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ABSTRACT: The Bloch-Siegert (B-S) B 1+ mapping method has been shown to be fast and accurate, yet it suffers from high Specific Absorption Rate (SAR) and moderately long echo time. An adiabatic RF pulse design is introduced here for optimizing the off-resonant B-S RF pulse to achieve more B-S B 1+ measurement sensitivity for a given pulse width. The extra sensitivity can be used for higher angle-to-noise ratio B 1+ maps or traded off for faster scans. Using numerical simulations and phantom experiments, it is shown that a numerically optimized 2-ms adiabatic B-S pulse is 2.5 times more efficient than a conventional 6-ms Fermi-shaped B-S pulse. The adiabatic B-S pulse performance is validated in a phantom, and in vivo brain B 1+ mapping at 3T and 7T are shown. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
Magnetic Resonance in Medicine 10/2012; · 2.96 Impact Factor
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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. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
Magnetic Resonance in Medicine 03/2012; 68(5):1553-62. · 2.96 Impact Factor
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ABSTRACT: The Bloch-Siegert (B-S) method of B ₁⁺ mapping has been shown to be fast and accurate, yet has high SAR and moderately long TE. These limitations can lengthen scan times and incur signal loss due to B(0) inhomogeneity, particularly at high field. The B-S method relies on applying a band-limited off-resonant B-S radiofrequency pulse to induce a B ₁⁺-dependent frequency-shift for resonant spins. A method for optimizing the B-S radiofrequency pulse is presented here, which maximizes B-S B ₁⁺ measurement sensitivity for a given SAR and T(2) . A 4-ms optimized pulse is shown to have 35% less SAR compared with the conventional 6-ms Fermi pulse while still improving B ₁⁺ map angle-to-noise ratio by 22%. The optimized pulse performance is validated both in phantom and in vivo brain imaging at 7 T.
Magnetic Resonance in Medicine 12/2011; 68(3):857-62. · 2.96 Impact Factor
