Andreas Sigfridsson

Bayesian multipoint velocity encoding for concurrent flow and turbulence mapping.

An approach to efficiently measure three‐dimensional velocity vector fields and turbulent kinetic energy of blood flow is presented. Multipoint phase‐contrast imaging is used in combination with Bayesian analysis to map both mean and fluctuating velocities over a large dynamic range and for practically relevant signal‐to‐noise ratios. It is demonstrated that the approach permits significant spatiotemporal undersampling to allow for clinically acceptable scan times. Using numerical simulations and in vitro measurements in aortic valve phantoms, it is shown that for given scan time, Bayesian multipoint velocity encoding provides consistently lower errors of velocity and turbulent kinetic energy over a larger dynamic range relative to previous methods. In vitro, significant differences in both peak velocity and turbulent kinetic energy between the aortic CoreValve (150 cm/s, 293 J/m3) and the St. Jude Medical mechanical valve (120 cm/s, 149 J/m3) were found. Comparison of peak turbulent kinetic energy measured in a patient with aortic stenosis (950 J/m3) and in a patient with an implanted aortic CoreValve (540 J/m3) revealed considerable differences relative to the values detected in healthy subjects (149 ± 12 J/m3) indicating the potential of the method to provide a comprehensive hemodynamic assessment of valve performance in vivo. Magn Reson Med, 2013.

Mapping mean and fluctuating velocities by Bayesian multipoint MR velocity encoding-validation against 3D particle tracking velocimetry

To validate Bayesian multipoint MR velocity encoding against particle tracking velocimetry for measuring velocity vector fields and fluctuating velocities in a realistic aortic model.

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

Retrospectively gated intra-cardiac 4D flow CMR using spiral k-space trajectories

Background Time-resolved three-dimensional phase contrast CMR (4D flow) is a powerful tool for hemodynamic assessment in the cardiovascular system. However, long scan times have hindered the application of the method in many cases. By using spiral readout trajectories, improved efficiency provides a means of reducing scan times without decreasing SNR. Furthermore, spiral acquisition offers increased robustness in areas with accelerating flow. Spiral readouts have previously been used for rapid 4D flow measurements in the aorta using prospective gating [1]. Using retrospective gating, the entire cardiac cycle is covered, which allows analysis of late diastole and tracking of blood over a complete cardiac cycle. These are crucial for cardiac 4D flow studies, and allow for pathline based data quality assessment. The aim of this work is to develop a retrospectively gated 4D flow sequence using a stack of spiral readouts for the measurement of intra-cardiac velocities.

Assessment of energy loss across aortic valves using accelerated CMR multi-point flow measurements

Summary A novel approach for evaluating the performance of artificial or diseased heart valves is presented and applied on in-vitro as well as in-vivo aortic valve data. The method, which is based on turbulence and flow measurements, provides a measure to assess and compare energy dissipation under varying flow conditions. Background Diseased or artificial heart valves possibly lead to turbulent flow and regurgitation, both increasing the workload of the heart. Current measures for valve assessment, i.e. effective orifice area, only indirectly and partially correlate with the energy loss due to the valve [1]. Phase-Contrast MRI makes it possible to directly quantify these energy losses, and by relating them to kinetic energy of the flow ap arameter describing the hemodynamic performance of the valve can be obtained. Methods

Hyperpolarized metabolic imaging of myocardial ischemia-reperfusion in a small-animal model at 9.4T

Background Dynamic hyperpolarization (DNP) of Carbon-13 (13C) allows in-vivo assessment of metabolic processes. The aims of the present study were to establish a living rat model of ischemia-reperfusion and to study the metabolic changes in the myocardium following short periods of coronary artery occlusion using intravenous injection of hyperpolarized 1-13C pyruvate. Methods An inflatable balloon was secured around the left coronary artery of Sprague Dawley rats. 5-7 days after surgery rats were placed in a Bruker Biospec 9.4T small animal MR system. To enhance bicarbonate signal, animals received an iv glucose/potassium infusion. The tubing of the occluder was connected to extension tubing to allow occlusion while the animal remained in the bore of the MR system. A custom-built multi-sample DNP polarizer (1) was used to polarize samples (25.4 μ L[ 1- 13 C]-pyruvic acid and 13.5 mM trityl radical doped with 1.5 mM Dotarem). The rats were injected with 1.4 ml DNP solution at 4 time points: baseline, reperfusion directly after 15 minutes of coronary occlusion, after 30 minutes of reperfusion, after 60 minutes of reperfusion. A subgroup of rats underwent repeat imaging 1 week after ischemia. Metabolic data were acquired with a multiband pulse in combination with a multi-echo single-shot EPI readout (2). Hearts were removed and stained to delineate the area at risk (AAR), and for myocardial infarction. Results

Multi-echo single-shot EPI for hyperpolarized 13C cardiac metabolic imaging of small animals

Background Cardiac metabolic imaging based on hyperpolarized 13C-labeled pyruvate shows great potential for assessing the metabolic changes that the heart undergoes during ischemia [1]. Rodent animal models offer unique opportunities to study ischemic processes, however, methods based on spectral-spatial excitation [2] of the individual metabolites is challenging due to the large minimal slice thickness that can be achieved with available gradient systems. A thick slice introduces both signal dephasing over the slice and errors due to partial volume effects. In this work, we explore multi-echo measurements using single-shot echo-planar imaging (EPI) readouts for thin slice dynamic cardiac metabolic imaging of small animals.