A multiple-pulse sequence for improved selective excitation in magnetic resonance imaging

Area of Science:

• Magnetic Resonance Imaging
• Radiofrequency Pulse Design

Background:

• Conventional selective excitation methods often rely on Fourier analysis for radiofrequency (rf) pulse envelope design.
• Fourier analysis provides accurate slice shape prediction only for small flip angles due to nonlinear spin system behavior.

Purpose of the Study:

• Introduce a new framework for selective excitation offering simpler design and superior performance.
• Enhance phase characteristics and slice definition compared to conventional methods.

Main Methods:

• Employing additional excitation pulses in sequence with a conventional pulse.
• Utilizing backward Bloch equation solving to determine the required initial spin distribution.
• Designing a "setup" pulse based on the residual distribution and Fourier analysis.
• Incorporating an opposite polarity gradient during the setup pulse to eliminate the need for a refocusing interval.

Main Results:

• Computer simulations verified the multiple-pulse excitation sequence's efficacy.
• Demonstrated improved performance in phase characteristics and slice definition.
• Validated the method for both 90-degree and 180-degree excitations.

Conclusions:

• The new framework provides a more robust and accurate method for selective excitation.
• Offers significant advantages over traditional Fourier-based approaches, especially for larger flip angles.
• Paves the way for improved magnetic resonance imaging slice selection and spatial manipulation.