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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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Zampini MA, Sijbers J, Verhoye M, Garipov R. A preparation pulse for fast steady state approach in Actual Flip angle Imaging. Med Phys 2024; 51:306-318. [PMID: 37480220 DOI: 10.1002/mp.16624] [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: 02/23/2023] [Revised: 05/26/2023] [Accepted: 06/20/2023] [Indexed: 07/23/2023] Open
Abstract
BACKGROUND Actual Flip angle Imaging (AFI) is a sequence used for B1 mapping, also embedded in the Variable flip angle with AFI for simultaneous estimation of T1 , B1 and equilibrium magnetization. PURPOSE To investigate the design of a preparation module for AFI to allow a fast approach to steady state (SS) without requiring the use of dummy acquisitions. METHODS The features of a preparation module with a B1 insensitive adiabatic pulse, spoiler gradients, and a recovery timeT r e c $T_{rec}$ were studied with simulations and validated via experiments and acquired with different k-space traveling strategies. The robustness of the flip angle of the preparation pulse on the acquired signal is studied. RESULTS When a 90° adiabatic pulse is used, the forthcomingT r e c $T_{rec}$ can be expressed as a function of repetition times and AFI flip angle only asTR 1 ( n + cos α ) / ( 1 - cos 2 α ) $\mathrm{TR_1}(n+\cos \alpha )/(1-\cos ^2\alpha )$ , where n represents the ratio between the two repetition times of AFI. The robustness of the method is demonstrated by showing that using the values further away from 90° still allows for a faster approach to SS than the use of dummy pulses. CONCLUSIONS The preparation module is particularly advantageous for low flip angles, as well as for AFI sequences that sample the center of the k-space early in the sequence, such as centric ordering acquisitions, and for ultrafast EPI-based AFI methods, thus allowing to reduce scanner overhead time.
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Affiliation(s)
- Marco Andrea Zampini
- MR Solutions Ltd., Ashbourne House, Guildford, Surrey, UK
- Bio-Imaging Lab, Department of Biomedical Sciences, University of Antwerp, Belgium
| | - Jan Sijbers
- imec-Vision Lab, Department of Physics, University of Antwerp, Belgium
- μNEURO Research Centre of Excellence, University of Antwerp, Belgium
| | - Marleen Verhoye
- Bio-Imaging Lab, Department of Biomedical Sciences, University of Antwerp, Belgium
- μNEURO Research Centre of Excellence, University of Antwerp, Belgium
| | - Ruslan Garipov
- MR Solutions Ltd., Ashbourne House, Guildford, Surrey, UK
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de Buck MHS, Kent JL, Jezzard P, Hess AT. Head-and-neck multichannel B1 + mapping and RF shimming of the carotid arteries using a 7T parallel-transmit head coil. Magn Reson Med 2024; 91:190-204. [PMID: 37794847 PMCID: PMC10962593 DOI: 10.1002/mrm.29845] [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: 03/14/2023] [Revised: 08/01/2023] [Accepted: 08/10/2023] [Indexed: 10/06/2023]
Abstract
PURPOSE Neurovascular MRI suffers from a rapid drop in B1 + into the neck when using transmit head coils at 7 T. One solution to improving B1 + magnitude in the major feeding arteries in the neck is to use custom RF shims on parallel-transmit head coils. However, calculating such shims requires robust multichannel B1 + maps in both the head and the neck, which is challenging due to low RF penetration into the neck, limited dynamic range of multichannel B1 + mapping techniques, and B0 sensitivity. We therefore sought a robust, large-dynamic-range, parallel-transmit field mapping protocol and tested whether RF shimming can improve carotid artery B1 + magnitude in practice. METHODS A pipeline is presented that combines B1 + mapping data acquired using circularly polarized (CP) and CP2-mode RF shims at multiple voltages. The pipeline was evaluated by comparing the predicted and measured B1 + for multiple random transmit shims, and by assessing the ability of RF shimming to increase B1 + in the carotid arteries. RESULTS The proposed method achieved good agreement between predicted and measured B1 + in both the head and the neck. The B1 + magnitude in the carotid arteries can be increased by 43% using tailored RF shims or by 37% using universal RF shims, while also improving the RF homogeneity compared with CP mode. CONCLUSION B1 + in the neck can be increased using RF shims calculated from multichannel B1 + maps in both the head and the neck. This can be achieved using universal phase-only RF shims, facilitating easy implementation in existing sequences.
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Affiliation(s)
- Matthijs H. S. de Buck
- FMRIB Division, Nuffield Department of Clinical Neurosciences, Wellcome Centre for Integrative NeuroimagingUniversity of OxfordOxfordUK
| | - James L. Kent
- FMRIB Division, Nuffield Department of Clinical Neurosciences, Wellcome Centre for Integrative NeuroimagingUniversity of OxfordOxfordUK
| | - Peter Jezzard
- FMRIB Division, Nuffield Department of Clinical Neurosciences, Wellcome Centre for Integrative NeuroimagingUniversity of OxfordOxfordUK
| | - Aaron T. Hess
- FMRIB Division, Nuffield Department of Clinical Neurosciences, Wellcome Centre for Integrative NeuroimagingUniversity of OxfordOxfordUK
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