Parallel Transmission for Ultrahigh Field MRI

Abstract

Magnetic resonance imaging (MRI) has been driven toward ultrahigh magnetic fields (UHF) in order to benefit from correspondingly higher signal-to-noise ratio and spectral resolution. Technological challenges associated with UHF, such as increased radiofrequency (RF) energy deposition and RF excitation inhomogeneity, limit realization of the full potential of these benefits. Parallel RF transmission (pTx) enables decreases in the inhomogeneity of RF excitations and in RF energy deposition by using multiple-transmit RF coils driven independently and operating simultaneously. pTx plays a fundamental role in UHF MRI by bringing the potential applications of UHF into reality. In this review article, we review the recent developments in pTx pulse design and RF safety in pTx. Simultaneous multislice imaging and inner volume imaging using pTx are reviewed with a focus on UHF applications. Emerging pTx design approaches using improved pTx design frameworks and calibrations are reviewed together with calibration-free approaches that remove the necessity of time-consuming calibrations necessary for successful pTx. Lastly, we focus on the safety of pTx that is improved by using intersubject variability analysis, proactively managing pTx and temperature-based pTx approaches.

Optimization of adiabatic pulses for pulsed arterial spin labeling at 7 tesla: Comparison with pseudo‐continuous arterial spin labeling

“…It is worth noting that pTx has proved to be a powerful tool for UHF experiments. [24][25][26][27] The advantage of pTx can be beneficial to 7T ASL imaging. Take Ref.…”

k‐Space Domain Parallel Transmit Pulse Design

“…Finally, while simple Tikhonov RF power regularization was used in the designs presented here, more sophisticated regularization could be incorporated to, eg, control peak RF power via adaptive regularization, 27 or to enforce array compression by projecting the weights into the null space of a compression matrix, 28 among other applications. 9,10 In such designs, it would be beneficial to pre-compute and store the lower triangular elements of the S H S matrices so they need not be recomputed as the regularization changes over iterations. Peak power could also be controlled in k-space domain designs using parallel transmission VERSE 29 or the iterative re-VERSEit technique.…”

“…The spatial domain method is mathematically straightforward and enables flexible modeling and compensation of effects such as off‐resonance, and incorporation of regions of interest. Since its introduction, most small‐tip‐angle parallel transmission studies have been formulated in the spatial domain 9,10 …”

“…Since its introduction, most small-tip-angle parallel transmission studies have been formulated in the spatial domain. 9,10 Despite the advantages of the spatial domain approach, it can have prohibitive computational requirements when the number of coils and the number of dimensions become large, for example in three-dimensional 11,12 or spectral-spatial pulse designs, [13][14][15][16] when there is a large number of coils, 17 and when off-resonance compensation is used. The spatial domain system matrix typically has dimensions N s × (N c ⋅ N t ), where N s is the number of spatial and spectral locations defined in the target excitation pattern, N c is the number of transmit channels, and N t is the number of time points in the pulse.…”

Local SAR compression with overestimation control to reduce maximum relative SAR overestimation and improve multi-channel RF array performance

“…While single-channel and dual-channel transmit systems are still standard in clinical MRI systems, multi-channel parallel transmit (pTx) radiofrequency (RF) systems are often used at ultra-high field (UHF). Not only are these systems necessary to cope with the inhomogeneity introduced by the short wavelength of the RF signals [ 1 , 2 ], these systems also offer more flexibility in excitation, especially at UHF [ 3 , 4 ]. Examples of the techniques utilizing pTx systems are RF shimming [ 5 , 6 ], kT-points [ 7 ], 2D spokes [ 8 ], 3D tailored radiofrequency pulses [ 9 ], Transmit SENSE [ 10 , 11 ], and TIAMO [ 12 ].…”