Parallel transmission is a very promising candidate technology to mitigate the inevitable radiofrequency (RF) field inhomogeneity in magnetic resonance imaging (MRI) at ultra-high field (UHF). For the first few years, pulse design utilizing this technique was expressed as a least squares problem with crude power regularizations aimed at controlling the specific absorption rate (SAR), hence the patient safety. This approach being suboptimal for many applications sensitive mostly to the magnitude of the spin excitation, and not its phase, the magnitude least squares (MLS) problem then was first formulated in 2007. Despite its importance and the availability of other powerful numerical optimization methods, the MLS problem yet has been faced almost exclusively by the pulse designer with the so-called variable exchange method. In this paper, we investigate various two-stage strategies consisting of different initializations and nonlinear programming approaches, and incorporate directly the strict SAR and hardware constraints. Several schemes such as sequential quadratic programming (SQP), interior point (I-P) methods, semidefinite programming (SDP) and magnitude squared least squares (MSLS) relaxations are studied both in the small and large tip angle regimes with RF and static field maps obtained in-vivo on a human brain at 7 Tesla. Convergence and robustness of the different approaches are analyzed, and recommendations to tackle this specific problem are finally given. Small tip angle and inversion pulses are returned in a few seconds and in under a minute respectively while respecting the constraints, allowing the use of the proposed approach in routine.

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This work demonstrates that the adopted formalism based on optimal control, combined with the kT -point method, successfully enables three-dimensional T2 -weighted brain imaging at 7T devoid of artifacts resulting from B1 (+) inhomogeneity.

Purpose: To investigate, via numerical simulations, the compliance of the specific absorption rate (SAR) versus temperature guidelines for the human head in magnetic resonance imaging procedures utilizing parallel transmission at high field. Materials and Methods:A combination of finite element and finite-difference time-domain methods was used to calculate the evolution of the temperature distribution in the human head for a large number of parallel transmission scenarios. The computations were performed on a new model containing 20 anatomical structures.Results: Among all the radiofrequency field exposure schemes simulated, the recommended 39 C maximum local temperature was never exceeded when the local 10-g average SAR threshold was reached. On the other hand, the maximum temperature barely complied with its guideline when the global SAR reached 3.2 W/kg. The maximal temperature in the eye could very well rise by more than 1 C in both cases. Conclusion:Considering parallel transmission, the recommended values of local 10-g SAR may remain a relevant metric to ensure that the local temperature inside the human head never exceeds 39 C, although it can lead to rises larger than 1 C in the eye. Monitoring temperature instead of SAR can provide increased flexibility in pulse design for parallel transmission.

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While T1 measurements present multiple challenges (robustness, acquisition time), the recently proposed MP2RAGE sequence (magnetization‐prepared two rapid acquisition gradient echoes) has opened new perspectives to characterize tissue microstructure changes occurring in a pathological or developmental context. Extensively used for brain studies, it was herein adapted to investigate the cervical spinal cord (SC) at 3 T. By integrating Bloch equations, the MP2RAGE sequence parameters were chosen to optimize SC gray matter/white matter (GM/WM) T1 contrast with sub‐millimetric resolution, a scan time less than 10 min and a reliable T1 determination with minimal B1+ variation effect, within a range of values compatible with different pathologies and surrounding structures. The residual B1+ effect on T1 values was corrected using a look‐up‐table approach and B1+ mapping. The accuracy of B1+‐corrected T1 measurements was assessed on a phantom with respect to conventional inversion recovery. In vivo MP2RAGE acquisitions were performed on five young (28.8 ± 4.3 years old) and five elderly (60.2 ± 2.9 years old) volunteers and analyzed using a template‐based approach. Phantom experiments led to high agreements between inversion‐recovery spin‐echo and MP2RAGE‐based T1 values (R2 = 0.997). In vivo T1 values for cervical WM, anterior GM (aGM), posterior sensory tracts (PSTs) and lateral motor tracts (LMTs) were 917 ± 29 s, 934 ± 33 ms, 920 ± 37 ms and 877 ± 35 ms, respectively, with all subjects and cervical levels considered. Significant differences were observed between aGM and LMTs, and between LMTs and PSTs, in agreement with the literature. Repeated T1 measurements demonstrated high reproducibility of the MP2RAGE in the SC (variation coefficient < 5% in all regions of interest). Finally, preliminary assessment of age‐related SC tissue microstructure variation additionally showed evidence of SC atrophy and slight trends of T1 decrease with age in all regions. Overall, this study shows that fast, robust and accurate sub‐millimetric resolution T1 mapping in the cervical SC using the MP2RAGE sequence is possible, paving the way for future multi‐centric and longitudinal clinical studies investigating the pathological cord.