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 B1 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 three-dimensional MRI experiments at high field. Copyright © 2011 Wiley Periodicals, Inc.