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

To create a frequency-modulated pulse for slice selection of arbitrary, uniform flip angle even when B₀ and B₁ are inhomogeneous, without utilizing B₀ gradients THEORY AND METHODS: An amplitude-modulated hyperbolic secant pulse (AM_HS1) 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 AM_HS1 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 B₀ and B₁ 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 AM_HS1 demonstrated slice inversion despite B₀ and B₁ 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 B₁ gradient was demonstrated on phantoms using a pair of AM_HS1 pulses transmitted with a surface coil. By using a low-flip angle AM_HS1 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 B₀ and B₁ are nonuniform.