Nikolai I. AvdievichAnton V. NikulinLoreen RuhmArthur W. Magill...
13页
查看更多>>摘要：Important issues in designing radiofrequency (RF) coils for human head imaging at ultra‐high field (UHF; ≥7?T) are the inhomogeneity and longitudinal coverage (along the magnet axis) of the transmit (Tx) RF field. Both the homogeneity and coverage produced by Tx volume coils can be improved by means of three‐dimensional (3D) RF shimming, which requires the use of multirow Tx‐arrays. In addition, according to recent findings of the ultimate intrinsic signal‐to‐noise ratio (UISNR) theory, the loop‐only receive (Rx) arrays do not provide optimal SNR near the brain center at UHF. The latter can be obtained by combining complementary conductive structures carrying different current patterns (e.g., loops and dipole antennas). In this work, we developed, constructed, and evaluated a novel 32‐element hybrid array design for human head imaging at 7?T. The array consists of 16 transceiver loops placed in two rows circumscribing the head and 16 folded‐end Rx‐only dipoles positioned in the centers of loops. By placing all elements in a single layer, we increased RF power deposition into the tissue and, thus, preserved the Tx‐efficiency. Using this hybrid design also simplifies the coil structure by minimizing the total number of array elements. The array demonstrated whole brain coverage, 3D RF shimming capability, and high SNR. It provided ~15% higher SNR near the brain center and, depending on the RF shim mode, from 20% to 40% higher Tx‐efficiency than a common commercial head array coil.
查看更多>>摘要：In myocardial T1 mapping, undesirable motion poses significant challenges because uncorrected motion can affect T1 estimation accuracy and cause incorrect diagnosis. In this study, we propose and evaluate a motion correction method for myocardial T1 mapping using self‐supervised deep learning based registration with contrast separation (SDRAP). A sparse coding based method was first proposed to separate the contrast component from T1‐weighted (T1w) images. Then, a self‐supervised deep neural network with cross‐correlation (SDRAP‐CC) or mutual information as the registration similarity measurement was developed to register contrast separated images, after which signal fitting was performed on the motion corrected T1w images to generate motion corrected T1 maps. The registration network was trained and tested in 80 healthy volunteers with images acquired using the modified Look–Locker inversion recovery (MOLLI) sequence. The proposed SDRAP was compared with the free form deformation (FFD) registration method regarding (1) Dice similarity coefficient (DSC) and mean boundary error (MBE) of myocardium contours, (2) T1 value and standard deviation (SD) of T1 fitting, (3) subjective evaluation score for overall image quality and motion artifact level, and (4) computation time. Results showed that SDRAP‐CC achieved the highest DSC of 85.0?±?3.9% and the lowest MBE of 0.92?±?0.25?mm among the methods compared. Additionally, SDRAP‐CC performed the best by resulting in lower SD value (28.1?±?17.6?ms) and higher subjective image quality scores (3.30?±?0.79 for overall quality and 3.53?±?0.68 for motion artifact) evaluated by a cardiologist. The proposed SDRAP took only 0.52?s to register one slice of MOLLI images, achieving about sevenfold acceleration over FFD (3.7?s/slice).
查看更多>>摘要：Tubular atrophy and fibrosis are pathological changes that determine the prognosis of kidney disease induced by acute kidney injury (AKI). We aimed to evaluate multiple magnetic resonance imaging (MRI) parameters, including pool size ratio (PSR) from quantitative magnetization transfer, relaxation rates, and measures from spin‐lock imaging ( R1ρ and Sρ), for assessing the pathological changes associated with AKI‐induced kidney disease. Eight‐week‐old male C57BL/6?J mice first underwent unilateral ischemia reperfusion injury (IRI) induced by reperfusion after 45 min of ischemia. They were imaged using a 7T MRI system 56?days after the injury. Paraffin tissue sections were stained using Masson trichrome and picrosirius red to identify histopathological changes such as tubular atrophy and fibrosis. Histology detected extensive tubular atrophy and moderate fibrosis in the cortex and outer stripe of the outer medulla (CR?+?OSOM) and more prominent fibrosis in the inner stripe of the outer medulla (ISOM) of IRI kidneys. In the CR?+?OSOM region, evident decreases in PSR, R1, R2, R1ρ, and Sρ showed in IRI compared with contralateral kidneys, with PSR and Sρ exhibiting the most significant changes. In addition, the exchange parameter Sρ dropped by the largest degree among all the MRI parameters, while R2* increased significantly. In the ISOM of IRI kidneys, PSR increased while Sρ kept decreasing. R2, R1ρ, and R2* all increased due to more severe fibrosis in this region. Among MRI measures, PSR and R1ρ showed the highest detectability of renal changes no matter whether tubular atrophy or fibrosis dominated. R2* and Sρ could be more specific to a single pathological event than other MRI measures because only R2* increased and Sρ decreased consistently when either fibrosis or tubular atrophy dominated, and their correlations with fibrosis scores were higher than other MRI measures. Multiparametric MRI may enable a more comprehensive analysis of histopathological changes following AKI.
Clerio F. AzevedoElizabeth R. JenistaYodying KaolawanichHan W. Kim...
12页
查看更多>>摘要：Myocardial lipomatous metaplasia, which can serve as substrate for ventricular arrhythmias, is usually composed of regions in which there is an admixture of fat and nonfat tissue. Although dedicated sequences for the detection of fat are available, it would be time‐consuming and burdensome to routinely use these techniques to image the entire heart of all patients as part of a typical cardiac MRI exam. Conventional steady‐state free‐precession (SSFP) cine imaging is insensitive to detecting myocardial regions with partial fatty infiltration. We developed an optimization process for SSFP imaging to set fat signal consistently “out‐of‐phase” with water throughout the heart, so that intramyocardial regions with partial volume fat would be detected as paradoxically dark regions. The optimized SSFP sequence was evaluated using a fat phantom, through simulations, and in 50 consecutive patients undergoing clinical cardiac MRI. Findings were validated using standard Dixon gradient‐recalled‐echo (GRE) imaging as the reference. Phantom studies of test tubes with diverse fat concentrations demonstrated good agreement between measured signal intensity and simulated values calculated using Bloch equations. In patients, a line of signal cancellation at the interface between myocardium and epicardial fat was noted in all cases, confirming that SSFP images were consistently out‐of‐phase throughout the entire heart. Intramyocardial dark regions identified on out‐of‐phase SSFP images were entirely dark throughout in 33 patients (66%) and displayed an India‐ink pattern in 17 (34%). In all cases, dark intramyocardial regions were also seen in the same locations on out‐of‐phase GRE and were absent on in‐phase GRE, confirming that these regions represent areas with partial fat. In conclusion, if appropriately optimized, SSFP cine imaging allows for consistent detection of myocardial fatty metaplasia in patients undergoing routine clinical cardiac MRI without the need for additional image acquisitions using dedicated fat‐specific sequences.
Johanna DorstTamas BorbathLoreen RuhmAnke Henning...
14页
查看更多>>摘要：A method to estimate phosphorus (31P) transversal relaxation times (T2s) of coupled spin systems is demonstrated. Additionally, intracellular and extracellular pH and relaxation‐corrected metabolite concentrations are reported. Echo time (TE) series of 31P metabolite spectra were acquired using stimulated echo acquisition mode (STEAM) localization. Spectra were fitted using LCModel with accurately modeled Versatile Simulation, Pulses and Analysis (VeSPA) basis sets accounting for J‐evolution of the coupled spin systems. T2s were estimated by fitting a single exponential two‐parameter model across the TE series. Fitted inorganic phosphate frequencies were used to calculate pH, and estimated relaxation times were used to determine the relaxation‐corrected brain metabolite concentrations on an assumption of 3 mM γ‐ATP. The method was demonstrated in healthy human brain at a field strength of 9.4?T. T2 times of ATP and nicotinamide adenine dinucleotide (NAD) were shortest between 8 and 20?ms, followed by T2s of inorganic phosphate between 25 and 50?ms, and phosphocreatine with a T2 of 100?ms. Phosphomonoesters and phosphodiesters had the longest T2s of about 130?ms. The measured T2s are comparable with literature values and fit in a decreasing trend with increasing field strengths. Calculated pHs and metabolite concentrations are also comparable with literature values.
Alfonso MastropietroDaniel ProcissiElisa ScalcoGiovanna Rizzo...
13页
查看更多>>摘要：Extraction of intravoxel incoherent motion (IVIM) parameters from noisy diffusion‐weighted (DW) images using a biexponential fitting model is computationally challenging, and the reliability of the estimated perfusion‐related quantities represents a limitation of this technique. Artificial intelligence can overcome the current limitations and be a suitable solution to advance use of this technique in both preclinical and clinical settings. The purpose of this work was to develop a deep neural network (DNN) approach, trained on numerical simulated phantoms with different signal to noise ratios (SNRs), to improve IVIM parameter estimation. The proposed approach is based on a supervised fully connected DNN having 3 hidden layers, 18 inputs and 3 targets with standardized values. 14?×?103 simulated DW images, based on a Shepp–Logan phantom, were randomly generated with varying SNRs (ranging from 10 to 100). 7?×?103 images (1000 for each SNR) were used for training. Performance accuracy was assessed in simulated images and the proposed approach was compared with the state‐of‐the‐art Bayesian approach and other DNN algorithms. The DNN approach was also evaluated in vivo on a high‐field MRI preclinical scanner. Our DNN approach showed an overall improvement in accuracy when compared with the Bayesian approach and other DNN methods in most of the simulated conditions. The in vivo results demonstrated the feasibility of the proposed approach in real settings and generated quantitative results comparable to those obtained using the Bayesian and unsupervised approaches, especially for D and f, and with lower variability in homogeneous regions. The DNN architecture proposed in this work outlines two innovative features as compared with other studies: (1) the use of standardized targets to improve the estimation of parameters, and (2) the implementation of a single DNN to enhance the IVIM fitting at different SNRs, providing a valuable alternative tool to compute IVIM parameters in conditions of high background noise.
查看更多>>摘要：Phosphorus (31P‐) MRS in vivo enables detection and quantification of important phosphorus‐containing metabolites in biological tissues. 31P‐MRS of the normal spleen is challenging due to the relatively small volume and the larger distance between the spleen and surface coil. However, reference spectra of the healthy spleen are invaluable in studies of splenic malignancies and benign causes of splenomegaly, as well as in the study of its physiology. The purpose of this work was to investigate the feasibility of localized 31P‐MRS of healthy spleen in situ in a clinically acceptable measurement time using a clinical 3?T MR scanner. In this work, 31P spectra of five healthy volunteers were measured using single‐voxel image‐selected in vivo spectroscopy (ISIS). The measurement sequence was augmented by broadband proton decoupling and nuclear Overhauser effect enhancement. It is demonstrated that localized 31P‐MRS of the spleen in situ using single‐voxel ISIS is feasible on a clinical 3?T scanner in a clinically acceptable acquisition time. However, results have to be corrected for the transmitter excitation profile, and chemical shift displacement errors need to be taken into consideration during placement of the volume of interest. Results presented here could be used as a reference in future studies of splenomegaly caused by haematological malignancies.
Kyung Min NamArjan D. HendriksVincent O. BoerDennis W. J. Klomp...
13页
查看更多>>摘要：The increased signal‐to‐noise ratio (SNR) and chemical shift dispersion at high magnetic fields (≥7?T) have enabled neuro‐metabolic imaging at high spatial resolutions. To avoid very long acquisition times with conventional magnetic resonance spectroscopic imaging (MRSI) phase‐encoding schemes, solutions such as pulse‐acquire or free induction decay (FID) sequences with short repetition time and inner volume selection methods with acceleration (echo‐planar spectroscopic imaging [EPSI]), have been proposed. With the inner volume selection methods, limited spatial coverage of the brain and long echo times may still impede clinical implementation. FID‐MRSI sequences benefit from a short echo time and have a high SNR per time unit; however, contamination from strong extra‐cranial lipid signals remains a problem that can hinder correct metabolite quantification. L2‐regularization can be applied to remove lipid signals in cases with high spatial resolution and accurate prior knowledge. In this work, we developed an accelerated two‐dimensional (2D) FID‐MRSI sequence using an echo‐planar readout and investigated the performance of lipid suppression by L2‐regularization, an external crusher coil, and the combination of these two methods to compare the resulting spectral quality in three subjects. The reduction factor of lipid suppression using the crusher coil alone varies from 2 to 7 in the lipid region of the brain boundary. For the combination of the two methods, the average lipid area inside the brain was reduced by 2% to 38% compared with that of unsuppressed lipids, depending on the subject's region of interest. 2D FID‐EPSI with external lipid crushing and L2‐regularization provides high in‐plane coverage and is suitable for investigating brain metabolite distributions at high fields.
Alfredo L. Lopez KolkovskyPierre G. CarlierBenjamin MartyMartin Meyerspeer...
24页
查看更多>>摘要：Magnetic resonance signals from different nuclei can be excited or received at the same time,rendering simultaneous or rapidly interleaved multi‐nuclear acquisitions feasible. The advan‐tages are a reduction of total scan time compared to sequential multi‐nuclear acquisitions or that additional information from heteronuclear data is obtained at thesame time and anatomical position. Information content can be qualitatively increased by delivering a more comprehensive MR‐based picture of a transient state (such as an exercise bout). Also, combiningnon‐proton MR acquisitions with 1Hinformation (e.g., dynamic shim updates and motion correction) can be used to improve data quality during long scans and benefits image coregistration. This work reviews the literature on interleaved and simultaneous multi‐nuclear MRI and MRS in vivo. Prominent use cases for this methodology in clinical and research applications are brain and muscle, but studies have also been carried out in other targets, including the lung, knee, breast and heart. Simultaneous multi‐nuclear measurements in the liver and kidney have also been performed, but exclusively in rodents. In this review, a consistent nomenclature is proposed, to help clarify the terminology used for this principle throughout the literature on in‐vivo MR. An overview covers the basic principles, the technical requirements on the MR scanner and the implementations realised either by MR system vendors or research groups, from the early days until today. Considerations regarding the multi‐tuned RF coils required and heteronuclear polarisation interactions are briefly discussed, and fields for future in‐vivo applications for interleaved multi‐nuclear MR pulse sequences are identified.
NSTL
Wiley