Radiofrequency pulse designs for three‐dimensional MRI providing uniform tipping in inhomogeneous B₁ fields

Abstract

Although high-field MRI offers increased signal-to-noise, the nonuniform tipping produced by conventional radiofrequency (RF) pulses leads to spatially dependent contrast and suboptimal signal-to-noise, thus complicating the interpretation of the MR images. For structural imaging, three-dimensional sequences that do not make use of frequency-selective RF pulses have become popular. Therefore, the aim of this research was to develop non-slice-selective (NSS) RF pulses with immunity to both amplitude of (excitation) RF field (B 1 ) inhomogeneity and resonance offset. To accomplish this, an optimization routine based on optimal control theory was used to design new NSS pulses with desired ranges of immunity to B 1 inhomogeneity and resonance offset. The design allows the phase of transverse magnetization produced by the pulses to vary. Although the emphasis is on shallow tip designs, new designs for 30°, 60°, 90°, and 180°NSS RF pulses are also provided. These larger tip angle pulses are compared with recently published NSS pulses. Evidence is presented that the pulses presented in this article have equivalent performance but are shorter than the recently published pulses. Although the NSS pulses generate higher specific absorption rates and larger magnetization transfer effects than the rectangular pulses they replace, they nevertheless show promise for threedimensional MRI experiments at high field. Magn Reson Med 66:1254-1266,

Training Schrödinger’s cat: quantum optimal control

“…It also includes an optimal control toolbox to facilitate robust experiment design [100]. MAT-PULSE [329,330], DYNAMO [94] and QuTiP [102] are versatile Matlab and Python based simulation and optimization programs. Other software packages for the simulation and/or optimization of spin dynamics include SI-MONE [331], OCTOPUS(SI) [332,333], Gamma [334] and SPINEVOLUTION [335].…”

“…Motivated by magnetic resonance imaging applications, optimal control pulses for MRI have been developed [88,[324][325][326]330,[397][398][399][400]. Applications include improved spatially selective excitation schemes [106,401,402], pulses with minimal radio-frequency (RF) power and pulses that counteract RF inhomogeneity in parallel transmission at ultra-high field [330,398].…”

“…Applications include improved spatially selective excitation schemes [106,401,402], pulses with minimal radio-frequency (RF) power and pulses that counteract RF inhomogeneity in parallel transmission at ultra-high field [330,398]. For chemical exchange saturation transfer (CEST) imaging, chemical exchange effects were taken into account in pulse sequence optimizaion [403].…”

Time-optimal polarization transfer from an electron spin to a nuclear spin

“…These methods have been successfully applied in NMR [92,93] to designing broad-band [94][95][96] and decoupling pulse sequences [97][98][99][100][101][102]. They have also been utilized in magnetic resonance imaging [25,[103][104][105] and electron paramagnetic resonance [106].…”

Slice-selective excitation with -insensitive composite pulses

“…The challenge of mitigating such signal variations in order to better realize the potential of high-field imaging systems has sparked a renewed interest in the design of RF pulses that invoke a magnetization response insensitive to the $B_1^{+}$ field strength [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Due to non-selective frequency profiles and non-linear transverse magnetization phase in the through-slice direction, many such pulses are limited in application to whole-volume acquisitions.…”