The Experts below are selected from a list of 52968 Experts worldwide ranked by ideXlab platform
Lawrence L Wald - One of the best experts on this subject based on the ideXlab platform.
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Parallel Transmission to reduce absorbed power around deep brain stimulation devices in mri impact of number and arrangement of transmit channels
Magnetic Resonance in Medicine, 2020Co-Authors: Bastien Guerin, Leonardo M Angelone, Darin D Dougherty, Lawrence L WaldAbstract:PURPOSE To assess the mean and variance performance of Parallel Transmission (pTx) coils for reduction of the absorbed power around electrodes (APAE) in patients implanted with deep brain stimulation (DBS) devices. METHODS We simulated 4 pTx coils (8 and 16 channels, head and body coils) and a birdcage body coil. We characterized the RF safety risk using the APAE, which is the integral of the deposited power (in Watts) in a small cylindrical volume of brain tissue surrounding the electrode tips. We assessed the APAE mean and variance by simulation of 5 realistic DBS patient models that include the full DBS implant length, extracranial loops, and implanted pulse generator. RESULTS PTx coils with 8 (16) channels were able to reduce the APAE by >18× (>169×) compared to the birdcage coil in average for all patient models, at no cost in term of flip angle uniformity or global specific absorption rate (SAR). Moreover, local pTx coils performed significantly better than body arrays. CONCLUSION PTx is a possible solution to the problem of RF heating of DBS patients when performing MRI, but the large interpatient variability of the APAE indicates that patient-specific safety monitoring may be needed.
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robust time shifted spoke pulse design in the presence of large b0 variations with simultaneous reduction of through plane dephasing b1 effects and the specific absorption rate using Parallel Transmission
Magnetic Resonance in Medicine, 2016Co-Authors: Bastien Guerin, Andrew V Stenger, Lawrence L Wald, Angel Torradocarvajal, Jason P Stockmann, Mehran BaboliAbstract:Purpose To design Parallel Transmission spokes pulses with time-shifted profiles for joint mitigation of intensity variations due to B1+ effects, signal loss due to through-plane dephasing, and the specific absorption rate (SAR) at 7T. Methods We derived a slice-averaged small tip angle (SA-STA) approximation of the magnetization signal at echo time that depends on the B1+ transmit profiles, the through-slice B0 gradient and the amplitude and time-shifts of the spoke waveforms. We minimize a magnitude least-squares objective based on this signal equation using a fast interior-point approach with analytical expressions of the Jacobian and Hessian. Results Our algorithm runs in less than three minutes for the design of two-spoke pulses subject to hundreds of local SAR constraints. On a B0/B1+ head phantom, joint optimization of the channel-dependent time-shifts and spoke amplitudes allowed signal recovery in high-B0 regions at no increase of SAR. Although the method creates uniform magnetization profiles (ie, uniform intensity), the flip angle varies across the image, which makes it ill-suited to T1-weighted applications. Conclusions The SA-STA approach presented in this study is best suited to T2*-weighted applications with long echo times that require signal recovery around high B0 regions. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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general design approach and practical realization of decoupling matrices for Parallel Transmission coils
Magnetic Resonance in Medicine, 2016Co-Authors: Zohaib Mahmood, Elfar Adalsteinsson, Lawrence L Wald, Bastien Guerin, Patrick Mcdaniel, Boris Keil, Markus Vester, Luca DanielAbstract:Purpose In a coupled Parallel transmit (pTx) array, the power delivered to a channel is partially distributed to other channels because of coupling. This power is dissipated in circulators resulting in a significant reduction in power efficiency. In this study, a technique for designing robust decoupling matrices interfaced between the RF amplifiers and the coils is proposed. The decoupling matrices ensure that most forward power is delivered to the load without loss of encoding capabilities of the pTx array. Theory and Methods The decoupling condition requires that the impedance matrix seen by the power amplifiers is a diagonal matrix whose entries match the characteristic impedance of the power amplifiers. In this work, the impedance matrix of the coupled coils is diagonalized by a successive multiplication by its eigenvectors. A general design procedure and software are developed to generate automatically the hardware that implements diagonalization using passive components. Results The general design method is demonstrated by decoupling two example Parallel transmit arrays. Our decoupling matrices achieve better than −20 db decoupling in both cases. Conclusion A robust framework for designing decoupling matrices for pTx arrays is presented and validated. The proposed decoupling strategy theoretically scales to any arbitrary number of channels. Magn Reson Med 76:329–339, 2016. © 2015 Wiley Periodicals, Inc.
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Parallel Transmission pulse design with explicit control for the specific absorption rate in the presence of radiofrequency errors
Magnetic Resonance in Medicine, 2016Co-Authors: Adrian Martin, Elfar Adalsteinsson, Lawrence L Wald, Emanuele Schiavi, Yigitcan Eryaman, J L Herraiz, Borjan Gagoski, Bastien GuerinAbstract:Purpose A new framework for the design of Parallel transmit (pTx) pulses is presented introducing constraints for local and global specific absorption rate (SAR) in the presence of errors in the radiofrequency (RF) transmit chain. Methods The first step is the design of a pTx RF pulse with explicit constraints for global and local SAR. Then, the worst possible SAR associated with that pulse due to RF Transmission errors (“worst-case SAR”) is calculated. Finally, this information is used to re-calculate the pulse with lower SAR constraints, iterating this procedure until its worst-case SAR is within safety limits. Results Analysis of an actual pTx RF transmit chain revealed amplitude errors as high as 8% (20%) and phase errors above 3° (15°) for spokes (spiral) pulses. Simulations show that using the proposed framework, pulses can be designed with controlled “worst-case SAR” in the presence of errors of this magnitude at minor cost of the excitation profile quality. Conclusion Our worst-case SAR-constrained pTx design strategy yields pulses with local and global SAR within the safety limits even in the presence of RF Transmission errors. This strategy is a natural way to incorporate SAR safety factors in the design of pTx pulses. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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design of Parallel Transmission pulses for simultaneous multislice with explicit control for peak power and local specific absorption rate
Magnetic Resonance in Medicine, 2015Co-Authors: Bastien Guerin, Andrew V Stenger, Lawrence L Wald, Kawin Setsompop, Benedikt A PoserAbstract:Purpose To design Parallel transmit (pTx) simultaneous multislice (SMS) spokes pulses with explicit control for peak power and local and global specific absorption rate (SAR). Methods We design SMS pTx least-squares and magnitude least squares spokes pulses while constraining local SAR using the virtual observation points (VOPs) compression of SAR matrices. We evaluate our approach in simulations of a head (7T) and a body (3T) coil with eight channels arranged in two z-rows. Results For many of our simulations, control of average power by Tikhonov regularization of the SMS pTx spokes pulse design yielded pulses that violated hardware and SAR safety limits. On the other hand, control of peak power alone yielded pulses that violated local SAR limits. Pulses optimized with control of both local SAR and peak power satisfied all constraints and therefore had the best excitation performance under limited power and SAR constraints. These results extend our previous results for single slice pTx excitations but are more pronounced because of the large power demands and SAR of SMS pulses. Conclusions Explicit control of local SAR and peak power is required to generate optimal SMS pTx excitations satisfying both the system's hardware limits and regulatory safety limits. Magn Reson Med 73:1946–1953, 2015. © 2014 Wiley Periodicals, Inc.
Nicolas Boulant - One of the best experts on this subject based on the ideXlab platform.
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optimizing bold sensitivity in the 7t human connectome project resting state fmri protocol using plug and play Parallel Transmission
NeuroImage, 2019Co-Authors: Vincent Gras, Benedikt A Poser, Raphael Tomitricot, Nicolas BoulantAbstract:The Human Connectome Project (HCP) has a 7T component that aims to study the human brain's organization and function with high spatial and temporal resolution fMRI and diffusion-weighted acquisitions. For whole brain applications at 7T, a major weakness however remains the heterogeneity of the radiofrequency Transmission field (B1+ ), which prevents from achieving an optimal signal and contrast homogeneously throughout the brain. In this work, we use Parallel Transmission (pTX) Universal Pulses (UP) to improve the flip angle homogeneity and demonstrate their application to highly accelerated multi-band EPI (MB5 and GRAPPA2, as prescribed in the 7T HCP protocol) sequence, but also to acquire at 7T B1+ -artefact-free T1 - and T2 -weighted anatomical scans used in the pre-processing pipeline of the HCP protocol. As compared to typical implementations of pTX, the proposed solution is fully operator-independent and allows "plug and play" exploitation of the benefits offered by multi-channel Transmission. Validation in five healthy adults shows that the proposed technique achieves a flip angle homogeneity comparable to that of a clinical 3 T system. Compared to standard single-channel Transmission, the use of UPs at 7T yielded up to a two-fold increase of the temporal signal-to-noise ratio in the temporal lobes as well as improved detection of functional connectivity in the brain regions most strongly affected by B1+ inhomogeneity.
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signal domain optimization metrics for mprage rf pulse design in Parallel Transmission at 7 tesla
Magnetic Resonance in Medicine, 2016Co-Authors: Vincent Gras, Alexis Amadon, Alexandre Vignaud, Franck Mauconduit, Michel Luong, Le D Bihan, Nicolas BoulantAbstract:Purpose Standard radiofrequency pulse design strategies focus on minimizing the deviation of the flip angle from a target value, which is sufficient but not necessary for signal homogeneity. An alternative approach, based directly on the signal, here is proposed for the MPRAGE sequence, and is developed in the Parallel Transmission framework with the use of the kT-points parametrization. Methods The flip angle-homogenizing and the proposed methods were investigated numerically under explicit power and specific absorption rate constraints and tested experimentally in vivo on a 7 T Parallel Transmission system enabling real time local specific absorption rate monitoring. Radiofrequency pulse performance was assessed by a careful analysis of the signal and contrast between white and gray matter. Results Despite a slight reduction of the flip angle uniformity, an improved signal and contrast homogeneity with a significant reduction of the specific absorption rate was achieved with the proposed metric in comparison with standard pulse designs. Conclusion The proposed joint optimization of the inversion and excitation pulses enables significant reduction of the specific absorption rate in the MPRAGE sequence while preserving image quality. The work reported thus unveils a possible direction to increase the potential of ultra-high field MRI and Parallel Transmission. Magn Reson Med 76:1431–1442, 2016. © 2015 International Society for Magnetic Resonance in Medicine
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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, 2016Co-Authors: Nicolas Boulant, Kamil Ugurbil, Xiaoping Wu, Gregor Adriany, Sebastian Schmitter, Pierrefrancois Van De MoorteleAbstract: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.
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radiofrequency pulse design in Parallel Transmission under strict temperature constraints
Magnetic Resonance in Medicine, 2014Co-Authors: Nicolas Boulant, Aurelien Massire, Alexis Amadon, Alexandre VignaudAbstract:Purpose To gain radiofrequency (RF) pulse performance by directly addressing the temperature constraints, as opposed to the specific absorption rate (SAR) constraints, in Parallel Transmission at ultra-high field. Methods The magnitude least-squares RF pulse design problem under hard SAR constraints was solved repeatedly by using the virtual observation points and an active-set algorithm. The SAR constraints were updated at each iteration based on the result of a thermal simulation. The numerical study was performed for an SAR-demanding and simplified time of flight sequence using B1 and ΔB0 maps obtained in vivo on a human brain at 7T. Results The proposed adjustment of the SAR constraints combined with an active-set algorithm provided higher flexibility in RF pulse design within a reasonable time. The modifications of those constraints acted directly upon the thermal response as desired. Conclusion Although further confidence in the thermal models is needed, this study shows that RF pulse design under strict temperature constraints is within reach, allowing better RF pulse performance and faster acquisitions at ultra-high fields at the cost of higher sequence complexity. Magn Reson Med 72:679–688, 2014. © 2013 Wiley Periodicals, Inc.
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on variant strategies to solve the magnitude least squares optimization problem in Parallel Transmission pulse design and under strict sar and power constraints
IEEE Transactions on Medical Imaging, 2014Co-Authors: Andres Hoyosidrobo, Alexis Amadon, Aurelien Massire, Pierre Weiss, Nicolas BoulantAbstract:Parallel Transmission is a very promising candidate technology to mitigate the inevitable radio-frequency (RF) field inhomogeneity in magnetic resonance imaging at ultra-high field. For the first few years, pulse design utilizing this technique was expressed as a least squares problem with crude power regularizations aimed at controlling the specific absorption rate (SAR), hence the patient safety. This approach being suboptimal for many applications sensitive mostly to the magnitude of the spin excitation, and not its phase, the magnitude least squares (MLS) problem then was first formulated in 2007. Despite its importance and the availability of other powerful numerical optimization methods, the MLS problem yet has been faced almost exclusively by the pulse designer with the so-called variable exchange method. In this paper, we investigate various two-stage strategies consisting of different initializations and nonlinear programming approaches, and incorporate directly the strict SAR and hardware constraints. Several schemes such as sequential quadratic programming, interior point methods, semidefinite programming and magnitude squared least squares relaxations are studied both in the small and large tip angle regimes with RF and static field maps obtained in vivo on a human brain at 7T. Convergence and robustness of the different approaches are analyzed, and recommendations to tackle this specific problem are finally given. Small tip angle and inversion pulses are returned in a few seconds and in under a minute respectively while respecting the constraints, allowing the use of the proposed approach in routine.
Bastien Guerin - One of the best experts on this subject based on the ideXlab platform.
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Parallel Transmission to reduce absorbed power around deep brain stimulation devices in mri impact of number and arrangement of transmit channels
Magnetic Resonance in Medicine, 2020Co-Authors: Bastien Guerin, Leonardo M Angelone, Darin D Dougherty, Lawrence L WaldAbstract:PURPOSE To assess the mean and variance performance of Parallel Transmission (pTx) coils for reduction of the absorbed power around electrodes (APAE) in patients implanted with deep brain stimulation (DBS) devices. METHODS We simulated 4 pTx coils (8 and 16 channels, head and body coils) and a birdcage body coil. We characterized the RF safety risk using the APAE, which is the integral of the deposited power (in Watts) in a small cylindrical volume of brain tissue surrounding the electrode tips. We assessed the APAE mean and variance by simulation of 5 realistic DBS patient models that include the full DBS implant length, extracranial loops, and implanted pulse generator. RESULTS PTx coils with 8 (16) channels were able to reduce the APAE by >18× (>169×) compared to the birdcage coil in average for all patient models, at no cost in term of flip angle uniformity or global specific absorption rate (SAR). Moreover, local pTx coils performed significantly better than body arrays. CONCLUSION PTx is a possible solution to the problem of RF heating of DBS patients when performing MRI, but the large interpatient variability of the APAE indicates that patient-specific safety monitoring may be needed.
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robust time shifted spoke pulse design in the presence of large b0 variations with simultaneous reduction of through plane dephasing b1 effects and the specific absorption rate using Parallel Transmission
Magnetic Resonance in Medicine, 2016Co-Authors: Bastien Guerin, Andrew V Stenger, Lawrence L Wald, Angel Torradocarvajal, Jason P Stockmann, Mehran BaboliAbstract:Purpose To design Parallel Transmission spokes pulses with time-shifted profiles for joint mitigation of intensity variations due to B1+ effects, signal loss due to through-plane dephasing, and the specific absorption rate (SAR) at 7T. Methods We derived a slice-averaged small tip angle (SA-STA) approximation of the magnetization signal at echo time that depends on the B1+ transmit profiles, the through-slice B0 gradient and the amplitude and time-shifts of the spoke waveforms. We minimize a magnitude least-squares objective based on this signal equation using a fast interior-point approach with analytical expressions of the Jacobian and Hessian. Results Our algorithm runs in less than three minutes for the design of two-spoke pulses subject to hundreds of local SAR constraints. On a B0/B1+ head phantom, joint optimization of the channel-dependent time-shifts and spoke amplitudes allowed signal recovery in high-B0 regions at no increase of SAR. Although the method creates uniform magnetization profiles (ie, uniform intensity), the flip angle varies across the image, which makes it ill-suited to T1-weighted applications. Conclusions The SA-STA approach presented in this study is best suited to T2*-weighted applications with long echo times that require signal recovery around high B0 regions. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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general design approach and practical realization of decoupling matrices for Parallel Transmission coils
Magnetic Resonance in Medicine, 2016Co-Authors: Zohaib Mahmood, Elfar Adalsteinsson, Lawrence L Wald, Bastien Guerin, Patrick Mcdaniel, Boris Keil, Markus Vester, Luca DanielAbstract:Purpose In a coupled Parallel transmit (pTx) array, the power delivered to a channel is partially distributed to other channels because of coupling. This power is dissipated in circulators resulting in a significant reduction in power efficiency. In this study, a technique for designing robust decoupling matrices interfaced between the RF amplifiers and the coils is proposed. The decoupling matrices ensure that most forward power is delivered to the load without loss of encoding capabilities of the pTx array. Theory and Methods The decoupling condition requires that the impedance matrix seen by the power amplifiers is a diagonal matrix whose entries match the characteristic impedance of the power amplifiers. In this work, the impedance matrix of the coupled coils is diagonalized by a successive multiplication by its eigenvectors. A general design procedure and software are developed to generate automatically the hardware that implements diagonalization using passive components. Results The general design method is demonstrated by decoupling two example Parallel transmit arrays. Our decoupling matrices achieve better than −20 db decoupling in both cases. Conclusion A robust framework for designing decoupling matrices for pTx arrays is presented and validated. The proposed decoupling strategy theoretically scales to any arbitrary number of channels. Magn Reson Med 76:329–339, 2016. © 2015 Wiley Periodicals, Inc.
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Parallel Transmission pulse design with explicit control for the specific absorption rate in the presence of radiofrequency errors
Magnetic Resonance in Medicine, 2016Co-Authors: Adrian Martin, Elfar Adalsteinsson, Lawrence L Wald, Emanuele Schiavi, Yigitcan Eryaman, J L Herraiz, Borjan Gagoski, Bastien GuerinAbstract:Purpose A new framework for the design of Parallel transmit (pTx) pulses is presented introducing constraints for local and global specific absorption rate (SAR) in the presence of errors in the radiofrequency (RF) transmit chain. Methods The first step is the design of a pTx RF pulse with explicit constraints for global and local SAR. Then, the worst possible SAR associated with that pulse due to RF Transmission errors (“worst-case SAR”) is calculated. Finally, this information is used to re-calculate the pulse with lower SAR constraints, iterating this procedure until its worst-case SAR is within safety limits. Results Analysis of an actual pTx RF transmit chain revealed amplitude errors as high as 8% (20%) and phase errors above 3° (15°) for spokes (spiral) pulses. Simulations show that using the proposed framework, pulses can be designed with controlled “worst-case SAR” in the presence of errors of this magnitude at minor cost of the excitation profile quality. Conclusion Our worst-case SAR-constrained pTx design strategy yields pulses with local and global SAR within the safety limits even in the presence of RF Transmission errors. This strategy is a natural way to incorporate SAR safety factors in the design of pTx pulses. Magn Reson Med, 2015. © 2015 Wiley Periodicals, Inc.
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design of Parallel Transmission pulses for simultaneous multislice with explicit control for peak power and local specific absorption rate
Magnetic Resonance in Medicine, 2015Co-Authors: Bastien Guerin, Andrew V Stenger, Lawrence L Wald, Kawin Setsompop, Benedikt A PoserAbstract:Purpose To design Parallel transmit (pTx) simultaneous multislice (SMS) spokes pulses with explicit control for peak power and local and global specific absorption rate (SAR). Methods We design SMS pTx least-squares and magnitude least squares spokes pulses while constraining local SAR using the virtual observation points (VOPs) compression of SAR matrices. We evaluate our approach in simulations of a head (7T) and a body (3T) coil with eight channels arranged in two z-rows. Results For many of our simulations, control of average power by Tikhonov regularization of the SMS pTx spokes pulse design yielded pulses that violated hardware and SAR safety limits. On the other hand, control of peak power alone yielded pulses that violated local SAR limits. Pulses optimized with control of both local SAR and peak power satisfied all constraints and therefore had the best excitation performance under limited power and SAR constraints. These results extend our previous results for single slice pTx excitations but are more pronounced because of the large power demands and SAR of SMS pulses. Conclusions Explicit control of local SAR and peak power is required to generate optimal SMS pTx excitations satisfying both the system's hardware limits and regulatory safety limits. Magn Reson Med 73:1946–1953, 2015. © 2014 Wiley Periodicals, Inc.
Alexis Amadon - One of the best experts on this subject based on the ideXlab platform.
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stacked magnetic resonators for mri rf coils decoupling
Journal of Magnetic Resonance, 2017Co-Authors: Elodie Georget, Alexis Amadon, Alexandre Vignaud, Franck Mauconduit, Michel Luong, Eric Giacomini, Edouard Chazel, Guillaume Ferrand, Stefan Enoch, Gerard TayebAbstract:Parallel Transmission is a very promising method to tackle B 1 + field inhomogeneities at ultrahigh field in magnetic resonant imaging (MRI). This technique is however limited by the mutual coupling between the radiating elements. Here we propose to solve this problem by designing a passive magneto-electric resonator that we here refer to as stacked magnetic resonator (SMR). By combining numerical and experimental methodologies, we prove that this passive solution allows an efficient decoupling of active elements of a phased-array coil. We demonstrate the ability of this technique to significantly reduce by more than 10 dB the coupling preserving the quality of images compared to ideally isolated linear resonators on a spherical salty agar gel phantom in a 7 T MRI scanner.
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signal domain optimization metrics for mprage rf pulse design in Parallel Transmission at 7 tesla
Magnetic Resonance in Medicine, 2016Co-Authors: Vincent Gras, Alexis Amadon, Alexandre Vignaud, Franck Mauconduit, Michel Luong, Le D Bihan, Nicolas BoulantAbstract:Purpose Standard radiofrequency pulse design strategies focus on minimizing the deviation of the flip angle from a target value, which is sufficient but not necessary for signal homogeneity. An alternative approach, based directly on the signal, here is proposed for the MPRAGE sequence, and is developed in the Parallel Transmission framework with the use of the kT-points parametrization. Methods The flip angle-homogenizing and the proposed methods were investigated numerically under explicit power and specific absorption rate constraints and tested experimentally in vivo on a 7 T Parallel Transmission system enabling real time local specific absorption rate monitoring. Radiofrequency pulse performance was assessed by a careful analysis of the signal and contrast between white and gray matter. Results Despite a slight reduction of the flip angle uniformity, an improved signal and contrast homogeneity with a significant reduction of the specific absorption rate was achieved with the proposed metric in comparison with standard pulse designs. Conclusion The proposed joint optimization of the inversion and excitation pulses enables significant reduction of the specific absorption rate in the MPRAGE sequence while preserving image quality. The work reported thus unveils a possible direction to increase the potential of ultra-high field MRI and Parallel Transmission. Magn Reson Med 76:1431–1442, 2016. © 2015 International Society for Magnetic Resonance in Medicine
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radiofrequency pulse design in Parallel Transmission under strict temperature constraints
Magnetic Resonance in Medicine, 2014Co-Authors: Nicolas Boulant, Aurelien Massire, Alexis Amadon, Alexandre VignaudAbstract:Purpose To gain radiofrequency (RF) pulse performance by directly addressing the temperature constraints, as opposed to the specific absorption rate (SAR) constraints, in Parallel Transmission at ultra-high field. Methods The magnitude least-squares RF pulse design problem under hard SAR constraints was solved repeatedly by using the virtual observation points and an active-set algorithm. The SAR constraints were updated at each iteration based on the result of a thermal simulation. The numerical study was performed for an SAR-demanding and simplified time of flight sequence using B1 and ΔB0 maps obtained in vivo on a human brain at 7T. Results The proposed adjustment of the SAR constraints combined with an active-set algorithm provided higher flexibility in RF pulse design within a reasonable time. The modifications of those constraints acted directly upon the thermal response as desired. Conclusion Although further confidence in the thermal models is needed, this study shows that RF pulse design under strict temperature constraints is within reach, allowing better RF pulse performance and faster acquisitions at ultra-high fields at the cost of higher sequence complexity. Magn Reson Med 72:679–688, 2014. © 2013 Wiley Periodicals, Inc.
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on variant strategies to solve the magnitude least squares optimization problem in Parallel Transmission pulse design and under strict sar and power constraints
IEEE Transactions on Medical Imaging, 2014Co-Authors: Andres Hoyosidrobo, Alexis Amadon, Aurelien Massire, Pierre Weiss, Nicolas BoulantAbstract:Parallel Transmission is a very promising candidate technology to mitigate the inevitable radio-frequency (RF) field inhomogeneity in magnetic resonance imaging at ultra-high field. For the first few years, pulse design utilizing this technique was expressed as a least squares problem with crude power regularizations aimed at controlling the specific absorption rate (SAR), hence the patient safety. This approach being suboptimal for many applications sensitive mostly to the magnitude of the spin excitation, and not its phase, the magnitude least squares (MLS) problem then was first formulated in 2007. Despite its importance and the availability of other powerful numerical optimization methods, the MLS problem yet has been faced almost exclusively by the pulse designer with the so-called variable exchange method. In this paper, we investigate various two-stage strategies consisting of different initializations and nonlinear programming approaches, and incorporate directly the strict SAR and hardware constraints. Several schemes such as sequential quadratic programming, interior point methods, semidefinite programming and magnitude squared least squares relaxations are studied both in the small and large tip angle regimes with RF and static field maps obtained in vivo on a human brain at 7T. Convergence and robustness of the different approaches are analyzed, and recommendations to tackle this specific problem are finally given. Small tip angle and inversion pulses are returned in a few seconds and in under a minute respectively while respecting the constraints, allowing the use of the proposed approach in routine.
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design of non selective refocusing pulses with phase free rotation axis by gradient ascent pulse engineering algorithm in Parallel Transmission at 7 t
Journal of Magnetic Resonance, 2013Co-Authors: Aurelien Massire, Alexis Amadon, Alexandre Vignaud, Martijn A Cloos, Denis Le Bihan, Nicolas BoulantAbstract:Abstract At ultra-high magnetic field (⩾7 T), B1 and ΔB0 non-uniformities cause undesired inhomogeneities in image signal and contrast. Tailored radiofrequency pulses exploiting Parallel Transmission have been shown to mitigate these phenomena. However, the design of large flip angle excitations, a prerequisite for many clinical applications, remains challenging due the non-linearity of the Bloch equation. In this work, we explore the potential of gradient ascent pulse engineering to design non-selective spin-echo refocusing pulses that simultaneously mitigate severe B1 and ΔB0 non-uniformities. The originality of the method lays in the optimization of the rotation matrices themselves as opposed to magnetization states. Consequently, the commonly used linear class of large tip angle approximation can be eliminated from the optimization procedure. This approach, combined with optimal control, provides additional degrees of freedom by relaxing the phase constraint on the rotation axis, and allows the derivative of the performance criterion to be found analytically. The method was experimentally validated on an 8-channel transmit array at 7 T, using a water phantom with B1 and ΔB0 inhomogeneities similar to those encountered in the human brain. For the first time in MRI, the rotation matrix itself on every voxel was measured by using Quantum Process Tomography. The results are complemented with a series of spin-echo measurements comparing the proposed method against commonly used alternatives. Both experiments confirm very good performance, while simultaneously maintaining a low energy deposition and pulse duration compared to well-known adiabatic solutions.
Alexandre Vignaud - One of the best experts on this subject based on the ideXlab platform.
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stacked magnetic resonators for mri rf coils decoupling
Journal of Magnetic Resonance, 2017Co-Authors: Elodie Georget, Alexis Amadon, Alexandre Vignaud, Franck Mauconduit, Michel Luong, Eric Giacomini, Edouard Chazel, Guillaume Ferrand, Stefan Enoch, Gerard TayebAbstract:Parallel Transmission is a very promising method to tackle B 1 + field inhomogeneities at ultrahigh field in magnetic resonant imaging (MRI). This technique is however limited by the mutual coupling between the radiating elements. Here we propose to solve this problem by designing a passive magneto-electric resonator that we here refer to as stacked magnetic resonator (SMR). By combining numerical and experimental methodologies, we prove that this passive solution allows an efficient decoupling of active elements of a phased-array coil. We demonstrate the ability of this technique to significantly reduce by more than 10 dB the coupling preserving the quality of images compared to ideally isolated linear resonators on a spherical salty agar gel phantom in a 7 T MRI scanner.
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signal domain optimization metrics for mprage rf pulse design in Parallel Transmission at 7 tesla
Magnetic Resonance in Medicine, 2016Co-Authors: Vincent Gras, Alexis Amadon, Alexandre Vignaud, Franck Mauconduit, Michel Luong, Le D Bihan, Nicolas BoulantAbstract:Purpose Standard radiofrequency pulse design strategies focus on minimizing the deviation of the flip angle from a target value, which is sufficient but not necessary for signal homogeneity. An alternative approach, based directly on the signal, here is proposed for the MPRAGE sequence, and is developed in the Parallel Transmission framework with the use of the kT-points parametrization. Methods The flip angle-homogenizing and the proposed methods were investigated numerically under explicit power and specific absorption rate constraints and tested experimentally in vivo on a 7 T Parallel Transmission system enabling real time local specific absorption rate monitoring. Radiofrequency pulse performance was assessed by a careful analysis of the signal and contrast between white and gray matter. Results Despite a slight reduction of the flip angle uniformity, an improved signal and contrast homogeneity with a significant reduction of the specific absorption rate was achieved with the proposed metric in comparison with standard pulse designs. Conclusion The proposed joint optimization of the inversion and excitation pulses enables significant reduction of the specific absorption rate in the MPRAGE sequence while preserving image quality. The work reported thus unveils a possible direction to increase the potential of ultra-high field MRI and Parallel Transmission. Magn Reson Med 76:1431–1442, 2016. © 2015 International Society for Magnetic Resonance in Medicine
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radiofrequency pulse design in Parallel Transmission under strict temperature constraints
Magnetic Resonance in Medicine, 2014Co-Authors: Nicolas Boulant, Aurelien Massire, Alexis Amadon, Alexandre VignaudAbstract:Purpose To gain radiofrequency (RF) pulse performance by directly addressing the temperature constraints, as opposed to the specific absorption rate (SAR) constraints, in Parallel Transmission at ultra-high field. Methods The magnitude least-squares RF pulse design problem under hard SAR constraints was solved repeatedly by using the virtual observation points and an active-set algorithm. The SAR constraints were updated at each iteration based on the result of a thermal simulation. The numerical study was performed for an SAR-demanding and simplified time of flight sequence using B1 and ΔB0 maps obtained in vivo on a human brain at 7T. Results The proposed adjustment of the SAR constraints combined with an active-set algorithm provided higher flexibility in RF pulse design within a reasonable time. The modifications of those constraints acted directly upon the thermal response as desired. Conclusion Although further confidence in the thermal models is needed, this study shows that RF pulse design under strict temperature constraints is within reach, allowing better RF pulse performance and faster acquisitions at ultra-high fields at the cost of higher sequence complexity. Magn Reson Med 72:679–688, 2014. © 2013 Wiley Periodicals, Inc.
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design of non selective refocusing pulses with phase free rotation axis by gradient ascent pulse engineering algorithm in Parallel Transmission at 7 t
Journal of Magnetic Resonance, 2013Co-Authors: Aurelien Massire, Alexis Amadon, Alexandre Vignaud, Martijn A Cloos, Denis Le Bihan, Nicolas BoulantAbstract:Abstract At ultra-high magnetic field (⩾7 T), B1 and ΔB0 non-uniformities cause undesired inhomogeneities in image signal and contrast. Tailored radiofrequency pulses exploiting Parallel Transmission have been shown to mitigate these phenomena. However, the design of large flip angle excitations, a prerequisite for many clinical applications, remains challenging due the non-linearity of the Bloch equation. In this work, we explore the potential of gradient ascent pulse engineering to design non-selective spin-echo refocusing pulses that simultaneously mitigate severe B1 and ΔB0 non-uniformities. The originality of the method lays in the optimization of the rotation matrices themselves as opposed to magnetization states. Consequently, the commonly used linear class of large tip angle approximation can be eliminated from the optimization procedure. This approach, combined with optimal control, provides additional degrees of freedom by relaxing the phase constraint on the rotation axis, and allows the derivative of the performance criterion to be found analytically. The method was experimentally validated on an 8-channel transmit array at 7 T, using a water phantom with B1 and ΔB0 inhomogeneities similar to those encountered in the human brain. For the first time in MRI, the rotation matrix itself on every voxel was measured by using Quantum Process Tomography. The results are complemented with a series of spin-echo measurements comparing the proposed method against commonly used alternatives. Both experiments confirm very good performance, while simultaneously maintaining a low energy deposition and pulse duration compared to well-known adiabatic solutions.
