Broadband selective excitation radiofrequency pulses for optimized localization in vivo

Abstract

Purpose: The aim of the study is to optimize the performance of localized ¹ H MRS sequences at 3T, using the entire spin system of N-acetyl aspartate (NAA) as an example of the large chemical shift spread of all the metabolites routinely detected in vivo, including the amide region. We specifically focus on the design of the suitable broadband excitation radiofrequency (RF) pulses to minimize chemical shift artifacts.

Methods: The performance of the excitation and refocusing pulse shapes is evaluated with respect to NAA localization. Two new excitation RF pulses are developed to achieve optimized performance in the brain using single-voxel ¹ H MRS at 3T. Numerical simulations and in vivo experiments are carried out to demonstrate the performance of the RF pulses.

Results: New excitation RF pulses with the same B₁ requirements but larger excitation bandwidth (up to a factor of 2) are shown to significantly reduce localization artifacts. The large frequency spread of the entire NAA spin system necessitates the use of broadband excitation and refocusing pulses for MRS at 3T.

Conclusion: To minimize chemical shift artifacts of metabolic compounds with spins in the amide area (>5 ppm) at 3T it is important to use broadband excitation and refocusing pulses.

Keywords: NAA; RF pulses; amide; brain; chemical shift; chemical shift displacement error; selective excitation.