Magnetic Resonance In Medicine Research Articles

Self-gated golden-angle spiral 4D flow MRI.

PurposeThe acquisition of 4D flow magnetic resonance imaging (MRI) in cardiovascular applications has recently made large progress toward clinical feasibility. The need for simultaneous compensation of cardiac and breathing motion still poses a challenge for widespread clinical use. Especially, breathing motion, addressed by gating approaches, can lead to unpredictable and long scan times. The current work proposes a time‐efficient self‐gated 4D flow sequence that exploits up to 100% of the acquired data and operates at a predictable scan time.MethodsA self‐gated golden‐angle spiral 4D flow sequence was implemented and tested in 10 volunteers. Data were retrospectively binned into respiratory and cardiac states and reconstructed using a conjugate‐gradient sensitivity encoding reconstruction. Net flow curves, stroke volumes, and peak flow in the aorta were evaluated and compared to a conventional Cartesian 4D flow sequence. Additionally, flow quantities reconstructed from 50% to 100% of the self‐gated 4D flow data were compared.ResultsSelf‐gating signals for respiratory and cardiac motion were extracted for all volunteers. Flow quantities were in agreement with the standard Cartesian scan. Mean differences in stroke volumes and peak flow of 7.6 ± 11.5 and 4.0 ± 79.9 mL/s were obtained, respectively. By retrospectively increasing breathing navigator efficiency while decreasing acquisition times (15:06–07:33 minutes), 50% of the acquired data were sufficient to measure stroke volumes with errors under 9.6 mL.ConclusionThe feasibility to acquire respiratory and cardiac self‐gated 4D flow data at a predictable scan time was demonstrated. Magn Reson Med 80:904–913, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

In vivo estimation of transverse relaxation time constant (T2 ) of 17 human brain metabolites at 3T.

The transverse relaxation times T2 of 17 metabolites in vivo at 3T is reported and region specific differences are addressed. An echo-time series protocol was applied to one, two, or three volumes of interest with different fraction of white and gray matter including a total number of 106 healthy volunteers and acquiring a total number of 128 spectra. The data were fitted with the 2D fitting tool ProFit2, which included individual line shape modeling for all metabolites and allowed the T2 calculation of 28 moieties of 17 metabolites. The T2 of 10 metabolites and their moieties have been reported for the first time. Region specific T2 differences in white and gray matter enriched tissue occur in 16 of 17 metabolites examined including single resonance lines and coupled spin systems. The relaxation time T2 is regions specific and has to be considered when applying tissue composition correction for internal water referencing. Magn Reson Med 80:452-461, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Probing the microscopic environment of 23 Na ions in brain tissue by MRI: On the accuracy of different sampling schemes for the determination of rapid, biexponential T2* decay at low signal-to-noise ratio.

To investigate and to reduce influences on the determination of the short and long apparent transverse relaxation times ( T2,s*, T2,l*) of 23 Na in vivo with respect to signal sampling. The accuracy of T2* determination was analyzed in simulations for five different sampling schemes. The influence of noise in the parameter fit was investigated for three different models. A dedicated sampling scheme was developed for brain parenchyma by numerically optimizing the parameter estimation. This scheme was compared in vivo to linear sampling at 7T. For the considered sampling schemes, T2,s* / T2,l* exhibit an average bias of 3% / 4% with a variation of 25% / 15% based on simulations with previously published T2* values. The accuracy could be improved with the optimized sampling scheme by strongly averaging the earliest sample. A fitting model with constant noise floor can increase accuracy while additional fitting of a noise term is only beneficial in case of sampling until late echo time > 80 ms. T2* values in white matter were determined to be T2,s* = 5.1 ± 0.8 / 4.2 ± 0.4 ms and T2,l* = 35.7 ± 2.4 / 34.4 ± 1.5 ms using linear/optimized sampling. Voxel-wise T2* determination of 23 Na is feasible in vivo. However, sampling and fitting methods have to be chosen carefully to retrieve accurate results. Magn Reson Med 80:571-584, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

A bayesian method for accelerated magnetic resonance elastography of the liver.

PurposeMagnetic resonance elastography is a noninvasive tool for quantifying soft tissue stiffness. Magnetic resonance elastography has been adopted as a clinical method for staging liver fibrosis. However, the application of liver magnetic resonance elastography requires multiple lengthy breath‐holds. We propose a new data acquisition and processing method to reduce magnetic resonance elastography scan time.MethodsA Bayesian image reconstruction method that uses transform sparsity and magnitude consistency across different phase offsets to recover images from highly undersampled data is proposed. The method is validated using retrospectively down‐sampled phantom data and prospectively down‐sampled in vivo data (N = 86).ResultsThe proposed technique allows accurate quantification of mean liver stiffness up to an acceleration factor of R = 6, enabling acquisition of a slice in 4.3 s. Bland‐Altman analysis indicates that the proposed technique (R = 6) has a bias of −0.04 kPa and limits of agreement of −0.36 to + 0.28 kPa when compared with traditional generalized autocalibrating partial parallel acquisition reconstruction (R = 1.4).ConclusionBy exploiting transform sparsity and magnitude consistency, accurate quantification of mean stiffness in the liver can be obtained at an acceleration rate of up to R = 6. This potentially enables the collection of three to four liver slices, as per clinical protocol, within a single breath‐hold. Magn Reson Med 80:1178–1188, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Improved liver R2* mapping by pixel-wise curve fitting with adaptive neighborhood regularization.

To improve liver R2* mapping by incorporating adaptive neighborhood regularization into pixel-wise curve fitting. Magnetic resonance imaging R2* mapping remains challenging because of the serial images with low signal-to-noise ratio. In this study, we proposed to exploit the neighboring pixels as regularization terms and adaptively determine the regularization parameters according to the interpixel signal similarity. The proposed algorithm, called the pixel-wise curve fitting with adaptive neighborhood regularization (PCANR), was compared with the conventional nonlinear least squares (NLS) and nonlocal means filter-based NLS algorithms on simulated, phantom, and in vivo data. Visually, the PCANR algorithm generates R2* maps with significantly reduced noise and well-preserved tiny structures. Quantitatively, the PCANR algorithm produces R2* maps with lower root mean square errors at varying R2* values and signal-to-noise-ratio levels compared with the NLS and nonlocal means filter-based NLS algorithms. For the high R2* values under low signal-to-noise-ratio levels, the PCANR algorithm outperforms the NLS and nonlocal means filter-based NLS algorithms in the accuracy and precision, in terms of mean and standard deviation of R2* measurements in selected region of interests, respectively. The PCANR algorithm can reduce the effect of noise on liver R2* mapping, and the improved measurement precision will benefit the assessment of hepatic iron in clinical practice. Magn Reson Med 80:792-801, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Interleaved susceptibility-weighted and FLAIR MRI for imaging lesion-penetrating veins in multiple sclerosis.

PurposeTo simultaneously image brain lesions and veins in multiple sclerosis.MethodsAn interleaved sequence was developed to simultaneously acquire 3‐dimensional ‐weighted (or susceptibility‐weighted (SW)) and fluid‐attenuated inversion recovery (FLAIR) images on a 3T MRI system. The pulse sequence parameters were calculated to minimize signal perturbation from steady state while maintaining acceptable image contrast and scan time. Fifteen multiple sclerosis patients were enrolled in this prospective study and underwent a standard multiple sclerosis imaging protocol. In addition, SW and FLAIR images were acquired separately and also in an interleaved manner. The SW and FLAIR images were combined into one image to visualize lesions and penetrating veins. The contrast ratios between white matter lesions and penetrating veins were compared between the interleaved sequence and the individual noninterleaved acquisitions.ResultsInterleaved scanning of the FLAIR and the SW pulse sequences was achieved, producing aligned images, and with similar image contrast as in the noninterleaved images. A total of 1076 lesions were identified in all patients on the combined SW‐FLAIR image, of which 968 lesions (90%) had visible penetrating veins. Lesion‐to‐vein contrast ratio was 32.7 ± 17.9 (mean ± standard deviation) for the interleaved sequence compared with 28.1 ± 13.7 using the separate acquisitions (P < 0.001).ConclusionThe feasibility of interleaved acquisition of SW and FLAIR images was demonstrated. This sequence provides self‐registered images and facilitates the visualization of veins in brain lesions. Magn Reson Med 80:1132–1137, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

William G. Bradley, Jr, MD, PhD, FACR (1942-2017).

William G. Bradley, Jr, MD, PhD, FACR (1942-2017).

Characterization of macromolecular baseline of human brain using metabolite cycled semi-LASER at 9.4T.

Macromolecular resonances (MM) arise mainly from cytosolic proteins and overlap with metabolites, influencing metabolite quantification. Macromolecules can serve as valuable biomarkers for diseases and pathologies. The objectives of this study were to characterize MM at 9.4T in the human brain (occipital and left parietal lobe) and to describe the RF coil setup used for MM acquisition in the two regions. An adiabatic inversion pulse was optimised for metabolite nulling at 9.4T using double inversion recovery and was combined for the first time with metabolite cycled (MC) semi-LASER and appropriate coil configuration. MM spectra (seven volunteers) from two brain locations were averaged and smoothed creating MM templates, which were then parametrized using simulated Voigt-shaped lines within LCModel. Quantification was performed on individual data sets, including corrections for different tissue composition and the T1 and T2 relaxation of water. Our coil configuration method resulted in efficient B1+ (>30 T/√kW) for both brain regions. The 15 MM components were detected and quantified in MM baselines of the two brain areas. No significant differences in concentration levels of MM between different regions were found. Two new MM peaks were reported (M7 & M8). Double inversion, which was combined with MC semi-LASER, enabled the acquisition of high spectral resolution MM spectra for both brain regions at 9.4T. The 15 MM components were detected and quantified. Two new MM peaks were reported for the first time (M7 & M8) and preliminarily assigned to β-methylene protons of aspartyl-groups. Magn Reson Med 80:462-473, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Millimeter spatial resolution in vivo sodium MRI of the human eye at 7 T using a dedicated radiofrequency transceiver array.

The aim of this study was to achieve millimeter spatial resolution sodium in vivo MRI of the human eye at 7 T using a dedicated six-channel transceiver array. We present a detailed description of the radiofrequency coil design, along with electromagnetic field and specific absorption ratio simulations, data validation, and in vivo application. Electromagnetic field and specific absorption ratio simulations were performed. Transmit field uniformity was optimized by using a multi-objective genetic algorithm. Transmit field mapping was conducted using a phase-sensitive method. An in vivo feasibility study was carried out with 3-dimensional density-adapted projection reconstruction imaging technique. Measured transmit field distribution agrees well with the one obtained from simulations. The specific absorption ratio simulations confirm that the radiofrequency coil is safe for clinical use. Our radiofrequency coil is light and conforms to an average human head. High spatial resolution (nominal 1.4 and 1.0 mm isotropic) sodium in vivo images of the human eye were acquired within scan times suitable for clinical applications (∼ 10 min). Three most important eye compartments in the context of sodium physiology were clearly delineated in all of the images: the vitreous humor, the aqueous humor, and the lens. Our results provide encouragement for further clinical studies. The implications for research into eye diseases including ocular melanoma, cataract, and glaucoma are discussed. Magn Reson Med 80:672-684, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Imaging macrophage distribution and density in mammary tumors and lung metastases using fluorine-19 MRI cell tracking.

PurposeThe presence of tumor‐associated macrophages (TAMs) correlates with breast cancer progression and metastatic spread. Metastasis‐associated macrophages (MAMs) are also recruited to distant sites, where they support metastatic growth. In this study, we demonstrate that in vivo fluorine‐19 (19F)‐based MRI cell tracking can evaluate the density and distribution of macrophages within murine breast cancer tumors and associated metastases.MethodsThree murine breast cancer cell lines with different metastatic potentials (4T1, 168FARN, and 67NR) were implanted into the mammary fat pad in mice. In vivo whole body 19F MRI was performed on tumor‐bearing mice 24 hours post‐intravenous injection of a perfluorocarbon (PFC) agent, which was taken up by macrophages in situ.ResultsTAMs were detected mainly in the periphery of primary tumors, and higher numbers of TAMs were detected in the more aggressive 4T1 tumors. Tumors had significantly greater 19F spins/mm3 when they were smaller, suggesting more TAM infiltration in early‐stage tumors. 19F signal was observed within lung metastases in mice with 4T1 tumors, and fluorescence microscopy confirmed the presence of PFC‐positive macrophages.ConclusionThis study shows for the first time proof of the ability to use MRI cell tracking to visualize MAMs in the lungs. The ability to detect and monitor the number of TAMs in individual tumors with 19F MRI would allow for identification of breast tumors with heavy infiltration of TAMs and could be used as a biomarker for decisions about how to best treat these patients as well as for monitoring responses to therapy. Magn Reson Med 80:1138–1147, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Dynamic coronary MR angiography in a pig model with hyperpolarized water.

PurposeTo investigate dynamic coronary MR angiography using hyperpolarized water as a positive contrast agent. Hyperpolarization can increase the signal by several orders of magnitude, and has recently been translated to human cardiac application. The aim was to achieve large 1H signal enhancement to allow high‐resolution imaging of the coronary arteries.MethodsProtons in D2O were hyperpolarized by dissolution dynamic nuclear polarization. A total of 18 mL of hyperpolarized water was injected into the coronary arteries of healthy pigs (N = 9; 3 injections in 3 animals). The MRI images were acquired with a gradient‐echo sequence in an oblique slab covering the main left coronary arteries with 0.55 mm in‐plane resolution. The acquisition time was 870 ms per frame.ResultsA more than 200‐fold signal enhancement compared with thermally polarized water at 3 T was obtained. Coronary angiographic images with a signal‐to‐noise ratio from the left main stem of 269 ± 169 and coronary sharpness from the proximal left anterior descending coronary artery of 0.31 ± 0.086 mm−1 were obtained. Dynamic images were acquired over a 10 s time window.ConclusionHyperpolarized water MR angiography of the coronary arteries in a large animal model with high signal‐to‐noise ratio and high spatial and temporal resolution was obtained. Magn Reson Med 80:1165–1169, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

CEST‐Dixon for human breast lesion characterization at 3 T: A preliminary study

PurposeChemical exchange saturation transfer (CEST) MRI for breast lesion characterization is promising. However, artifacts are prone to develop in breast CEST imaging as a result of strong lipid signals. The aims of the study are (i) to develop and validate the CEST‐Dixon imaging sequence for simultaneous water‐fat separation and B0 mapping; and (ii) use the CEST‐Dixon method to characterize suspicious lesions in patients undergoing percutaneous biopsy.MethodsThe gradient‐echo multi‐echo Dixon acquisition is used to create fat‐free CEST and B0 maps. The sequence has been validated in phantoms and in vivo. Five healthy volunteers and 10 patients were scanned to compare the CEST contrast in three frequency ranges centered at 1, 2, and 3.5 ppm. The correlation between the CEST contrast and pathology markers (tumor type, estrogen receptor (ER) status, and Ki‐67) was also investigated by stratifying the patients into ER‐negative invasive ductal carcinoma (IDC) (more aggressive), ER‐positive IDC (less aggressive), and benign groups.ResultsThe CEST‐Dixon sequence shows homogenous fat removal in the water‐only images. The ER‐negative IDC tissues display a trend to higher CEST contrast in all three frequency ranges, whereas the ER‐positive IDC, benign, and normal tissues have lower CEST contrast. No significant differences were observed among the ER‐positive IDC, benign, and normal tissues. Of the three frequencies ranges, the CEST contrasts at 1 ppm are high in the ER‐negative IDC group, have the largest difference among the ER‐negative IDC and the other groups, and have the highest correlation with Ki‐67.ConclusionBreast CEST‐Dixon imaging shows potential to differentiate more aggressive from less aggressive cancers. Magn Reson Med 80:895–903, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Improving parallel imaging by jointly reconstructing multi-contrast data.

To develop parallel imaging techniques that simultaneously exploit coil sensitivity encoding, image phase prior information, similarities across multiple images, and complementary k-space sampling for highly accelerated data acquisition. We introduce joint virtual coil (JVC)-generalized autocalibrating partially parallel acquisitions (GRAPPA) to jointly reconstruct data acquired with different contrast preparations, and show its application in 2D, 3D, and simultaneous multi-slice (SMS) acquisitions. We extend the joint parallel imaging concept to exploit limited support and smooth phase constraints through Joint (J-) LORAKS formulation. J-LORAKS allows joint parallel imaging from limited autocalibration signal region, as well as permitting partial Fourier sampling and calibrationless reconstruction. We demonstrate highly accelerated 2D balanced steady-state free precession with phase cycling, SMS multi-echo spin echo, 3D multi-echo magnetization-prepared rapid gradient echo, and multi-echo gradient recalled echo acquisitions in vivo. Compared to conventional GRAPPA, proposed joint acquisition/reconstruction techniques provide more than 2-fold reduction in reconstruction error. JVC-GRAPPA takes advantage of additional spatial encoding from phase information and image similarity, and employs different sampling patterns across acquisitions. J-LORAKS achieves a more parsimonious low-rank representation of local k-space by considering multiple images as additional coils. Both approaches provide dramatic improvement in artifact and noise mitigation over conventional single-contrast parallel imaging reconstruction. Magn Reson Med 80:619-632, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Development of methods and feasibility of using hyperpolarized carbon-13 imaging data for evaluating brain metabolism in patient studies.

PurposeHyperpolarized carbon‐13 (13C) metabolic imaging is a noninvasive imaging modality for evaluating real‐time metabolism. The purpose of this study was to develop and implement experimental strategies for using [1‐13C]pyruvate to probe in vivo metabolism for patients with brain tumors and other neurological diseases.MethodsThe 13C radiofrequency coils and pulse sequences were tested in a phantom and were performed using a 3 Tesla whole‐body scanner. Samples of [1‐13C]pyruvate were polarized using a SPINlab system. Dynamic 13C data were acquired from 8 patients previously diagnosed with brain tumors, who had received treatment and were being followed with serial magnetic resonance scans.ResultsThe phantom studies produced good‐quality spectra with a reduction in signal intensity in the center attributed to the reception profiles of the 13C receive coils. Dynamic data obtained from a 3‐cm slice through a patient's brain following injection with [1‐13C]pyruvate showed the anticipated arrival of the agent, its conversion to lactate and bicarbonate, and subsequent reduction in signal intensity. A similar temporal pattern was observed in 2D dynamic patient studies, with signals corresponding to pyruvate, lactate, and bicarbonate being in normal appearing brain, but only pyruvate and lactate being detected in regions corresponding to the anatomical lesion. Physiological monitoring and follow‐up confirmed that there were no adverse events associated with the injection.ConclusionThis study has presented the first application of hyperpolarized 13C metabolic imaging in patients with brain tumor and demonstrated the safety and feasibility of using hyperpolarized [1‐13C]pyruvate to evaluate in vivo brain metabolism. Magn Reson Med 80:864–873, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Noise properties of proton density fat fraction estimated using chemical shift-encoded MRI.

The purpose of this work is to characterize the noise distribution of proton density fat fraction (PDFF) measured using chemical shift-encoded MRI, and to provide alternative strategies to reduce bias in PDFF estimation. We derived the probability density function for PDFF estimated using chemical shift-encoded MRI, and found it to exhibit an asymmetric noise distribution that contributes to signal-to-noise-ratio dependent bias. To study PDFF noise bias, we performed (at 1.5 T) numerical simulations, phantom acquisitions, and a retrospective in vivo experiment. In each experiment, we compared the performance of three statistics (mean, median, and maximum likelihood estimator) in estimating the PDFF in a region of interest. We demonstrated the presence of the asymmetric noise distribution in simulations, phantoms, and in vivo. In each experiment we demonstrated that both the median and proposed maximum likelihood estimator statistics outperformed the mean statistic in mitigating noise-related bias for low signal-to-noise-ratio acquisitions. Characterization of the noise distribution of PDFF estimated using chemical shift-encoded MRI enabled new strategies based on median and maximum likelihood estimator statistics to mitigate noise-related bias for accurate PDFF measurement from a region of interest. Such strategies are important for quantitative chemical shift-encoded MRI applications that typically operate in low signal-to-noise-ratio regimes. Magn Reson Med 80:685-695, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Fast nonlinear susceptibility inversion with variational regularization.

Quantitative susceptibility mapping can be performed through the minimization of a function consisting of data fidelity and regularization terms. For data consistency, a Gaussian-phase noise distribution is often assumed, which breaks down when the signal-to-noise ratio is low. A previously proposed alternative is to use a nonlinear data fidelity term, which reduces streaking artifacts, mitigates noise amplification, and results in more accurate susceptibility estimates. We hereby present a novel algorithm that solves the nonlinear functional while achieving computation speeds comparable to those for a linear formulation. We developed a nonlinear quantitative susceptibility mapping algorithm (fast nonlinear susceptibility inversion) based on the variable splitting and alternating direction method of multipliers, in which the problem is split into simpler subproblems with closed-form solutions and a decoupled nonlinear inversion hereby solved with a Newton-Raphson iterative procedure. Fast nonlinear susceptibility inversion performance was assessed using numerical phantom and in vivo experiments, and was compared against the nonlinear morphology-enabled dipole inversion method. Fast nonlinear susceptibility inversion achieves similar accuracy to nonlinear morphology-enabled dipole inversion but with significantly improved computational efficiency. The proposed method enables accurate reconstructions in a fraction of the time required by state-of-the-art quantitative susceptibility mapping methods. Magn Reson Med 80:814-821, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Dynamic measurement of oxygen extraction fraction using a multiecho asymmetric spin echo (MASE) pulse sequence.

PurposeThe oxygen extraction fraction (OEF), an important parameter for assessing brain oxygen metabolism, is a key marker of brain health. Dynamic measurement of the OEF is currently limited by the long acquisition time. In this study, a noninvasive method for rapidly and effectively assessing dynamic changes in the OEF during brain stimulation is proposed.MethodsA novel pulse sequence using the multi‐echo asymmetric spin echo with an echo‐planar imaging acquisition scheme was developed to improve the temporal resolution of functional OEF measurements. A theoretical formulation that describes the multi‐echo asymmetric spin echo signal dephasing phenomena in the presence of deoxygenated hemoglobin was established. A functional motor experiment was performed using human subjects, in which both OEF maps using the multi‐echo asymmetric spin echo sequence and blood oxygenation level–dependent maps using a ‐weighted gradient‐echo echo‐planar imaging sequence were generated and compared.ResultsThe activation maps for both OEF and blood oxygenation level–dependent maps revealed significant changes in the motor cortex. Our results, which were consistent with the previous reports, showed that the average relative changes in the OEF and blood oxygenation level–dependent signal were −22% and 2.0%, respectively.ConclusionObtaining dynamic OEF measurements during functional experiments with the proposed multi‐echo asymmetric spin echo method is feasible. Magn Reson Med 80:1118–1124, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

High resolution hyperpolarized 13 C MRSI using SPICE at 9.4T.

To test the feasibility of using the SPICE (SPectroscopic Imaging by exploiting spatiospectral CorrElation) technique, which uses the partial separability of spectroscopic data, for high resolution hyperpolarized (HP) 13 C spectroscopic imaging. Numerical simulations were performed to investigate the impact of transient HP signals on SPICE reconstruction. Furthermore, spectroscopic imaging exams from SPICE and conventional EPSI (echo-planar spectroscopic imaging) were simulated for comparison. For in vivo experiments, HP 13 C SPICE was performed in a mouse kidney by means of the injection of HP [1-13 C] pyruvate at 9.4T. The variation of lactate/pyruvate from the simulated SPICE was less than 4% under various factors that affect the transient HP signal, suggesting that the impact is negligible. We found that while HP 13 C EPSI was limited to the low signal-to-noise ratio (SNR) of lactate, these limitations were mitigated through HP 13 C SPICE, facilitating the improved SNR of lactate and the distinction of tissues. Acquisition of a high resolution HP 13 C spectroscopic image was possible for the in vivo experiments. With the fine structural information, the acquired image showed higher signal of pyruvate and lactate in the renal cortices than in the medullas, which is known to be attributed to higher activity of lactate dehydrogenase. The feasibility of HP 13 C SPICE was investigated. Simulation studies were conducted and in vivo experiments were performed in the mouse kidney at 9.4T. Results confirmed that a high resolution HP 13 C spectroscopic image with adequate spectral resolution can be obtained. Magn Reson Med 80:703-710, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Simultaneous quantitative susceptibility mapping (QSM) and R2* for high iron concentration quantification with 3D ultrashort echo time sequences: An echo dependence study.

To evaluate the echo dependence of 3D ultrashort echo time (TE) quantitative susceptibility mapping (3D UTE-QSM) and effective transverse relaxation rate ( R2*) measurement in the setting of high concentrations of iron oxide nanoparticles. A phantom study with iron concentrations ranging from 2 to 22 mM was performed using a 3D UTE Cones sequence. Simultaneous QSM processing with morphology-enabled dipole inversion (MEDI) and R2* single exponential fitting was conducted offline with the acquired 3D UTE data. The dependence of UTE-QSM and R2* on echo spacing (ΔTE) and first TE (TE1 ) was investigated. A linear relationship was observed between UTE-QSM measurement and iron concentration up to 22 mM only, with the minimal TE1 of 0.032 ms and ΔTE of less than 0.1 ms. A linear relationship was observed between R2* and iron concentration up to 22 mM only when TE1 was less than 0.132 ms and ΔTE was less than 1.2 ms. UTE-QSM with MEDI processing showed strong dependence on ΔTE and TE1 , especially at high iron concentrations. UTE-QSM is more sensitive than R2* measurement to TE selection. Both an ultrashort TE1 and a small ΔTE are needed to achieve accurate QSM for high iron concentrations. Magn Reson Med 79:2315-2322, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Nonrigid active shape model-based registration framework for motion correction of cardiac T1 mapping.

Accurate reconstruction of myocardial T1 maps from a series of T1 -weighted images consists of cardiac motions induced from breathing and diaphragmatic drifts. We propose and evaluate a new framework based on active shape models to correct for motion in myocardial T1 maps. Multiple appearance models were built at different inversion time intervals to model the blood-myocardium contrast and brightness changes during the longitudinal relaxation. Myocardial inner and outer borders were automatically segmented using the built models, and the extracted contours were used to register the T1 -weighted images. Data acquired from 210 patients using a free-breathing acquisition protocol were used to train and evaluate the proposed framework. Two independent readers evaluated the quality of the T1 maps before and after correction using a four-point score. The mean absolute distance and Dice index were used to validate the registration process. The testing data set from 180 patients at 5 short axial slices showed a significant decrease of mean absolute distance (from 3.3 ± 1.6 to 2.3 ± 0.8 mm, P < 0.001) and increase of Dice (from 0.89 ± 0.08 to 0.94 ± 0.4%, P < 0.001) before and after correction, respectively. The T1 map quality improved in 70 ± 0.3% of the motion-affected maps after correction. Motion-corrupted segments of the myocardium reduced from 21.8 to 8.5% (P < 0.001) after correction. The proposed method for nonrigid registration of T1 -weighted images allows T1 measurements in more myocardial segments by reducing motion-induced T1 estimation errors in myocardial segments. Magn Reson Med 80:780-791, 2018. © 2018 International Society for Magnetic Resonance in Medicine.