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Aigner CS, Sánchez Alarcon MF, D'Astous A, Alonso-Ortiz E, Cohen-Adad J, Schmitter S. Calibration-free parallel transmission of the cervical, thoracic, and lumbar spinal cord at 7T. Magn Reson Med 2024; 92:1496-1510. [PMID: 38733068 DOI: 10.1002/mrm.30137] [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: 02/16/2024] [Revised: 04/16/2024] [Accepted: 04/16/2024] [Indexed: 05/13/2024]
Abstract
PURPOSE To address the limitations of spinal cord imaging at ultra-high field (UHF) due to time-consuming parallel transmit (pTx) adjustments. This study introduces calibration-free offline computed universal shim modes that can be applied seamlessly for different pTx RF coils and spinal cord target regions, substantially enhancing spinal cord imaging efficiency at UHF. METHODS A library of channel-wise relativeB 1 + $$ {B}_1^{+} $$ maps for the cervical spinal cord (six datasets) and thoracic and lumbar spinal cord (nine datasets) was constructed to optimize transmit homogeneity and efficiency for these regions. A tailored B0 shim was optimized for the cervical spine to enhance spatial magnetic field homogeneity further. The performance of the universal shims was validated using absolute saturation basedB 1 + $$ {B}_1^{+} $$ mapping and high-resolution 2D and 3D multi-echo gradient-recalled echo (GRE) data to assess the image quality. RESULTS The proposed universal shims demonstrated a 50% improvement inB 1 + $$ {B}_1^{+} $$ efficiency compared to the default (zero phase) shim mode.B 1 + $$ {B}_1^{+} $$ homogeneity was also improved by 20%. The optimized universal shims achieved performance comparable to subject-specific pTx adjustments, while eliminating the need for lengthy pTx calibration times, saving about 10 min per experiment. CONCLUSION The development of universal shims represents a significant advance by eliminating time-consuming subject-specific pTx adjustments. This approach is expected to make UHF spinal cord imaging more accessible and user-friendly, particularly for non-pTx experts.
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Affiliation(s)
- Christoph S Aigner
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Manuel F Sánchez Alarcon
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
- Working Group on Cardiovascular Magnetic Resonance, Experimental and Clinical Research Center, A Joint Cooperation between the Charité Medical Faculty and the Max-Delbrück Center for Molecular Medicine, Berlin, Germany
| | - Alexandre D'Astous
- NeuroPoly Lab, Institute of Biomedical Engineering, Polytechnique Montréal, Montréal, Quebec, Canada
- Centre de recherche du CHU Sainte-Justine, Université de Montréal, Montréal, Quebec, Canada
| | - Eva Alonso-Ortiz
- Functional Neuroimaging Unit, CRIUGM, Université de Montréal, Montréal, Quebec, Canada
- Mila-Quebec AI Institute, Montréal, Quebec, Canada
| | - Julien Cohen-Adad
- Functional Neuroimaging Unit, CRIUGM, Université de Montréal, Montréal, Quebec, Canada
- Mila-Quebec AI Institute, Montréal, Quebec, Canada
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Sebastian Schmitter
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
- Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
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Egger N, Nagelstraßer S, Wildenberg S, Bitz A, Ruck L, Herrler J, Meixner CR, Kimmlingen R, Lanz T, Schmitter S, Uder M, Nagel AM. Accelerated B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping and robust parallel transmit pulse design for heart and prostate imaging at 7 T. Magn Reson Med 2024. [PMID: 38888143 DOI: 10.1002/mrm.30185] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/29/2023] [Revised: 04/26/2024] [Accepted: 05/17/2024] [Indexed: 06/20/2024]
Abstract
PURPOSE To investigate the impact of reduced k-space sampling onB 1 + $$ {\mathrm{B}}_1^{+} $$ mapping and the resulting impact on phase shimming and dynamic/universal parallel transmit (pTx) RF pulse design. METHODS Channel-wise 3DB 1 + $$ {\mathrm{B}}_1^{+} $$ maps were measured at 7 T in 35 and 23 healthy subjects for the heart and prostate region, respectively. With theseB 1 + $$ {\mathrm{B}}_1^{+} $$ maps, universal phase shims optimizing homogeneity andB 1 + $$ {\mathrm{B}}_1^{+} $$ efficiency were designed for heart and prostate imaging. In addition, universal 4kT-point pulses were designed for the heart. Subsequently, individual phase shims and individual 4kT-pulses were designed based onB 1 + $$ {\mathrm{B}}_1^{+} $$ maps with different acceleration factors and tested on the original maps. The performance of the pulses was compared by evaluating their coefficients of variation (CoV),B 1 + $$ {\mathrm{B}}_1^{+} $$ efficiencies and specific energy doses (SED). Furthermore, validation measurements were carried out for one heart and one prostate subject. RESULTS For both organs, the universal phase shims showed significantly higherB 1 + $$ {\mathrm{B}}_1^{+} $$ efficiencies and lower CoVs compared to the vendor provided default shim, but could still be improved with individual phase shims based on acceleratedB 1 + $$ {\mathrm{B}}_1^{+} $$ maps (acquisition time = 30 s). In the heart, the universal 4kT-pulse achieved significantly lower CoVs than tailored phase shims. Tailored 4kT-pulses based on acceleratedB 1 + $$ {\mathrm{B}}_1^{+} $$ maps resulted in even further reduced CoVs or a 2.5-fold reduction in SED at the same CoVs as the universal 4kT-pulse. CONCLUSION AcceleratedB 1 + $$ {\mathrm{B}}_1^{+} $$ maps can be used for the design of tailored pTx pulses for prostate and cardiac imaging at 7 T, which further improve homogeneity,B 1 + $$ {\mathrm{B}}_1^{+} $$ efficiency, or SED compared to universal pulses.
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Affiliation(s)
- Nico Egger
- Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
| | - Sophia Nagelstraßer
- Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
| | - Saskia Wildenberg
- Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
- Electrical Engineering and Information Technology, University of Applied Sciences - FH Aachen, Aachen, Germany
| | - Andreas Bitz
- Electrical Engineering and Information Technology, University of Applied Sciences - FH Aachen, Aachen, Germany
| | - Laurent Ruck
- Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
| | | | | | | | | | - Sebastian Schmitter
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Michael Uder
- Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
| | - Armin Michael Nagel
- Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
- Division of Medical Physics in Radiology, German Cancer Research Centre (DKFZ), Heidelberg, Germany
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Kilic T, Liebig P, Demirel OB, Herrler J, Nagel AM, Ugurbil K, Akçakaya M. Unsupervised deep learning with convolutional neural networks for static parallel transmit design: A retrospective study. Magn Reson Med 2024; 91:2498-2507. [PMID: 38247050 PMCID: PMC10997461 DOI: 10.1002/mrm.30014] [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: 04/07/2023] [Revised: 12/28/2023] [Accepted: 12/29/2023] [Indexed: 01/23/2024]
Abstract
PURPOSE To mitigateB 1 + $$ {B}_1^{+} $$ inhomogeneity at 7T for multi-channel transmit arrays using unsupervised deep learning with convolutional neural networks (CNNs). METHODS Deep learning parallel transmit (pTx) pulse design has received attention, but such methods have relied on supervised training and did not use CNNs for multi-channelB 1 + $$ {B}_1^{+} $$ maps. In this work, we introduce an alternative approach that facilitates the use of CNNs with multi-channelB 1 + $$ {B}_1^{+} $$ maps while performing unsupervised training. The multi-channelB 1 + $$ {B}_1^{+} $$ maps are concatenated along the spatial dimension to enable shift-equivariant processing amenable to CNNs. Training is performed in an unsupervised manner using a physics-driven loss function that minimizes the discrepancy of the Bloch simulation with the target magnetization, which eliminates the calculation of reference transmit RF weights. The training database comprises 3824 2D sagittal, multi-channelB 1 + $$ {B}_1^{+} $$ maps of the healthy human brain from 143 subjects.B 1 + $$ {B}_1^{+} $$ data were acquired at 7T using an 8Tx/32Rx head coil. The proposed method is compared to the unregularized magnitude least-squares (MLS) solution for the target magnetization in static pTx design. RESULTS The proposed method outperformed the unregularized MLS solution for RMS error and coefficient-of-variation and had comparable energy consumption. Additionally, the proposed method did not show local phase singularities leading to distinct holes in the resulting magnetization unlike the unregularized MLS solution. CONCLUSION Proposed unsupervised deep learning with CNNs performs better than unregularized MLS in static pTx for speed and robustness.
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Affiliation(s)
- Toygan Kilic
- Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota, USA
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | | | - Omer Burak Demirel
- Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota, USA
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | | | - Armin M Nagel
- Institute of Radiology, University Hospital Erlangen, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen, Germany
- Division of Medical Physics in Radiology, German Cancer Research Centre (DKFZ), Heidelberg, Germany
| | - Kamil Ugurbil
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
| | - Mehmet Akçakaya
- Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota, USA
- Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA
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Williams SN, McElhinney P, Gunamony S. Ultra-high field MRI: parallel-transmit arrays and RF pulse design. Phys Med Biol 2023; 68. [PMID: 36410046 DOI: 10.1088/1361-6560/aca4b7] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/28/2022] [Accepted: 11/21/2022] [Indexed: 11/22/2022]
Abstract
This paper reviews the field of multiple or parallel radiofrequency (RF) transmission for magnetic resonance imaging (MRI). Currently the use of ultra-high field (UHF) MRI at 7 tesla and above is gaining popularity, yet faces challenges with non-uniformity of the RF field and higher RF power deposition. Since its introduction in the early 2000s, parallel transmission (pTx) has been recognized as a powerful tool for accelerating spatially selective RF pulses and combating the challenges associated with RF inhomogeneity at UHF. We provide a survey of the types of dedicated RF coils used commonly for pTx and the important modeling of the coil behavior by electromagnetic (EM) field simulations. We also discuss the additional safety considerations involved with pTx such as the specific absorption rate (SAR) and how to manage them. We then describe the application of pTx with RF pulse design, including a practical guide to popular methods. Finally, we conclude with a description of the current and future prospects for pTx, particularly its potential for routine clinical use.
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Affiliation(s)
- Sydney N Williams
- Imaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom
| | - Paul McElhinney
- Imaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom
| | - Shajan Gunamony
- Imaging Centre of Excellence, University of Glasgow, Glasgow, United Kingdom.,MR CoilTech Limited, Glasgow, United Kingdom
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Aigner CS, Dietrich S, Schaeffter T, Schmitter S. Calibration-free pTx of the human heart at 7T via 3D universal pulses. Magn Reson Med 2021; 87:70-84. [PMID: 34399002 DOI: 10.1002/mrm.28952] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2021] [Revised: 07/19/2021] [Accepted: 07/19/2021] [Indexed: 02/03/2023]
Abstract
PURPOSE MRI at ultra-high fields in the human body is highly challenging and requires lengthy calibration times to compensate for spatially heterogeneous B 1 + profiles. This study investigates the feasibility of using pre-computed universal pulses for calibration-free homogeneous 3D flip angle distribution in the human heart at 7T. METHODS Twenty-two channel-wise 3D B 1 + data sets were acquired under free-breathing in 19 subjects to generate a library for an offline universal pulse (UP) design (group 1: 12 males [M] and 7 females [F], 21-66 years, 19.8-28.3 kg/m2 ). Three of these subjects (2M/1F, 21-33 years, 20.8-23.6 kg/m2 ) were re-scanned on different days. A 4kT-points UP optimized for the 22 channel-wise 3D B 1 + data sets in group 1 (UP22-4kT) is proposed and applied at 7T in 9 new and unseen subjects (group 2: 4M/5F, 25-56 years, 19.5-35.3 kg/m2 ). Multiple tailored and universal static and dynamic parallel-transmit (pTx) pulses were designed and evaluated for different permutations of the B 1 + data sets in group 1 and 2. RESULTS The proposed UP22-4kT provides low B 1 + variation in all subjects, seen and unseen, without severe signal drops. Experimental data at 7T acquired with UP22-4kT shows comparable image quality as data acquired with tailored-4kT pulses and demonstrates successful calibration-free pTx of the human heart. CONCLUSION UP22-4kT allows for calibration-free homogeneous flip angle distributions across the human heart at 7T. Large inter-subject variations because of sex, age, and body mass index are well tolerated. The proposed universal pulse removes the need for lengthy (10-15 min) calibration scans and therefore has the potential to bring body imaging at 7T closer to the clinical application.
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Affiliation(s)
| | - Sebastian Dietrich
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany
| | - Tobias Schaeffter
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany.,Division of Imaging Sciences and Biomedical Engineering, King's College London, London, United Kingdom
| | - Sebastian Schmitter
- Physikalisch-Technische Bundesanstalt (PTB), Braunschweig and Berlin, Germany.,Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, Minnesota, USA.,Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany
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