Abstract
Parallel transmission (pTx) technology, despite its great potential to mitigate the transmit field inhomogeneity problem in magnetic resonance imaging at ultra-high field (UHF), suffers from a cumbersome calibration procedure, thereby making the approach problematic for routine use. The purpose of this work is to demonstrate on two different 7T systems respectively equipped with 8-transmit-channel RF coils from two different suppliers (Rapid-Biomed and Nova Medical), the benefit of so-called universal pulses (UP), optimized to produce uniform excitations in the brain in a population of adults and making unnecessary the calibration procedures mentioned above. Non-selective and slice-selective UPs were designed to return homogeneous excitation profiles throughout the brain simultaneously on a group of ten subjects, which then were subsequently tested on ten additional volunteers in magnetization prepared rapid gradient echo (MPRAGE) and multi-slice gradient echo (2D GRE) protocols. The results were additionally compared experimentally with the standard non-pTx circularly-polarized (CP) mode, and in simulation with subject-specific tailored excitations. For both pulse types and both coils, the UP mode returned a better signal and contrast homogeneity than the CP mode. Retrospective analysis of the flip angle (FA) suggests that the FA deviation from the nominal FA on average over a healthy adult population does not exceed 11% with the calibration-free parallel-transmit pulses whereas it goes beyond 25% with the CP mode. As a result the universal pulses designed in this work confirm their relevance in 3D and 2D protocols with commercially available equipment. Plug-and-play pTx implementations henceforth become accessible to exploit with more flexibility the potential of UHF for brain imaging.
Highlights
Magnetic Resonance Imaging (MRI) of the brain has improved considerably in the last decade with the advent of ultra-high field (UHF) scanners (B0 ! 7T)
Similar comparisons are provided for the 2D gradient recalled echo (GRE) protocol in Figs 4 and 5
The flip angle (FA)-NRMSE comparisons summarized in Table 1 show that the proposed universal pulses (UP) approach clearly outperforms the CP and RF shim modes in 3D while for the slice-selective 30 ̊ pulses, the whole brain performance (3D-FA-NRMSE) of UPs is at least comparable to the subject and slice specific RF-shims
Summary
Magnetic Resonance Imaging (MRI) of the brain has improved considerably in the last decade with the advent of ultra-high field (UHF) scanners (B0 ! 7T). The use of high field strengths comes with a dramatic increase in the radiofrequency (RF) field inhomogeneity which can severely degrade imaging performance. Within this context, parallel transmission (pTx) [1,2] has revealed a great potential in mitigating the transmit field inhomogeneity problem in the human head at field strengths equal or larger than 7T [3,4,5,6,7,8,9,10,11]. While pRx increases image SNR and encodes spatial information via the reception profile, pTx provides additional degrees of freedom to shape the total excitation profile and to reduce the Specific Absorption Rate (SAR), a critical limiting measure at UHF
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