Rf-inhomogeneity compensation using method of Fourier synthesis

Area of Science:

• Magnetic Resonance Imaging
• Pulse Sequence Design
• Radiofrequency Engineering

Background:

• Radiofrequency (rf) amplitude inhomogeneity is a significant challenge in Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI).
• This inhomogeneity, particularly with surface coils in in-vivo studies, leads to variations in flip angles and degraded image quality.
• Existing methods struggle to compensate for wide ranges of rf-amplitude variations.

Purpose of the Study:

• To develop a novel method for designing composite pulses that are robust to radiofrequency (rf) amplitude inhomogeneity.
• To demonstrate the versatility of this method for various pulse types, including excitation, inversion, and arbitrary flip angles.
• To enable amplitude-selective excitation capabilities within the designed pulses.

Main Methods:

• The proposed method, termed Fourier synthesis, designs composite pulses by synthesizing their frequency-domain representation.
• This approach allows for the direct incorporation of robustness to rf-amplitude variations during the design process.
• The method is general and can be adapted for different pulse functionalities and spatial selectivities.

Main Results:

• Experimental validation demonstrated effective rf-compensation over a 20-decibel (dB) range of rf-amplitude variation.
• The designed pulses maintained their intended performance despite significant rf-inhomogeneity.
• The method successfully generated amplitude-selective excitation pulses.

Conclusions:

• Fourier synthesis offers a robust and versatile approach to designing composite pulses for NMR and MRI.
• This method effectively mitigates the detrimental effects of rf-amplitude inhomogeneity, particularly in scenarios involving surface coils.
• The technique is expected to enhance the reliability and quality of in-vivo NMR studies.