Design of universal parallel-transmit refocusing kT-point pulses and application to 3D T2-weighted imaging at 7T
Vincent Gras
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorAlexandre Vignaud
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorAlexis Amadon
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorDenis Le Bihan
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorTony Stöcker
German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany
Search for more papers by this authorCorresponding Author
Nicolas Boulant
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Correspondence to: Nicolas Boulant, Ph.D., CEA Saclay, 91191 Gif sur Yvette Cedex, France. E-mail: [email protected]Search for more papers by this authorVincent Gras
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorAlexandre Vignaud
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorAlexis Amadon
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorDenis Le Bihan
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Search for more papers by this authorTony Stöcker
German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany
Search for more papers by this authorCorresponding Author
Nicolas Boulant
CEA, DRF, Joliot, NeuroSpin, Unirs, CEA Saclay, Gif sur Yvette, France
Correspondence to: Nicolas Boulant, Ph.D., CEA Saclay, 91191 Gif sur Yvette Cedex, France. E-mail: [email protected]Search for more papers by this authorAbstract
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.
Supporting Information
Additional Supporting Information may be found in the online version of this article.
| Filename | Description |
|---|---|
| mrm27001-sup-0001-suppinfo01.avi3.8 MB | Video S1. Comparison of the CP mode and the UP mode for the T2-weighted SPACE sequence. The video provides a comparison of the CP mode (left image) and the UP mode (right image) for the T2-weighted SPACE sequence for subjects 1–5. The introduction of UPs allows restoring the signal and the contrast in all brain areas where the CP mode often fails (i.e., the cerebellum, occipital lobe, and left and right temporal lobes). |
| mrm27001-sup-0002-suppinfo02.docx189.9 KB | Fig. S1. Simulations of the T2-weighted SPACE signal taking into account pulse imperfections. Retrospective control of (a) the transmit field map of the CP mode, (b) the SPACE signal for the CP mode, and (c) the SPACE signal obtained with the universal pulses. The SPACE signal simulations consist in (1) computing in each voxel the exact propagator of all pulses (90° excitation and refocusing pulses, i.e., ETL+1 pulses) and (2) reproducing in each voxel the nominal signal calculation of Mugler et al. (23) while taking the exact rotation matrices instead of the idealized ones for the pulse propagators. For simplicity, the signal was taken as the echo amplitude at the k-space origin. The maximum of the CP mode transmit field is noticeably higher for subject 2 (0.21 µT/V) that for the other subjects (∼0.18 µT/V). The SPACE signal simulation of the CP mode confirms the pronounced signal dropout in the midbrain in subject 2, which disappears with the application of UPs. |
Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
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