2014 Poster Sessions : Lowering the B1 Threshold for BEAR B1 Mapping

Student Name : Kalina Jordanova
Advisor : Dwight Nishimura
Research Areas: Information Systems
The accurate measurement of the nonuniform transmit radiofrequency field is useful for a variety of applications in magnetic resonance imaging, such as calibrating the scanner transmit system and evaluating coil performance. The radiofrequency field excitation amplitude (B1) is often obtained by acquiring a B1 map. The recently proposed BEAR method is a phase-based B1 mapping method, which has a linear phase sensitivity to variations in B1 and is insensitive to off-resonance frequency variations. The method relies on two hyperbolic secant (HS1) pulses operating in their adiabatic regime, which limits the range of B1 that can be measured due to the adiabatic threshold of the pulses. Here, we redesign the BEAR method to use HSn adiabatic pulses, which have lower adiabatic thresholds but result in a phase that is sensitive to off-resonance frequency. We optimize the HSn pulse parameters to greatly reduce the phase sensitivity to off-resonance frequency, resulting in a B1 mapping method which can be used to reliably measure lower nominal peak B1 values than with the original BEAR method. We validate the performance of BEAR with HSn pulses via simulation and in vivo at 3T, and show that for n <= 8, accurate B1 maps can be acquired using lower nominal peak B1 values.

Kalina Jordanova is a Ph.D. candidate in the department of Electrical Engineering at Stanford University. She is advised by Dr. Dwight Nishimura in the Magnetic Resonance Systems Research Laboratory. Her research focuses on using computational tools to improve magnetic resonance imaging techniques. Her work is supported by a National Science Foundation Graduate Research Fellowship and a Stanford Graduate Fellowship. Kalina received her B.S. in Applied and Engineering Physics from Cornell University in 2009.