SA2RAGE: A new sequence for fast B1+-mapping

• Published in: Magnetic resonance in medicine Vol. 67; no. 6; pp. 1609 - 1619
• Main Authors: Eggenschwiler, Florent, Kober, Tobias, Magill, Arthur W., Gruetter, Rolf, Marques, José P.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01.06.2012
• Subjects: Algorithms; Brain - anatomy & histology; B 1+-mapping; B 1+-shimming; Humans; Image Enhancement - methods; Image Interpretation, Computer-Assisted - methods; Magnetic Resonance Imaging - methods; Reproducibility of Results; SA2RAGE; Sensitivity and Specificity; Signal Processing, Computer-Assisted
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.23145

At high magnetic field strengths (≥3T), the radiofrequency wavelength used in MRI is of the same order of magnitude of (or smaller than) the typical sample size, making transmit magnetic field (B 1+) inhomogeneities more prominent. Methods such as radiofrequency‐shimming and transmit SENSE have been proposed to mitigate these undesirable effects. A prerequisite for such approaches is an accurate and rapid characterization of the B 1+ field in the organ of interest. In this work, a new phase‐sensitive three‐dimensional B 1+‐mapping technique is introduced that allows the acquisition of a 64 × 64 × 8 B 1+‐map in ∼20 s, yielding an accurate mapping of the relative B 1+ with a 10‐fold dynamic range (0.2–2 times the nominal B 1+). Moreover, the predominant use of low flip angle excitations in the presented sequence minimizes specific absorption rate, which is an important asset for in vivo B 1+‐shimming procedures at high magnetic fields. The proposed methodology was validated in phantom experiments and demonstrated good results in phantom and human B 1+‐shimming using an 8‐channel transmit‐receive array. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.