Regularized B1+ Map Estimation in MRI

Amanda K. Funai, Jeffrey A. Fessler, William Grissom, Douglas C. Noll

Abstract: A challenge in MR imaging is that RF transmit coils produce non-uniform field strengths, so an excitation pulse will produce tip angles that vary substantially from the desired tip angle over the field of view. For parallel transmit excitation (using a coil array), it is important to have a map of the B1+ field strength (and phase) for RF pulse design. Standard B1+ map estimation methods perform poorly in image regions with low spin density. This paper describes a regularized method for B1+ map estimation using MR scans for each coil and for two or more tip angles. Using these scans and exploiting the fact that maps are generally smooth, the iterative algorithm estimates both the magnitude and phase at each coil's B1+ map. Results from both simulations and real MR data show significant improvements over conventional unregularized methods for B1 + mapping.