Fast Large-Tip-Angle Multidimensional and Parallel RF Pulse Design in MRI

• Published in: IEEE transactions on medical imaging Vol. 28; no. 10; pp. 1548 - 1559
• Main Authors: Grissom, W.A., Dan Xu, Kerr, A.B., Fessler, J.A., Noll, D.C.
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.10.2009; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Algorithm design and analysis; Algorithms; Computer Simulation; Equations; Fast Fourier transforms; Fourier Analysis; Fourier transforms; Information systems; Large-tip-angle RF pulse design; Linear approximation; Linear Models; Magnetic resonance imaging; magnetic resonance imaging (MRI); Magnetic Resonance Imaging - methods; multidimensional excitation; Multidimensional systems; Nonuniform electric fields; Optimal control; parallel excitation; Radio frequency; Radio Waves; RF pulse design; Signal Processing, Computer-Assisted
• ISSN: 0278-0062; 1558-254X; 1558-254X
• DOI: 10.1109/TMI.2009.2020064

Large-tip-angle multidimensional radio-frequency (RF) pulse design is a difficult problem, due to the nonlinear response of magnetization to applied RF at large tip-angles. In parallel excitation, multidimensional RF pulse design is further complicated by the possibility for transmit field patterns to change between subjects, requiring pulses to be designed rapidly while a subject lies in the scanner. To accelerate pulse design, we introduce a fast version of the optimal control method for large-tip-angle parallel excitation. The new method is based on a novel approach to analytically linearizing the Bloch equation about a large-tip-angle RF pulse, which results in an approximate linear model for the perturbations created by adding a small-tip-angle pulse to a large-tip-angle pulse. The linear model can be evaluated rapidly using nonuniform fast Fourier transforms, and we apply it iteratively to produce a sequence of pulse updates that improve excitation accuracy. We achieve drastic reductions in design time and memory requirements compared to conventional optimal control, while producing pulses of similar accuracy. The new method can also compensate for nonidealities such as main field inhomogeneties.