Tailored RF pulse optimization for magnetization inversion at ultra high field

TL;DR

This paper presents a genetic algorithm-based method to optimize RF pulses for uniform magnetization inversion at 7 Tesla, addressing RF inhomogeneity issues in ultra-high field MRI.

Contribution

It introduces a tailored RF pulse design approach that accounts for transmit field heterogeneity using a genetic algorithm, improving inversion uniformity at 7 T.

Findings

Achieved uniform inversion over typical RF amplitude ranges at 7 T

Maintained feasible pulse length and amplitude for practical use

Enhanced image quality in sequences requiring inversion pulses

Abstract

The radiofrequency (RF) transmit field is severely inhomogeneous at ultrahigh field due to both RF penetration and RF coil design issues. This particularly impairs image quality for sequences that use inversion pulses such as magnetization prepared rapid acquisition gradient echo and limits the use of quantitative arterial spin labeling sequences such as flow-attenuated inversion recovery. Here we have used a search algorithm to produce inversion pulses tailored to take into account the heterogeneity of the RF transmit field at 7 T. This created a slice selective inversion pulse that worked well (good slice profile and uniform inversion) over the range of RF amplitudes typically obtained in the head at 7 T while still maintaining an experimentally achievable pulse length and pulse amplitude in the brain at 7 T. The pulses used were based on the frequency offset correction inversion technique, as well as time dilation of functions, but the RF amplitude, frequency sweep, and gradient functions were all generated using a genetic algorithm with an evaluation function that took into account both the desired inversion profile and the transmit field inhomogeneity.