Designing B₁-Selective Pulses by Frequency Modulating in a Second Rotating Frame.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Pulse Sequence Design
• Radiofrequency Engineering

Background:

• Slice selection in MRI typically relies on magnetic field gradients (B0 gradients).
• Inhomogeneities in static magnetic fields (B0) and radiofrequency fields (B1) can degrade slice selection accuracy.
• Existing methods often struggle to maintain uniform flip angles under these challenging conditions.

Purpose of the Study:

• To develop a frequency-modulated pulse for robust slice selection.
• To achieve arbitrary, uniform flip angles independent of B0 and B1 inhomogeneities.
• To eliminate the need for B0 gradients in slice selection.

Main Methods:

• Derived an amplitude-modulated hyperbolic secant pulse (AMHS1) based on Hoult's B1-selective imaging.
• Utilized B1y as a gradient and B1x for selective excitation, with frequency sweep achieved via time-dependent amplitude modulation of B1x.
• Investigated resilience to B0 and B1 inhomogeneities through simulations.
• Demonstrated slice- and slab-selective imaging experimentally using surface coils.

Main Results:

• Simulations confirmed AMHS1's ability to perform slice inversion despite B0 and B1 inhomogeneities.
• A correction method involving scaling B1x(t) was developed to address flip angle variations caused by simultaneous B1x and B1y gradients.
• Slice selection using only a B1 gradient was successfully demonstrated on phantoms.
• Slice- and slab-selective excitation was achieved in 3D gradient-echo imaging of rat brain in vivo at 9.4T using low-flip angle AMHS1 pulses.

Conclusions:

• Frequency modulation in a second rotating frame enables B1-selective excitation and inversion.
• The developed method is effective even in the presence of nonuniform B0 and B1 fields.
• This technique offers a promising alternative for precise slice selection in challenging MRI scenarios.