B1 Estimation Using Adiabatic Refocusing: BEAR

PurposeAccurate measurement of the nonuniform transmit radiofrequency field is useful for many applications in magnetic resonance imaging, such as calibrating the scanner transmit system, evaluating coil performance, and improving image quality and quantitation. The radiofrequency field excitation amplitude (B-1) is often obtained by acquiring a B-1 map. In this study, a new B-1 mapping method is proposed. Theory and MethodsThe use of two adiabatic full passage pulses with different magnitudes applied as successive refocusing pulses results in a linear relationship between phase and B-1 field strength that is insensitive to the repetition time, off-resonance effects, T-1, and T-2. Using this method, B-1 mapping can be localized to a slice or three-dimensional (3D) volume, with a spin-echo acquisition that is appropriate for fast projection measurements. ResultsThis new method is shown to agree well with the Bloch-Siegert B-1 mapping method for both phantom and in vivo B-1 measurements at 1.5T, 3T, and 7T. The method's ability to acquire accurate projection B-1 measurements is also demonstrated. ConclusionThis method's high dynamic range, ability to make fast projection measurements, and linear quantitative relationship between phase and B-1 make it an ideal candidate for use in robust transmitter gain calibration. Magn Reson Med 72:1302-1310, 2014. (c) 2013 Wiley Periodicals, Inc.