Ultra-high field MRI: parallel-transmit arrays and RF pulse design

Abstract

This paper reviews the field of multiple or parallel radiofrequency (RF) transmission for magnetic resonance imaging (MRI). Currently the use of ultra-high field (UHF) MRI at 7 tesla and above is gaining popularity, yet faces challenges with non-uniformity of the RF field and higher RF power deposition. Since its introduction in the early 2000s, parallel transmission (pTx) has been recognized as a powerful tool for accelerating spatially selective RF pulses and combating the challenges associated with RF inhomogeneity at UHF. We provide a survey of the types of dedicated RF coils used commonly for pTx and the important modeling of the coil behavior by electromagnetic (EM) field simulations. We also discuss the additional safety considerations involved with pTx such as the specific absorption rate (SAR) and how to manage them. We then describe the application of pTx with RF pulse design, including a practical guide to popular methods. Finally, we conclude with a description of the current and future prospects for pTx, particularly its potential for routine clinical use.

Scaling the mountains: what lies above 7 Tesla magnetic resonance?

“…This includes implementation of multi-channel transmit-receive RF arrays, approaching the ultimate signal-to-noise ratio, improvements in RF transmission field distribution and enhanced SAR prediction and management [28]. The tools to implement and use parallel transmission methods, like the kT points approach, are now well documented, enabling easier implementation and dissemination across a wide range of sites and users [29][30][31]. Novel RF array designs include optimal combinations of loop-dipole building blocks, as well as dielectric resonators, with numerous studies demonstrating their added value at 7 T [32] and 10.5 T. This Special Issue highlights the merits of transmit RF arrays.…”

Electromagnetic and RF pulse design simulation based optimization of an eight‐channel loop array for 11.7T brain imaging

“…Each loop was segmented to accommodate a total of 14 evenly distributed capacitors (13 fixed capacitors and one trimmer) to tune the loops to 499.415 MHz, which corresponds to a segment length of $$$ \lambda /15 $$$. This was empirically determined to ensure that the coil tuning is not dominated by the capacitive coupling to the sample 34 . The array elements were matched to 50 Ω through a balanced matching circuit 35 .…”

“…The power budget analysis was conducted to identify all the loss mechanisms within the transmit array. 34,37 This offers insights into the sources of losses and implements methods to minimize controllable losses. A snapshot of the loss mechanisms is shown in Figure 2.…”

5T magnetic resonance imaging: radio frequency hardware and initial brain imaging

“…Furthermore, as the radio frequency (RF) wavelength becomes shorter than the dimensions of the object being imaged, the B 1 + field shows inhomogeneity due to constructive and destructive magnetic field interferences in ultrahigh field MRI scanners (15,16). This problem has been solved by using parallel transmit RF coils with B 1 + phase and/or amplitude shimming (17)(18)(19), and pulse sequences such as transmit sensitivity encoding (20)(21)(22)(23), spoke pulses (24)(25)(26)(27), and k-T pulses (28). These advances generate uniform flip angles over an imaged region.…”

“…This transfer is defined by the specific absorption rate (SAR, measured in W/kg) and SAR increases quadratically with the strength of the main magnetic field ( 30 , 31 ). Measures that can be taken to reduce SAR have been described ( 17 , 32 , 33 ). Nevertheless, for only head MRI, International Electrotechnical Commission (IEC) guidelines recommend a global SAR limit of 3.2 W/kg for body transmit coils and a local SAR limit of 10 W/kg for local transmit coils in the normal operating mode ( 34 ).…”