1
|
Yetisir F, Abaci Turk E, Adalsteinsson E, Wald LL, Grant PE. Local SAR management strategies to use two-channel RF shimming for fetal MRI at 3 T. Magn Reson Med 2024; 91:1165-1178. [PMID: 37929768 PMCID: PMC10843691 DOI: 10.1002/mrm.29913] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/12/2023] [Revised: 10/10/2023] [Accepted: 10/16/2023] [Indexed: 11/07/2023]
Abstract
PURPOSE This study evaluates the imaging performance of two-channel RF-shimming for fetal MRI at 3 T using four different local specific absorption rate (SAR) management strategies. METHODS Due to the ambiguity of safe local SAR levels for fetal MRI, local SAR limits for RF shimming were determined based on either each individual's own SAR levels in standard imaging mode (CP mode) or the maximum SAR level observed across seven pregnant body models in CP mode. Local SAR was constrained either indirectly by further constraining the whole-body SAR (wbSAR) or directly by using subject-specific local SAR models. Each strategy was evaluated by the improvement of the transmit field efficiency (average |B1 + |) and nonuniformity (|B1 + | variation) inside the fetus compared with CP mode for the same wbSAR. RESULTS Constraining wbSAR when using RF shimming decreases B1 + efficiency inside the fetus compared with CP mode (by 12%-30% on average), making it inefficient for SAR management. Using subject-specific models with SAR limits based on each individual's own CP mode SAR value, B1 + efficiency and nonuniformity are improved on average by 6% and 13% across seven pregnant models. In contrast, using SAR limits based on maximum CP mode SAR values across seven models, B1 + efficiency and nonuniformity are improved by 13% and 25%, compared with the best achievable improvement without SAR constraints: 15% and 26%. CONCLUSION Two-channel RF-shimming can safely and significantly improve the transmit field inside the fetus when subject-specific models are used with local SAR limits based on maximum CP mode SAR levels in the pregnant population.
Collapse
Affiliation(s)
- Filiz Yetisir
- Fetal-Neonatal Neuroimaging & Developmental Science Center, Boston Children’s Hospital, Boston, MA, USA
| | - Esra Abaci Turk
- Fetal-Neonatal Neuroimaging & Developmental Science Center, Boston Children’s Hospital, Boston, MA, USA
- Department of Pediatrics, Harvard Medical School, Boston, MA 02115, USA
| | - Elfar Adalsteinsson
- Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, USA
- Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA
| | - Lawrence L. Wald
- Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA, USA
- Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, USA
- Department of Radiology, Harvard Medical School, Boston, MA, USA
| | - P. Ellen Grant
- Fetal-Neonatal Neuroimaging & Developmental Science Center, Boston Children’s Hospital, Boston, MA, USA
- Department of Pediatrics, Harvard Medical School, Boston, MA 02115, USA
- Department of Radiology, Harvard Medical School, Boston, MA, USA
| |
Collapse
|
2
|
Ivanov D, De Martino F, Formisano E, Fritz FJ, Goebel R, Huber L, Kashyap S, Kemper VG, Kurban D, Roebroeck A, Sengupta S, Sorger B, Tse DHY, Uludağ K, Wiggins CJ, Poser BA. Magnetic resonance imaging at 9.4 T: the Maastricht journey. MAGMA (NEW YORK, N.Y.) 2023; 36:159-173. [PMID: 37081247 PMCID: PMC10140139 DOI: 10.1007/s10334-023-01080-4] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/29/2022] [Revised: 03/18/2023] [Accepted: 03/20/2023] [Indexed: 04/22/2023]
Abstract
The 9.4 T scanner in Maastricht is a whole-body magnet with head gradients and parallel RF transmit capability. At the time of the design, it was conceptualized to be one of the best fMRI scanners in the world, but it has also been used for anatomical and diffusion imaging. 9.4 T offers increases in sensitivity and contrast, but the technical ultra-high field (UHF) challenges, such as field inhomogeneities and constraints set by RF power deposition, are exacerbated compared to 7 T. This article reviews some of the 9.4 T work done in Maastricht. Functional imaging experiments included blood oxygenation level-dependent (BOLD) and blood-volume weighted (VASO) fMRI using different readouts. BOLD benefits from shorter T2* at 9.4 T while VASO from longer T1. We show examples of both ex vivo and in vivo anatomical imaging. For many applications, pTx and optimized coils are essential to harness the full potential of 9.4 T. Our experience shows that, while considerable effort was required compared to our 7 T scanner, we could obtain high-quality anatomical and functional data, which illustrates the potential of MR acquisitions at even higher field strengths. The practical challenges of working with a relatively unique system are also discussed.
Collapse
Affiliation(s)
- Dimo Ivanov
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands.
| | - Federico De Martino
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Elia Formisano
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Francisco J Fritz
- Institute of Systems Neuroscience, Center for Experimental Medicine, University Medical Center Hamburg-Eppendorf (UKE), Hamburg, Germany
| | - Rainer Goebel
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Laurentius Huber
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Sriranga Kashyap
- Krembil Brain Institute, University Health Network, Toronto, ON, Canada
| | - Valentin G Kemper
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Denizhan Kurban
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Alard Roebroeck
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | | | - Bettina Sorger
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| | - Desmond H Y Tse
- Scannexus BV, Oxfordlaan 55, 6229 EV, Maastricht, The Netherlands
| | - Kâmil Uludağ
- Krembil Brain Institute, Koerner Scientist in MR Imaging, University Health Network Toronto, Toronto, ON, Canada
- Department of Medical Biophysics, University of Toronto, Toronto, ON, Canada
- Center for Neuroscience Imaging Research, Institute for Basic Science and Department of Biomedical Engineering, Sungkyunkwan University, Suwon, Republic of Korea
| | - Christopher J Wiggins
- Imaging Core Facility (INM-ICF), Institut für Neurowissenschaften und Medizin, Forschungszentrum Jülich GmbH, 52425, Jülich, Germany
| | - Benedikt A Poser
- Faculty of Psychology and Neuroscience, Maastricht University, Universiteitssingel 40, 6229 ER, Maastricht, The Netherlands
| |
Collapse
|
3
|
Bosch D, Bause J, Geldschläger O, Scheffler K. Optimized ultrahigh field parallel transmission workflow using rapid presaturated TurboFLASH transmit field mapping with a three-dimensional centric single-shot readout. Magn Reson Med 2022; 89:322-330. [PMID: 36120984 DOI: 10.1002/mrm.29459] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2021] [Revised: 07/27/2022] [Accepted: 08/22/2022] [Indexed: 11/07/2022]
Abstract
PURPOSE To evaluate the usage of three-dimensional (3D) presaturated TurboFLASH (satTFL) for B 1 + $$ {\mathrm{B}}_1^{+} $$ and B 0 $$ {\mathrm{B}}_0 $$ mapping on single channel and parallel transmission (pTx) systems. METHODS B 1 + $$ {\mathrm{B}}_1^{+} $$ maps recorded with 3D satTFL were compared to maps from three other 3D B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping sequences in an agar phantom. Furthermore, individual-channel B 1 + $$ {\mathrm{B}}_1^{+} $$ maps of 18 human subjects were recorded with 3D satTFL using B 1 + $$ {\mathrm{B}}_1^{+} $$ interferometry. A neural network was trained for masking of the maps. RESULTS Out of the sequences compared satTFL was the only one with a mapping range exceeding well over 90°. In regions with lower flip angles there was high correspondence between satTFL and AFI. DREAM and double angle method also showed high qualitative similarity, however the magnitude differed from the other two measurements. The individual-channel B 1 + $$ {\mathrm{B}}_1^{+} $$ maps were successfully used for pTx pulse calculation in a separate study. CONCLUSION 3D satTFL can record high-quality B 1 + $$ {\mathrm{B}}_1^{+} $$ maps with a high dynamic range in a short time. Correspondence with AFI maps is high, while measurement duration is reduced drastically.
Collapse
Affiliation(s)
- Dario Bosch
- High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
- Department for Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany
| | - Jonas Bause
- High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Ole Geldschläger
- High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
| | - Klaus Scheffler
- High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany
- Department for Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany
| |
Collapse
|
4
|
Ma X, Uğurbil K, Wu X. Mitigating transmit‐B
1
artifacts by predicting parallel transmission images with deep learning: A feasibility study using high‐resolution whole‐brain diffusion at 7 Tesla. Magn Reson Med 2022; 88:727-741. [PMID: 35403237 PMCID: PMC9324974 DOI: 10.1002/mrm.29238] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2021] [Revised: 02/24/2022] [Accepted: 02/28/2022] [Indexed: 11/12/2022]
Abstract
Purpose To propose a novel deep learning (DL) approach to transmit‐B1 (B1+)‐artifact mitigation without direct use of parallel transmission (pTx), by predicting pTx images from single‐channel transmission (sTx) images. Methods A deep encoder–decoder convolutional neural network was constructed and trained to learn the mapping from sTx to pTx images. The feasibility was demonstrated using 7 T Human‐Connectome Project (HCP)‐style diffusion MRI. The training dataset comprised images acquired on 5 healthy subjects using commercial Nova RF coils. Relevant hyperparameters were tuned with a nested cross‐validation, and the generalization performance evaluated using a regular cross‐validation. Results Our DL method effectively improved the image quality for sTx images by restoring the signal dropout, with quality measures (including normalized root‐mean‐square error, peak SNR, and structural similarity index measure) improved in most brain regions. The improved image quality was translated into improved performances for diffusion tensor imaging analysis; our method improved accuracy for fractional anisotropy and mean diffusivity estimations, reduced the angular errors of principal eigenvectors, and improved the fiber orientation delineation relative to sTx images. Moreover, the final DL model trained on data of all 5 subjects was successfully used to predict pTx images for unseen new subjects (randomly selected from the 7 T HCP database), effectively recovering the signal dropout and improving color‐coded fractional anisotropy maps with largely reduced noise levels. Conclusion The proposed DL method has potential to provide images with reduced B1+ artifacts in healthy subjects even when pTx resources are inaccessible on the user side.
Collapse
Affiliation(s)
- Xiaodong Ma
- Center for Magnetic Resonance Research, Radiology, Medical School University of Minnesota Minneapolis Minnesota USA
| | - Kâmil Uğurbil
- Center for Magnetic Resonance Research, Radiology, Medical School University of Minnesota Minneapolis Minnesota USA
| | - Xiaoping Wu
- Center for Magnetic Resonance Research, Radiology, Medical School University of Minnesota Minneapolis Minnesota USA
| |
Collapse
|
5
|
Eberhardt B, Poser BA, Shah NJ, Felder J. B1 field map synthesis with generative deep learning used in the design of parallel-transmit RF pulses for ultra-high field MRI. Z Med Phys 2022; 32:334-345. [PMID: 35144850 PMCID: PMC9948838 DOI: 10.1016/j.zemedi.2021.12.003] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2021] [Revised: 11/27/2021] [Accepted: 12/23/2021] [Indexed: 10/19/2022]
Abstract
Spoke trajectory parallel transmit (pTX) excitation in ultra-high field MRI enables B1+ inhomogeneities arising from the shortened RF wavelength in biological tissue to be mitigated. To this end, current RF excitation pulse design algorithms either employ the acquisition of field maps with subsequent non-linear optimization or a universal approach applying robust pre-computed pulses. We suggest and evaluate an intermediate method that uses a subset of acquired field maps combined with generative machine learning models to reduce the pulse calibration time while offering more tailored excitation than robust pulses (RP). The possibility of employing image-to-image translation and semantic image synthesis machine learning models based on generative adversarial networks (GANs) to deduce the missing field maps is examined. Additionally, an RF pulse design that employs a predictive machine learning model to find solutions for the non-linear (two-spokes) pulse design problem is investigated. As a proof of concept, we present simulation results obtained with the suggested machine learning approaches that were trained on a limited data-set, acquired in vivo. The achieved excitation homogeneity based on a subset of half of the B1+ maps acquired in the calibration scans and half of the B1+ maps synthesized with GANs is comparable with state of the art pulse design methods when using the full set of calibration data while halving the total calibration time. By employing RP dictionaries or machine-learning RF pulse predictions, the total calibration time can be reduced significantly as these methods take only seconds or milliseconds per slice, respectively.
Collapse
Affiliation(s)
- Boris Eberhardt
- Institute of Neuroscience and Medicine 4, Forschungszentrum Jülich, Jüich, Germany; RWTH Aachen University, Aachen, Germany.
| | - Benedikt A. Poser
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands
| | - N. Jon Shah
- Institute of Neuroscience and Medicine 4, Forschungszentrum Jülich, Jüich, Germany,Institute of Neuroscience and Medicine 11, Forschungszentrum Jülich, Jülich, Germany,Department of Neurology, RWTH Aachen University, Aachen, Germany,JARA-BRAIN, Translational Medicine, Aachen, Germany
| | - Jörg Felder
- Institute of Neuroscience and Medicine 4, Forschungszentrum Jülich, Jüich, Germany; RWTH Aachen University, Aachen, Germany.
| |
Collapse
|
6
|
Paez A, Gu C, Cao Z. Robust RF shimming and small-tip-angle multispoke pulse design with finite-difference regularization. Magn Reson Med 2021; 86:1472-1481. [PMID: 33934406 DOI: 10.1002/mrm.28820] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2020] [Revised: 03/29/2021] [Accepted: 04/07/2021] [Indexed: 01/07/2023]
Abstract
PURPOSE A new regularizer is proposed for the magnitude least-squares optimization algorithm, to ensure robust parallel transmit RF shimming and small-tip-angle multispoke pulse designs for ultrahigh-field MRI. METHODS A finite-difference regularization term is activated as an additional regularizer in the iterative magnitude-least-squares based pulse design algorithm when an unwanted flip angle null distribution is detected. Both simulated and experimental B 1 + maps from different transmit arrays and different human subjects at 7 T were used to evaluate the proposed algorithm. The algorithm was further demonstrated in experiment with dynamic multislice RF shimming for a single-shot gradient-echo EPI for human functional MRI at 7 T. RESULTS The proposed finite-difference regularizer effectively prevented excitation null to be formed for RF shimming and small-tip-angle multispoke pulses, and improved the latter with a monotonic trade-off relationship between flip angle error and RF power. The proposed algorithm was demonstrated to be effective with several head-array geometries by simulation and with a commercial head array with 12 healthy human subjects by experiment. During a functional MRI scan at 7 T with dynamic RF shimming, the proposed algorithm ensured high image SNR throughout the human brain, compared with near-complete local signal loss by the conventional magnitude-least-squares algorithm. CONCLUSION Using finite-difference regularization to avoid unwanted solutions, the robustness of RF shimming and small-tip-angle multispoke pulse design algorithms are improved, with better flip angle homogeneity and a monotonic trade-off relationship between flip angle error and RF power.
Collapse
Affiliation(s)
- Adrian Paez
- F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland, USA.,Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA
| | - Chunming Gu
- F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, Maryland, USA.,Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.,Department of Biomedical Engineering, Johns Hopkins University, Baltimore, Maryland, USA
| | - Zhipeng Cao
- Vanderbilt University Institute of Imaging Science, Nashville, Tennessee, USA.,Department of Radiology, Vanderbilt University Medical Center, Nashville, Tennessee, USA
| |
Collapse
|
7
|
Eberhardt B, Poser BA, Shah NJ, Felder J. Application of Evolution Strategies to the Design of SAR Efficient Parallel Transmit Multi-Spoke Pulses for Ultra-High Field MRI. IEEE TRANSACTIONS ON MEDICAL IMAGING 2020; 39:4225-4236. [PMID: 32763849 DOI: 10.1109/tmi.2020.3013982] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
We present an evolution-strategy based approach to solve the magnitude least squares (MLS) design problem of low flip-angle slice-selective parallel transmit RF pulses for ultra-high field MRI using SAR and peak-RF-constraints. A combined transmit k-space trajectory and RF pulse weight optimization is proposed in two algorithmic steps. The first step is a coarse grid search to find an initial solution that fulfills all constraints for the subsequent multistage optimization. This avoids convergence to the next nearest local minimum. The second step attempts to refine the results using multiple evolution strategies. We compare the performance of our approach with the non-convex optimization methods described in the literature. The proposed algorithm converges for phantom and in vivo data and only requires an initial estimate of the range of suitable regularization parameters. It demonstrates improved excitation homogeneity compared to published spoke-design methods and allows optimization for homogeneity with a subsequent reduction in the SAR burden. Moreover, excitation homogeneity and the SAR burden can be balanced against each other, enabling a further reduction in SAR at the cost of minor relaxations in excitation homogeneity. This feature makes the algorithm a good candidate for SAR limited sequences in ultra-high field imaging. The algorithm is validated using phantom and in vivo measurements obtained with a 16-channel transmit array at 9.4T.
Collapse
|
8
|
A Platform for 4-Channel Parallel Transmission MRI at 3 T: Demonstration of Reduced Radiofrequency Heating in a Test Object Containing an Implanted Wire. J Med Biol Eng 2019. [DOI: 10.1007/s40846-019-00478-7] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
|
9
|
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.
Collapse
Affiliation(s)
- Cem M. Deniz
- Center for Advanced Imaging Innovation and Research (CAI2R) and Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, NY
- RF Test Labs, LLC, New York, NY
| |
Collapse
|
10
|
Ladd ME, Bachert P, Meyerspeer M, Moser E, Nagel AM, Norris DG, Schmitter S, Speck O, Straub S, Zaiss M. Pros and cons of ultra-high-field MRI/MRS for human application. PROGRESS IN NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY 2018; 109:1-50. [PMID: 30527132 DOI: 10.1016/j.pnmrs.2018.06.001] [Citation(s) in RCA: 267] [Impact Index Per Article: 44.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/06/2018] [Revised: 06/06/2018] [Accepted: 06/07/2018] [Indexed: 05/08/2023]
Abstract
Magnetic resonance imaging and spectroscopic techniques are widely used in humans both for clinical diagnostic applications and in basic research areas such as cognitive neuroimaging. In recent years, new human MR systems have become available operating at static magnetic fields of 7 T or higher (≥300 MHz proton frequency). Imaging human-sized objects at such high frequencies presents several challenges including non-uniform radiofrequency fields, enhanced susceptibility artifacts, and higher radiofrequency energy deposition in the tissue. On the other side of the scale are gains in signal-to-noise or contrast-to-noise ratio that allow finer structures to be visualized and smaller physiological effects to be detected. This review presents an overview of some of the latest methodological developments in human ultra-high field MRI/MRS as well as associated clinical and scientific applications. Emphasis is given to techniques that particularly benefit from the changing physical characteristics at high magnetic fields, including susceptibility-weighted imaging and phase-contrast techniques, imaging with X-nuclei, MR spectroscopy, CEST imaging, as well as functional MRI. In addition, more general methodological developments such as parallel transmission and motion correction will be discussed that are required to leverage the full potential of higher magnetic fields, and an overview of relevant physiological considerations of human high magnetic field exposure is provided.
Collapse
Affiliation(s)
- Mark E Ladd
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; Faculty of Medicine, University of Heidelberg, Heidelberg, Germany; Faculty of Physics and Astronomy, University of Heidelberg, Heidelberg, Germany; Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Essen, Germany.
| | - Peter Bachert
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; Faculty of Physics and Astronomy, University of Heidelberg, Heidelberg, Germany.
| | - Martin Meyerspeer
- Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria; MR Center of Excellence, Medical University of Vienna, Vienna, Austria.
| | - Ewald Moser
- Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria; MR Center of Excellence, Medical University of Vienna, Vienna, Austria.
| | - Armin M Nagel
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany.
| | - David G Norris
- Donders Institute for Brain, Cognition and Behaviour, Radboud University Nijmegen, Nijmegen, Netherlands; Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Essen, Germany.
| | - Sebastian Schmitter
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany; Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany.
| | - Oliver Speck
- Department of Biomedical Magnetic Resonance, Otto-von-Guericke-University Magdeburg, Magdeburg, Germany; German Center for Neurodegenerative Diseases, Magdeburg, Germany; Center for Behavioural Brain Sciences, Magdeburg, Germany; Leibniz Institute for Neurobiology, Magdeburg, Germany.
| | - Sina Straub
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
| | - Moritz Zaiss
- High-Field Magnetic Resonance Center, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany.
| |
Collapse
|
11
|
Wu X, Auerbach EJ, Vu AT, Moeller S, Van de Moortele PF, Yacoub E, Uğurbil K. Human Connectome Project-style resting-state functional MRI at 7 Tesla using radiofrequency parallel transmission. Neuroimage 2018; 184:396-408. [PMID: 30237033 DOI: 10.1016/j.neuroimage.2018.09.038] [Citation(s) in RCA: 20] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2018] [Revised: 09/13/2018] [Accepted: 09/15/2018] [Indexed: 01/16/2023] Open
Abstract
We investigate the utility of radiofrequency (RF) parallel transmission (pTx) for whole-brain resting-state functional MRI (rfMRI) acquisition at 7 Tesla (7T). To this end, Human Connectome Project (HCP)-style data acquisitions were chosen as a showcase example. Five healthy subjects were scanned in pTx and single-channel transmit (1Tx) modes. The pTx data were acquired using a prototype 16-channel transmit system and a commercially available Nova 8-channel transmit 32-channel receive RF head coil. Additionally, pTx single-spoke multiband (MB) pulses were designed to image sagittal slices. HCP-style 7T rfMRI data (1.6-mm isotropic resolution, 5-fold slice and 2-fold in-plane acceleration, 3600 image volumes and ∼ 1-h scan) were acquired with pTx and the results were compared to those acquired with the original 7T HCP rfMRI protocol. The use of pTx significantly improved flip-angle uniformity across the brain, with coefficient of variation (i.e., std/mean) of whole-brain flip-angle distribution reduced on average by ∼39%. This in turn yielded ∼17% increase in group temporal SNR (tSNR) as averaged across the entire brain and ∼10% increase in group functional contrast-to-noise ratio (fCNR) as averaged across the grayordinate space (including cortical surfaces and subcortical voxels). Furthermore, when placing a seed in either the posterior parietal lobe or putamen to estimate seed-based dense connectome, the increase in fCNR was observed to translate into stronger correlation of the seed with the rest of the grayordinate space. We have demonstrated the utility of pTx for slice-accelerated high-resolution whole-brain rfMRI at 7T; as compared to current state-of-the-art, the use of pTx improves flip-angle uniformity, increases tSNR, enhances fCNR and strengthens functional connectivity estimation.
Collapse
Affiliation(s)
- Xiaoping Wu
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, MN, United States.
| | - Edward J Auerbach
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, MN, United States
| | - An T Vu
- Center for Imaging of Neurodegenerative Diseases, VA Healthcare System, San Francisco, CA, United States
| | - Steen Moeller
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, MN, United States
| | | | - Essa Yacoub
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, MN, United States
| | - Kâmil Uğurbil
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, MN, United States
| |
Collapse
|
12
|
Brunheim S, Gratz M, Johst S, Bitz AK, Fiedler TM, Ladd ME, Quick HH, Orzada S. Fast and accurate multi-channel B1+ mapping based on the TIAMO technique for 7T UHF body MRI. Magn Reson Med 2018; 79:2652-2664. [PMID: 28994132 DOI: 10.1002/mrm.26925] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/13/2017] [Revised: 08/21/2017] [Accepted: 08/26/2017] [Indexed: 01/11/2023]
Abstract
PURPOSE Current methods for mitigation of transmit field B1+ inhomogeneities at ultrahigh field (UHF) MRI by multi-channel radiofrequency (RF) shimming rely on accurate B1+ mapping. This can be time consuming when many RF channels have to be mapped for in vivo body MRI, where the B1 maps should ideally be acquired within a single breath-hold. Therefore, a new B1+ mapping technique (B1TIAMO) is proposed. METHODS The performance of this technique is validated against an established method (DREAM) in phantom measurements for a cylindrical head phantom with an 8-channel transmit/receive (Tx/Rx) array. Furthermore, measurements for a 32-channel Tx/Rx remote array are conducted in a large body phantom and the |B1+| map reliability is validated against simulations of the transmit RF field distribution. Finally, in vivo results of this new mapping technique for human abdomen are presented. RESULTS For the head phantom (8-channel Tx/Rx coil), the single |B1+| comparison between B1 TIAMO, the direct DREAM measurements, and simulation data showed good agreement with 10-19% difference. For the large body phantom (32-channel Tx/Rx coil), B1TIAMO matched the RF field simulations well. CONCLUSION The results demonstrate the potential to acquire 32 accurate single-channel B1+ maps for large field-of-view body imaging within only a single breath-hold of 16 s at 7T UHF MRI. Magn Reson Med 79:2652-2664, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
Collapse
Affiliation(s)
- Sascha Brunheim
- Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Essen, Germany
- High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany
| | - Marcel Gratz
- Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Essen, Germany
- High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany
| | - Sören Johst
- Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Essen, Germany
| | - Andreas K Bitz
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
- Electromagnetic Theory and Applied Mathematics, Faculty of Electrical Engineering and Information Technology, FH Aachen-University of Applied Sciences, Aachen, Germany
| | - Thomas M Fiedler
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
| | - Mark E Ladd
- Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Essen, Germany
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
| | - Harald H Quick
- Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Essen, Germany
- High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany
| | - Stephan Orzada
- Erwin L. Hahn Institute for Magnetic Resonance Imaging, University of Duisburg-Essen, Essen, Germany
| |
Collapse
|
13
|
Wu X, Auerbach EJ, Vu AT, Moeller S, Lenglet C, Schmitter S, Van de Moortele PF, Yacoub E, Uğurbil K. High-resolution whole-brain diffusion MRI at 7T using radiofrequency parallel transmission. Magn Reson Med 2018; 80:1857-1870. [PMID: 29603381 DOI: 10.1002/mrm.27189] [Citation(s) in RCA: 32] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2017] [Revised: 02/20/2018] [Accepted: 03/02/2018] [Indexed: 12/21/2022]
Abstract
PURPOSE Investigating the utility of RF parallel transmission (pTx) for Human Connectome Project (HCP)-style whole-brain diffusion MRI (dMRI) data at 7 Tesla (7T). METHODS Healthy subjects were scanned in pTx and single-transmit (1Tx) modes. Multiband (MB), single-spoke pTx pulses were designed to image sagittal slices. HCP-style dMRI data (i.e., 1.05-mm resolutions, MB2, b-values = 1000/2000 s/mm2 , 286 images and 40-min scan) and data with higher accelerations (MB3 and MB4) were acquired with pTx. RESULTS pTx significantly improved flip-angle detected signal uniformity across the brain, yielding ∼19% increase in temporal SNR (tSNR) averaged over the brain relative to 1Tx. This allowed significantly enhanced estimation of multiple fiber orientations (with ∼21% decrease in dispersion) in HCP-style 7T dMRI datasets. Additionally, pTx pulses achieved substantially lower power deposition, permitting higher accelerations, enabling collection of the same data in 2/3 and 1/2 the scan time or of more data in the same scan time. CONCLUSION pTx provides a solution to two major limitations for slice-accelerated high-resolution whole-brain dMRI at 7T; it improves flip-angle uniformity, and enables higher slice acceleration relative to current state-of-the-art. As such, pTx provides significant advantages for rapid acquisition of high-quality, high-resolution truly whole-brain dMRI data.
Collapse
Affiliation(s)
- Xiaoping Wu
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, Minnesota
| | - Edward J Auerbach
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, Minnesota
| | - An T Vu
- Center for Imaging of Neurodegenerative Diseases, VA Healthcare System, San Francisco, California
| | - Steen Moeller
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, Minnesota
| | - Christophe Lenglet
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, Minnesota
| | - Sebastian Schmitter
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, Minnesota.,Physikalisch-Technische Bundesanstalt, Berlin, Germany
| | | | - Essa Yacoub
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, Minnesota
| | - Kâmil Uğurbil
- Center for Magnetic Resonance Research, Radiology, Medical School, University of Minnesota, Minneapolis, Minnesota
| |
Collapse
|
14
|
Poser BA, Setsompop K. Pulse sequences and parallel imaging for high spatiotemporal resolution MRI at ultra-high field. Neuroimage 2018; 168:101-118. [PMID: 28392492 PMCID: PMC5630499 DOI: 10.1016/j.neuroimage.2017.04.006] [Citation(s) in RCA: 42] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2016] [Revised: 04/01/2017] [Accepted: 04/03/2017] [Indexed: 12/18/2022] Open
Abstract
The SNR and CNR benefits of ultra-high field (UHF) have helped push the envelope of achievable spatial resolution in MRI. For applications based on susceptibility contrast where there is a large CNR gain, high quality sub-millimeter resolution imaging is now being routinely performed, particularly in fMRI and phase imaging/QSM. This has enabled the study of structure and function of very fine-scale structures in the brain. UHF has also helped push the spatial resolution of many other MRI applications as will be outlined in this review. However, this push in resolution comes at a cost of a large encoding burden leading to very lengthy scans. Developments in parallel imaging with controlled aliasing and the move away from 2D slice-by-slice imaging to much more SNR-efficient simultaneous multi-slice (SMS) and 3D acquisitions have helped address this issue. In particular, these developments have revolutionized the efficiency of UHF MRI to enable high spatiotemporal resolution imaging at an order of magnitude faster acquisition. In addition to describing the main approaches to these techniques, this review will also outline important key practical considerations in using these methods in practice. Furthermore, new RF pulse design to tackle the B1+ and SAR issues of UHF and the increased SAR and power requirement of SMS RF pulses will also be touched upon. Finally, an outlook into new developments of smart encoding in more dimensions, particularly through using better temporal/across-contrast encoding and reconstruction will be described. Just as controlled aliasing fully exploits spatial encoding in parallel imaging to provide large multiplicative gains in accelerations, the complimentary use of these new approaches in temporal and across-contrast encoding are expected to provide exciting opportunities for further large gains in efficiency to further push the spatiotemporal resolution of MRI.
Collapse
Affiliation(s)
- Benedikt A Poser
- Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands.
| | - Kawin Setsompop
- Department of Radiology, Harvard Medical School, Boston, MA, USA; Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, USA
| |
Collapse
|
15
|
Pfaffenrot V, Brunheim S, Rietsch SHG, Koopmans PJ, Ernst TM, Kraff O, Orzada S, Quick HH. An 8/15-channel Tx/Rx head neck RF coil combination with region-specific B1+ shimming for whole-brain MRI focused on the cerebellum at 7T. Magn Reson Med 2018; 80:1252-1265. [DOI: 10.1002/mrm.27125] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2017] [Revised: 01/17/2018] [Accepted: 01/18/2018] [Indexed: 12/12/2022]
Affiliation(s)
- Viktor Pfaffenrot
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
- High Field and Hybrid MR Imaging, University Hospital Essen; University of Duisburg-Essen; Essen Germany
| | - Sascha Brunheim
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
- High Field and Hybrid MR Imaging, University Hospital Essen; University of Duisburg-Essen; Essen Germany
| | - Stefan H. G. Rietsch
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
- High Field and Hybrid MR Imaging, University Hospital Essen; University of Duisburg-Essen; Essen Germany
| | - Peter J. Koopmans
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
- High Field and Hybrid MR Imaging, University Hospital Essen; University of Duisburg-Essen; Essen Germany
| | - Thomas M. Ernst
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
- Department of Neurology, University Hospital Essen; University of Duisburg-Essen; Essen Germany
| | - Oliver Kraff
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
| | - Stephan Orzada
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
| | - Harald H. Quick
- Erwin L. Hahn Institute for Magnetic Resonance Imaging; University of Duisburg-Essen; Essen Germany
- High Field and Hybrid MR Imaging, University Hospital Essen; University of Duisburg-Essen; Essen Germany
| |
Collapse
|
16
|
Gras V, Mauconduit F, Vignaud A, Amadon A, Le Bihan D, Stöcker T, Boulant N. Design of universal parallel-transmit refocusing k T -point pulses and application to 3D T 2 -weighted imaging at 7T. Magn Reson Med 2017; 80:53-65. [PMID: 29193250 DOI: 10.1002/mrm.27001] [Citation(s) in RCA: 30] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2017] [Revised: 10/02/2017] [Accepted: 10/16/2017] [Indexed: 12/18/2022]
Abstract
PURPOSE T2 -weighted sequences are particularly sensitive to the radiofrequency (RF) field inhomogeneity problem at ultra-high-field because of the errors accumulated by the imperfections of the train of refocusing pulses. As parallel transmission (pTx) has proved particularly useful to counteract RF heterogeneities, universal pulses were recently demonstrated to save precious time and computational efforts by skipping B1 calibration and online RF pulse tailoring. Here, we report a universal RF pulse design for non-selective refocusing pulses to mitigate the RF inhomogeneity problem at 7T in turbo spin-echo sequences with variable flip angles. METHOD Average Hamiltonian theory was used to synthetize a single non-selective refocusing pulse with pTx while optimizing its scaling properties in the presence of static field offsets. The design was performed under explicit power and specific absorption rate constraints on a database of 10 subjects using a 8Tx-32Rx commercial coil at 7T. To validate the proposed design, the RF pulses were tested in simulation and applied in vivo on 5 additional test subjects. RESULTS The root-mean-square rotation angle error (RA-NRMSE) evaluation and experimental data demonstrated great improvement with the proposed universal pulses (RA-NRMSE ∼8%) compared to the standard circularly polarized mode of excitation (RA-NRMSE ∼26%). CONCLUSION This work further completes the spectrum of 3D universal pulses to mitigate RF field inhomogeneity throughout all 3D MRI sequences without any pTx calibration. The approach returns a single pulse that can be scaled to match the desired flip angle train, thereby increasing the modularity of the proposed plug and play approach. Magn Reson Med 80:53-65, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
Collapse
Affiliation(s)
- Vincent Gras
- CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
| | | | - Alexandre Vignaud
- CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
| | - Alexis Amadon
- CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
| | - Denis Le Bihan
- CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
| | - Tony Stöcker
- German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany
| | - Nicolas Boulant
- CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
| |
Collapse
|
17
|
Gras V, Boland M, Vignaud A, Ferrand G, Amadon A, Mauconduit F, Le Bihan D, Stöcker T, Boulant N. Homogeneous non-selective and slice-selective parallel-transmit excitations at 7 Tesla with universal pulses: A validation study on two commercial RF coils. PLoS One 2017; 12:e0183562. [PMID: 28827835 PMCID: PMC5565195 DOI: 10.1371/journal.pone.0183562] [Citation(s) in RCA: 33] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/18/2017] [Accepted: 08/07/2017] [Indexed: 11/21/2022] Open
Abstract
Parallel transmission (pTx) technology, despite its great potential to mitigate the transmit field inhomogeneity problem in magnetic resonance imaging at ultra-high field (UHF), suffers from a cumbersome calibration procedure, thereby making the approach problematic for routine use. The purpose of this work is to demonstrate on two different 7T systems respectively equipped with 8-transmit-channel RF coils from two different suppliers (Rapid-Biomed and Nova Medical), the benefit of so-called universal pulses (UP), optimized to produce uniform excitations in the brain in a population of adults and making unnecessary the calibration procedures mentioned above. Non-selective and slice-selective UPs were designed to return homogeneous excitation profiles throughout the brain simultaneously on a group of ten subjects, which then were subsequently tested on ten additional volunteers in magnetization prepared rapid gradient echo (MPRAGE) and multi-slice gradient echo (2D GRE) protocols. The results were additionally compared experimentally with the standard non-pTx circularly-polarized (CP) mode, and in simulation with subject-specific tailored excitations. For both pulse types and both coils, the UP mode returned a better signal and contrast homogeneity than the CP mode. Retrospective analysis of the flip angle (FA) suggests that the FA deviation from the nominal FA on average over a healthy adult population does not exceed 11% with the calibration-free parallel-transmit pulses whereas it goes beyond 25% with the CP mode. As a result the universal pulses designed in this work confirm their relevance in 3D and 2D protocols with commercially available equipment. Plug-and-play pTx implementations henceforth become accessible to exploit with more flexibility the potential of UHF for brain imaging.
Collapse
Affiliation(s)
- Vincent Gras
- CEA/DRF/Joliot/NeuroSpin/Unirs, Gif sur Yvette, France
| | - Markus Boland
- German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany
| | | | | | - Alexis Amadon
- CEA/DRF/Joliot/NeuroSpin/Unirs, Gif sur Yvette, France
| | | | | | - Tony Stöcker
- German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany
| | - Nicolas Boulant
- CEA/DRF/Joliot/NeuroSpin/Unirs, Gif sur Yvette, France
- * E-mail:
| |
Collapse
|
18
|
Variable slice thickness (VAST) EPI for the reduction of susceptibility artifacts in whole-brain GE-EPI at 7 Tesla. MAGNETIC RESONANCE MATERIALS IN PHYSICS BIOLOGY AND MEDICINE 2017; 30:591-607. [DOI: 10.1007/s10334-017-0641-0] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/23/2017] [Revised: 06/23/2017] [Accepted: 06/26/2017] [Indexed: 01/11/2023]
|
19
|
Tse DH, da Silva NA, Poser BA, Shah NJ. B1+ inhomogeneity mitigation in CEST using parallel transmission. Magn Reson Med 2017; 78:2216-2225. [DOI: 10.1002/mrm.26624] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2016] [Revised: 12/05/2016] [Accepted: 01/07/2017] [Indexed: 12/18/2022]
Affiliation(s)
- Desmond H.Y. Tse
- Faculty of Psychology and Neuroscience; Maastricht University; Maastricht The Netherlands
| | - Nuno Andre da Silva
- Institute of Neuroscience and Medicine-4, Forschungszentrum Juelich GmbH, Wilhelm-Johnen-Strasse; Juelich Germany
| | - Benedikt A. Poser
- Faculty of Psychology and Neuroscience; Maastricht University; Maastricht The Netherlands
| | - N. Jon Shah
- Institute of Neuroscience and Medicine-4, Forschungszentrum Juelich GmbH, Wilhelm-Johnen-Strasse; Juelich Germany
- Department of Neurology; Faculty of Medicine, RWTH Aachen University, JARA; Aachen Germany
- Department of Electrical and Computer Systems Engineering; and Monash Biomedical Imaging, School of Psychological Sciences, Monash University; Melbourne Victoria Australia
| |
Collapse
|
20
|
Gras V, Vignaud A, Amadon A, Mauconduit F, Le Bihan D, Boulant N. New method to characterize and correct with sub-µs precision gradient delays in bipolar multispoke RF pulses. Magn Reson Med 2017; 78:2194-2202. [PMID: 28112827 DOI: 10.1002/mrm.26614] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2016] [Revised: 12/28/2016] [Accepted: 12/28/2016] [Indexed: 11/07/2022]
Abstract
PURPOSE Small gradient delays with respect to radiofrequency (RF) events can have disastrous effects on the performance of bipolar spokes RF pulses used in parallel transmission (pTx). In this work, we propose a new method to characterize and correct this delay with sub-µs precision. METHODS By determining experimentally the phase Δφ producing a 0 ° flip angle excitation in a α0°-α180°+Δφ bipolar two-spoke pulse configuration at multiple slice locations, we demonstrate the possibility of deducing the underlying gradient delay with precision. The technique also suggests prospectively compensating for the same delay by altering the phase of the second pulse. The approach was tested with a multislice gradient echo sequence on a phantom and on one healthy volunteer at 7 Tesla. RESULTS Application of the method returned an accuracy of approximately 50 ns on the gradient delay measurement, a performance shown in fact to be desirable for high-performance pTx 2D applications. Phase corrections of up to 180 ° on the second spoke RF pulse in the bipolar configuration allowed us to obtain similar performance as for unipolar designs, yet with significantly shorter excitations. CONCLUSIONS A simple and accurate gradient-delay calibration method was proposed that offers the possibility of using bipolar multispoke pulses in multislice protocols. Magn Reson Med 78:2194-2202, 2017. © 2017 International Society for Magnetic Resonance in Medicine.
Collapse
Affiliation(s)
- Vincent Gras
- CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France
| | | | - Alexis Amadon
- CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France
| | | | - Denis Le Bihan
- CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France
| | - Nicolas Boulant
- CEA, DRF, I2BM, NeuroSpin, Unirs, Gif-sur-Yvette, Cedex, France
| |
Collapse
|
21
|
Tse DHY, Wiggins CJ, Poser BA. Estimating and eliminating the excitation errors in bipolar gradient composite excitations caused by radiofrequency-gradient delay: Example of bipolar spokes pulses in parallel transmission. Magn Reson Med 2016; 78:1883-1890. [PMID: 28019035 PMCID: PMC6084356 DOI: 10.1002/mrm.26586] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2016] [Revised: 11/23/2016] [Accepted: 11/23/2016] [Indexed: 11/29/2022]
Abstract
Purpose To eliminate a slice‐position–dependent excitation error commonly observed in bipolar‐gradient composite excitations such as spokes pulses in parallel transmission. Theory and Methods An undesired timing delay between subpulses in the composite pulse and their bipolar slice‐selective gradient is hypothesized to cause the error. A mathematical model is presented here to relate this mismatch to an induced slice‐position–dependent phase difference between the subpulses. A new navigator method is proposed to measure the timing mismatch and eliminate the error. This is demonstrated at 7 Tesla with flip‐angle maps measured by a presaturation turbo‐flash sequence and in vivo images acquired by a simultaneous multislice/echo‐planar imaging (SMS‐EPI) sequence. Results Error‐free flip‐angle maps were obtained in two ways: 1) by correcting the time delay directly and 2) by applying the corresponding slice‐position–dependent phase differences to the subpulses. This confirms the validity of the mathematical description. The radiofrequency (RF)‐gradient delay measured by the navigator method was of 6.3 μs, which agreed well with the estimate from flip‐angle maps at different delay times. By applying the timing correction, accurately excited EPI images were acquired with bipolar dual‐spokes SMS‐2 excitations. Conclusion An effective correction is proposed to mitigate slice‐position–dependent errors in bipolar composite excitations caused by undesired RF‐gradient timing delays. Magn Reson Med 78:1883–1890, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.
Collapse
Affiliation(s)
- Desmond H Y Tse
- Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| | | | - Benedikt A Poser
- Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands
| |
Collapse
|