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Actual flip‐angle imaging in the pulsed steady state: A method for rapid three‐dimensional mapping of the transmitted radiofrequency field
Author(s) -
Yarnykh Vasily L.
Publication year - 2007
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.21120
Subject(s) - flip angle , pulse (music) , signal (programming language) , excitation , nuclear magnetic resonance , physics , relaxation (psychology) , pulse sequence , field (mathematics) , steady state (chemistry) , optics , chemistry , mathematics , magnetic resonance imaging , computer science , medicine , quantum mechanics , detector , pure mathematics , radiology , programming language
A new method has been developed for fast image‐based measurements of the transmitted radiofrequency (RF) field. The method employs an actual flip‐angle imaging (AFI) pulse sequence that consists of two identical RF pulses followed by two delays of different duration (TR 1 < TR 2 ). After each pulse, a gradient‐echo (GRE) signal is acquired. It has been shown theoretically and experimentally that if delays TR 1 and TR 2 are sufficiently short and the transverse magnetization is completely spoiled, the ratio r = S 2 / S 1 of signal intensities S 1 and S 2 , acquired at the beginning of the time intervals TR 1 and TR 2 , depends on the flip angle (FA) of applied pulses as r = (1 + n * cos(FA))/( n + cos(FA)), where n = TR 2 /TR 1 . The method allows fast 3D implementation and provides accurate B 1 measurements that are highly insensitive to T 1 . The unique feature of the AFI method is that it uses a pulsed steady‐state signal acquisition. This overcomes the limitation of previous methods that required long relaxation delays between sequence repetitions. The method has been shown to be useful for time‐efficient whole‐body B 1 mapping and correction of T 1 maps obtained using a variable FA technique in the presence of nonuniform RF excitation. Magn Reson Med 57:192–200, 2007. © 2006 Wiley‐Liss, Inc.