The Experts below are selected from a list of 240 Experts worldwide ranked by ideXlab platform

Malcolm H Levitt - One of the best experts on this subject based on the ideXlab platform.

  • recycling and imaging of nuclear singlet hyperpolarization
    Journal of the American Chemical Society, 2013
    Co-Authors: Giuseppe Pileio, Sean Bowen, Christoffer Laustsen, Michael C D Tayler, Joseph T Hillcousins, Lynda J Brown, Richard C D Brown, Jan Henrik Ardenkjaerlarsen, Malcolm H Levitt
    Abstract:

    The strong enhancement of NMR signals achieved by hyperpolarization decays, at best, with a time constant of a few minutes. Here, we show that a combination of long-lived singlet states, molecular design, magnetic field cycling, and specific Radiofrequency Pulse sequences allows repeated observation of the same batch of polarized nuclei over a period of 30 min and more. We report a recycling protocol in which the enhanced nuclear polarization achieved by dissolution-DNP is observed with full intensity and then returned to singlet order. MRI experiments may be run on a portion of the available spin polarization, while the remaining is preserved and made available for a later use. An analogy is drawn with a “spin bank” or “resealable container” in which highly polarized spin order may be deposited and retrieved.

  • truncated dipolar recoupling in solid state nuclear magnetic resonance
    Chemical Physics Letters, 2006
    Co-Authors: Ildefonso Marinmontesinos, Giulia Mollica, Marina Carravetta, Axel Gansmuller, Giuseppe Pileio, Matthias Bechmann, Angelika Sebald, Malcolm H Levitt
    Abstract:

    We describe a solid-state NMR concept for the estimation of individual spin–spin couplings in strongly-coupled homonuclear spin systems. A Radiofrequency Pulse sequence, synchronised with the magic-angle sample rotation recouples zero-quantum dipolar interactions as well as a frequency-dispersing interaction such as the chemical shift anisotropy. The combination of these two recoupled interactions causes the spin system to behave in an approximately weakly-coupled fashion. Individual spin–spin couplings may then be disentangled by using frequency-selective Radiofrequency Pulses. Theoretical results and numerical simulations are compared with experimental data for the 13C nuclei in [2H7,13C3,15N]-l-alanine.

  • symmetry principles for the design of Radiofrequency Pulse sequences in the nuclear magnetic resonance of rotating solids
    Chemical Physics Letters, 2000
    Co-Authors: Marina Carravetta, Andreas Brinkmann, Mattias Eden, Xin Zhao, Malcolm H Levitt
    Abstract:

    Some new symmetry theorems are presented which simplify the task of designing multiple-Pulse radio-frequency Pulse sequences in magic-angle-spinning solid-state NMR. The symmetry theorems apply to sequences denoted RN n , which n consists of N repetitions of a Pulse sequence element R, alternating in phase between the values "pnrN. Each R element ideally rotates the spins by an angle p about the rotating frame x-axis. The entire R N n sequence is timed to span n n rotational periods. Applications are presented for homonuclear double-quantum and zero-quantum recoupling, heteronuclear decoupling and heteronuclear recoupling. q 2000 Elsevier Science B.V. All rights reserved.

  • efficient dipolar recoupling in the nmr of rotating solids a sevenfold symmetric Radiofrequency Pulse sequence
    Chemical Physics Letters, 1995
    Co-Authors: Y K Lee, N D Kurur, M Helmle, Ole G Johannessen, Niels Chr Nielsen, Malcolm H Levitt
    Abstract:

    Abstract A new Radiofrequency Pulse sequence is introduced for the efficient reintroduction of magnetic dipolar couplings in the magic-angle-spinning NMR of solids. The sequence involves seven phase-shifted Radiofrequency Pulse cycles, timed to span two rotational periods of the sample. Double-quantum coherences are excited with high efficiency in a rotating powder sample of zinc acetate- 13 C 2 .

Pierrefrancois Van De Moortele - One of the best experts on this subject based on the ideXlab platform.

  • direct control of the temperature rise in parallel transmission by means of temperature virtual observation points simulations at 10 5 tesla
    Magnetic Resonance in Medicine, 2016
    Co-Authors: Nicolas Boulant, Sebastian Schmitter, Kâmil Ugurbil, Xiaoping Wu, Gregor Adriany, Pierrefrancois Van De Moortele
    Abstract:

    Purpose A method using parallel transmission to mitigate B1+ inhomogeneity while explicitly constraining the temperature rise is reported and compared with a more traditional SAR-constrained Pulse design. Methods Finite difference time domain simulations are performed on a numerical human head model and for a 16-channel coil at 10.5 Tesla. Based on a set of presimulations, a virtual observation point compression model for the temperature rise is derived. This compact representation is then used in a nonlinear programming algorithm for Pulse design under explicit temperature rise constraints. Results In the example of a time-of-flight sequence, Radiofrequency Pulse performance in some cases is increased by a factor of two compared with SAR-constrained Pulses, while temperature rise is directly and efficiently controlled. Pulse performance can be gained by relaxing the SAR constraints, but at the expense of a loss of direct control on temperature. Conclusion Given the importance of accurate safety control at ultrahigh field and the lack of direct correspondence between SAR and temperature, this work motivates the need for thorough thermal studies in normal in vivo conditions. The tools presented here will possibly contribute to safer and more efficient MR exams. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.

  • seven tesla time of flight angiography using a 16 channel parallel transmit system with power constrained 3 dimensional spoke Radiofrequency Pulse design
    Investigative Radiology, 2014
    Co-Authors: Sebastian Schmitter, Edward J Auerbach, Kâmil Ugurbil, Gregor Adriany, Josef Pfeuffer, Michael Hamm, Pierrefrancois Van De Moortele
    Abstract:

    OBJECTIVES Ultrahigh magnetic fields of 7 T or higher have proven to significantly enhance the contrast in time-of-flight (TOF) imaging, one of the most commonly used non-contrast-enhanced magnetic resonance angiography techniques. Compared with lower field strength, however, the required Radiofrequency (RF) power is increased at 7 T and the contrast obtained with a conventional head transmit RF coil is typically spatially heterogeneous.In this work, we addressed the contrast heterogeneity in multislab TOF acquisitions by optimizing the excitation flip angle homogeneity while constraining the RF power using 3-dimensional tailored RF Pulses ("spokes") with a 16-channel parallel transmission system and a 16-channel transceiver head coil. MATERIALS AND METHODS We investigated in simulations and in vivo experiments flip angle homogeneity and angiogram quality with a same 3-slab TOF protocol for different excitations including 1-, 2-, and 3-spoke parallel transmit RF Pulses and compared the results with a circularly polarized (CP) phase setting similar to a birdcage excitation. B1 and B0 calibration maps were obtained in multiple slices, and the RF Pulse for each slab was designed on the basis of 3 calibration slices located at the bottom/middle/top of each slab, respectively. By design, all excitations were computed to generate the same total RF power for the same flip angle. In 8 subjects, we quantified the excitation homogeneity and the distribution of the RF power to individual channels. In addition, we investigated the consequences of local flip angle variations at the junction between adjacent slabs as well as the impact of ΔB0 on image quality. RESULTS The flip angle heterogeneity, expressed as the coefficient of variation, averaged over all volunteers and all slabs could be reduced from 29.4% for CP mode excitation to 14.1% for a 1-spoke excitation and to 7.3% for 2-spoke excitations. A separate detailed analysis shows only a marginal improvement for 3-spoke compared with the 2-spoke excitation. The strong improvement in flip angle homogeneity particularly impacted the junction between adjacent TOF slabs, where significant residual artifacts observed with 1-spoke excitation could be efficiently mitigated using a 2-spoke excitation with same RF power and same average flip angle. Although the total RF power is maintained at the same level than that in CP mode excitation, the energy distribution is fairly heterogeneous through the 16 transmit channels for 1- and 2-spoke excitations, with the highest energy for 1 channel being a factor of 2.4 (1 spoke) and 2.2 (2 spokes) higher than that in CP mode. In vivo experiments demonstrated the necessity for including ΔB0 spatial variations during 2-spoke RF Pulse design, particularly in areas with strong local susceptibility variations such as the lower frontal lobe. CONCLUSIONS Significant improvement in excitation fidelity leading to improved TOF contrast, particularly in the brain periphery, as well as smooth slab transitions can be achieved with 2-spoke excitation while maintaining the same excitation energy as that in CP mode. These results suggest that expanding parallel transmit methods, including the use of multidimensional spatially selective excitation, will also be very beneficial for other techniques, such as perfusion imaging.

  • cardiac imaging at 7 tesla single and two spoke Radiofrequency Pulse design with 16 channel parallel excitation
    Magnetic Resonance in Medicine, 2013
    Co-Authors: Sebastian Schmitter, Lance Delabarre, Andreas Greiser, Dingxin Wang, Edward J Auerbach, Thomas J Vaughan, Kâmil Ugurbil, Pierrefrancois Van De Moortele
    Abstract:

    Purpose Higher signal to noise ratio (SNR) and improved contrast have been demonstrated at ultra-high magnetic fields (≥7 Tesla [T]) in multiple targets, often with multi-channel transmit methods to address the deleterious impact on tissue contrast due to spatial variations in B1+ profiles. When imaging the heart at 7T, however, respiratory and cardiac motion, as well as B0 inhomogeneity, greatly increase the methodological challenge. In this study we compare two-spoke parallel transmit (pTX) RF Pulses with static B1+ shimming in cardiac imaging at 7T. Methods Using a 16-channel pTX system, slice-selective two-spoke pTX Pulses and static B1+ shimming were applied in cardiac CINE imaging. B1+ and B0 mapping required modified cardiac triggered sequences. Excitation homogeneity and RF energy were compared in different imaging orientations. Results Two-spoke Pulses provide higher excitation homogeneity than B1+ shimming, especially in the more challenging posterior region of the heart. The peak value of channel-wise RF energy was reduced, allowing for a higher flip angle, hence increased tissue contrast. Image quality with two-spoke excitation proved to be stable throughout the entire cardiac cycle. Conclusion Two-spoke pTX excitation has been successfully demonstrated in the human heart at 7T, with improved image quality and reduced RF Pulse energy when compared with B1+ shimming. Magn Reson Med 70:1210–1219, 2013. © 2013 Wiley Periodicals, Inc.

  • contrast enhancement in tof cerebral angiography at 7 t using saturation and mt Pulses under sar constraints impact of verse and sparse Pulses
    Magnetic Resonance in Medicine, 2012
    Co-Authors: Sebastian Schmitter, Edward J Auerbach, Kâmil Ugurbil, Michael Bock, Soren Johst, Pierrefrancois Van De Moortele
    Abstract:

    Cerebral three-dimensional time of flight (TOF) angiography significantly benefits from ultrahigh fields, mainly due to higher signal-to-noise ratio and to longer T1 relaxation time of static brain tissues; however, specific absorption rate (SAR) significantly increases with B0. Thus, additional Radiofrequency Pulses commonly used at lower field strengths to improve TOF contrast such as saturation of venous signal and improved background suppression by magnetization transfer typically cannot be used at higher fields. In this work, we aimed at reducing SAR for each Radiofrequency Pulse category in a TOF sequence. We use the variable-rate selective excitation principle for the slab selective TOF excitation as well as the venous saturation Radiofrequency Pulses. In addition, magnetization transfer Pulses are implemented by sparsely applying the Pulses only during acquisition of the central k-space lines to limit their SAR contribution. Image quality, angiographic contrast, and SAR reduction were investigated as a function of variable-rate selective excitation parameters and of the total number of magnetization transfer Pulses applied. Based on these results, a TOF protocol was generated that increases the angiographic contrast by more than 50% and reduces subcutaneous fat signal while keeping the resulting SAR within regulatory limits. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.

Marina Carravetta - One of the best experts on this subject based on the ideXlab platform.

  • truncated dipolar recoupling in solid state nuclear magnetic resonance
    Chemical Physics Letters, 2006
    Co-Authors: Ildefonso Marinmontesinos, Giulia Mollica, Marina Carravetta, Axel Gansmuller, Giuseppe Pileio, Matthias Bechmann, Angelika Sebald, Malcolm H Levitt
    Abstract:

    We describe a solid-state NMR concept for the estimation of individual spin–spin couplings in strongly-coupled homonuclear spin systems. A Radiofrequency Pulse sequence, synchronised with the magic-angle sample rotation recouples zero-quantum dipolar interactions as well as a frequency-dispersing interaction such as the chemical shift anisotropy. The combination of these two recoupled interactions causes the spin system to behave in an approximately weakly-coupled fashion. Individual spin–spin couplings may then be disentangled by using frequency-selective Radiofrequency Pulses. Theoretical results and numerical simulations are compared with experimental data for the 13C nuclei in [2H7,13C3,15N]-l-alanine.

  • symmetry principles for the design of Radiofrequency Pulse sequences in the nuclear magnetic resonance of rotating solids
    Chemical Physics Letters, 2000
    Co-Authors: Marina Carravetta, Andreas Brinkmann, Mattias Eden, Xin Zhao, Malcolm H Levitt
    Abstract:

    Some new symmetry theorems are presented which simplify the task of designing multiple-Pulse radio-frequency Pulse sequences in magic-angle-spinning solid-state NMR. The symmetry theorems apply to sequences denoted RN n , which n consists of N repetitions of a Pulse sequence element R, alternating in phase between the values "pnrN. Each R element ideally rotates the spins by an angle p about the rotating frame x-axis. The entire R N n sequence is timed to span n n rotational periods. Applications are presented for homonuclear double-quantum and zero-quantum recoupling, heteronuclear decoupling and heteronuclear recoupling. q 2000 Elsevier Science B.V. All rights reserved.

Sebastian Schmitter - One of the best experts on this subject based on the ideXlab platform.

  • direct control of the temperature rise in parallel transmission by means of temperature virtual observation points simulations at 10 5 tesla
    Magnetic Resonance in Medicine, 2016
    Co-Authors: Nicolas Boulant, Sebastian Schmitter, Kâmil Ugurbil, Xiaoping Wu, Gregor Adriany, Pierrefrancois Van De Moortele
    Abstract:

    Purpose A method using parallel transmission to mitigate B1+ inhomogeneity while explicitly constraining the temperature rise is reported and compared with a more traditional SAR-constrained Pulse design. Methods Finite difference time domain simulations are performed on a numerical human head model and for a 16-channel coil at 10.5 Tesla. Based on a set of presimulations, a virtual observation point compression model for the temperature rise is derived. This compact representation is then used in a nonlinear programming algorithm for Pulse design under explicit temperature rise constraints. Results In the example of a time-of-flight sequence, Radiofrequency Pulse performance in some cases is increased by a factor of two compared with SAR-constrained Pulses, while temperature rise is directly and efficiently controlled. Pulse performance can be gained by relaxing the SAR constraints, but at the expense of a loss of direct control on temperature. Conclusion Given the importance of accurate safety control at ultrahigh field and the lack of direct correspondence between SAR and temperature, this work motivates the need for thorough thermal studies in normal in vivo conditions. The tools presented here will possibly contribute to safer and more efficient MR exams. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.

  • seven tesla time of flight angiography using a 16 channel parallel transmit system with power constrained 3 dimensional spoke Radiofrequency Pulse design
    Investigative Radiology, 2014
    Co-Authors: Sebastian Schmitter, Edward J Auerbach, Kâmil Ugurbil, Gregor Adriany, Josef Pfeuffer, Michael Hamm, Pierrefrancois Van De Moortele
    Abstract:

    OBJECTIVES Ultrahigh magnetic fields of 7 T or higher have proven to significantly enhance the contrast in time-of-flight (TOF) imaging, one of the most commonly used non-contrast-enhanced magnetic resonance angiography techniques. Compared with lower field strength, however, the required Radiofrequency (RF) power is increased at 7 T and the contrast obtained with a conventional head transmit RF coil is typically spatially heterogeneous.In this work, we addressed the contrast heterogeneity in multislab TOF acquisitions by optimizing the excitation flip angle homogeneity while constraining the RF power using 3-dimensional tailored RF Pulses ("spokes") with a 16-channel parallel transmission system and a 16-channel transceiver head coil. MATERIALS AND METHODS We investigated in simulations and in vivo experiments flip angle homogeneity and angiogram quality with a same 3-slab TOF protocol for different excitations including 1-, 2-, and 3-spoke parallel transmit RF Pulses and compared the results with a circularly polarized (CP) phase setting similar to a birdcage excitation. B1 and B0 calibration maps were obtained in multiple slices, and the RF Pulse for each slab was designed on the basis of 3 calibration slices located at the bottom/middle/top of each slab, respectively. By design, all excitations were computed to generate the same total RF power for the same flip angle. In 8 subjects, we quantified the excitation homogeneity and the distribution of the RF power to individual channels. In addition, we investigated the consequences of local flip angle variations at the junction between adjacent slabs as well as the impact of ΔB0 on image quality. RESULTS The flip angle heterogeneity, expressed as the coefficient of variation, averaged over all volunteers and all slabs could be reduced from 29.4% for CP mode excitation to 14.1% for a 1-spoke excitation and to 7.3% for 2-spoke excitations. A separate detailed analysis shows only a marginal improvement for 3-spoke compared with the 2-spoke excitation. The strong improvement in flip angle homogeneity particularly impacted the junction between adjacent TOF slabs, where significant residual artifacts observed with 1-spoke excitation could be efficiently mitigated using a 2-spoke excitation with same RF power and same average flip angle. Although the total RF power is maintained at the same level than that in CP mode excitation, the energy distribution is fairly heterogeneous through the 16 transmit channels for 1- and 2-spoke excitations, with the highest energy for 1 channel being a factor of 2.4 (1 spoke) and 2.2 (2 spokes) higher than that in CP mode. In vivo experiments demonstrated the necessity for including ΔB0 spatial variations during 2-spoke RF Pulse design, particularly in areas with strong local susceptibility variations such as the lower frontal lobe. CONCLUSIONS Significant improvement in excitation fidelity leading to improved TOF contrast, particularly in the brain periphery, as well as smooth slab transitions can be achieved with 2-spoke excitation while maintaining the same excitation energy as that in CP mode. These results suggest that expanding parallel transmit methods, including the use of multidimensional spatially selective excitation, will also be very beneficial for other techniques, such as perfusion imaging.

  • cardiac imaging at 7 tesla single and two spoke Radiofrequency Pulse design with 16 channel parallel excitation
    Magnetic Resonance in Medicine, 2013
    Co-Authors: Sebastian Schmitter, Lance Delabarre, Andreas Greiser, Dingxin Wang, Edward J Auerbach, Thomas J Vaughan, Kâmil Ugurbil, Pierrefrancois Van De Moortele
    Abstract:

    Purpose Higher signal to noise ratio (SNR) and improved contrast have been demonstrated at ultra-high magnetic fields (≥7 Tesla [T]) in multiple targets, often with multi-channel transmit methods to address the deleterious impact on tissue contrast due to spatial variations in B1+ profiles. When imaging the heart at 7T, however, respiratory and cardiac motion, as well as B0 inhomogeneity, greatly increase the methodological challenge. In this study we compare two-spoke parallel transmit (pTX) RF Pulses with static B1+ shimming in cardiac imaging at 7T. Methods Using a 16-channel pTX system, slice-selective two-spoke pTX Pulses and static B1+ shimming were applied in cardiac CINE imaging. B1+ and B0 mapping required modified cardiac triggered sequences. Excitation homogeneity and RF energy were compared in different imaging orientations. Results Two-spoke Pulses provide higher excitation homogeneity than B1+ shimming, especially in the more challenging posterior region of the heart. The peak value of channel-wise RF energy was reduced, allowing for a higher flip angle, hence increased tissue contrast. Image quality with two-spoke excitation proved to be stable throughout the entire cardiac cycle. Conclusion Two-spoke pTX excitation has been successfully demonstrated in the human heart at 7T, with improved image quality and reduced RF Pulse energy when compared with B1+ shimming. Magn Reson Med 70:1210–1219, 2013. © 2013 Wiley Periodicals, Inc.

  • contrast enhancement in tof cerebral angiography at 7 t using saturation and mt Pulses under sar constraints impact of verse and sparse Pulses
    Magnetic Resonance in Medicine, 2012
    Co-Authors: Sebastian Schmitter, Edward J Auerbach, Kâmil Ugurbil, Michael Bock, Soren Johst, Pierrefrancois Van De Moortele
    Abstract:

    Cerebral three-dimensional time of flight (TOF) angiography significantly benefits from ultrahigh fields, mainly due to higher signal-to-noise ratio and to longer T1 relaxation time of static brain tissues; however, specific absorption rate (SAR) significantly increases with B0. Thus, additional Radiofrequency Pulses commonly used at lower field strengths to improve TOF contrast such as saturation of venous signal and improved background suppression by magnetization transfer typically cannot be used at higher fields. In this work, we aimed at reducing SAR for each Radiofrequency Pulse category in a TOF sequence. We use the variable-rate selective excitation principle for the slab selective TOF excitation as well as the venous saturation Radiofrequency Pulses. In addition, magnetization transfer Pulses are implemented by sparsely applying the Pulses only during acquisition of the central k-space lines to limit their SAR contribution. Image quality, angiographic contrast, and SAR reduction were investigated as a function of variable-rate selective excitation parameters and of the total number of magnetization transfer Pulses applied. Based on these results, a TOF protocol was generated that increases the angiographic contrast by more than 50% and reduces subcutaneous fat signal while keeping the resulting SAR within regulatory limits. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.

Andrew V Stenger - One of the best experts on this subject based on the ideXlab platform.

  • spectral decomposition of susceptibility artifacts for spectral spatial Radiofrequency Pulse design
    Magnetic Resonance in Medicine, 2012
    Co-Authors: Cungeng Yang, Benedikt A Poser, Weiran Deng, Andrew V Stenger
    Abstract:

    Susceptibility induced signal loss is a limitation in gradient echo functional MRI. The through-plane artifact in axial slices is particularly problematic due to the inferior position of air cavities in the brain. Spectral-spatial Radiofrequency Pulses have recently been shown to reduce signal loss in a single excitation. The Pulses were successfully demonstrated assuming a linear relationship between susceptibility gradient and frequency, however, the exact frequency and spatial distribution of the susceptibility gradient in the brain is unknown. We present a spiral spectroscopic imaging sequence with a time-shifted Radiofrequency Pulse that can spectrally decompose the through-plane susceptibility gradient for spectral-spatial Radiofrequency Pulse design. Maps of the through-plane susceptibility gradient as a function of frequency were generated for the human brain at 3T. We found that the linear relationship holds well for the whole brain with an optimal slope of -1.0 μT/m/Hz.

  • accelerated multidimensional Radiofrequency Pulse design for parallel transmission using concurrent computation on multiple graphics processing units
    Magnetic Resonance in Medicine, 2011
    Co-Authors: Weiran Deng, Cungeng Yang, Andrew V Stenger
    Abstract:

    Multidimensional Radiofrequency (RF) Pulses are of current interest because of their promise for improving high-field imaging and for optimizing parallel transmission methods. One major drawback is that the computation time of numerically designed multidimensional RF Pulses increases rapidly with their resolution and number of transmitters. This is critical because the construction of multidimensional RF Pulses often needs to be in real time. The use of graphics processing units for computations is a recent approach for accelerating image reconstruction applications. We propose the use of graphics processing units for the design of multidimensional RF Pulses including the utilization of parallel transmitters. Using a desktop computer with four NVIDIA Tesla C1060 computing processors, we found acceleration factors on the order of 20 for standard eight-transmitter two-dimensional spiral RF Pulses with a 64 × 64 excitation resolution and a 10-μsec dwell time. We also show that even greater acceleration factors can be achieved for more complex RF Pulses.

  • fast kz three dimensional tailored Radiofrequency Pulse for reduced b1 inhomogeneity
    Magnetic Resonance in Medicine, 2006
    Co-Authors: Suwit Saekho, Chun Yu Yip, Douglas C Noll, Fernando E Boada, Andrew V Stenger
    Abstract:

    This article presents a small-flip-angle, three-dimensional tailored RF Pulse that excites thin slices with an adjustable quadratic in-plane spatial variation. The quadratic spatial variation helps to compensate for the loss in image uniformity using a volume coil at 3 T due to the wavelike properties of the RF field. The Pulse is based on a novel "fast-kz" design that uses a series of slice-select subPulses along kz and phase encoding "blips" along kx-ky. The method is demonstrated by acquiring a series of 5-mm-thick T2-weighted images of the human brain at 3 T using Pulses 4.8 ms in length with a 45 degrees flip angle.