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B 1 mapping of short T 2 * spins using a 3D radial gradient echo sequence
Author(s) -
Kobayashi Naoharu,
Garwood Michael
Publication year - 2014
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.24817
Subject(s) - dephasing , flip angle , sinc function , spins , physics , pulse sequence , excitation , algorithm , offset (computer science) , nuclear magnetic resonance , optics , computational physics , mathematics , computer science , mathematical analysis , magnetic resonance imaging , medicine , quantum mechanics , radiology , condensed matter physics , programming language
Purpose To develop a method to acquire a radiofrequency (B 1 ) field map when the signal has a short T 2 *. Theory and Methods The method is based on the actual flip angle imaging (AFI) technique and a radial 3D gradient‐echo sequence known as COncurrent Dephasing and Excitation (CODE), which preserves short T 2 * signals. CODE was implemented with Gradient‐modulated Offset‐Independent Adiabaticity (GOIA) pulses to obtain high estimation sensitivity with AFI. The correlation method, which removes the quadratic phase from the frequency‐modulated pulse excitation, was modified to handle gradient‐modulated pulses. Validity of the modified correlation procedure was tested by Bloch simulations. CODE experiments with sinc, hyperbolic secant, and GOIA pulses were performed in order to see effects from the frequency and gradient modulation. Finally, GOIA‐CODE AFI was conducted and compared with conventional AFI with 3D gradient echo (GRE). Results The modified correlation method developed to accommodate frequency and gradient modulations of GOIA performed well as judged by the minimal impact on reconstructed image quality. GOIA‐CODE AFI provided flip angle maps consistent with those measured by GRE AFI when the T 2 * was long (>2 ms) and continued to perform well for short T 2 * signals. Conclusion The proposed technique provides a means to obtain a 3D B 1 field map when imaging spins with short T 2 * . Magn Reson Med 71:1689–1699, 2014. © 2013 Wiley Periodicals, Inc .