Second, channels’ electric fields depend on the same factors, including load position. First, the issues described previously cannot be corrected retrospectively without motion-resolved B 1 + maps, which are not available. Motion artifacts are often addressed through retrospective correction 22, 24- 27 however, this is problematic for several reasons. In contrast with these approaches, data-driven approaches inherently incorporate these changes. However, interactions between channels’ highly nonuniform transmit fields at 7 T, 23 especially with pTx, indicate that dynamic motion-induced field changes cannot be overlooked. Neither approach considered dynamic motion-related field changes (e.g., changes in coil loading, shifting susceptibility gradients in tissue), as their effects were deemed minimal at 3 T. 22 Extrapolated maps were used for retrospective correction. used radial basis functions to extrapolate channel sensitivities to voxel locations outside of the head, providing sensitivity information for all voxels in the FOV, regardless of head position. partially overcame motion-related effects on the (receive) B 1 field by simply reorienting coils’ measured sensitivity maps using a Euclidean transformation.
Because flip angle (and therefore the acquired signal) depends on B 1 +, displacements of approximately 5° have been found to cause an excitation error of 12%–19% (percent of target flip angle) when using pTx at 7 T, 12 with larger movements causing larger flip angle–related artifacts.Ī few approaches have been proposed to correct motion-related RF field changes. 15 Large head movements (exceeding 20 mm/degree) often occur among certain clinical populations, 16, 17 elderly, 18 and pediatric 19, 20 subjects. 12- 14 The former is often overlooked, whereas the latter is commonly reported. Moreover, these methods do not address the dependence of B 1 + on load position, leading to unpredictable pulse performance in cases of different initial subject positioning 11 and/or within-scan head motion. Additionally, the database approach is problematic in cases in which an individual is an outlier with respect to anatomies represented in the database. Tailored pulses typically yield lower normalized RMS error (nRMSE) of flip angle compared with UPs (7% vs 11% in Gras et al 7). However, the intersubject robustness of UPs comes at a cost to flip-angle uniformity. Plug-and-play usability of UPs in pTx has led to the method’s growing popularity. The designed pulse (a minimum error solution for excitation over multiple B 1 + distributions) is therefore assumed to work fairly well for any individual subject without the need for B 1 + mapping. An underlying assumption is that the range in head geometry and composition across human subjects is relatively constrained, implying that B 1 + distributions are similarly constrained. 10 Intersubject robustness is achieved by designing a UP (offline) to minimize error across a small database of representative subjects. Geometrical and compositional differences between human subjects are partly addressed in alternative, nontailored approaches such as universal pulses (UPs), 7, 8 SmartPulse, 9 and fast online-customized pTx pulses. However, channels’ electromagnetic fields (including B 1 +) and their interference patterns depend critically upon the object being imaged (i.e., the coil load), including its position, geometry, and composition. For optimal tailored pulse performance, the measured B 1 + distributions must match those present at the time of pulse playout. 3 Tailored pulse design incorporates the measured transmit sensitivities (B 1 +) of each pTx channel, achieving a homogeneous flip angle across specified slices or regions. Parallel transmission (pTx) of RF pulses through independently controlled channels can help to overcome B 1 nonuniformity seen in the head at 7 T, 1, 2 particularly when tailored pulses are used.
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