Designing B 1 ‐Selective Pulses by Frequency Modulating in a Second Rotating Frame

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.