2019
Deniz, Cem M
Parallel Transmission for Ultrahigh Field MRI Journal Article
In: Topics in Magnetic Resonance Imaging, vol. 28, no. 3, pp. 159-171, 2019.
@article{Deniz2019,
title = {Parallel Transmission for Ultrahigh Field MRI},
author = {Cem M Deniz},
doi = {10.1097/RMR.0000000000000204},
year = {2019},
date = {2019-06-01},
journal = {Topics in Magnetic Resonance Imaging},
volume = {28},
number = {3},
pages = {159-171},
abstract = {Magnetic resonance imaging (MRI) has been driven toward ultrahigh magnetic fields (UHF) in order to benefit from correspondingly higher signal-to-noise ratio and spectral resolution. Technological challenges associated with UHF, such as increased radiofrequency (RF) energy deposition and RF excitation inhomogeneity, limit realization of the full potential of these benefits. Parallel RF transmission (pTx) enables decreases in the inhomogeneity of RF excitations and in RF energy deposition by using multiple-transmit RF coils driven independently and operating simultaneously. pTx plays a fundamental role in UHF MRI by bringing the potential applications of UHF into reality. In this review article, we review the recent developments in pTx pulse design and RF safety in pTx. Simultaneous multislice imaging and inner volume imaging using pTx are reviewed with a focus on UHF applications. Emerging pTx design approaches using improved pTx design frameworks and calibrations are reviewed together with calibration-free approaches that remove the necessity of time-consuming calibrations necessary for successful pTx. Lastly, we focus on the safety of pTx that is improved by using intersubject variability analysis, proactively managing pTx and temperature-based pTx approaches.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Magnetic resonance imaging (MRI) has been driven toward ultrahigh magnetic fields (UHF) in order to benefit from correspondingly higher signal-to-noise ratio and spectral resolution. Technological challenges associated with UHF, such as increased radiofrequency (RF) energy deposition and RF excitation inhomogeneity, limit realization of the full potential of these benefits. Parallel RF transmission (pTx) enables decreases in the inhomogeneity of RF excitations and in RF energy deposition by using multiple-transmit RF coils driven independently and operating simultaneously. pTx plays a fundamental role in UHF MRI by bringing the potential applications of UHF into reality. In this review article, we review the recent developments in pTx pulse design and RF safety in pTx. Simultaneous multislice imaging and inner volume imaging using pTx are reviewed with a focus on UHF applications. Emerging pTx design approaches using improved pTx design frameworks and calibrations are reviewed together with calibration-free approaches that remove the necessity of time-consuming calibrations necessary for successful pTx. Lastly, we focus on the safety of pTx that is improved by using intersubject variability analysis, proactively managing pTx and temperature-based pTx approaches.
2018
Deniz, Cem M; Xiang, Siyuan; Hallyburton, Spencer; Welbeck, Arakua; Babb, James S; Honig, Stephen; Cho, Kyunghyun; Chang, Gregory
Segmentation of the Proximal Femur from MR Images using Deep Convolutional Neural Networks Journal Article
In: Scientific Reports, vol. 8, no. 16485, 2018.
@article{Deniz2017b,
title = {Segmentation of the Proximal Femur from MR Images using Deep Convolutional Neural Networks},
author = {Cem M Deniz and Siyuan Xiang and Spencer Hallyburton and Arakua Welbeck and James S Babb and Stephen Honig and Kyunghyun Cho and Gregory Chang},
doi = {10.1038/s41598-018-34817-6},
year = {2018},
date = {2018-11-07},
journal = {Scientific Reports},
volume = {8},
number = {16485},
abstract = {Magnetic resonance imaging (MRI) has been proposed as a complimentary method to measure bone quality and assess fracture risk. However, manual segmentation of MR images of bone is time-consuming, limiting the use of MRI measurements in the clinical practice. The purpose of this paper is to present an automatic proximal femur segmentation method that is based on deep convolutional neural networks (CNNs). This study had institutional review board approval and written informed consent was obtained from all subjects. A dataset of volumetric structural MR images of the proximal femur from 86 subjects were manually-segmented by an expert. We performed experiments by training two different CNN architectures with multiple number of initial feature maps, layers and dilation rates, and tested their segmentation performance against the gold standard of manual segmentations using four-fold cross-validation. Automatic segmentation of the proximal femur using CNNs achieved a high dice similarity score of 0.95 ± 0.02 with precision = 0.95 ± 0.02, and recall = 0.95 ± 0.03. The high segmentation accuracy provided by CNNs has the potential to help bring the use of structural MRI measurements of bone quality into clinical practice for management of osteoporosis.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Magnetic resonance imaging (MRI) has been proposed as a complimentary method to measure bone quality and assess fracture risk. However, manual segmentation of MR images of bone is time-consuming, limiting the use of MRI measurements in the clinical practice. The purpose of this paper is to present an automatic proximal femur segmentation method that is based on deep convolutional neural networks (CNNs). This study had institutional review board approval and written informed consent was obtained from all subjects. A dataset of volumetric structural MR images of the proximal femur from 86 subjects were manually-segmented by an expert. We performed experiments by training two different CNN architectures with multiple number of initial feature maps, layers and dilation rates, and tested their segmentation performance against the gold standard of manual segmentations using four-fold cross-validation. Automatic segmentation of the proximal femur using CNNs achieved a high dice similarity score of 0.95 ± 0.02 with precision = 0.95 ± 0.02, and recall = 0.95 ± 0.03. The high segmentation accuracy provided by CNNs has the potential to help bring the use of structural MRI measurements of bone quality into clinical practice for management of osteoporosis.
Alon, Leeor; Lattanzi, Riccardo; Lakshmanan, Karthik; Brown, Ryan; Deniz, Cem M; Sodickson, Daniel K; Collins, Christopher M
Transverse slot antennas for high field MRI Journal Article
In: Magnetic Resonance in Medicine, vol. 80, no. 3, pp. 1233–1242, 2018, ISSN: 07403194.
@article{Alon2018,
title = {Transverse slot antennas for high field MRI},
author = {Leeor Alon and Riccardo Lattanzi and Karthik Lakshmanan and Ryan Brown and Cem M Deniz and Daniel K Sodickson and Christopher M Collins},
doi = {10.1002/mrm.27095},
issn = {07403194},
year = {2018},
date = {2018-09-01},
journal = {Magnetic Resonance in Medicine},
volume = {80},
number = {3},
pages = {1233--1242},
abstract = {Purpose
Introduce a novel coil design using an electrically long transversely oriented slot in a conductive sheet.
Theory and Methods
Theoretical considerations, numerical simulations, and experimental measurements are presented for transverse slot antennas as compared with electric dipole antennas.
Results
Simulations show improved central and average transmit and receive efficiency, as well as larger coverage in the transverse plane, for a single slot as compared to a single dipole element. Experiments on a body phantom confirm the simulation results for a slot antenna relative to a dipole, demonstrating a large region of relatively high sensitivity and homogeneity. Images in a human subject also show a large imaging volume for a single slot and six slot antenna array. High central transmit efficiency was observed for slot arrays relative to dipole arrays.
Conclusion
Transverse slots can exhibit improved sensitivity and larger field of view compared with traditional conductive dipoles. Simulations and experiments indicate high potential for slot antennas in high field MRI.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Purpose
Introduce a novel coil design using an electrically long transversely oriented slot in a conductive sheet.
Theory and Methods
Theoretical considerations, numerical simulations, and experimental measurements are presented for transverse slot antennas as compared with electric dipole antennas.
Results
Simulations show improved central and average transmit and receive efficiency, as well as larger coverage in the transverse plane, for a single slot as compared to a single dipole element. Experiments on a body phantom confirm the simulation results for a slot antenna relative to a dipole, demonstrating a large region of relatively high sensitivity and homogeneity. Images in a human subject also show a large imaging volume for a single slot and six slot antenna array. High central transmit efficiency was observed for slot arrays relative to dipole arrays.
Conclusion
Transverse slots can exhibit improved sensitivity and larger field of view compared with traditional conductive dipoles. Simulations and experiments indicate high potential for slot antennas in high field MRI.
Introduce a novel coil design using an electrically long transversely oriented slot in a conductive sheet.
Theory and Methods
Theoretical considerations, numerical simulations, and experimental measurements are presented for transverse slot antennas as compared with electric dipole antennas.
Results
Simulations show improved central and average transmit and receive efficiency, as well as larger coverage in the transverse plane, for a single slot as compared to a single dipole element. Experiments on a body phantom confirm the simulation results for a slot antenna relative to a dipole, demonstrating a large region of relatively high sensitivity and homogeneity. Images in a human subject also show a large imaging volume for a single slot and six slot antenna array. High central transmit efficiency was observed for slot arrays relative to dipole arrays.
Conclusion
Transverse slots can exhibit improved sensitivity and larger field of view compared with traditional conductive dipoles. Simulations and experiments indicate high potential for slot antennas in high field MRI.
Ianniello, Carlotta; de Zwart, Jacco A; Duan, Qi; Deniz, Cem M; Alon, Leeor; Lee, Jae-Seung; Lattanzi, Riccardo; Brown, Ryan
Synthesized tissue-equivalent dielectric phantoms using salt and polyvinylpyrrolidone solutions Journal Article
In: Magnetic Resonance in Medicine, vol. 80, no. 1, pp. 413–419, 2018, ISSN: 07403194.
@article{Ianniello2017,
title = {Synthesized tissue-equivalent dielectric phantoms using salt and polyvinylpyrrolidone solutions},
author = {Carlotta Ianniello and Jacco A de Zwart and Qi Duan and Cem M Deniz and Leeor Alon and Jae-Seung Lee and Riccardo Lattanzi and Ryan Brown},
url = {http://doi.wiley.com/10.1002/mrm.27005},
doi = {10.1002/mrm.27005},
issn = {07403194},
year = {2018},
date = {2018-07-01},
journal = {Magnetic Resonance in Medicine},
volume = {80},
number = {1},
pages = {413--419},
abstract = {Purpose
To explore the use of polyvinylpyrrolidone (PVP) for simulated materials with tissue‐equivalent dielectric properties.
Methods
PVP and salt were used to control, respectively, relative permittivity and electrical conductivity in a collection of 63 samples with a range of solute concentrations. Their dielectric properties were measured with a commercial probe and fitted to a 3D polynomial in order to establish an empirical recipe. The material's thermal properties and MR spectra were measured.
Results
The empirical polynomial recipe (available at https://www.amri.ninds.nih.gov/cgi-bin/phantomrecipe) provides the PVP and salt concentrations required for dielectric materials with permittivity and electrical conductivity values between approximately 45 and 78, and 0.1 to 2 siemens per meter, respectively, from 50 MHz to 4.5 GHz. The second‐ (solute concentrations) and seventh‐ (frequency) order polynomial recipe provided less than 2.5% relative error between the measured and target properties. PVP side peaks in the spectra were minor and unaffected by temperature changes.
Conclusion
PVP‐based phantoms are easy to prepare and nontoxic, and their semitransparency makes air bubbles easy to identify. The polymer can be used to create simulated material with a range of dielectric properties, negligible spectral side peaks, and long T2 relaxation time, which are favorable in many MR applications. },
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Purpose
To explore the use of polyvinylpyrrolidone (PVP) for simulated materials with tissue‐equivalent dielectric properties.
Methods
PVP and salt were used to control, respectively, relative permittivity and electrical conductivity in a collection of 63 samples with a range of solute concentrations. Their dielectric properties were measured with a commercial probe and fitted to a 3D polynomial in order to establish an empirical recipe. The material's thermal properties and MR spectra were measured.
Results
The empirical polynomial recipe (available at https://www.amri.ninds.nih.gov/cgi-bin/phantomrecipe) provides the PVP and salt concentrations required for dielectric materials with permittivity and electrical conductivity values between approximately 45 and 78, and 0.1 to 2 siemens per meter, respectively, from 50 MHz to 4.5 GHz. The second‐ (solute concentrations) and seventh‐ (frequency) order polynomial recipe provided less than 2.5% relative error between the measured and target properties. PVP side peaks in the spectra were minor and unaffected by temperature changes.
Conclusion
PVP‐based phantoms are easy to prepare and nontoxic, and their semitransparency makes air bubbles easy to identify. The polymer can be used to create simulated material with a range of dielectric properties, negligible spectral side peaks, and long T2 relaxation time, which are favorable in many MR applications.
To explore the use of polyvinylpyrrolidone (PVP) for simulated materials with tissue‐equivalent dielectric properties.
Methods
PVP and salt were used to control, respectively, relative permittivity and electrical conductivity in a collection of 63 samples with a range of solute concentrations. Their dielectric properties were measured with a commercial probe and fitted to a 3D polynomial in order to establish an empirical recipe. The material's thermal properties and MR spectra were measured.
Results
The empirical polynomial recipe (available at https://www.amri.ninds.nih.gov/cgi-bin/phantomrecipe) provides the PVP and salt concentrations required for dielectric materials with permittivity and electrical conductivity values between approximately 45 and 78, and 0.1 to 2 siemens per meter, respectively, from 50 MHz to 4.5 GHz. The second‐ (solute concentrations) and seventh‐ (frequency) order polynomial recipe provided less than 2.5% relative error between the measured and target properties. PVP side peaks in the spectra were minor and unaffected by temperature changes.
Conclusion
PVP‐based phantoms are easy to prepare and nontoxic, and their semitransparency makes air bubbles easy to identify. The polymer can be used to create simulated material with a range of dielectric properties, negligible spectral side peaks, and long T2 relaxation time, which are favorable in many MR applications.
Vaidya, Manushka V; Deniz, Cem M; Collins, Christopher M; Sodickson, Daniel K; Lattanzi, Riccardo
Manipulating transmit and receive sensitivities of radiofrequency surface coils using shielded and unshielded high-permittivity materials Journal Article
In: Magnetic Resonance Materials in Physics, Biology and Medicine, vol. 31, no. 3, pp. 355–366, 2018, ISSN: 0968-5243.
@article{Vaidya2017,
title = {Manipulating transmit and receive sensitivities of radiofrequency surface coils using shielded and unshielded high-permittivity materials},
author = {Manushka V Vaidya and Cem M Deniz and Christopher M Collins and Daniel K Sodickson and Riccardo Lattanzi},
url = {http://link.springer.com/10.1007/s10334-017-0657-5},
doi = {10.1007/s10334-017-0657-5},
issn = {0968-5243},
year = {2018},
date = {2018-06-01},
journal = {Magnetic Resonance Materials in Physics, Biology and Medicine},
volume = {31},
number = {3},
pages = {355--366},
abstract = {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.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
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.
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.
Vaidya, Manushka V; Lazar, Mariana; Deniz, Cem M; Haemer, Gillian G; Chen, Gang; Bruno, Mary; Sodickson, Daniel K; Lattanzi, Riccardo; Collins, Christopher M
Improved detection of fMRI activation in the cerebellum at 7T with dielectric pads extending the imaging region of a commercial head coil Journal Article
In: Journal of Magnetic Resonance Imaging, 2018, ISSN: 10531807.
@article{Vaidya2018,
title = {Improved detection of fMRI activation in the cerebellum at 7T with dielectric pads extending the imaging region of a commercial head coil},
author = {Manushka V Vaidya and Mariana Lazar and Cem M Deniz and Gillian G Haemer and Gang Chen and Mary Bruno and Daniel K Sodickson and Riccardo Lattanzi and Christopher M Collins},
url = {http://doi.wiley.com/10.1002/jmri.25936},
doi = {10.1002/jmri.25936},
issn = {10531807},
year = {2018},
date = {2018-01-01},
journal = {Journal of Magnetic Resonance Imaging},
abstract = {Background
There is growing interest in detecting cerebro‐cerebellar circuits, which requires adequate blood oxygenation level dependent contrast and signal‐to‐noise ratio (SNR) throughout the brain. Although 7T scanners offer increased SNR, coverage of commercial head coils is currently limited to the cerebrum.
Purpose
To improve cerebellar functional MRI (fMRI) at 7T with high permittivity material (HPM) pads extending the sensitivity of a commercial coil.
Study Type
Simulations were used to determine HPM pad configuration and assess radiofrequency (RF) safety. In vivo experiments were performed to evaluate RF field distributions and SNR and assess improvements of cerebellar fMRI.
Subjects
Eight healthy volunteers enrolled in a prospective motor fMRI study with and without HPM.
Field Strength/Sequence
Gradient echo (GRE) echo planar imaging for fMRI, turbo FLASH for flip angle mapping, GRE sequence for SNR maps, and T1‐weighted MPRAGE were acquired with and without HPM pads at 7T.
Assessment
Field maps, SNR maps, and anatomical images were evaluated for coverage. Simulation results were used to assess SAR levels of the experiment. Activation data from fMRI experiments were compared with and without HPM pads.
Statistical Tests
fMRI data were analyzed using FEAT FSL for each subject followed by group level analysis using paired t‐test of acquisitions with and without HPM.
Results
Simulations showed 52% improvement in transmit efficiency in cerebellum with HPM and SAR levels well below recommended limits. Experiments showed 27% improvement in SNR in cerebellum and improvement in coverage on T1‐weighted images. fMRI showed greater cerebellar activation in individual subjects with the HPM pad present (Z > = 4), especially in inferior slices of cerebellum, with 59% average increase in number of activated voxels in the cerebellum. Group‐level analysis showed improved functional activation (Z > = 2.3) in cerebellar regions with HPM pads without loss of measured activation elsewhere.
Data Conclusion
HPM pads can improve cerebellar fMRI at 7T with a commonly‐used head coil without compromising RF safety.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Background
There is growing interest in detecting cerebro‐cerebellar circuits, which requires adequate blood oxygenation level dependent contrast and signal‐to‐noise ratio (SNR) throughout the brain. Although 7T scanners offer increased SNR, coverage of commercial head coils is currently limited to the cerebrum.
Purpose
To improve cerebellar functional MRI (fMRI) at 7T with high permittivity material (HPM) pads extending the sensitivity of a commercial coil.
Study Type
Simulations were used to determine HPM pad configuration and assess radiofrequency (RF) safety. In vivo experiments were performed to evaluate RF field distributions and SNR and assess improvements of cerebellar fMRI.
Subjects
Eight healthy volunteers enrolled in a prospective motor fMRI study with and without HPM.
Field Strength/Sequence
Gradient echo (GRE) echo planar imaging for fMRI, turbo FLASH for flip angle mapping, GRE sequence for SNR maps, and T1‐weighted MPRAGE were acquired with and without HPM pads at 7T.
Assessment
Field maps, SNR maps, and anatomical images were evaluated for coverage. Simulation results were used to assess SAR levels of the experiment. Activation data from fMRI experiments were compared with and without HPM pads.
Statistical Tests
fMRI data were analyzed using FEAT FSL for each subject followed by group level analysis using paired t‐test of acquisitions with and without HPM.
Results
Simulations showed 52% improvement in transmit efficiency in cerebellum with HPM and SAR levels well below recommended limits. Experiments showed 27% improvement in SNR in cerebellum and improvement in coverage on T1‐weighted images. fMRI showed greater cerebellar activation in individual subjects with the HPM pad present (Z > = 4), especially in inferior slices of cerebellum, with 59% average increase in number of activated voxels in the cerebellum. Group‐level analysis showed improved functional activation (Z > = 2.3) in cerebellar regions with HPM pads without loss of measured activation elsewhere.
Data Conclusion
HPM pads can improve cerebellar fMRI at 7T with a commonly‐used head coil without compromising RF safety.
There is growing interest in detecting cerebro‐cerebellar circuits, which requires adequate blood oxygenation level dependent contrast and signal‐to‐noise ratio (SNR) throughout the brain. Although 7T scanners offer increased SNR, coverage of commercial head coils is currently limited to the cerebrum.
Purpose
To improve cerebellar functional MRI (fMRI) at 7T with high permittivity material (HPM) pads extending the sensitivity of a commercial coil.
Study Type
Simulations were used to determine HPM pad configuration and assess radiofrequency (RF) safety. In vivo experiments were performed to evaluate RF field distributions and SNR and assess improvements of cerebellar fMRI.
Subjects
Eight healthy volunteers enrolled in a prospective motor fMRI study with and without HPM.
Field Strength/Sequence
Gradient echo (GRE) echo planar imaging for fMRI, turbo FLASH for flip angle mapping, GRE sequence for SNR maps, and T1‐weighted MPRAGE were acquired with and without HPM pads at 7T.
Assessment
Field maps, SNR maps, and anatomical images were evaluated for coverage. Simulation results were used to assess SAR levels of the experiment. Activation data from fMRI experiments were compared with and without HPM pads.
Statistical Tests
fMRI data were analyzed using FEAT FSL for each subject followed by group level analysis using paired t‐test of acquisitions with and without HPM.
Results
Simulations showed 52% improvement in transmit efficiency in cerebellum with HPM and SAR levels well below recommended limits. Experiments showed 27% improvement in SNR in cerebellum and improvement in coverage on T1‐weighted images. fMRI showed greater cerebellar activation in individual subjects with the HPM pad present (Z > = 4), especially in inferior slices of cerebellum, with 59% average increase in number of activated voxels in the cerebellum. Group‐level analysis showed improved functional activation (Z > = 2.3) in cerebellar regions with HPM pads without loss of measured activation elsewhere.
Data Conclusion
HPM pads can improve cerebellar fMRI at 7T with a commonly‐used head coil without compromising RF safety.
2017
Deniz, Cem M; Carluccio, Giuseppe; Collins, Christopher
Parallel transmission RF pulse design with strict temperature constraints Journal Article
In: NMR in Biomedicine, vol. 30, no. 5, pp. e3694, 2017, ISSN: 09523480.
@article{Deniz2017p,
title = {Parallel transmission RF pulse design with strict temperature constraints},
author = {Cem M Deniz and Giuseppe Carluccio and Christopher Collins},
url = {http://doi.wiley.com/10.1002/nbm.3694},
doi = {10.1002/nbm.3694},
issn = {09523480},
year = {2017},
date = {2017-05-01},
journal = {NMR in Biomedicine},
volume = {30},
number = {5},
pages = {e3694},
abstract = {RF safety in parallel transmission (pTx) is generally ensured by imposing specific absorption rate (SAR) limits during pTx RF pulse design. There is increasing interest in using temperature to ensure safety in MRI. In this work, we present a local temperature correlation matrix formalism and apply it to impose strict constraints on maximum absolute temperature in pTx RF pulse design for head and hip regions. Electromagnetic field simulations were performed on the head and hip of virtual body models. Temperature correlation matrices were calculated for four different exposure durations ranging between 6 and 24 min using simulated fields and body‐specific constants. Parallel transmission RF pulses were designed using either SAR or temperature constraints, and compared with each other and unconstrained RF pulse design in terms of excitation fidelity and safety. The use of temperature correlation matrices resulted in better excitation fidelity compared with the use of SAR in parallel transmission RF pulse design (for the 6 min exposure period, 8.8% versus 21.0% for the head and 28.0% versus 32.2% for the hip region). As RF exposure duration increases (from 6 min to 24 min), the benefit of using temperature correlation matrices on RF pulse design diminishes. However, the safety of the subject is always guaranteed (the maximum temperature was equal to 39°C). This trend was observed in both head and hip regions, where the perfusion rates are very different.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
RF safety in parallel transmission (pTx) is generally ensured by imposing specific absorption rate (SAR) limits during pTx RF pulse design. There is increasing interest in using temperature to ensure safety in MRI. In this work, we present a local temperature correlation matrix formalism and apply it to impose strict constraints on maximum absolute temperature in pTx RF pulse design for head and hip regions. Electromagnetic field simulations were performed on the head and hip of virtual body models. Temperature correlation matrices were calculated for four different exposure durations ranging between 6 and 24 min using simulated fields and body‐specific constants. Parallel transmission RF pulses were designed using either SAR or temperature constraints, and compared with each other and unconstrained RF pulse design in terms of excitation fidelity and safety. The use of temperature correlation matrices resulted in better excitation fidelity compared with the use of SAR in parallel transmission RF pulse design (for the 6 min exposure period, 8.8% versus 21.0% for the head and 28.0% versus 32.2% for the hip region). As RF exposure duration increases (from 6 min to 24 min), the benefit of using temperature correlation matrices on RF pulse design diminishes. However, the safety of the subject is always guaranteed (the maximum temperature was equal to 39°C). This trend was observed in both head and hip regions, where the perfusion rates are very different.
Alon, Leeor; Gabriel, Sami; Cho, Gene Y; Brown, Ryan; Deniz, Cem M
Prospects for Millimeter-Wave Compliance Measurement Technologies [Measurements Corner] Journal Article
In: IEEE Antennas and Propagation Magazine, vol. 59, no. 2, pp. 115–125, 2017, ISSN: 1045-9243.
@article{Alon2017,
title = {Prospects for Millimeter-Wave Compliance Measurement Technologies [Measurements Corner]},
author = {Leeor Alon and Sami Gabriel and Gene Y Cho and Ryan Brown and Cem M Deniz},
url = {http://ieeexplore.ieee.org/document/7892071/},
doi = {10.1109/MAP.2017.2655530},
issn = {1045-9243},
year = {2017},
date = {2017-04-01},
journal = {IEEE Antennas and Propagation Magazine},
volume = {59},
number = {2},
pages = {115--125},
abstract = {In recent years, there has been an increasing interest in millimeterwave (mm-wave) technologies operating at frequencies between 10 and 300 GHz. mm-wave communication provides higher transfer rates that will be employed in the next-generation wireless networks. However, mm-wave device exposure compliance assessment is challenging due to a reduced wavelength and an increased number of antennas. In this article, we discuss physical features of future mmwave communication networks and describe the requirements for proper exposure compliance assessment. Leading technologies that can be prospectively used for mm-wave exposure compliance are surveyed. Their pros and cons with respect to operating frequency, availability, sensitivity, and speed are discussed.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In recent years, there has been an increasing interest in millimeterwave (mm-wave) technologies operating at frequencies between 10 and 300 GHz. mm-wave communication provides higher transfer rates that will be employed in the next-generation wireless networks. However, mm-wave device exposure compliance assessment is challenging due to a reduced wavelength and an increased number of antennas. In this article, we discuss physical features of future mmwave communication networks and describe the requirements for proper exposure compliance assessment. Leading technologies that can be prospectively used for mm-wave exposure compliance are surveyed. Their pros and cons with respect to operating frequency, availability, sensitivity, and speed are discussed.
2016
Alon, Leeor; Sodickson, Daniel K; Deniz, Cem M
Heat equation inversion framework for average SAR calculation from magnetic resonance thermal imaging Journal Article
In: Bioelectromagnetics, vol. 37, no. 7, pp. 493–503, 2016, ISSN: 01978462.
@article{Alon2016b,
title = {Heat equation inversion framework for average SAR calculation from magnetic resonance thermal imaging},
author = {Leeor Alon and Daniel K Sodickson and Cem M Deniz},
url = {http://doi.wiley.com/10.1002/bem.21996},
doi = {10.1002/bem.21996},
issn = {01978462},
year = {2016},
date = {2016-10-01},
journal = {Bioelectromagnetics},
volume = {37},
number = {7},
pages = {493--503},
abstract = {Deposition of radiofrequency (RF) energy can be quantified via electric field or temperature change measurements. Magnetic resonance imaging has been used as a tool to measure three dimensional small temperature changes associated with RF radiation exposure. When duration of RF exposure is long, conversion from temperature change to specific absorption rate (SAR) is nontrivial due to prominent heat-diffusion and conduction effects. In this work, we demonstrated a method for calculation of SAR via an inversion of the heat equation including heat-diffusion and conduction effects. This method utilizes high-resolution three dimensional magnetic resonance temperature images and measured thermal properties of the phantom to achieve accurate calculation of SAR. Accuracy of the proposed method was analyzed with respect to operating frequency of a dipole antenna and parameters used in heat equation inversion. Bioelectromagnetics. 37:493-503, 2016. textcopyright 2016 Wiley Periodicals, Inc.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Deposition of radiofrequency (RF) energy can be quantified via electric field or temperature change measurements. Magnetic resonance imaging has been used as a tool to measure three dimensional small temperature changes associated with RF radiation exposure. When duration of RF exposure is long, conversion from temperature change to specific absorption rate (SAR) is nontrivial due to prominent heat-diffusion and conduction effects. In this work, we demonstrated a method for calculation of SAR via an inversion of the heat equation including heat-diffusion and conduction effects. This method utilizes high-resolution three dimensional magnetic resonance temperature images and measured thermal properties of the phantom to achieve accurate calculation of SAR. Accuracy of the proposed method was analyzed with respect to operating frequency of a dipole antenna and parameters used in heat equation inversion. Bioelectromagnetics. 37:493-503, 2016. textcopyright 2016 Wiley Periodicals, Inc.
Deniz, Cem M; Alon, Leeor; Brown, Ryan; Zhu, Yudong
Subject- and resource-specific monitoring and proactive management of parallel radiofrequency transmission Journal Article
In: Magnetic Resonance in Medicine, vol. 76, no. 1, pp. 20–31, 2016, ISSN: 07403194.
@article{Deniz2015a,
title = {Subject- and resource-specific monitoring and proactive management of parallel radiofrequency transmission},
author = {Cem M Deniz and Leeor Alon and Ryan Brown and Yudong Zhu},
url = {http://doi.wiley.com/10.1002/mrm.25828},
doi = {10.1002/mrm.25828},
issn = {07403194},
year = {2016},
date = {2016-07-01},
journal = {Magnetic Resonance in Medicine},
volume = {76},
number = {1},
pages = {20--31},
abstract = {Purpose
Develop a practical comprehensive package for proactive management of parallel radiofrequency (RF) transmission.
Methods
With a constrained optimization framework and predictive models from a prescan based multichannel calibration, we presented a method supporting design and optimization of parallel RF excitation pulses that accurately obey the forward/reflected peak and average power limits of the RF power amplifiers in parallel transmit imaging experiments and Bloch simulations. Moreover, local SAR limits were incorporated into the parallel RF excitation pulses using electromagnetic field simulations. Virtual transmit coils concept for minimization of reflected power (effecting subject‐specific matching) was additionally demonstrated by leveraging experimentally calibrated power models.
Results
Incorporation of experimentally calibrated power prediction models resulted in accurate compliance with prescribed hardware and global specific absorption rate (SAR) limits. Incorporation of spatial average 10 g SAR models, facilitated by simplifying numerical approximations, provided assurance of patient safety. RF pulses designed with various constraints demonstrated excellent excitation fidelity—the normalized root‐mean‐square error of the simulated excitation profiles was 2.6% for the fully constrained pulses, comparable to that of the unconstrained pulses. An RF shimming example showed a reduction of the reflected‐to‐forward power ratio to 1.7% from a conventional approach's 8.1%.
Conclusion
Using the presented RF pulse design method, effective proactive management of the multifaceted power and SAR limits was demonstrated in experimental and simulation studies.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Purpose
Develop a practical comprehensive package for proactive management of parallel radiofrequency (RF) transmission.
Methods
With a constrained optimization framework and predictive models from a prescan based multichannel calibration, we presented a method supporting design and optimization of parallel RF excitation pulses that accurately obey the forward/reflected peak and average power limits of the RF power amplifiers in parallel transmit imaging experiments and Bloch simulations. Moreover, local SAR limits were incorporated into the parallel RF excitation pulses using electromagnetic field simulations. Virtual transmit coils concept for minimization of reflected power (effecting subject‐specific matching) was additionally demonstrated by leveraging experimentally calibrated power models.
Results
Incorporation of experimentally calibrated power prediction models resulted in accurate compliance with prescribed hardware and global specific absorption rate (SAR) limits. Incorporation of spatial average 10 g SAR models, facilitated by simplifying numerical approximations, provided assurance of patient safety. RF pulses designed with various constraints demonstrated excellent excitation fidelity—the normalized root‐mean‐square error of the simulated excitation profiles was 2.6% for the fully constrained pulses, comparable to that of the unconstrained pulses. An RF shimming example showed a reduction of the reflected‐to‐forward power ratio to 1.7% from a conventional approach's 8.1%.
Conclusion
Using the presented RF pulse design method, effective proactive management of the multifaceted power and SAR limits was demonstrated in experimental and simulation studies.
Develop a practical comprehensive package for proactive management of parallel radiofrequency (RF) transmission.
Methods
With a constrained optimization framework and predictive models from a prescan based multichannel calibration, we presented a method supporting design and optimization of parallel RF excitation pulses that accurately obey the forward/reflected peak and average power limits of the RF power amplifiers in parallel transmit imaging experiments and Bloch simulations. Moreover, local SAR limits were incorporated into the parallel RF excitation pulses using electromagnetic field simulations. Virtual transmit coils concept for minimization of reflected power (effecting subject‐specific matching) was additionally demonstrated by leveraging experimentally calibrated power models.
Results
Incorporation of experimentally calibrated power prediction models resulted in accurate compliance with prescribed hardware and global specific absorption rate (SAR) limits. Incorporation of spatial average 10 g SAR models, facilitated by simplifying numerical approximations, provided assurance of patient safety. RF pulses designed with various constraints demonstrated excellent excitation fidelity—the normalized root‐mean‐square error of the simulated excitation profiles was 2.6% for the fully constrained pulses, comparable to that of the unconstrained pulses. An RF shimming example showed a reduction of the reflected‐to‐forward power ratio to 1.7% from a conventional approach's 8.1%.
Conclusion
Using the presented RF pulse design method, effective proactive management of the multifaceted power and SAR limits was demonstrated in experimental and simulation studies.