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PASTeUR: Package of Anatomical Sequences using parallel Transmission UniveRsal kT-point pulses1Neurospin, CEA, Université Paris-Saclay, Gif-sur-Yvette, France, 2Siemens Healthineers, Saint Denis, France, 3German Center for Neurodegenerative Diseases (DZNE), Bonn, Germany, 4Department of Cognitive Neuroscience, Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, Netherlands, 5Scannexus, Maastricht, Netherlands, 6Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States
Synopsis
Despite its power to counteract the inevitable radiofrequency field inhomogeneity problem at ultra-high field, parallel transmission has failed to be embraced by the community in routine due to a cumbersome workflow. Universal pulses have shown great potential to circumvent this problem by providing plug and play solutions. Here we validate a package of 3D anatomical sequences for a given commercial coil covering multiple contrasts for use in clinical routine and including, thanks to their versatility, very few pulse solutions. The utilization of universal kT-points enables direct embedding of these pulses in the sequences and easy handling of the power/SAR limits.
Introduction
Parallel transmission (pTx) is the most promising technology to mitigate the radiofrequency (RF) field inhomogeneity problem at ultra-high field because of its versatility and its ability to tame the SAR. One major obstacle however has been the inherent cumbersome workflow involving subject-based calibration (field map measurements), data processing and online pulse design, incurring to the user a significant time penalty. Universal pulses (UPs) were proposed to bypass entirely the calibration procedure by providing plug and play pTx solutions at no cost for the user1. They are based on an offline pulse design performed on a database of different subject field maps to be robust with respect to intersubject variability. Non-selective and selective pulses this way were shown to counteract RF field inhomogeneity in the human brain at 7T for several applications and at a mild cost in performance compared to the tailored-based approach2-4. Here we report the integration of non-selective universal kT-points pulses5 in a package named PASTeUR, covering the 3D GRE, MPRAGE, SPACE, FLAIR with T2-preparation and DIR sequences, and handling SAR and RF coil power limits. As a first step, the package was developed for compatibility with the Siemens step 2.3 protected mode.Methods
All sequences incorporated non-selective kT-point pulses5 designed on a database of 20 subject field maps2 acquired on a 7T Siemens (Siemens Healthcare, Erlangen, Germany) Magnetom scanner equipped with the Nova (Nova Medical, Wilmington, MA, USA) 8Tx-32Rx pTx coil. A second order optimization scheme with explicit constraints and with simultaneous optimization of the k-space trajectory was employed for pulse design6. The 3D GRE embeds 3 scalable pulses (maximum flip angles of 10, 20 and 60°) of different durations (570, 800 and 1160 µs respectively) to handle different energy demands. The MPRAGE integrates a 3.68 ms-inversion pulse, designed with a GPU-based Bloch simulator, and a small tip angle pulse (570 µs) reaching up to 8°. The readout of the SPACE, FLAIR and DIR sequences was built upon a single 1.04 ms-long refocusing kT-point pulse that can likewise be scaled to match a given flip angle train3. The inversions for the DIR are the same as for the MPRAGE. The T2-preparation for the FLAIR is constituted of a 90°, delay, 180°, delay and 90° pulse. These pulses were designed independently due to the phase coherence constraint imposed among them. All designs were performed to be compatible with Siemens protected mode step 2.3, i.e. with peak amplitude limits of 165 V and average power limits of 1.5 W per channel and 8 W total for the coil of interest. Given the low number of pulse solutions (1, for the GRE and SPACE sequences and 2 for the MPRAGE, 3D FLAIR and 3D DIR sequences), power assessment could simply be made by calculating their respective energies and weighting them with their corresponding duty cycles. The sequence thereby easily forbids parameters that would exceed the 6-min time average power limits to prevent scan abortion during runtime. The package was tested in vivo on 3 healthy volunteers at 7T.Results
Table 1 summarizes the durations and flip angle normalized root mean square errors (FA-NRMSE) calculated over the 20 subjects of the database and for all pulses employed in the package. Figures 1 and 2 report 3 orthogonal view brain images on 1 volunteer acquired with the MPRAGE, SPACE, FLAIR with T2 preparation, and DIR sequences, for the CP mode and universal pulse solutions respectively. The CP mode is clearly outperformed by the universal pulse solutions with no pTx-specific procedure for the user, yielding images virtually-free of B1+ artefacts.Conclusion
We have reported in this work the development and validation of a package of anatomical sequences directly embedding calibration-free pTx solutions to mitigate the RF field inhomogeneity problem in brain imaging at 7T. For easier handling of SAR/power constraints, the package for the moment conforms to the Siemens protected mode of operation where only peak and average power limits need to be fulfilled. Future work includes the use of validated Virtual Observation Points for less conservative SAR assessments and higher power limits. Yet, for the package of sequences presented here, these limits did not constitute a serious constraint, thanks mostly to the coil transmit efficiency. Finally, although this package was tested here on a small number of volunteers to propose useful default protocols, individual sequences with the same universal pulses cumulate close to a 50 volunteers experience across 4 different sites2-4,7. They have never failed to suppress the ubiquitous B1+ artefacts observed with the CP excitation mode.Acknowledgements
The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Program (FP7/2013-2018), ERC Grant Agreement n. 309674. B.A.P. is funded by the Netherlands Organization for Scientific Research (NWO 016.Vidi.178.052) and the National Institute of Health (R01MH111444, PI Feinberg). X.W. and K.U. were supported by NIH grants U01 EB025144 and P41 EB015894.References
[1] Gras V, Vignaud A, Amadon A, Le Bihan D, Boulant N. Universal pulses: A new concept for calibration-free parallel transmission. Magnetic Resonance in Medicine 2017;77:635–643 doi: 10.1002/mrm.26148.
[2] Gras V, Boland M, Vignaud A, et al. 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 doi: 10.1371/journal.pone.0183562.
[3] Gras V, Mauconduit F, Vignaud A, et al. Design of universal parallel-transmit refocusing k T -point pulses and application to 3D T 2 -weighted imaging at 7T: Universal Pulse Design of 3D Refocusing Pulses. Magnetic Resonance in Medicine 2018;80:53–65 doi: 10.1002/mrm.27001.
[4] Pracht E, Gras V, Boulant N, Stöcker T. Whole Brain FLAIR Imaging at 7T Employing Universal Pulses. In: Proceedings of the 26th Annual Meeting of ISMRM. Paris; 2018. p. Abstract 585.
[5] Cloos MA, Boulant N, Luong M, et al. kT-points: Short three-dimensional tailored RF pulses for flip-angle homogenization over an extended volume. Magn Reson Med 2012;67:72–80 doi: 10.1002/mrm.22978.
[6] 1. Gras V, Luong M, Amadon A, Boulant N. Joint design of kT-points trajectories and {RF} pulses under explicit {SAR} and power constraints in the large flip angle regime. Journal of Magnetic Resonance 2015;261:181 – 189 doi: http://dx.doi.org/10.1016/j.jmr.2015.10.017.
[7] Wu X, Gras V, Vignaud, A, et al. The travelling pulses: multicenter evaluation of universal pulses at 7T. In: Proceedings of the 26th Annual Meeting of ISMRM. Paris; 2018.
[8] Boulant N, Le Bihan D, Amadon A. Strongly modulating pulses: a new method for tackling RF inhomogeneity problems at high fields. Magnetic Resonance in Medicine 2008;68:701–708.
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