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Design of universal parallel-transmit refocusing kT -point pulses and application to 3D T2 -weighted imaging at 7T.

Abstract：

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.

journal_name

Magn Reson Med

journal_title

Magnetic resonance in medicine

authors

Gras V,Mauconduit F,Vignaud A,Amadon A,Le Bihan D,Stöcker T,Boulant N

doi

10.1002/mrm.27001

subject

Has Abstract

pub_date

2018-07-01 00:00:00

pages

53-65

issue

eissn

0740-3194

issn

1522-2594

journal_volume

pub_type

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Design of universal parallel-transmit refocusing kT -point pulses and application to 3D T2 -weighted imaging at 7T.