title: Small-tip-angle spokes pulse design using interleaved greedy and local optimization methods.
abstract: Current spokes pulse design methods can be grouped into methods based either on sparse approximation or on iterative local (gradient descent-based) optimization of the transverse-plane spatial frequency locations visited by the spokes. These two classes of methods have complementary strengths and weaknesses: sparse approximation-based methods perform an efficient search over a large swath of candidate spatial frequency locations but most are incompatible with off-resonance compensation, multifrequency designs, and target phase relaxation, while local methods can accommodate off-resonance and target phase relaxation but are sensitive to initialization and suboptimal local cost function minima. This article introduces a method that interleaves local iterations, which optimize the radiofrequency pulses, target phase patterns, and spatial frequency locations, with a greedy method to choose new locations. Simulations and experiments at 3 and 7 T show that the method consistently produces single- and multifrequency spokes pulses with lower flip angle inhomogeneity compared to current methods.
authors.affiliation: GE Global Research, Munich, Germany. will.grissom@vanderbilt.edu
authors.forename: William A
authors.initials: WA
authors.lastname: Grissom
authors.forename: Mohammad-Mehdi
authors.initials: MM
authors.lastname: Khalighi
authors.forename: Laura I
authors.initials: LI
authors.lastname: Sacolick
authors.forename: Brian K
authors.initials: BK
authors.lastname: Rutt
authors.forename: Mika W
authors.initials: MW
authors.lastname: Vogel
mesh_terms: D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted