1 week after ischemia reperfusion. The conversion of pyruvate into lactate and bicarbonate was imaged by using dedicated MR sequences alongside wall motion and delayed enhancement imaging. After imaging, the heart was removed and stained to delineate the area at risk (AAR). Differences between AAR and remote myocardium were assessed by using a repeated measures analysis of variance and a post hoc Bonferroni multiple comparison test. RESULTS Data were collected in 12 animals. Occlusion led to hypokinesia of the anterior or anterolateral segments of the myocardium. At reperfusion, the average lactate-to-bicarbonate ratio increased in the AAR relative to that at baseline (from 1.93 ± 0.48 to 3.01 ± 0.74, P < .001) and was significantly higher when compared with that in the remote area (1.91 ± 0.38, P < .001). In the 60 minutes after occlusion, the lactate-to-bicarbonate ratio in the AAR recovered but was still elevated relative to that in the remote area. One week after ischemia-reperfusion, no difference between AAR and remote area could be detected. CONCLUSION Hyperpolarized metabolic MR imaging can be used to successfully detect acute changes in [1-(13)C]pyruvate metabolism after ischemia-reperfusion, thereby enabling in vivo monitoring of the metabolic effects of reperfusion strategies.

Accelerating hyperpolarized metabolic imaging of the heart by exploiting spatiotemporal correlations

Hyperpolarized 13C‐labeled pyruvate is a promising tool to investigate cardiac metabolism. It has been shown that changes in substrate metabolism occur following the induction of ischemia. To investigate the metabolic changes that are confined to spatial regions, high spatiotemporal resolution is required. The present work exploits both spatial and temporal correlations using k–t principal component analysis (PCA) to undersample the spatiotemporal domain, thereby speeding up data acquisition. A numerical model was implemented to investigate optimal acquisition and reconstruction parameters for pyruvate, lactate and bicarbonate maps of the heart. Subsequently, prospectively undersampled in vivo data on rat hearts were acquired using a combination of spectral–spatial signal excitation and a variable‐density single‐shot echo planar readout. Using five‐fold k–t PCA, a spatial resolution of 1 × 1 mm2 at a temporal resolution of 3 s was achieved. Copyright

Iterative k-t principal component analysis with nonrigid motion correction for dynamic three-dimensional cardiac perfusion imaging

In this study, an iterative k‐t principal component analysis (PCA) algorithm with nonrigid frame‐to‐frame motion correction is proposed for dynamic contrast‐enhanced three‐dimensional perfusion imaging.

A multisample dissolution dynamic nuclear polarization system for serial injections in small animals

Several in vivo applications of dissolution dynamic nuclear polarization (DNP) require rapid successive injections of hyperpolarized substrates. Here we present the design and performance of a custom‐built multisample dissolution DNP setup for small animal research.

First fusion and combined evaluation of 3D-CMR perfusion with 3D-MR coronary angiography

Introduction Myocardial perfusion and the status of the coronary arteries are the two major parameters for the characterization of coronary artery disease (CAD) and for guiding therapeutical interventions. It has been shown that hybrid imaging strategies to acquire both parameters such as SPECT with CT-angiography provide an added value for clinical decision making in the treatment of CAD[1]. Thus the 2014 ESC Guidelines for the first time recommend hybrid imaging for planning myocardial revascularization[2]. However, SPECT and CT expose the patient to ionizing radiation and, in large prospective trials, SPECT showed inferior sensitivity to detect CAD when compared with CMR-perfusion[3]. Therefore, the aim of this study was to investigate the feasibility and potential added value of MR-based hybrid imaging by the combined assessment and fusion of 3D-MR coronary angiography (MRCA) with a 3D-CMR perfusion sequence.