2013
Alon, Leeor; Deniz, Cem M; Brown, Ryan; Sodickson, Daniel K; Zhu, Yudong
Method for in situ characterization of radiofrequency heating in parallel transmit MRI Journal Article
In: Magnetic Resonance in Medicine, vol. 69, no. 5, pp. 1457–1465, 2013, ISSN: 07403194.
@article{Alon2013a,
title = {Method for in situ characterization of radiofrequency heating in parallel transmit MRI},
author = {Leeor Alon and Cem M Deniz and Ryan Brown and Daniel K Sodickson and Yudong Zhu},
url = {http://doi.wiley.com/10.1002/mrm.24374},
doi = {10.1002/mrm.24374},
issn = {07403194},
year = {2013},
date = {2013-05-01},
journal = {Magnetic Resonance in Medicine},
volume = {69},
number = {5},
pages = {1457--1465},
abstract = {In ultra-high-field magnetic resonance imaging, parallel radiofrequency (RF) transmission presents both opportunities and challenges for specific absorption rate management. On one hand, parallel transmission provides flexibility in tailoring electric fields in the body while facilitating magnetization profile control. On the other hand, it increases the complexity of energy deposition as well as possibly exacerbating local specific absorption rate by improper design or delivery of RF pulses. This study shows that the information needed to characterize RF heating in parallel transmission is contained within a local power correlation matrix. Building upon a calibration scheme involving a finite number of magnetic resonance thermometry measurements, this work establishes a way of estimating the local power correlation matrix. Determination of this matrix allows prediction of temperature change for an arbitrary parallel transmit RF pulse. In the case of a three transmit coil MR experiment in a phantom, determination and validation of the power correlation matrix were conducted in less than 200 min with induced temperature changes of textless4 degrees C. Further optimization and adaptation are possible, and simulations evaluating potential feasibility for in vivo use are presented. The method allows general characteristics indicative of RF coil/pulse safety determined in situ.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In ultra-high-field magnetic resonance imaging, parallel radiofrequency (RF) transmission presents both opportunities and challenges for specific absorption rate management. On one hand, parallel transmission provides flexibility in tailoring electric fields in the body while facilitating magnetization profile control. On the other hand, it increases the complexity of energy deposition as well as possibly exacerbating local specific absorption rate by improper design or delivery of RF pulses. This study shows that the information needed to characterize RF heating in parallel transmission is contained within a local power correlation matrix. Building upon a calibration scheme involving a finite number of magnetic resonance thermometry measurements, this work establishes a way of estimating the local power correlation matrix. Determination of this matrix allows prediction of temperature change for an arbitrary parallel transmit RF pulse. In the case of a three transmit coil MR experiment in a phantom, determination and validation of the power correlation matrix were conducted in less than 200 min with induced temperature changes of textless4 degrees C. Further optimization and adaptation are possible, and simulations evaluating potential feasibility for in vivo use are presented. The method allows general characteristics indicative of RF coil/pulse safety determined in situ.
Deniz, Cem M; Brown, Ryan; Lattanzi, Riccardo; Alon, Leeor; Sodickson, Daniel K; Zhu, Yudong
Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla Journal Article
In: Magnetic Resonance in Medicine, vol. 69, no. 5, pp. 1379–1388, 2013, ISSN: 07403194.
@article{Deniz2013,
title = {Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla},
author = {Cem M Deniz and Ryan Brown and Riccardo Lattanzi and Leeor Alon and Daniel K Sodickson and Yudong Zhu},
url = {http://doi.wiley.com/10.1002/mrm.24377},
doi = {10.1002/mrm.24377},
issn = {07403194},
year = {2013},
date = {2013-05-01},
journal = {Magnetic Resonance in Medicine},
volume = {69},
number = {5},
pages = {1379--1388},
abstract = {Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high-frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power-demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal-to-noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high-frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power-demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal-to-noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging.
2012
Zhu, Yudong; Alon, Leeor; Deniz, Cem M; Brown, Ryan; Sodickson, Daniel K
System and SAR characterization in parallel RF transmission Journal Article
In: Magnetic Resonance in Medicine, vol. 67, no. 5, pp. 1367–1378, 2012, ISSN: 07403194.
@article{Zhu2012,
title = {System and SAR characterization in parallel RF transmission},
author = {Yudong Zhu and Leeor Alon and Cem M Deniz and Ryan Brown and Daniel K Sodickson},
url = {http://doi.wiley.com/10.1002/mrm.23126},
doi = {10.1002/mrm.23126},
issn = {07403194},
year = {2012},
date = {2012-05-01},
journal = {Magnetic Resonance in Medicine},
volume = {67},
number = {5},
pages = {1367--1378},
abstract = {The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The markedly increased degrees of freedom introduced by parallel radiofrequency transmission presents both opportunities and challenges for specific absorption rate (SAR) management. On one hand they enable E-field tailoring and SAR reduction while facilitating excitation profile control. On other hand they increase the complexity of SAR behavior and the risk of inadvertently exacerbating SAR by improper design or playout of radiofrequency pulses. The substantial subject-dependency of SAR in high field magnetic resonance can be a compounding factor. Building upon a linear system concept and a calibration scheme involving a finite number of in situ measurements, this work establishes a clinically applicable method for characterizing global SAR behavior as well as channel-by-channel power transmission. The method offers a unique capability of predicting, for any excitation, the SAR and power consequences that are specific to the subject to be scanned and the MRI hardware. The method was validated in simulation and experimental studies, showing promise as the foundation to a prospective paradigm where power and SAR are not only monitored but, through prediction-guided optimization, proactively managed.
Deniz, Cem M; Alon, Leeor; Brown, Ryan; Sodickson, Daniel K; Zhu, Yudong
Specific absorption rate benefits of including measured electric field interactions in parallel excitation pulse design Journal Article
In: Magnetic Resonance in Medicine, vol. 67, no. 1, pp. 164–174, 2012, ISSN: 07403194.
@article{Deniz2012,
title = {Specific absorption rate benefits of including measured electric field interactions in parallel excitation pulse design},
author = {Cem M Deniz and Leeor Alon and Ryan Brown and Daniel K Sodickson and Yudong Zhu},
url = {http://doi.wiley.com/10.1002/mrm.23004},
doi = {10.1002/mrm.23004},
issn = {07403194},
year = {2012},
date = {2012-01-01},
journal = {Magnetic Resonance in Medicine},
volume = {67},
number = {1},
pages = {164--174},
abstract = {Specific absorption rate management and excitation fidelity are key aspects of radiofrequency pulse design for parallel transmission at ultra-high magnetic field strength. The design of radiofrequency pulses for multiple channels is often based on the solution of regularized least-squares optimization problems for which a regularization term is typically selected to control the integrated or peak pulse waveform amplitude. Unlike single-channel transmission, the specific absorption rate of parallel transmission is significantly influenced by interferences between the electric fields associated with the individual transmission elements, which a conventional regularization term does not take into account. This work explores the effects upon specific absorption rate of incorporating experimentally measurable electric field interactions into parallel transmission pulse design. Results of numerical simulations and phantom experiments show that the global specific absorption rate during parallel transmission decreases when electric field interactions are incorporated into pulse design optimization. The results also show that knowledge of electric field interactions enables robust prediction of the net power delivered to the sample or subject by parallel radiofrequency pulses before they are played out on a scanner.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Specific absorption rate management and excitation fidelity are key aspects of radiofrequency pulse design for parallel transmission at ultra-high magnetic field strength. The design of radiofrequency pulses for multiple channels is often based on the solution of regularized least-squares optimization problems for which a regularization term is typically selected to control the integrated or peak pulse waveform amplitude. Unlike single-channel transmission, the specific absorption rate of parallel transmission is significantly influenced by interferences between the electric fields associated with the individual transmission elements, which a conventional regularization term does not take into account. This work explores the effects upon specific absorption rate of incorporating experimentally measurable electric field interactions into parallel transmission pulse design. Results of numerical simulations and phantom experiments show that the global specific absorption rate during parallel transmission decreases when electric field interactions are incorporated into pulse design optimization. The results also show that knowledge of electric field interactions enables robust prediction of the net power delivered to the sample or subject by parallel radiofrequency pulses before they are played out on a scanner.