共查询到20条相似文献,搜索用时 15 毫秒
1.
Ralf Lützkendorf Robin M. Heidemann Thorsten Feiweier Michael Luchtmann Sebastian Baecke Jörn Kaufmann Jörg Stadler Eike Budinger Johannes Bernarding 《Magma (New York, N.Y.)》2018,31(6):701-713
Objectives
We assessed the use of high-resolution ultra-high-field diffusion magnetic resonance imaging (dMRI) to determine neuronal fiber orientation density functions (fODFs) throughout the human brain, including gray matter (GM), white matter (WM), and small intertwined structures in the cerebellopontine region.Materials and methods
We acquired 7-T whole-brain dMRI data of 23 volunteers with 1.4-mm isotropic resolution; fODFs were estimated using constrained spherical deconvolution.Results
High-resolution fODFs enabled a detailed view of the intravoxel distributions of fiber populations in the whole brain. In the brainstem region, the fODF of the extra- and intrapontine parts of the trigeminus could be resolved. Intrapontine trigeminal fiber populations were crossed in a network-like fashion by fiber populations of the surrounding cerebellopontine tracts. In cortical GM, additional evidence was found that in parts of primary somatosensory cortex, fODFs seem to be oriented less perpendicular to the cortical surface than in GM of motor, premotor, and secondary somatosensory cortices.Conclusion
With 7-T MRI being introduced into clinical routine, high-resolution dMRI and derived measures such as fODFs can serve to characterize fine-scale anatomic structures as a prerequisite to detecting pathologies in GM and small or intertwined WM tracts.2.
Alexander C. Wright Jonathon H. Yoder Edward J. Vresilovic Dawn M. Elliott 《Magma (New York, N.Y.)》2016,29(4):711-722
Objective
Here we develop a three-dimensional analytic model for MR image contrast of collagen lamellae in the annulus fibrosus of the intervertebral disc of the spine, based on the dependence of the MRI signal on collagen fiber orientation.Materials and methods
High-resolution MRI scans were performed at 1.5 and 7 T on intact whole disc specimens from ovine, bovine, and human spines. An analytic model that approximates the three-dimensional curvature of the disc lamellae was developed to explain inter-lamellar contrast and intensity variations in the annulus. The model is based on the known anisotropic dipolar relaxation of water in tissues with ordered collagen.Results
Simulated MRI data were generated that reproduced many features of the actual MRI data. The calculated inter-lamellar image contrast demonstrated a strong dependence on the collagen fiber angle and on the circumferential location within the annulus.Conclusion
This analytic model may be useful for interpreting MR images of the disc and for predicting experimental conditions that will optimize MR image contrast in the annulus fibrosus.3.
Lukas Reinhold Buschle Christian H. Ziener Ke Zhang Volker J. F. Sturm Thomas Kampf Artur Hahn Gergely Solecki Frank Winkler Martin Bendszus Sabine Heiland Heinz-Peter Schlemmer Felix T. Kurz 《Magma (New York, N.Y.)》2018,31(4):531-551
Objectives
Spin dephasing of the local magnetization in blood vessel networks can be described in the static dephasing regime (where diffusion effects may be ignored) by the established model of Yablonskiy and Haacke. However, for small capillary radii, diffusion phenomena for spin-bearing particles are not negligible.Material and methods
In this work, we include diffusion effects for a set of randomly distributed capillaries and provide analytical expressions for the transverse relaxation times T2* and T2 in the strong collision approximation and the Gaussian approximation that relate MR signal properties with microstructural parameters such as the mean local capillary radius.Results
Theoretical results are numerically validated with random walk simulations and are used to calculate capillary radius distribution maps for glioblastoma mouse brains at 9.4 T. For representative tumor regions, the capillary maps reveal a relative increase of mean radius for tumor tissue towards healthy brain tissue of \(128 \pm 23 \%\) (p < 0.001).Conclusion
The presented method may be used to quantify angiogenesis or the effects of antiangiogenic therapy in tumors whose growth is associated with significant microvascular changes.4.
Victor Adalid André Döring Sreenath Pruthviraj Kyathanahally Christine Sandra Bolliger Chris Boesch Roland Kreis 《Magma (New York, N.Y.)》2017,30(5):429-448
Objective
Simultaneous modeling of true 2-D spectroscopy data, or more generally, interrelated spectral datasets has been described previously and is useful for quantitative magnetic resonance spectroscopy applications. In this study, a combined method of reference-lineshape enhanced model fitting and two-dimensional prior-knowledge fitting for the case of diffusion weighted MR spectroscopy is presented.Materials and methods
Time-dependent field distortions determined from a water reference are applied to the spectral bases used in linear-combination modeling of interrelated spectra. This was implemented together with a simultaneous spectral and diffusion model fitting in the previously described Fitting Tool for Arrays of Interrelated Datasets (FiTAID), where prior knowledge conditions and restraints can be enforced in two dimensions.Results
The benefit in terms of increased accuracy and precision of parameters is illustrated with examples from Monte Carlo simulations, in vitro and in vivo human brain scans for one- and two-dimensional datasets from 2-D separation, inversion recovery and diffusion-weighted spectroscopy (DWS). For DWS, it was found that acquisitions could be substantially shortened.Conclusion
It is shown that inclusion of a measured lineshape into modeling of interrelated MR spectra is beneficial and can be combined also with simultaneous spectral and diffusion modeling.5.
Daniel Giese Kilian Weiss Bettina Baeßler Navid Madershahian Yeong-Hoon Choi David Maintz Alexander C. Bunck 《Magma (New York, N.Y.)》2018,31(1):165-172
Objectives
The objective of the current work was to evaluate flow and turbulent kinetic energy in different transcatheter aortic valve implants using highly undersampled time-resolved multi-point 3-directional phase-contrast measurements (4D Flow MRI) in an in vitro setup.Materials and methods
A pulsatile flow setup was used with a compliant tubing mimicking a stiff left ventricular outflow tract and ascending aorta. Five different implants were measured using a highly undersampled multi-point 4D Flow MRI sequence. Velocities and turbulent kinetic energy values were analysed and compared.Results
Strong variations of turbulent kinetic energy distributions between the valves were observed. Maximum turbulent kinetic energy values ranged from 100 to over 500 J/m3 while through-plane velocities were similar between all valves.Conclusion
Highly accelerated 4D Flow MRI for the measurement of velocities and turbulent kinetic energy values allowed for the assessment of hemodynamic parameters in five different implant models. The presented setup, measurement protocol and analysis methods provides an efficient approach to compare different valve implants and could aid future novel valve designs.6.
Houchun H. Hu Amber L. Pokorney Niccolo Stefani Jonathan M. Chia Jeffrey H. Miller 《Magma (New York, N.Y.)》2017,30(2):107-112
Objective
We demonstrate the potential clinical utility of a 4D non-gadolinium dynamic angiography technique based on arterial spin-labeling called contrast inherent inflow enhanced multi-phase angiography (CINEMA) in pediatric patients.Materials and Methods
CINEMA was qualitatively compared to conventional time-of-flight (TOF) angiography in a cohort of 31 pediatric patients at 3 Tesla.Results
CINEMA data were successfully acquired and reconstructed in all patients with no image artifacts. There were no cases where CINEMA was rated inferior to TOF in depicting intracranial vessel conspicuity. In 19 cases, CINEMA was rated equivalent to TOF and in the 12 remaining cases CINEMA was rated superior to TOF.Conclusion
There is a steadily rising concern in adults and children over the potential effects of intracranial deposition of gadolinium. CINEMA is therefore a viable alternative in dynamic neurovascular imaging.7.
Yuhong Peng Chao Zou Yangzi Qiao Changjun Tie Qian Wan Rui Jiang Chuanli Cheng Dong Liang Hairong Zheng Faqi Li Xin Liu 《Magma (New York, N.Y.)》2018,31(6):771-779
Purpose
Real-time monitoring is important for the safety and effectiveness of high-intensity focused ultrasound (HIFU) therapy. Magnetic resonance imaging is the preferred imaging modality for HIFU monitoring, with its unique capability of temperature imaging. For real-time temperature imaging, higher temporal resolution and larger spatial coverage are needed. In this study, a sequence based on the echo-shifted RF-spoiled gradient echo (GRE) with simultaneous multi-slice (SMS) imaging was designed for fast temperature imaging.Methods
A phantom experiment was conducted to evaluate the accuracy of the echo-shifted sequence using a fluorescent fiber thermometer as reference. The temperature uncertainty of the echo-shifted sequence was compared with the traditional GRE sequence at room temperature through the ex vivo porcine muscle. Finally, the ex vivo porcine liver tissue experiment using HIFU heating was performed to demonstrate that the spatial coverage was increased without decreasing temporal resolution.Results
The echo-shifted sequence had a better temperature uncertainty performance compared with the traditional GRE sequence with the same temporal resolution. The ex vivo heating experiment confirmed that by combining the SMS technique and echo-shifted sequence, the spatial coverage was increased without decreasing the temporal resolution while maintaining high temperature measurement precision.Conclusion
The proposed technique was validated as an effective real-time method for monitoring HIFU therapy.8.
Object
Recent advances have allowed oscillating gradient (OG) diffusion MRI to infer the sizes of micron-scale axon diameters. Here the effects on the precision of the inferred diameters are studied when reducing the number of images collected to reduce imaging time for clinical feasibility.Materials and methods
Monte Carlo simulations of cosine OG sequences (50–1000 Hz) using a two-compartment model on a parallel cylinder (diameters 1–5 μm) geometry were conducted. Temporal diffusion spectroscopy was used to infer axon diameters. Three different gradient sets were simulated with different combinations of gradient strengths.Results
Five frequencies were adequate for d = 3–5 μm with single-sized cylinders and for effective mean axon diameters greater than 2 μm for cylinders with a distributions of diameters. There was some improvement in precision for d = 1–2 μm with 10 frequencies. It is better to repeat measurements at higher gradient strengths than to use a range of gradient strengths. The improvement tended to be greatest when using fewer frequencies and was especially noticeable at very high gradient strengths.Conclusion
Images can be collected with fewer gradient strengths and frequencies without sacrificing the precision of the measurements. This could be useful in reducing imaging time so that OG techniques can be used in clinical settings.9.
Maximilian N. Voelker Oliver Kraff Daniel Brenner Astrid Wollrab Oliver Weinberger Moritz C. Berger Simon Robinson Wolfgang Bogner Christopher Wiggins Robert Trampel Tony Stöcker Thoralf Niendorf Harald H. Quick David G. Norris Mark E. Ladd Oliver Speck 《Magma (New York, N.Y.)》2016,29(3):399-415
Objective
This study evaluates the inter-site and intra-site reproducibility of 7 Tesla brain imaging and compares it to literature values for other field strengths.Materials and methods
The same two subjects were imaged at eight different 7 T sites. MP2RAGE, TSE, TOF, SWI, EPI as well as B1 and B0 field maps were analyzed quantitatively to assess inter-site reproducibility. Intra-site reproducibility was measured with rescans at three sites.Results
Quantitative measures of MP2RAGE scans showed high agreement. Inter-site and intra-site reproducibility errors were comparable to 1.5 and 3 T. Other sequences also showed high reproducibility between the sites, but differences were also revealed. The different RF coils used were the main source for systematic differences between the sites.Conclusion
Our results show for the first time that multi-center brain imaging studies of the supratentorial brain can be performed at 7 T with high reproducibility and similar reliability as at 3T. This study develops the basis for future large-scale 7 T multi-site studies.10.
Jason Langley Daniel E. Huddleston Christine J. Liu Xiaoping Hu 《Magma (New York, N.Y.)》2017,30(2):121-125
Objectives
The purpose of this study was to assess the reproducibility of substantia nigra pars compacta (SNpc) and locus coeruleus (LC) delineation and measurement with neuromelanin-sensitive MRI.Materials and methods
Eleven subjects underwent two neuromelanin-sensitive MRI scans. SNpc and LC volumes were extracted for each scan. Reproducibility of volume and magnetization transfer contrast measurements in SNpc and LC was assessed using intraclass correlation coefficients (ICC) and dice similarity coefficients (DSC).Results
SNpc and LC volume measurements showed excellent reproducibility (SNpc-ICC: 0.94, p < 0.001; LC-ICC: 0.96, p < 0.001). SNpc and LC were accurately delineated between scans (SNpc-DSC: 0.80 ± 0.03; LC-DSC: 0.63 ± 0.07).Conclusion
Neuromelanin-sensitive MRI can consistently delineate SNpc and LC.11.
Muhammad G. Saleh Jamie Near Alqadafi Alhamud Frances Robertson André J. W. van der Kouwe Ernesta M. Meintjes 《Magma (New York, N.Y.)》2016,29(6):863-874
Objective
Measuring the pure form of GABA has become increasingly important because of its association with behaviour and certain pathologies. The aim of this study was to assess the reproducibility of GABA measurements using a shim and motion navigated MEGA-SPECIAL sequence with LCModel, jMRUI and GANNET software.Materials and methods
Motion and shim navigated MEGA-SPECIAL scans were acquired in 20 healthy subjects. Two acquisitions were performed for each of two regions: the anterior cingulate (ACC) and medial-parietal (PAR) cortices. Absolute GABA concentration (\({\text{GABA}}_{{{\text{H}}_{2} {\text{O}}}}\)) and GABA-to-Creatine ratio (GABA/Cr) were quantified using the three software packages.Results
Using the within-subject coefficient of variation (CVws) as an index, reproducibility for both GABAH20 and GABA/Cr ranged from 13 to 22 % in the ACC and 13 to 18 % in PAR using the three software packages.Conclusion
Based on CVws, GABA concentrations in both the ACC and PAR are reproducible using a shim and motion navigated MEGA-SPECIAL sequence with any of the three software packages, thus demonstrating the ability to quantify the pure form of GABA using these software in studies relating GABA to pathology and healthy behaviour.12.
Object
The objective of this study is to propose a modified VARiable PROjection (VARPRO) algorithm specifically tailored for fitting the intravoxel incoherent motion (IVIM) model to diffusion-weighted magnetic resonance imaging (DW-MRI) data from locally advanced rectal cancer (LARC).Materials and methods
The proposed algorithm is compared with classical non-linear least squares (NLLS) analysis using the Levenberg-Marquardt (LM) algorithm and with two recently proposed algorithms for ‘segmented’ analysis. These latter two comprise two consecutive steps: first, a subset of parameters is estimated using a portion of data; second, the remaining parameters are estimated using the whole data and the previous estimates. The comparison between the algorithms was based on the \(R^2\) goodness-of-fit measure: performance analysis was carried out on real data obtained by DW-MRI on 40 LARC patients.Results
The performance of the proposed algorithm was higher than that of LM in 64 % of cases; ‘segmented’ methods were poorer than our algorithm in 100 % of cases.Conclusion
The proposed modified VARPRO algorithm can lead to better fit of the IVIM model to LARC DW-MRI data compared to other techniques.13.
Sheryl L. Herrera Morgan E. Mercredi Richard Buist Melanie Martin 《Magma (New York, N.Y.)》2018,31(5):609-620
Object
Most early methods to infer axon diameter distributions using magnetic resonance imaging (MRI) used single diffusion encoding sequences such as pulsed gradient spin echo (SE) and are thus sensitive to axons of diameters > 5 μm. We previously simulated oscillating gradient (OG) SE sequences for diffusion spectroscopy to study smaller axons including the majority constituting cortical connections. That study suggested the model of constant extra-axonal diffusion breaks down at OG accessible frequencies. In this study we present data from phantoms to test a time-varying interstitial apparent diffusion coefficient.Materials and Methods
Diffusion spectra were measured in four samples from water packed around beads of diameters 3, 6 and 10 μm; and 151 μm diameter tubes. Surface-to-volume ratios, and diameters were inferred.Results
The bead pore radii estimates were 0.60±0.08 μm, 0.54±0.06 μm and 1.0±0.1 μm corresponding to bead diameters ranging from 2.9±0.4 μm to 5.3±0.7 μm, 2.6±0.3 μm to 4.8±0.6 μm, and 4.9±0.7 μm to 9±1 μm. The tube surface-to-volume ratio estimate was 0.06±0.02 μm?1 corresponding to a tube diameter of 180±70 μm.Conclusion
Interstitial models with OG inferred 3-10 μm bead diameters from 0.54±0.06 μm to 1.0±0.1 μm pore radii and 151 μm tube diameters from 0.06±0.02 μm?1 surface-to-volume ratios.14.
Andreas Reichert Michael Bock Simon Reiss Christiaan G. Overduin Jurgen J. Fütterer Axel Joachim Krafft 《Magma (New York, N.Y.)》2018,31(6):781-788
Objective
To accelerate a passive tracking sequence based on phase-only cross correlation (POCC) using simultaneous slice excitation.Methods
For magnetic resonance (MR)-guided biopsy procedures, passive markers have been proposed that can be automatically localized online using a POCC-based tracking sequence. To accelerate the sequence, a phase-offset multiplanar (POMP) excitation technique was implemented to acquire tracking images. In a phantom experiment, the POMP–POCC sequence was tested and compared with the sequential non-accelerated version in terms of duration and accuracy. Further, technical feasibility of the POMP–POCC sequence was tested in a patient undergoing a prostate biopsy.Results
The temporal resolution of the POMP–POCC tracking sequence is accelerated by 33% compared with the sequential approach. In phantom experiments, the POMP–POCC and sequential sequences yielded the same targeting accuracy of 1.6?±?0.7 mm. Technical proof of concept of the new sequence could be demonstrated in a successful in vivo prostate biopsy.Conclusion
POMP–POCC tracking can substantially reduce the duration of localization of passive markers in MR-guided needle interventions without compromising targeting accuracy.15.
Feng Jia Gerrit Schultz Frederik Testud Anna Masako Welz Hans Weber Sebastian Littin Huijun Yu Jürgen Hennig Maxim Zaitsev 《Magma (New York, N.Y.)》2016,29(1):59-73
Objective
In this paper, we present a new performance measure of a matrix coil (also known as multi-coil) from the perspective of efficient, local, non-linear encoding without explicitly considering target encoding fields.Materials and methods
An optimization problem based on a joint optimization for the non-linear encoding fields is formulated. Based on the derived objective function, a figure of merit of a matrix coil is defined, which is a generalization of a previously known resistive figure of merit for traditional gradient coils.Results
A cylindrical matrix coil design with a high number of elements is used to illustrate the proposed performance measure. The results are analyzed to reveal novel features of matrix coil designs, which allowed us to optimize coil parameters, such as number of coil elements. A comparison to a scaled, existing multi-coil is also provided to demonstrate the use of the proposed performance parameter.Conclusions
The assessment of a matrix gradient coil profits from using a single performance parameter that takes the local encoding performance of the coil into account in relation to the dissipated power.16.
Amandine Coum Lobna Ouldamer Fanny Noury Laurent Barantin Aymeric Saint-Hilaire Anne Vilde Philippe Bougnoux Giulio Gambarota 《Magma (New York, N.Y.)》2016,29(1):1-4
Objective
To evaluate the feasibility of in vivo measurement of the fatty acid (FA) composition of breast adipose tissue by MRS on a clinical platform.Material and methods
MRS experiments were performed at 3 T, using a STEAM sequence, on 25 patients diagnosed with breast cancer. MR spectra, acquired on healthy breast tissue, were analysed with the LCModel.Results
The measured values of the saturated fatty acid (SFA), mono-unsaturated fatty acid (MUFA) and poly-unsaturated fatty acid (PUFA) fractions were 23.8 ± 7.1 %, 55.4 ± 6.8 % and 20.8 ± 4.4 %, respectively.The values of SFA, MUFA and PUFA observed in the current study are in the same range as those found in two previous studies performed at 7 T.Conclusion
The results of the current study show that it is possible to quantify the fatty acid composition of breast tissue in vivo in a clinical setting (3 T).17.
Mads S. Vinding Daniel Brenner Desmond H. Y. Tse Sebastian Vellmer Thomas Vosegaard Dieter Suter Tony Stöcker Ivan I. Maximov 《Magma (New York, N.Y.)》2017,30(1):29-39
Objective
Ultrahigh field MRI provides great opportunities for medical diagnostics and research. However, ultrahigh field MRI also brings challenges, such as larger magnetic susceptibility induced field changes. Parallel-transmit radio-frequency pulses can ameliorate these complications while performing advanced tasks in routine applications. To address one class of such pulses, we propose an optimal-control algorithm as a tool for designing advanced multi-dimensional, large flip-angle, radio-frequency pulses. We contrast initial conditions, constraints, and field correction abilities against increasing pulse trajectory acceleration factors.Materials and methods
On an 8-channel 7T system, we demonstrate the quasi-Newton algorithm with pulse designs for reduced field-of-view imaging with an oil phantom and in vivo with scans of the human brain stem. We used echo-planar imaging with 2D spatial-selective pulses. Pulses are computed sufficiently rapid for routine applications.Results
Our dataset was quantitatively analyzed with the conventional mean-square-error metric and the structural-similarity index from image processing. Analysis of both full and reduced field-of-view scans benefit from utilizing both complementary measures.Conclusion
We obtained excellent outer-volume suppression with our proposed method, thus enabling reduced field-of-view imaging using pulse trajectory acceleration factors up to 4.18.
Elias Kellner Irina Mader Marco Reisert Horst Urbach Valerij Gennadevic Kiselev 《Magma (New York, N.Y.)》2018,31(3):439-448
Object
We aimed to modify our previously published method for arterial input function measurements for evaluation of cerebral perfusion (dynamic susceptibility contrast MRI) such that it can be applied in humans in a clinical setting.Materials and methods
Similarly to our previous work, a conventional measurement sequence for dynamic susceptibility contrast MRI is extended with an additional measurement slice at the neck. Measurement parameters at this slice were optimized for the blood signal (short echo time, background suppression, magnitude and phase images). Phase-based evaluation of the signal in the carotid arteries is used to obtain quantitative arterial input functions.Results
In all pilot measurements, quantitative arterial input functions were obtained. The resulting absolute perfusion parameters agree well with literature values (gray and white matter mean values of 46 and 24 mL/100 g/min, respectively, for cerebral blood flow and 3.0% and 1.6%, respectively, for cerebral blood volume).Conclusions
The proposed method has the potential to quantify arterial input functions in the carotid arteries from a direct measurement without any additional normalization.19.
Mustafa Çavuşoğlu Benjamin Emanuel Dietrich David Otto Brunner Markus Weiger Klaas Paul Pruessmann 《Magma (New York, N.Y.)》2017,30(5):473-488
Objectives
The accuracy and precision of the parallel RF excitations are highly dependent on the spatial and temporal fidelity of the magnetic fields involved in spin excitation. The consistency between the nominal and effective fields is typically limited by the imperfections of the employed hardware existing both in the gradient system and the RF chain. In this work, we experimentally presented highly improved spatially tailored parallel excitations by turning the native hardware accuracy challenge into a measurement and control problem using an advanced field camera technology to fully correct parallel RF transmission experiment.Materials and methods
An array of NMR field probes is used to measure the multiple channel RF pulses and gradient waveforms recording the high power RF pulses simultaneously with low frequency gradient fields on equal time basis. The recorded waveforms were integrated in RF pulse design for gradient trajectory correction, time imperfection compensation and introduction of iterative RF pre-emphasis.Results
Superior excitation accuracy was achieved. Two major applications were presented at 7 Tesla including multi-dimensional tailored RF pulses for spatially selective excitation and slice-selective spoke pulses for \(B_{1}^{ + }\) mitigation.Conclusion
Comprehensive field monitoring is a highly effective means of correcting for the field deviations during parallel transmit pulse design.20.
E. E. Sigmund S. H. Baete K. Patel D. Wang D. Stoffel R. Otazo P. Parasoglou J. Bencardino 《Magma (New York, N.Y.)》2018,31(5):599-608