Optimization and Validation of Methods for Mapping of the Radiofrequency Transmit Field at 3T

MRI techniques such as quantitative imaging and parallel transmit require precise knowledge of the radio‐frequency transmit field (B  1+ ). Three published methods were optimized for robust B  1+ mapping at 3T in the human brain: three‐dimensional (3D) actual flip angle imaging (AFI), 3D echo‐planar imaging (EPI), and two‐dimensional (2D) stimulated echo acquisition mode (STEAM). We performed a comprehensive comparison of the methods, focusing on artifacts, reproducibility, and accuracy compared to a reference 2D double angle method. For the 3D AFI method, the addition of flow‐compensated gradients for diffusion damping reduced the level of physiological artifacts and improved spoiling of transverse coherences. Correction of susceptibility‐induced artifacts alleviated image distortions and improved the accuracy of the 3D EPI imaging method. For the 2D STEAM method, averaging over multiple acquisitions reduced the impact of physiological noise and a new calibration method enhanced the accuracy of the B  1+ maps. After optimization, all methods yielded low noise B  1+ maps (below 2 percentage units), of the nominal flip angle value (p.u.) with a systematic bias less than 5 p.u. units. Full brain coverage was obtained in less than 5 min. The 3D AFI method required minimal postprocessing and showed little sensitivity to off‐resonance and physiological effects. The 3D EPI method showed the highest level of reproducibility. The 2D STEAM method was the most time‐efficient technique. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.

[1]  D. Hoult The solution of the bloch equations in the presence of a varying B1 field—An approach to selective pulse analysis , 1979 .

[2]  L. Bolinger,et al.  Mapping of the Radiofrequency Field , 1993 .

[3]  F. Franconi,et al.  Radiofrequency map of an NMR coil by imaging. , 1993, Magnetic resonance imaging.

[4]  J. Weaver,et al.  High resolution renal diffusion imaging using a modified steady‐state free precession sequence , 1995, Magnetic resonance in medicine.

[5]  Yihong Yang,et al.  Functional Magnetic Resonance Neuroimaging Data Acquisition Techniques , 1996, NeuroImage.

[6]  P Wach,et al.  Imaging of the active B1 field in vivo , 1996, Magnetic resonance in medicine.

[7]  J. Ashburner,et al.  Multimodal Image Coregistration and Partitioning—A Unified Framework , 1997, NeuroImage.

[8]  Michael B. Smith,et al.  SAR and B1 field distributions in a heterogeneous human head model within a birdcage coil , 1998, Magnetic resonance in medicine.

[9]  G. Bruce Pike,et al.  Standing-wave and RF penetration artifacts caused by elliptic geometry: an electrodynamic analysis of MRI , 1998, IEEE Transactions on Medical Imaging.

[10]  E M Haacke,et al.  Accurate determination of spin‐density and T1 in the presence of RF‐field inhomogeneities and flip‐angle miscalibration , 1998, Magnetic resonance in medicine.

[11]  Correction for B(1) and B(0) variations in quantitative T(2) measurements using MRI. , 2000 .

[12]  G. Pike,et al.  Correction for B1 and B0 variations in quantitative T2 measurements using MRI , 2000, Magnetic resonance in medicine.

[13]  G Helms,et al.  A precise and user‐independent quantification technique for regional comparison of single volume proton MR spectroscopy of the human brain , 2000, NMR in biomedicine.

[14]  Robert Turner,et al.  Image Distortion Correction in fMRI: A Quantitative Evaluation , 2002, NeuroImage.

[15]  R Turner,et al.  RF inhomogeneity compensation in structural brain imaging , 2002, Magnetic resonance in medicine.

[16]  Denis Le Bihan,et al.  Looking into the functional architecture of the brain with diffusion MRI , 2003, Nature Reviews Neuroscience.

[17]  R Turner,et al.  Optimisation of the 3D MDEFT sequence for anatomical brain imaging: technical implications at 1.5 and 3 T , 2004, NeuroImage.

[18]  Keith Heberlein,et al.  Inherent insensitivity to RF inhomogeneity in FLASH imaging , 2004, Magnetic resonance in medicine.

[19]  Klaas Nicolay,et al.  The effect of B1 field inhomogeneity and the nonselective inversion profile on the kinetics of FAIR‐based perfusion MRI , 2005, Magnetic resonance in medicine.

[20]  Maolin Qiu,et al.  In vivo method for correcting transmit/receive nonuniformities with phased array coils , 2005, Magnetic resonance in medicine.

[21]  Robert Turner,et al.  3D MDEFT imaging of the human brain at 4.7 T with reduced sensitivity to radiofrequency inhomogeneity , 2005, Magnetic resonance in medicine.

[22]  J. Pauly,et al.  Saturated double‐angle method for rapid B1+ mapping , 2006, Magnetic resonance in medicine.

[23]  Maolin Qiu,et al.  Factors influencing flip angle mapping in MRI: RF pulse shape, slice‐select gradients, off‐resonance excitation, and B0 inhomogeneities , 2006, Magnetic resonance in medicine.

[24]  Nikolaus Weiskopf,et al.  Optimal EPI parameters for reduction of susceptibility-induced BOLD sensitivity losses: A whole-brain analysis at 3 T and 1.5 T , 2006, NeuroImage.

[25]  M. Symms,et al.  A simple correction for B1 field errors in magnetization transfer ratio measurements. , 2006, Magnetic resonance imaging.

[26]  U Klose,et al.  Fast 3D radiofrequency field mapping using echo‐planar imaging , 2006, Magnetic resonance in medicine.

[27]  Ralf Deichmann,et al.  Improvement of the image quality of T1-weighted anatomical brain scans , 2006, NeuroImage.

[28]  Douglas C Noll,et al.  Reduction of transmitter B1 inhomogeneity with transmit SENSE slice‐select pulses , 2007, Magnetic resonance in medicine.

[29]  Bostjan Likar,et al.  A Review of Methods for Correction of Intensity Inhomogeneity in MRI , 2007, IEEE Transactions on Medical Imaging.

[30]  Vasily L Yarnykh,et al.  Actual flip‐angle imaging in the pulsed steady state: A method for rapid three‐dimensional mapping of the transmitted radiofrequency field , 2007, Magnetic resonance in medicine.

[31]  Optimal spoiling of the transverse magnetization in the Actual Flip-angle Imaging (AFI) sequence for fast B1 field mapping , 2007 .

[32]  Jan J W Lagendijk,et al.  Simultaneous B  1+ homogenization and specific absorption rate hotspot suppression using a magnetic resonance phased array transmit coil , 2007, Magnetic resonance in medicine.

[33]  P. Dechent,et al.  Quantitative FLASH MRI at 3T using a rational approximation of the Ernst equation , 2008, Magnetic resonance in medicine.

[34]  Nikolaus Weiskopf,et al.  Rapid Radiofrequency Field Mapping In Vivo Using Single-Shot STEAM MRI , 2008, Magnetic resonance in medicine.

[35]  G. Metzger,et al.  Local B1+ shimming for prostate imaging with transceiver arrays at 7T based on subject‐dependent transmit phase measurements , 2008, Magnetic resonance in medicine.

[36]  P. Dechent,et al.  High‐resolution maps of magnetization transfer with inherent correction for RF inhomogeneity and T1 relaxation obtained from 3D FLASH MRI , 2008, Magnetic resonance in medicine.

[37]  Kay Nehrke,et al.  On the steady‐state properties of actual flip angle imaging (AFI) , 2009, Magnetic resonance in medicine.

[38]  R. Deichmann,et al.  Influence of RF spoiling on the stability and accuracy of T1 mapping based on spoiled FLASH with varying flip angles , 2009, Magnetic resonance in medicine.