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

TL;DR

This paper introduces a new method for slice selection in MRI that works even when magnetic field inhomogeneities are present, using a novel pulse design.

Contribution

A new B1-selective pulse design using frequency modulation in a second rotating frame to enable slice selection without B0 gradients.

Findings

AMHS1 simulations showed robust slice inversion despite B0 and B1 inhomogeneities.

Slice and slab selection was experimentally demonstrated in phantoms and rat brains using surface coils.

Low-flip angle AMHS1 pulses enabled 3D gradient-echo imaging in vivo at 9.4 T.

Abstract

To create a frequency‐modulated pulse for slice selection of arbitrary, uniform flip angle even when B
0 and B
1 are inhomogeneous, without utilizing B0 gradients

An amplitude‐modulated hyperbolic secant pulse (AMHS1) was derived from Hoult's B1‐selective imaging method which utilizes one component, B
1y, as a gradient and another, B
1x, for selective excitation. The frequency sweep in AMHS1 was produced by a time‐dependent amplitude‐modulation of B
1x, defining the band of nutation frequencies to select along the B
1y gradient. Resilience of slice selection to B
0 and B
1 inhomogeneities was investigated by simulations. Slice‐ and slab‐selective imaging were demonstrated experimentally in phantoms and rat brain in vivo using surface coils.

Simulations of AMHS1 demonstrated slice inversion despite B
0 and B
1 inhomogeneities. When operating sub‐adiabatically to produce excitation flip angles < 180° with a single coil, the flip angle across the slice varied because both B
1x and B
1y gradients were present. This problem was corrected by scaling B
1x(t) by the normalized frequency‐sweep and phase‐modulation functions. Slice selection using only a B
1 gradient was demonstrated on phantoms using a pair of AMHS1 pulses transmitted with a surface coil. By using a low‐flip angle AMHS1 with B
1y refocusing lobes, slice‐ and slab‐selective excitation was realized in 3D gradient‐echo imaging of rat brain in vivo at 9.4 T.

By implementing frequency modulation in a second rotating frame, B1‐selective excitation and inversion are feasible, even when B
0 and B
1 are nonuniform.

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