Tailored excitation using nonlinear B0-shims.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Physics

Background:

• Radiofrequency (RF) flip angle inhomogeneity in high-field MRI causes spatial variations in image contrast and sensitivity.
• Multidimensional excitation improves homogeneity but is limited by long RF pulse durations.
• Parallel excitation offers reduced pulse durations but requires multiple RF channels.

Purpose of the Study:

• To propose and demonstrate an alternative method for shortening multidimensional excitation pulses in MRI.
• To improve RF flip angle homogeneity without parallel excitation.
• To enhance contrast and sensitivity in gradient echo (GE) MRI at high fields.

Main Methods:

• Utilized nonlinear spatial variations in the stationary (B(0)) magnetic field during a B(0)-sensitive excitation pulse.
• Applied B(0) shims with up to second-order spatial dependence.
• Demonstrated the method on 2D gradient echo (GE) MRI of the human brain at 7 Tesla (T).

Main Results:

• Reduced root-mean-squared flip angle variation from 20% to 11%.
• Achieved practical RF pulse lengths suitable for general GE imaging applications.
• Did not require parallel excitation.

Conclusions:

• The proposed method effectively shortens multidimensional excitation pulses by exploiting B(0) field variations.
• This approach enhances RF flip angle homogeneity, improving contrast and sensitivity in high-field GE MRI.
• The technique offers a practical alternative for improving image quality in clinical brain imaging.