1Actual Flip Angle Imaging (AFI)¶
Transmit radiofrequency field maps (B1+, or B1 for short) are used in diverse applications in MRI including: the study of electrical properties in tissues in vivo Sled & Pike, 1998Katscher et al., 2009, specific absorption rate (SAR) calculations Ibrahim et al., 2001, the calibration of quantitative T1 Deoni, 2007Boudreau et al., 2017 and T2 Sled & Pike, 2000 maps, better parameter estimation from magnetization transfer measurements Ropele et al., 2005Boudreau et al., 2018, B1 shimming to improve image quality at whole-body ultra high fields Bergen et al., 2007, or quality control of RF coils Yarnykh, 2007. Several B1 mapping techniques have been developed, and they can be broadly divided as magnitude-based and phase-based methods. The double angle method (DAM) is a saturation-recovery magnitude-based method that takes the ratio of the signal intensity of two magnitude images measured with different excitation flip angles Insko & Bolinger, 1993Stollberger & Wach, 1996. The Bloch-Siegert shift technique is a rapid phase-based method that encodes the B1 information into phase signal Sacolick et al., 2010. The actual flip-angle imaging (AFI) is a magnitude-based B1 mapping method that consists of a 3D acquisition that benefits from good anatomical coverage. In addition, this technique allows the acquisitions of whole-body (~7 min) and brain (~3 min) B1 maps leading to a feasible implementation in clinics Yarnykh, 2007. On the other hand, the AFI pulse sequence has certain constraints that need to be considered for this B1 mapping method to be widely deployed. Some of the limitations include the use of spoiler gradients that can give rise to prohibitive SAR values Sacolick et al., 2010, and the pulse sequence modifications on the MRI machine to implement the AFI method.
In this section, we will focus on presenting details about the AFI B1 mapping method. We will cover signal modeling, data fitting, the benefits and the pitfalls of the technique. The figures are generated using the qMRLab module for this method.
- Sled, J. G., & Pike, G. B. (1998). Standing-wave and RF penetration artifacts caused by elliptic geometry: an electrodynamic analysis of MRI. IEEE Trans. Med. Imaging, 17(4), 653–662.
- Katscher, U., Voigt, T., Findeklee, C., Vernickel, P., Nehrke, K., & Dössel, O. (2009). Determination of electric conductivity and local SAR via B1 mapping. IEEE Trans. Med. Imaging, 28(9), 1365–1374.
- Ibrahim, T. S., Lee, R., Baertlein, B. A., & Robitaille, P.-M. L. (2001). B1 field homogeneity and SAR calculations for the birdcage coil. Phys. Med. Biol., 46, 609–619.
- Deoni, S. C. L. (2007). High-resolution T1 mapping of the brain at 3T with driven equilibrium single pulse observation of T1 with high-speed incorporation of RF field inhomogeneities (DESPOT1-HIFI). J. Magn. Reson. Imaging, 26(4), 1106–1111.
- Boudreau, M., Tardif, C. L., Stikov, N., Sled, J. G., Lee, W., & Pike, G. B. (2017). B1 mapping for bias-correction in quantitative T1 imaging of the brain at 3T using standard pulse sequences. J. Magn. Reson. Imaging, 46(6), 1673–1682.