Fast three‐dimensional inner volume excitations using parallel transmission and optimized k‐space trajectories

Abstract

Purpose To design short parallel transmission (pTx) pulses for excitation of arbitrary three-dimensional (3D) magnetization patterns. Methods We propose a joint optimization of the pTx radiofrequency (RF) and gradient waveforms for excitation of arbitrary 3D magnetization patterns. Our optimization of the gradient waveforms is based on the parameterization of k-space trajectories (3D shells, stack-of-spirals, and cross) using a small number of shape parameters that are well-suited for optimization. The resulting trajectories are smooth and sample k-space efficiently with few turns while using the gradient system at maximum performance. Within each iteration of the k-space trajectory optimization, we solve a small tip angle least-squares RF pulse design problem. Our RF pulse optimization framework was evaluated both in Bloch simulations and experiments on a 7T scanner with eight transmit channels. Results Using an optimized 3D cross (shells) trajectory, we were able to excite a cube shape (brain shape) with 3.4% (6.2%) normalized root-mean-square error in less than 5 ms using eight pTx channels and a clinical gradient system (Gmax = 40 mT/m, Smax = 150 T/m/s). This compared with 4.7% (41.2%) error for the unoptimized 3D cross (shells) trajectory. Incorporation of B0 robustness in the pulse design significantly altered the k-space trajectory solutions. Conclusion Our joint gradient and RF optimization approach yields excellent excitation of 3D cube and brain shapes in less than 5 ms, which can be used for reduced field of view imaging and fat suppression in spectroscopy by excitation of the brain only.

Fast online‐customized (FOCUS) parallel transmission pulses: A combination of universal pulses and individual optimization

“…4,[10][11][12] Therefore, dynamic pTx pulses provide more degrees of freedom and, thus, higher potential to produce uniform FA patterns than static pTx pulses. Several techniques to design dynamic pTx pulses have been proposed including slice/slab selective, 13,14 tailored 3D volume excitation, 15,16 and nonselective 17,18 pulses.…”

In vivo B 0 field shimming methods for MRI at 7 T

“…ΔB 0 also has undesired effects on RF excitation pulses, including lipid saturation, RF inversion and VERSE pulses (Hargreaves et al, 2004). For 7 T functional brain studies, the most problematic effect from ΔB 0 during excitation is probably the contrast changes in T 1 -weighted MPRAGE in poorly shimmed areas which has compelled the use of off-resonance tolerant RF inversion pulses (Wrede et al, 2012) and parallel transmit pulse design optimizations which either utilize the ΔB 0 map during pulse design (Grissom et al, 2006) or trajectory design (Schneider et al, 2014; Davids et al, 2016) or optimize the design over a range of frequencies (Setsompop et al, 2009). …”

k‐Space Domain Parallel Transmit Pulse Design

“…This work was initially motivated by the observation that spatial domain parallel pulse designs can be very slow for 3D problems with large grid sizes, requiring many iterations with considerable computation per iteration. It is anticipated that the proposed k-space domain algorithm will be most useful for these types of large >2D problems, which include 3D spatial designs 11,12 and 2D and 3D spatial-spectral designs [13][14][15][16] where full matrix construction and inversion is infeasible due to the problem size, and an iterative design can require several minutes to solve. Furthermore, unlike an iterative spatial domain design the proposed algorithm does not need to be repeated if the target pattern changes.…”