Large dynamic range relative B1+ mapping

Purpose: Parallel transmission (PTx) requires knowledge of the (Formula presented.) produced by each element. However, (Formula presented.) mapping can be challenging when transmit fields exhibit large dynamic range. This study presents a method to produce high quality relative (Formula presented.) maps when this is the case. Theory and Methods: The proposed technique involves the acquisition of spoiled gradient echo (SPGR) images at multiple radiofrequency drive levels for each transmitter. The images are combined using knowledge of the SPGR signal equation using maximum likelihood estimation, yielding an image for each channel whose signal is proportional to the (Formula presented.) field strength. Relative (Formula presented.) maps are then obtained by taking image ratios. The method was tested using numerical simulations, phantom imaging, and through in vivo experiments. Results: The numerical simulations demonstrated that the proposed method can reconstruct relative transmit sensitivities over a wide range of (Formula presented.) amplitudes and at several SNR levels. The method was validated at 3 Tesla (T) by comparing it with an alternative (Formula presented.) mapping method, and demonstrated in vivo at 7T. Conclusion: Relative (Formula presented.) mapping in the presence of large dynamic range has been demonstrated through numerical simulations, phantom imaging at 3T and experimentally at 7T. The method will enable PTx to be applied in challenging imaging scenarios at ultrahigh field. Magn Reson Med 76:490–499, 2016.