Design of parallel transmission radiofrequency pulses robust against respiration in cardiac MRI at 7 Tesla
Corresponding Author
Sebastian Schmitter
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Correspondence to: Sebastian Schmitter, Ph.D., Center for Magnetic Resonance Research, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455. E-mail: [email protected]Search for more papers by this authorXiaoping Wu
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Search for more papers by this authorKâmil Uğurbil
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Search for more papers by this authorPierre-François Van de Moortele
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Search for more papers by this authorCorresponding Author
Sebastian Schmitter
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Correspondence to: Sebastian Schmitter, Ph.D., Center for Magnetic Resonance Research, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455. E-mail: [email protected]Search for more papers by this authorXiaoping Wu
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Search for more papers by this authorKâmil Uğurbil
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Search for more papers by this authorPierre-François Van de Moortele
University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA
Search for more papers by this authorAbstract
Purpose
Two-spoke parallel transmission (pTX) radiofrequency (RF) pulses have been demonstrated in cardiac MRI at 7T. However, current pulse designs rely on a single set of B1+/B0 maps that may not be valid for subsequent scans acquired at another phase of the respiration cycle because of organ displacement. Such mismatches may yield severe excitation profile degradation.
Methods
B1+/B0 maps were obtained, using 16 transmit channels at 7T, at three breath-hold positions: exhale, half-inhale, and inhale. Standard and robust RF pulses were designed using maps obtained at exhale only, and at multiple respiratory positions, respectively. Excitation patterns were analyzed for all positions using Bloch simulations. Flip-angle homogeneity was compared in vivo in cardiac CINE acquisitions.
Results
Standard one- and two-spoke pTX RF pulses are sensitive to breath-hold position, primarily due to B1+ alterations, with high dependency on excitation trajectory for two spokes. In vivo excitation inhomogeneity varied from nRMSE = 8.2% (exhale) up to 32.5% (inhale) with the standard design; much more stable results were obtained with the robust design with nRMSE = 9.1% (exhale) and 10.6% (inhale).
Conclusion
A new pTX RF pulse design robust against respiration induced variations of B1+/B0 maps is demonstrated and is expected to have a positive impact on cardiac MRI in breath-hold, free-breathing, and real-time acquisitions. Magn Reson Med 74:1291–1305, 2015. © 2014 Wiley Periodicals, Inc.
Supporting Information
Additional Supporting Information may be found in the online version of this article.
| Filename | Description |
|---|---|
| mrm25512-sup-0001-suppfigs.pdf1 MB | Supporting Figure S1. a) 1-spoke optimization curve between Energy (En) and for different tradeoff parameters λ. The diagram reveals two different L-curves, among which the curve closer to the origin describes acceptable solutions while the upwards shifted curve reflects solutions with a pronounced |B1+| minimum within the ROI. Two examples (setting 1 and 2) marked in a) are shown in b) for three different respiratory positions. Changes of nRMSE with different breathing positions are shown in c) for (c) and (d) as a function of for different λ values. For these diagrams, only the acceptable solutions (lower L-curve in a) are displayed. Relative changes of the diagrams shown in c) and d) are displayed in e) as ratio values and as a function of . Supporting Figure S2. Top row: En, Emax, and nRMSE values as a function of the two-spoke k space trajectories in a polar coordinate system defined by the spokes location. The optimization was performed using an electromagnetic simulation dataset of the same RF coil and similar nRMSE values were targeted for the optimization as used in-vivo. Bottom row: corresponding global and 10g average local SAR values for each spoke trajectory calculated for a 10° flip angle using same RF pulse durations and same duty cycle as utilized in vivo. |
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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