Second-order motion compensated PRESS for cardiac spectroscopy

Purpose Second‐order motion compensation for point‐resolved spectroscopy (PRESS) is proposed to allow for robust single‐voxel cardiac spectroscopy throughout the entire cardiac cycle and at various heart rates. Methods Bipolar FID spoiling gradient pairs compensating for first and second‐order motion were designed and implemented into a cardiac‐triggered PRESS sequence on a clinical MR system. A numerical three‐dimensional model of cardiac motion was used to optimize and validate the gradient waveforms. In vivo measurements in healthy volunteers were obtained to assess the signal‐to‐noise ratio (SNR) and triglyceride‐to‐water ratio (TG/W). SNR gains and variability of TG/W of the proposed approach were evaluated against a conventional PRESS sequence with optimized gradients. Results The proposed sequence increases the mean SNR by 32% (W) and 23% (TG) on average with significantly lower variability for different trigger delays. The variability of TG/W quantification over the cardiac cycle is significantly decreased with second‐order motion compensated PRESS when compared with conventional PRESS with reduced‐spoiler gradients (coefficient of variation: 0.1 ± 0.02 versus 0.37 ± 0.26). Conclusion Second‐order motion compensated PRESS effectively reduces cardiac motion–induced signal degradation during FID spoiling, providing higher SNR and less variability for TG/W quantification. The sequence is considered promising to assess the TG/W modulation during various interventions including pharmacologically induced stress. Magn Reson Med 77:57–64, 2017. © 2016 Wiley Periodicals, Inc

Joint image and field map estimation for multi‐echo hyperpolarized 13 C metabolic imaging of the heart

Purpose Image reconstruction of metabolic images from hyperpolarized 13C multi-echo data acquisition is sensitive to susceptibility-induced phase offsets, which are particularly challenging in the heart. A model-based framework for joint estimation of metabolite images and field map from echo shift–encoded data is proposed. Using simulations, it is demonstrated that correction of signal spilling due to incorrect decomposition of metabolites and geometrical distortions over a wide range of off-resonance gradients is possible. In vivo feasibility is illustrated using hyperpolarized [1-13C]pyruvate in the pig heart. Methods The model-based reconstruction for multi-echo, multicoil data was implemented as a nonconvex minimization problem jointly optimizing for metabolic images and B0. A comprehensive simulation framework for echo shift–encoded hyperpolarized [1-13C]pyruvate imaging was developed and applied to assess reconstruction performance and distortion correction of the proposed method. In vivo data were obtained in four pigs using hyperpolarized [1-13C]pyruvate on a clinical 3T MR system with a six-channel receiver coil. Dynamic images were acquired during suspended ventilation using cardiac-triggered multi-echo single-shot echo-planar imaging in short-axis orientation. Results Simulations revealed that off-resonance gradients up to ±0.26 ppm/pixel can be corrected for with reduced signal spilling and geometrical distortions yielding an accuracy of ≥90% in terms of Dice similarity index. In vivo, improved geometrical consistency (10% Dice improvement) compared to image reconstruction without field map correction and with reference to anatomical data was achieved. Conclusion Joint image and field map estimation allows addressing off-resonance-induced geometrical distortions and metabolite spilling in hyperpolarized 13C metabolic imaging of the heart

Joint image and field map estimation for multi-echo hyperpolarized 13C metabolic imaging of the heart

Purpose: Image reconstruction of metabolic images from hyperpolarized C multi-echo data acquisition is sensitive to susceptibility-induced phase offsets, which are particularly challenging in the heart. A model-based framework for joint estimation of metabolite images and field map from echo shift–encoded data is proposed. Using simulations, it is demonstrated that correction of signal spilling due to incorrect decomposition of metabolites and geometrical distortions over a wide range of off-resonance gradients is possible. In vivo feasibility is illustrated using hyperpolarized [1- C]pyruvate in the pig heart. Methods: The model-based reconstruction for multi-echo, multicoil data was implemented as a nonconvex minimization problem jointly optimizing for metabolic images and B . A comprehensive simulation framework for echo shift–encoded hyperpolarized [1- C]pyruvate imaging was developed and applied to assess reconstruction performance and distortion correction of the proposed method. In vivo data were obtained in four pigs using hyperpolarized [1- C]pyruvate on a clinical 3T MR system with a six-channel receiver coil. Dynamic images were acquired during suspended ventilation using cardiac-triggered multi-echo single-shot echo-planar imaging in short-axis orientation. Results: Simulations revealed that off-resonance gradients up to ±0.26 ppm/pixel can be corrected for with reduced signal spilling and geometrical distortions yielding an accuracy of ≥90% in terms of Dice similarity index. In vivo, improved geometrical consistency (10% Dice improvement) compared to image reconstruction without field map correction and with reference to anatomical data was achieved. Conclusion: Joint image and field map estimation allows addressing off-resonance-induced geometrical distortions and metabolite spilling in hyperpolarized C metabolic imaging of the heart. 13 13 13 13 13 0ISSN:0740-3194ISSN:1522-259

Navigator‐free metabolite‐cycled proton spectroscopy of the heart

Purpose Respiratory gating in cardiac water‐suppressed (WS) proton spectroscopy leads to long and unpredictable scan times. Metabolite cycling allows to perform frequency and phase correction on the water signal and, hence, offers an approach to navigator‐free cardiac spectroscopy with fixed scan time. The objective of the present study was to develop and implement navigator‐free metabolite‐cycled cardiac proton spectroscopy (MC nonav) and compare it with standard navigator‐gated WS (WS nav) and navigator‐free WS (WS nonav) measurements for the assessment of triglyceride‐to‐water ratios (TG/W) and creatine‐to‐water ratios (CR/W) in the intraventricular septum of the in vivo heart. Methods Navigator‐free metabolite‐cycled spectroscopy was implemented on a clinical 1.5T system. In vivo measurements were performed on 10 young and 5 older healthy volunteers to assess signal‐to‐noise ratio efficiency as well as TG/W and CR/W and the relative Cramér‐Rao lower bounds for CR. The performance of the metabolite‐cycled sequence was verified using simulations. Results On average, scan times of MC nonav were 3.4 times shorter compared with WS nav, while no significant bias for TG/W was observed (coefficient of variation = 14.0%). signal‐to‐noise ratio efficiency of both TG and CR increased for MC nonav compared with WS nav. Relative Cramér‐Rao lower bounds of CR decreased for MC nonav. Overall spectral quality was found comparable between MC nonav and WS nav, while it was inferior for WS nonav. Conclusion Navigator‐free metabolite‐cycled cardiac proton spectroscopy offers 3.4‐fold accelerated assessment of TG/W and CR/W in the heart with preserved spectral quality when compared with navigator‐gated WS scans

Considerations for hyperpolarized 13C MR at reduced field: Comparing 1.5T versus 3T

Purpose: In contrast to conventional MR, signal-to-noise ratio (SNR) is not linearly dependent on field strength in hyperpolarized MR, as polarization is generated outside the MR system. Moreover, field inhomogeneity-induced artifacts and other practical limitations associated with field strengths ≥ 3T are alleviated at lower fields. The potential of hyperpolarized 13C spectroscopy and imaging at 1.5T versus 3T is demonstrated in silico, in vitro, and in vivo for applications on clinical MR systems. Theory and Methods: Theoretical noise and SNR behavior at different field strengths are investigated based on simulations. A thorough field comparison between 1.5T and 3T is performed using thermal and hyperpolarized 13C spectroscopy and imaging. Cardiac in vivo data is obtained in pigs using hyperpolarized [1-13C] pyruvate spectroscopy and imaging at 1.5T and 3T. Results: Based on theoretical considerations and simulations, the SNR of hyperpolarized MR at identical acquisition bandwidths is independent of the field strength for typical coil setups, while adaptively changing the acquisition bandwidth proportional to the static magnetic field allows for net SNR gains of up to 40% at 1.5T compared to 3T. In vitro 13 C data verified these considerations with less than 7% deviation. In vivo feasibility of hyperpolarized [1-13 C] pyruvate dynamic metabolic spectroscopy and imaging at 1.5T is demonstrated in the pig heart with comparable SNR between 1.5T and 3T while B 0 artifacts are noticeably reduced at 1.5T. Conclusion: Hyperpolarized 13 C MR at lower field strengths is favorable in terms of SNR and off-resonance effects, which makes 1.5T a promising alternative to 3T, especially for clinical cardiac metabolic imaging.ISSN:0740-3194ISSN:1522-259

Free-breathing motion-informed locally low-rank quantitative 3D myocardial perfusion imaging

Purpose To propose respiratory motion-informed locally low-rank reconstruction (MI-LLR) for robust free-breathing single-bolus quantitative 3D myocardial perfusion CMR imaging. Simulation and in-vivo results are compared to locally low-rank (LLR) and compressed sensing reconstructions (CS) for reference. Methods Data were acquired using a 3D Cartesian pseudo-spiral in-out k-t undersampling scheme (R = 10) and reconstructed using MI-LLR, which encompasses two stages. In the first stage, approximate displacement fields are derived from an initial LLR reconstruction to feed a motion-compensated reference system to a second reconstruction stage, which reduces the rank of the inverse problem. For comparison, data were also reconstructed with LLR and frame-by-frame CS using wavelets as sparsifying transform (l1-wavelet). Reconstruction accuracy relative to ground truth was assessed using synthetic data for realistic ranges of breathing motion, heart rates, and SNRs. In-vivo experiments were conducted in healthy subjects at rest and during adenosine stress. Myocardial blood flow (MBF) maps were derived using a Fermi model. Results Improved uniformity of MBF maps with reduced local variations was achieved with MI-LLR. For rest and stress, intra-volunteer variation of absolute and relative MBF was lower in MI-LLR (±0.17 mL/g/min [26%] and ±1.07 mL/g/min [33%]) versus LLR (±0.19 mL/g/min [28%] and ±1.22 mL/g/min [36%]) and versus l1-wavelet (±1.17 mL/g/min [113%] and ±6.87 mL/g/min [115%]). At rest, intra-subject MBF variation was reduced significantly with MI-LLR. Conclusion The combination of pseudo-spiral Cartesian undersampling and dual-stage MI-LLR reconstruction improves free-breathing quantitative 3D myocardial perfusion CMR imaging under rest and stress condition.ISSN:0740-3194ISSN:1522-259

Free-breathing motion-informed locally low-rank quantitative 3D myocardial perfusion imaging

PURPOSE To propose respiratory motion-informed locally low-rank reconstruction (MI-LLR) for robust free-breathing single-bolus quantitative 3D myocardial perfusion CMR imaging. Simulation and in-vivo results are compared to locally low-rank (LLR) and compressed sensing reconstructions (CS) for reference. METHODS Data were acquired using a 3D Cartesian pseudo-spiral in-out k-t undersampling scheme (R = 10) and reconstructed using MI-LLR, which encompasses two stages. In the first stage, approximate displacement fields are derived from an initial LLR reconstruction to feed a motion-compensated reference system to a second reconstruction stage, which reduces the rank of the inverse problem. For comparison, data were also reconstructed with LLR and frame-by-frame CS using wavelets as sparsifying transform ( -wavelet). Reconstruction accuracy relative to ground truth was assessed using synthetic data for realistic ranges of breathing motion, heart rates, and SNRs. In-vivo experiments were conducted in healthy subjects at rest and during adenosine stress. Myocardial blood flow (MBF) maps were derived using a Fermi model. RESULTS Improved uniformity of MBF maps with reduced local variations was achieved with MI-LLR. For rest and stress, intra-volunteer variation of absolute and relative MBF was lower in MI-LLR (±0.17 mL/g/min [26%] and ±1.07 mL/g/min [33%]) versus LLR (±0.19 mL/g/min [28%] and ±1.22 mL/g/min [36%]) and versus -wavelet (±1.17 mL/g/min [113%] and ±6.87 mL/g/min [115%]). At rest, intra-subject MBF variation was reduced significantly with MI-LLR. CONCLUSION The combination of pseudo-spiral Cartesian undersampling and dual-stage MI-LLR reconstruction improves free-breathing quantitative 3D myocardial perfusion CMR imaging under rest and stress condition

Cardiac- versus diaphragm-based respiratory navigation for proton spectroscopy of the heart

OBJECTIVES To study inter-individual differences of the relation between diaphragm and heart motion, the objective of the present study was to implement respiratory navigation on the heart and compare it against the established method of navigator gating on the diaphragm for single-voxel cardiac H-MRS. MATERIALS AND METHODS H-MRS was performed on a 1.5T system in 19 healthy volunteers of mixed age (range 24-75 years). Spectra were recorded in a 6-8 ml voxel in the ventricular septum using a PRESS (point-resolved spectroscopy) sequence and ECG gating. Water-unsuppressed data acquired with pencil beam navigation on the heart were compared to data with navigation on the diaphragm. Water-suppressed data were obtained to assess triglyceride-to-water ratios. RESULTS Water phase and amplitude fluctuations for cardiac versus diaphragm navigation did not reveal significant differences. Both navigator positions provided comparable triglyceride-to-water ratios and gating efficiencies (coefficient of variation (CoV) 7.0%). The cardiac navigator showed a good reproducibility (CoV 5.2%). DISCUSSION Respiratory navigation on the heart does not convey an advantage over diaphragm-based navigator gating for cardiac H-MRS, but also no disadvantage. Consequently, cardiac and diaphragm respiratory navigation may be used interchangeably

Retrospective phase-based gating for cardiac proton spectroscopy with fixed scan time

BACKGROUND Respiratory motion is a major limiting factor for spectral quality and duration of in vivo proton MR spectroscopy of the heart. Prospective navigator gating is frequently applied to minimize the effects of respiratory motion, but scan durations are subject-dependent and hence difficult to predict. PURPOSE To implement cardiac proton MRS with fixed scan time by employing retrospective phase-based gating and to compare the proposed method to conventional navigator-gated MRS. STUDY TYPE Prospective. SUBJECTS Eighteen healthy volunteers (29.7 ± 7.8 years). FIELD STRENGTH/SEQUENCE 1.5, navigator-gated (16 averages without, 96 with water suppression [WS]) data acquisition as reference and navigator-free data acquisition with a fixed scan time (48 without WS, 304 with WS), cardiac-triggered point-resolved spectroscopy (PRESS). ASSESSMENT Navigator-free data acquisition with retrospective phase-based gating was compared with prospective navigator-gating. Navigator-free acquisition was repeated in 10 subjects to assess reproducibility. Scan time was assessed for prospective and retrospective gating. Retrospective phase-based gating was performed using a threshold based on the standard deviation (SD) of individual water (W) and triglyceride (TG) phases. STATISTICAL TESTS T-tests and Bland-Altman analysis. RESULTS The duration of the prospective navigator-gated scans ranged from 6:09 minutes to 21:50 minutes (mean 10:05 ± 3:46 min, gating efficiency 40.4 ± 10.5%), while data acquisition for retrospective phase-based gating had a fixed scan time of 11:44 minutes. Retrospective phase-based gating using a threshold of 1 × SD yielded a gating efficiency of 72.7 ± 4.3% and a coefficient of variation (CoV) of triglyceride-to-water ratios of 9.8% compared with the navigated reference. The intrasubject reproducibility of retrospective gating revealed a CoV of 9.5%. DATA CONCLUSION Cardiac proton MRS employing retrospective phase-based gating is feasible and provides reproducible assessment of TG/W in a fixed scan time. Since scan time is independent of respiratory motion, retrospective phase-based gating offers an approach to motion compensation with predictable exam time for proton MRS of the heart. LEVEL OF EVIDENCE 2 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:1973-1981

Impact of late gadolinium enhancement image resolution on neural network based automatic scar segmentation in cardiovascular magnetic resonance imaging

Background Automatic myocardial scar segmentation from late gadolinium enhancement (LGE) images using neural networks promises an alternative to time-consuming and observer-dependent semi-automatic approaches. However, alterations in data acquisition, reconstruction as well as post-processing may compromise network performance. The objective of the present work was to systematically assess network performance degradation due to a mismatch of point-spread function between training and testing data. Methods Thirty-six high-resolution (0.7x0.7x2.0mm3) LGE k-space datasets were acquired post-mortem in porcine models of myocardial infarction. The in-plane point-spread function and hence in-plane resolution Δx was retrospectively degraded using k-space lowpass filtering, while field-of-view and matrix size were kept constant. Manual segmentation of the left ventricle (LV) and healthy remote myocardium was performed to quantify location and area (% of myocardium) of scar by thresholding (≥ SD5 above remote). Three standard U-Nets were trained on training resolutions Δxtrain = 0.7, 1.2 and 1.7 mm to predict endo- and epicardial borders of LV myocardium and scar. The scar prediction of the three networks for varying test resolutions (Δxtest = 0.7 to 1.7 mm) was compared against the reference SD5 thresholding at 0.7 mm. Finally, a fourth network trained on a combination of resolutions (Δxtrain = 0.7 to 1.7 mm) was tested. Results The prediction of relative scar areas showed the highest precision when the resolution of the test data was identical to or close to the resolution used during training. The median fractional scar errors and precisions (IQR) from networks trained and tested on the same resolution were 0.0 percentage points (p.p.; (1.24 - 1.45), and -0.5 - 0.0 p.p. (2.00 – 3.25) for networks trained and tested on the most differing resolutions, respectively. Deploying the network trained on multiple resolutions resulted in reduced resolution dependency with median scar errors and IQRs of 0.0 p.p. (1.24 – 1.69) for all investigated test resolutions. Conclusion A mismatch of the imaging point-spread function between training and test data can lead to degradation of scar segmentation when using current U-Net architectures as demonstrated on LGE porcine myocardial infarction data. Training networks on multi-resolution data can alleviate the resolution dependency.ISSN:1097-6647ISSN:1532-429