Optimized ultrahigh field parallel transmission workflow 

Radiofrequency (RF) coils with multiple transmit elements, so-called array coils, are commonly used in ultrahigh field MRI. When using them for parallel transmit (pTx) applications, it is important to have accurate knowledge of the RF field distribution (B1+-Field) generated by each individual element. In addition, information about the spatial distribution of the static magnetic field (B0) is also often included, to account for spin dephasing during the RF pulse design. This data is typically recorded at the beginning of the experiment and used throughout to calculate pTx RF pulses or gradient trajectories. Therefore there is a need for accurate, fast and reliable calibration scans.

In this work, we implemented and analyzed how a three-dimensional 3D presaturated TurboFLASH (satTFL) sequence can be used in a pTx workflow as a robust and easy to use method. The goal of this study was to have a single sequence that allows for three-dimensional B1+ and B0 mapping, as well as for creating a tissue- and a brain mask. From this dataset, subject-specific RF pulses and gradient waveforms can be calculated. The method was compared with established methods for 3D mapping of B1+ fields, such as the double angle method (DAM), which requires a very long acquisition time (TA); actual flip angle imaging (AFI), which causes a substantial specific energy deposition (SED); and dual refocusing echo acquisition mode (DREAM), which covers a fairly limited dynamic range. While DAM, AFI and DREAM were described for 3D mapping before, the sat-TFL technique was mostly described for two-dimensional scans in the past.

Of the sequences compared, 3D satTFL was the only one with a mapping range exceeding well over 90°. The sequence was shown to provide accurate B1+ and B0 maps for an 8 channel pTx system within less than 2 minutes. The single channel maps were successfully used for pTx pulse calculation in a separate study.