Abstract
PURPOSE: 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 B(0) gradients THEORY AND METHODS: An amplitude-modulated hyperbolic secant pulse (AM(HS1)) was derived from Hoult's B(1)-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 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. RESULTS: Simulations of AM(HS1) 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 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. CONCLUSION: By implementing frequency modulation in a second rotating frame, B(1)-selective excitation and inversion are feasible, even when B(0) and B(1) are nonuniform.