Radio Frequency Pulse Designs for Magnetic Resonance at High Field

Abstract

The increased signal-to-noise ratio (SNR) that accompanies the use of stronger static fields in magnetic resonance imaging (MRI) has driven the use of increasingly higher fields ever since the technology's inception over thirty years ago. Currently, the potential clinical use of 7 Tesla (7 T) MRI scanners is under investigation. At such high fields, a number of complications prohibit practical realization of the potential gains in SNR and reduce image quality. One major problem is the inhomogeneity of radio frequency (RF) fields that originates when transmitted RF wavelengths are comparable to the dimensions of the human body. When this condition is fulfilled, significant RF attenuation and interference result, leading to spatially varying MR signal intensities.

This thesis addresses the challenge of nonuniform RF fields through the design of RF pulses that induced magnetization responses largely independent of underlying RF field variations. The advantages of this approach are that pulse designs are not patient-specific and can even be implemented across a range of static field strengths. Drawbacks to such strategies involve the limited degree to which uniform magnetization responses can be achieved given the practical restrictions of RF pulse power and duration. Pulse designs rely upon numerical optimization of RF modulation patterns composed of series of discrete sub-pulses. Such composite pulses have been designed for excitation, inversion, refocusing, and saturation and largely tolerate the field variations observed in the human brain at 7 T. In all cases, appropriate comparisons are made with established field-insensitive pulse designs, and performance as a function of pulse power and duration is considered in detail. Results indicate that use of numerically optimized composite pulses can greatly reduce the effects of nonuniform RF fields in almost any pulse sequence employed in the human brain at 7 T. In addition to field-insensitive composite pulses, this thesis includes an evaluation of RF field mapping protocols, the numerical optimization of adiabatic pulses, and the design of frequency-selective composite pulses for a number of applications at both low and high field strengths.