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Purpose To achieve whole-liver motion-corrected fat fraction (FF) and R2* quantification with a 3-minute free-breathing (FB) 3D radial isotropic acquisition, for increased organ coverage, ease-of-use, and patient comfort. Methods A FB 3D radial multiecho gradient-echo liver acquisition with integrated Pilot Tone (PT) navigation and NTE=8 echoes was reconstructed with a motion-correction algorithm based on focused navigation and guided by PT signals (PT-fNAV), with and without a denoising step. Fat fraction (FF) and R2* quantification using a graph cut algorithm was performed on the motion-corrected whole-liver multiecho volumes. Volunteer experiments (n=10) at 1.5T included reference 3D and 2D Cartesian breath-hold (BH) acquisitions. Image sharpness was assessed to evaluate the quality of motion correction with PT-fNAV, compared to a motion-resolved reconstruction. Fat-water images and parametric maps were compared to BH reference acquisitions following Cartesian trajectories, and to a routinely used clinical software (MRQuantiF). Results The image sharpness provided by PT-fNAV (with and without denoising) was similar in end-expiratory motion-resolved reconstructions. The 3D radial FB FF maps compared well with reference BH 3D Cartesian maps (bias +0.7%, limits of agreement (LOA) [-2.5; 4.0]%) and with 2D quantification with MRQuantiF (-0.2%, LOA [-1.1; 0.6]%). While expected visual deviations between proposed FB and reference BH R2* maps were observed, no significant differences were found in quantitative analyses. Conclusion A 3D radial technique with retrospective motion correction by PT-fNAV enabled FF and R2* quantification of the whole-liver at 1.5T. The FB whole-liver acquisition at isotropic spatial resolution compared in accuracy with BH techniques, enabling 3D assessment of steatosis in individuals with limited respiratory capabilities., Comment: 26 pages, 5 figures, under review

It was demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with ultraviolet (UV) light, enabling radical-free dissolution DNP. Although pyruvate is endogenous, an excess of additional pyruvate may perturb metabolic processes, making it potentially unsuitable as a polarizing agent when studying fatty acids or carbohydrate metabolism. Therefore, the aim of the study was to characterize solutions containing endogenously-occurring alternatives to pyruvate as UV-induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alpha-ketovalerate (AKV) and alpha-ketobutyrate (AKB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing AKV and AKB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with ESR and compared to pyruvate. The addition of 13C labeled substrates to the sample matrix altered the radical yield of the precursors. Using AKB increased the 13C-labeled glucose liquid state polarization to 16.3 +/- 1.3% compared with 13.3 +/- 1.5% obtained with pyruvate, and 8.9 +/- 2.1% with AKV. For [1-13C]butyric acid, polarization levels of 12.1 +/- 1.1% for AKV and 12.9 +/- 1.7% for AKB were achieved. Hyperpolarized [1-13C]butyrate metabolism in the heart revealed label incorporation into [1-13C]acetylcarnitine, [1-13C]acetoacetate, [1-13C]butyrylcarnitine, [5-13C]glutamate and [5-13C]citrate. This study demonstrates the potential of AKV and AKB as endogenous polarizing agents for in vivo radical-free hyperpolarized MRI. UV-induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality-control tests in clinical applications., Comment: 38 pages, 5 Tables, 8 Figures, Submitted to NMR in Biomedicine

Purpose: To develop SPARCQ (Signal Profile Asymmetries for Rapid Compartment Quantification), a novel approach to quantify fat fraction using asymmetries in the phase-cycled bSSFP profile. Methods: SPARCQ uses phase-cycling to obtain bSSFP frequency profiles, which display asymmetries in the presence of fat and water at certain repetition times. For each voxel, the measured signal profile is decomposed into a weighted sum of simulated profiles via multi-compartment dictionary matching. Each dictionary entry represents a single-compartment bSSFP profile with a specific off-resonance frequency and relaxation time ratio. Using the results of dictionary matching, the fractions of the different off-resonance components are extracted for each voxel, generating quantitative maps of water and fat fraction and banding-artifact-free images for the entire image volume. SPARCQ was validated using simulations, experiments in a water-fat phantom and in knees of healthy volunteers. Experimental results were compared with reference proton density fat fractions obtained with 1H-MRS (phantoms) and with multiecho gradient-echo MRI (phantoms and volunteers). SPARCQ repeatability was evaluated in six scan-rescan experiments. Results: Simulations showed that fat fraction quantification is accurate and robust for signal-to-noise ratios greater than 20. Phantom experiments demonstrated good agreement between SPARCQ and gold standard fat fractions. In volunteers, banding-artifact-free quantitative maps and water-fat-separated images obtained with SPARCQ and ME-GRE demonstrated the expected contrast between fatty and non-fatty tissues. The coefficient of repeatability of SPARCQ fat fraction was 0.0512. Conclusion: SPARCQ demonstrates potential for fat quantification using asymmetries in bSSFP profiles, and may be a promising alternative to conventional fat fraction quantification techniques., Comment: 30 pages, 8 figures, submitted to Magnetic Resonance in Medicine

Purpose: Designing a new T2 preparation (T2-Prep) module in order to simultaneously provide robust fat suppression and efficient T2 preparation without requiring an additional fat suppression module for T2-weighted imaging at 3T. Methods: The tip-down RF pulse of an adiabatic T2 preparation (T2-Prep) module was replaced by a custom-designed RF excitation pulse that induces a phase difference between water and fat, resulting in a simultaneous T2 preparation of water signals and the suppression of fat signals at the end of the module (now called a phaser adiabatic T2-Prep). Using numerical simulations, in vitro and in vivo ECG-triggered navigator gated acquisitions of the human heart, the blood, myocardium and fat signal-to-noise ratio and right coronary artery (RCA) vessel sharpness using this approach were compared against previously published conventional adiabatic T2-Prep approaches Results: Numerical simulations predicted an increased fat suppression bandwidth and decreased sensitivity against transmit magnetic field inhomogeneities using the proposed approach, while preserving the water T2 preparation capabilities. This was confirmed by the tissue signals acquired on the phantom and the in vivo MRA, which show similar blood and myocardium SNR and CNR and significantly reduced fat SNR compared to the other methods tested. As a result, the RCA conspicuity was significantly increased and the motion artifacts were visually decreased. Conclusion: A novel fat-suppressing T2-preparation method was developed and implemented that demonstrated robust fat suppression and increased vessel sharpness compared with conventional techniques, while preserving its T2 preparation capabilities., Comment: 23 pages, 5 figures, submitted to Magnetic Resonance in Medicine

Purpose: To implement, optimize and test fast interrupted steady-state (FISS) for natively fat-suppressed free-running 5D whole-heart MRI at 1.5T and 3T. Methods: FISS was implemented for fully self-gated free-running cardiac- and respiratory-motion-resolved radial imaging of the heart at 1.5T and 3T. Numerical simulations and phantom scans were performed to compare fat suppression characteristics and to determine parameter ranges (readouts per FISS module (NR) and repetition time (TR)) for effective fat suppression. Subsequently, free-running FISS data were collected in ten healthy volunteers. All acquisitions were compared with a continuous bSSFP version of the same sequence, and both fat suppression and scan times were analyzed. Results: Simulations demonstrate a variable width and location of suppression bands in FISS that was dependent on TR and NR. For a fat suppression bandwidth of 100Hz and NR below 8, simulations demonstrated that a TR between 2.2ms and 3.0ms is required at 1.5T while a range of 3.0ms to 3.5ms applies at 3T. Fat signal increases with NR. These findings were corroborated in phantom experiments. In volunteers, fat SNR was significantly decreased using FISS compared with bSSPF at both field strengths. After protocol optimization, high-resolution (1.1mm x 1.1mm x 1.1mm) 5D whole-heart free-running FISS can be performed with effective fat suppression in under 8 min at 1.5T and 3T at a modest scan time increase compared to bSSFP.Conclusion: An optimal FISS parameter range was determined enabling natively fat-suppressed 5D whole-heart free-running MRI with a single continuous scan at 1.5T and 3T, demonstrating potential for cardiac imaging and noncontrast angiography., Comment: Submitted to Magnetic Resonance in Medicine, 27 pages

Purpose: To develop a robust and flexible low power water excitation pulse that enables effective fat suppression at high magnetic field strength. Methods: A water excitation method that uses spatially non-selective pulses was optimized in numerical simulations, and implemented and tested in phantoms and healthy volunteers at 3T. The lipid insensitive binomial off-resonant excitation (LIBRE) pulse comprises two low power rectangular sub-pulses that have a variable frequency offset, phase offset and duration. The capability and extent of LIBRE fat suppression was quantitatively compared with conventional fat saturation (FS) and water excitation (WE) techniques. Results: LIBRE enables simultaneous water excitation and near complete fat suppression in large volumes at 3T as demonstrated by numerical simulations, and experiments. In phantoms and in human subjects, the frequency responses matched well with those from the numerical simulation. Comparing FS and WE, LIBRE demonstrated an improved robustness to magnetic field inhomogeneities, and a much more effectively suppressed fat signal. This applied for a range of pulse durations and pulses as short as 1.4 ms. Conclusion: A flexible water excitation method was developed that shows robust, near complete fat suppression at 3T., Comment: Published in Magnetic Resonance in Medicine, 35 pages