共查询到20条相似文献,搜索用时 78 毫秒
1.
Manushka V. Vaidya Cem M. Deniz Christopher M. Collins Daniel K. Sodickson Riccardo Lattanzi 《Magma (New York, N.Y.)》2018,31(3):355-366
Objective
To use high-permittivity materials (HPM) positioned near radiofrequency (RF) surface coils to manipulate transmit/receive field patterns.Materials and methods
A large HPM pad was placed below the RF coil to extend the field of view (FOV). The resulting signal-to-noise ratio (SNR) was compared with that of other coil configurations covering the same FOV in simulations and experiments at 7 T. Transmit/receive efficiency was evaluated when HPM discs with or without a partial shield were positioned at a distance from the coil. Finally, we evaluated the increase in transmit homogeneity for a four-channel array with HPM discs interposed between adjacent coil elements.Results
Various configurations of HPM increased SNR, transmit/receive efficiency, excitation/reception sensitivity overlap, and FOV when positioned near a surface coil. For a four-channel array driven in quadrature, shielded HPM discs enhanced the field below the discs as well as at the center of the sample as compared with other configurations with or without unshielded HPM discs.Conclusion
Strategically positioning HPM at a distance from a surface coil or array can increase the overlap between excitation/reception sensitivities, and extend the FOV of a single coil for reduction of the number of channels in an array while minimally affecting the SNR.2.
Local SAR management by RF Shimming: a simulation study with multiple human body models 总被引:2,自引:2,他引:0
Homann H Graesslin I Eggers H Nehrke K Vernickel P Katscher U Dössel O Börnert P 《Magma (New York, N.Y.)》2012,25(3):193-204
Object
Parallel transmission facilitates a relatively direct control of the RF transmit field. This is usually applied to improve the RF field homogeneity but might also allow a reduction of the specific absorption rate (SAR) to increase freedom in sequence design for high-field MRI. However, predicting the local SAR is challenging as it depends not only on the multi-channel drive but also on the individual patient.Materials and methods
The potential of RF shimming for SAR management is investigated for a 3?T body coil with eight independent transmit elements, based on Finite-Difference Time-Domain (FDTD) simulations. To address the patient-dependency of the SAR, nine human body models were generated from volunteer MR data and used in the simulations. A novel approach to RF shimming that enforces local SAR constraints is proposed.Results
RF shimming substantially reduced the local SAR, consistently for all volunteers. Using SAR constraints, a further SAR reduction could be achieved with only minor compromises in RF performance.Conclusion
Parallel transmission can become an important tool to control and manage the local SAR in the human body. The practical use of local SAR constraints is feasible with consistent results for a variety of body models. 相似文献3.
Jens Hoffmann Gunamony Shajan Klaus Scheffler Rolf Pohmann 《Magma (New York, N.Y.)》2014,27(5):373-386
Objective
To provide a numerical and experimental investigation of the static RF shimming capabilities in the human brain at 9.4 T using a dual-row transmit array.Materials and methods
A detailed numerical model of an existing 16-channel, inductively decoupled dual-row array was constructed using time-domain software together with circuit co-simulation. Experiments were conducted on a 9.4 T scanner. Investigation of RF shimming focused on B1 + homogeneity, efficiency and local specific absorption rate (SAR) when applied to large brain volumes and on a slice-by-slice basis.Results
Numerical results were consistent with experiments regarding component values, S-parameters and B1 + pattern, though the B1 + field was about 25 % weaker in measurements than simulations. Global shim settings were able to prevent B1 + field voids across the entire brain but the capability to simultaneously reduce inhomogeneities was limited. On a slice-by-slice basis, B1 + standard deviations of below 10 % without field dropouts could be achieved in axial, sagittal and coronal orientations across the brain, even with phase-only shimming, but decreased B1 + efficiency and SAR limitations must be considered.Conclusion
Dual-row transmit arrays facilitate flexible 3D RF management across the entire brain at 9.4 T in order to trade off B1 + homogeneity against power-efficiency and local SAR. 相似文献4.
Gerd Weidemann Frank Seifert Werner Hoffmann Harald Pfeiffer Reiner Seemann Bernd Ittermann 《Magma (New York, N.Y.)》2016,29(3):371-378
Objective
The power balance of multichannel transmit coils is a central consideration in assessing performance and safety issues. At ultrahigh fields, in addition to absorption and reflection, radiofrequency (RF) radiation into the far field becomes a concern.Materials and methods
We engineered a system for in situ measurement of complex-valued scattering parameter (S-parameter) matrices of multichannel transmit coils that allows for the calculation of the reflected and accepted power for arbitrary steering conditions. The radiated power from an RF coil inside a large single-mode waveguide couples to that mode. Finite-difference time-domain simulations were used for the calculations, and E-field probes were used to measure the electric field distribution, and hence the radiated power, in the waveguide. To test this concept, an eight-channel shielded-loop array for 7T imaging was studied inside a 280-cm-long cylindrical waveguide with a 60-cm diameter.Results
For a 7T parallel-transmit coil, the S-parameters were measured and the reflected power calculated as a function of steering conditions. Maximum radiated power was observed for the circularly polarized mode.Conclusion
A system was developed for in situ S-parameter measurements combined with a method for determining radiated power, allowing a complete assessment of the power balance of multichannel transmit coils at 7T.5.
Morwan Choli Martin Blaimer Felix A. Breuer Philipp Ehses Oliver Speck Andreas J. Bartsch Peter M. Jakob 《Magma (New York, N.Y.)》2013,26(4):411-418
Object
Clinical 3 T MRI systems are rapidly increasing and MRI systems with a static field of 7 T or even more have been installed. The RF power deposition is proportional to the square of the static magnetic field strength and is characterized by the specific absorption rate (SAR). Therefore, there exist defined safety limits to avoid heating of the patient. Here, we describe a hybrid method to significantly reduce the SAR compared to a turbo-spin-echo (TSE) sequence.Materials and methods
We investigate the potential benefits of a combined acquisition technique (CAT) for high-field neuroimaging at 3 and 7 T. The TSE/EPI CAT experiments were performed on volunteers and patients and compared with standard TSE and GRASE protocols. Problems and solutions regarding T2 weighted CAT imaging are discussed.Results
We present in vivo images with T2 and proton density contrast obtained on 3 and 7 T with significant SAR reduction (up to 60 %) compared with standard TSE. Image quality is comparable to TSE but CAT shows fewer artifacts than a GRASE sequence.Conclusion
CAT is a promising candidate for neuroimaging at high fields up to 7 T. The SAR reduction allows one to shorten the waiting time between two excitations or to image more slices thereby reducing the overall measurement time. 相似文献6.
Ralf Kartäusch Toni Driessle Thomas Kampf Thomas Christian Basse-Lüsebrink Uvo Christoph Hoelscher Peter Michael Jakob Florian Fidler Xavier Helluy 《Magma (New York, N.Y.)》2014,27(5):363-371
Objective
The present work introduces an alternative to the conventional \(B_{0}\) -gradient spatial phase encoding technique. By applying far off-resonant radiofrequency (RF) pulses, a spatially dependent phase shift is introduced to the on-resonant transverse magnetization. This so-called Bloch–Siegert (BS) phase shift has been recently used for \(B_{1}^{ + }\) -mapping. The current work presents the theoretical background for the BS spatial encoding technique (BS-SET) using RF-gradients.Materials and methods
Since the BS-gradient leads to nonlinear encoding, an adapted reconstruction method was developed to obtain undistorted images. To replace conventional phase encoding gradients, BS-SET was implemented in a two-dimensional (2D) spin echo sequence on a 0.5 T portable MR scanner.Results
A 2D spin echo (SE) measurement imaged along a single dimension using the BS-SET was compared to a conventional SE 2D measurement. The proposed reconstruction method yielded undistorted images.Conclusions
BS-gradients were demonstrated as a feasible option for spatial phase encoding. Furthermore, undistorted BS-SET images could be obtained using the proposed reconstruction method. 相似文献7.
Stefanie Buchenau Martin Haas Daniel Nicolas Splitthoff Jürgen Hennig Maxim Zaitsev 《Magma (New York, N.Y.)》2013,26(5):463-476
Object
The specific absorption rate (SAR) can be determined from radiofrequency transmit fields measured via magnetic resonance imaging.Materials and methods
The proposed method estimates the SAR solely from the complex transmit field (B 1 + ) by taking into account the particular properties of the electromagnetic field generated by an 8-channel transmit array. It is further based on an iterative consistency check between the measured B 1 + magnitude and an appropriate field estimate fulfilling Maxwell’s equations. For testing the method, simulations and phantom experiments were performed for a multi-transmit array at 3T using a cylindrical phantom.Results
The method’s robustness with respect to the assumptions made about electric tissue properties as well as its stability under different initial conditions regarding the signal phase was shown. A high sensitivity to signal noise was found. Robust reconstruction results were achieved including information from more than two transmit elements. The validity of the experimental results was confirmed by a qualitative comparison to simulated electromagnetic fields.Conclusions
The method allows the determination of the SAR as well as the transmit phase of the individual channels of a multi-transmit array. With additional B0 inhomogeneity measurements, a reconstruction of the receive phase is feasible independent of the receive coil type in use. 相似文献8.
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.9.
Alexander Brunner Jens Groebner Reiner Umathum Florian Maier Wolfhard Semmler Michael Bock 《Magma (New York, N.Y.)》2014,27(4):277-282
Background and methods
A commercial three-dimensional (3D) monitor was modified for use inside the scanner room to provide stereoscopic real-time visualization during magnetic resonance (MR)-guided interventions, and tested in a catheter-tracking phantom experiment at 1.5 T. Brightness, uniformity, radio frequency (RF) emissions and MR image interferences were measured.Results and discussion
Due to modifications, the center luminance of the 3D monitor was reduced by 14 %, and the addition of a Faraday shield further reduced the remaining luminance by 31 %. RF emissions could be effectively shielded; only a minor signal-to-noise ratio (SNR) decrease of 4.6 % was observed during imaging. During the tracking experiment, the 3D orientation of the catheter and vessel structures in the phantom could be visualized stereoscopically. 相似文献10.
Julia Stabinska Tom Cronenberg Hans-Jörg Wittsack Rotem Shlomo Lanzman Anja Müller-Lutz 《Magma (New York, N.Y.)》2017,30(5):505-516
Objectives
The goal of this study was to quantify CEST related parameters such as chemical exchange rate and fractional concentration of exchanging protons at a clinical 3T scanner. For this purpose, two CEST quantification approaches—the AREX metric (for ‘apparent exchange dependent relaxation’), and the AREX-based Ω-plot method were used. In addition, two different pulsed RF irradiation schemes, using Gaussian-shaped and spin-lock pulses, were compared.Materials and methods
Numerical simulations as well as MRI measurements in phantoms were performed. For simulations, the Bloch–McConnell equations were solved using a two-pool exchange model. MR experiments were performed on a clinical 3T MRI scanner using a cylindrical phantom filled with creatine solution at different pH values and different concentrations.Results
The validity of the Ω-plot method and the AREX approach using spin-lock preparation for determination of the quantitative CEST parameters was demonstrated. Especially promising results were achieved for the Ω-plot method when the spin-lock preparation was employed.Conclusion
Pulsed CEST at 3T could be used to quantify parameters such as exchange rate constants and concentrations of protons exchanging with free water. In the future this technique might be used to estimate the exchange rates and concentrations of biochemical substances in human tissues in vivo.11.
Objective and methods
A radiofrequency (RF) pulse design technique is presented that uses iterative constrained minimization to determine Fourier domain coefficients for an optimal time domain RF pulse. The design of new RF pulses is especially beneficial for field strengths of 7.0 T and above, where challenges pertaining to specific absorption rate (SAR) are exacerbated.Results and conclusion
A pair of 90° and 180° spin-echo pulses was designed to lower SAR without the need for a variable slice gradient. The optimized pulses were deployed to a 7.0 T human scanner to demonstrate a reduction in SAR while retaining signal-to-noise (SNR) ratio.12.
Sairamesh Raghuraman Joachim H. X. Schrauth Daniel L. Weber Frank Resmer Meike Haddad-Weber Felix A. Breuer Ulrich Nöth Peter M. Jakob Titus Lanz Daniel Haddad 《Magma (New York, N.Y.)》2013,26(2):215-228
Object
To construct an optimised, high-density receive array and a movement device to achieve dynamic imaging of the knee in orthopedic large animal models (e.g., minipigs) at 1.5 T.Materials and methods
A 13-channel RF receive array was constructed, and the crucial choice of the array element size (based on considerations like region of interest, geometry of the minipig’s knee, achievable signal-to-noise ratio, applicability of parallel imaging, etc.) was determined using the Q factors of loops with different sizes. A special movement device was constructed to guide and produce a reproducible motion of the minipig’s knee during acquisition.Results
The constructed array was electrically characterised and the reproducibility of the cyclic motion was validated. Snapshots of dynamic in vivo images taken at a temporal resolution (308 ms) are presented. Some of the fine internal structures within the minipig’s knee, like cruciate ligaments, are traced in the snapshots.Conclusion
This study is a step towards making dynamic imaging which can give additional information about joint injuries when static MRI is not able to give sufficient information, a routine clinical application. There, the combination of a high-density receive array and a movement device will be highly helpful in the diagnosis and therapy monitoring of knee injuries in the future. 相似文献13.
Groebner J Umathum R Bock M Krafft AJ Semmler W Rauschenberg J 《Magma (New York, N.Y.)》2011,24(6):315-322
Object
The EU directive on safety requirements (2004/40/EC) limits the exposure to time varying magnetic fields to dB /dt = 200 mT/s. This action value is not clearly defined as it considers only the temporal change of the magnitude of ${\vec {B}}$ . Thus, only the translational motion in the magnet??s fringe field is considered and rotations are neglected.Materials and methods
A magnetic field probe was constructed to simultaneously record the magnetic flux density ${\vec{B}}$ (x, y, z) with a 3-axis Hall sensor and the induced voltage due to movements with a set of three orthogonal coils. Voltages were converted into time-varying magnetic flux d ??(x, y, z)/dt serving as an exposition parameter for both translations and rotations. To separate the two types of motion, d B/dt was additionally calculated on the basis of the Hall sensor??s data. The calibrated probe was attached to the forehead of 8 healthcare workers and 17 MR physicists, and ${\vec {B}}$ and d??/dt were recorded during standard operating procedures at three different MR systems up to 7 T.Results
The maximum percentage of the translational motion referring the data including both translations and rotations amounts to 32%. During volunteer measurements, maximum exposure values of d??/dt = 21 mWb/s, dB/dt = 1.40 T/s and ${\left| {\vec {B}}\right|= 2.75}$ Twere found.Conclusion
The findings in this work indicate that both translations and rotations in the vicinity of an MR system should be taken into account, and that a single regulatory action level might not be sufficient. 相似文献14.
Hervé Saint-Jalmes Pierre-Antoine Eliat Johanne Bezy-Wendling Alejandro Bordelois Giulio Gambarota 《Magma (New York, N.Y.)》2014,27(5):419-424
Object
The ability to generate reference signals is of great benefit for quantitation of the magnetic resonance (MR) signal. The aim of the present study was to implement a dedicated experimental set-up to generate MR images of virtual phantoms.Materials and methods
Virtual phantoms of a given shape and signal intensity were designed and the k-space representation was generated. A waveform generator converted the k-space lines into a radiofrequency (RF) signal that was transmitted to the MR scanner bore by a dedicated RF coil. The k-space lines of the virtual phantom were played line-by-line in synchronization with the magnetic resonance imaging data acquisition.Results
Virtual phantoms of complex patterns were reproduced well in MR images without the presence of artifacts. Time-series measurements showed a coefficient of variation below 1 % for the signal intensity of the virtual phantoms. An excellent linearity (coefficient of determination r 2 = 0.997 as assessed by linear regression) was observed in the signal intensity of virtual phantoms.Conclusion
Virtual phantoms represent an attractive alternative to physical phantoms for providing a reference signal. MR images of virtual phantoms were here generated using a stand-alone, independent unit that can be employed with MR scanners from different vendors. 相似文献15.
Quantification and localization of contrast agents using delta relaxation enhanced magnetic resonance at 1.5?T 总被引:1,自引:1,他引:0
Object
Delta relaxation enhanced magnetic resonance (dreMR) is a new imaging technique based on the idea of cycling the magnetic field B 0 during an imaging sequence. The method determines the field dependency of the relaxation rate (relaxation dispersion dR 1/dB). This quantity is of particular interest in contrast agent imaging because the parameter can be used to determine contrast agent concentrations and increases the ability to localize the contrast agent.Materials and methods
In this paper dreMR imaging was implemented on a clinical 1.5?T MR scanner combining conventional MR imaging with fast field-cycling. Two improvements to dreMR theory are presented describing the quantification of contrast agent concentrations from dreMR data and a correction for field-cycling with finite ramp times.Results
Experiments demonstrate the use of the extended theory and show the measurement of contrast agent concentrations with the dreMR method. A second experiment performs localization of a contrast agent with a significant improvement in comparison to conventional imaging.Conclusion
dreMR imaging has been extended by a method to quantify contrast agent concentrations and improved for field-cycling with finite ramp times. Robust localization of contrast agents using dreMR imaging has been performed in a sample where conventional imaging delivers inconclusive results. 相似文献16.
Bart R. Steensma Ingmar J. Voogt Tim Leiner Peter R. Luijten Jesse Habets Dennis W. J. Klomp Cornelis A. T. van den Berg Alexander J. E. Raaijmakers 《Magma (New York, N.Y.)》2018,31(1):7-18
Objective
To demonstrate imaging performance for cardiac MR imaging at 7 T using a coil array of 8 transmit/receive dipole antennas and 16 receive loops.Materials and methods
An 8-channel dipole array was extended by adding 16 receive-only loops. Average power constraints were determined by electromagnetic simulations. Cine imaging was performed on eight healthy subjects. Geometrical factor (g-factor) maps were calculated to assess acceleration performance. Signal-to-noise ratio (SNR)-scaled images were reconstructed for different combinations of receive channels, to demonstrate the SNR benefits of combining loops and dipoles.Results
The overall image quality of the cardiac functional images was rated a 2.6 on a 4-point scale by two experienced radiologists. Imaging results at different acceleration factors demonstrate that acceleration factors up to 6 could be obtained while keeping the average g-factor below 1.27. SNR maps demonstrate that combining loops and dipoles provides a more than 50% enhancement of the SNR in the heart, compared to a situation where only loops or dipoles are used.Conclusion
This work demonstrates the performance of a combined loop/dipole array for cardiac imaging at 7 T. With this array, acceleration factors of 6 are possible without increasing the average g-factor in the heart beyond 1.27. Combining loops and dipoles in receive mode enhances the SNR compared to receiving with loops or dipoles only.17.
Noam Ben-Eliezer Eddy Solomon Elad Harel Nava Nevo Lucio Frydman 《Magma (New York, N.Y.)》2012,25(6):433-442
Object
An approach has been recently introduced for acquiring arbitrary 2D NMR spectra or images in a single scan, based on the use of frequency-swept RF pulses for the sequential excitation and acquisition of the spins response. This spatiotemporal-encoding (SPEN) approach enables a unique, voxel-by-voxel refocusing of all frequency shifts in the sample, for all instants throughout the data acquisition. The present study investigates the use of this full-refocusing aspect of SPEN-based imaging in the multi-shot MRI of objects, subject to sizable field inhomogeneities that complicate conventional imaging approaches.Materials and methods
2D MRI experiments were performed at 7 T on phantoms and on mice in vivo, focusing on imaging in proximity to metallic objects. Fully refocused SPEN-based spin echo imaging sequences were implemented, using both Cartesian and back-projection trajectories, and compared with k-space encoded spin echo imaging schemes collected on identical samples under equal bandwidths and acquisition timing conditions.Results
In all cases assayed, the fully refocused spatiotemporally encoded experiments evidenced a ca. 50?% reduction in signal dephasing in the proximity of the metal, as compared to analogous results stemming from the k-space encoded spin echo counterparts.Conclusion
The results in this study suggest that SPEN-based acquisition schemes carry the potential to overcome strong field inhomogeneities, of the kind that currently preclude high-field, high-resolution tissue characterizations in the neighborhood of metallic implants. 相似文献18.
Thérèse Barbier Sarra Aissani Nicolas Weber Cédric Pasquier Jacques Felblinger 《Magma (New York, N.Y.)》2018,31(5):677-688
Purpose
To evaluate the function of an active implantable medical device (AIMD) during magnetic resonance imaging (MRI) scans. The induced voltages caused by the switching of magnetic field gradients and rectified radio frequency (RF) pulse were measured, along with the AIMD stimulations.Materials and methods
An MRI-compatible voltage probe with a bandwidth of 0–40 kHz was designed. Measurements were carried out both on the bench with an overvoltage protection circuit commonly used for AIMD and with a pacemaker during MRI scans on a 1.5 T (64 MHz) MR scanner.Results
The sensor exhibits a measurement range of?±?15 V with an amplitude resolution of 7 mV and a temporal resolution of 10 µs. Rectification was measured on the bench with the overvoltage protection circuit. Linear proportionality was confirmed between the induced voltage and the magnetic field gradient slew rate. The pacemaker pacing was recorded successfully during MRI scans.Conclusion
The characteristics of this low-frequency voltage probe allow its use with extreme RF transmission power and magnetic field gradient positioning for MR safety test of AIMD during MRI scans.19.
Fuster-Garcia E Navarro C Vicente J Tortajada S García-Gómez JM Sáez C Calvar J Griffiths J Julià-Sapé M Howe FA Pujol J Peet AC Heerschap A Moreno-Torres A Martínez-Bisbal MC Martínez-Granados B Wesseling P Semmler W Capellades J Majós C Alberich-Bayarri A Capdevila A Monleón D Martí-Bonmatí L Arús C Celda B Robles M 《Magma (New York, N.Y.)》2011,24(1):35-42
Object
This study demonstrates that 3T SV-MRS data can be used with the currently available automatic brain tumour diagnostic classifiers which were trained on databases of 1.5T spectra. This will allow the existing large databases of 1.5T MRS data to be used for diagnostic classification of 3T spectra, and perhaps also the combination of 1.5T and 3T databases.Materials and methods
Brain tumour classifiers trained with 154 1.5T spectra to discriminate among high grade malignant tumours and common grade II glial tumours were evaluated with a subsequently-acquired set of 155 1.5T and 37 3T spectra. A similarity study between spectra and main brain tumour metabolite ratios for both field strengths (1.5T and 3T) was also performed.Results
Our results showed that classifiers trained with 1.5T samples had similar accuracy for both test datasets (0.87 ± 0.03 for 1.5T and 0.88 ± 0.03 for 3.0T). Moreover, non-significant differences were observed with most metabolite ratios and spectral patterns.Conclusion
These results encourage the use of existing classifiers based on 1.5T datasets for diagnosis with 3T 1H SV-MRS. The large 1.5T databases compiled throughout many years and the prediction models based on 1.5T acquisitions can therefore continue to be used with data from the new 3T instruments. 相似文献20.
Johannes Mallow Tim Herrmann Kyoung-Nam Kim Joerg Stadler Judith Mylius Michael Brosch Johannes Bernarding 《Magma (New York, N.Y.)》2013,26(4):389-400