A Detailed Quantitative Analysis of B1 Components at 1.5T & 3T

Xin Chen¹, Michael Steckner¹

While SAR is crucial for the RF safety control of an MR scan, B1rms (root mean square of the total B1 field ) serves as a supplemental safety metric. For example, B1rms is useful for defining the MR conditional labeling of implants. An unloaded quadrature driven (QD) birdcage coil generates nearly perfect B1 with circular polarization: B1+ (the tipping component) is homogeneous and B1- (the component rotating counter to the spin precession) is zero and thus |B1-|<<|B1+| is usually assumed. Loading the coil distorts the B1 field, causing B1+ and B1- components. While only B1+ is useful for MRI purposes, both components contribute to RF power deposition. In this work we use FDTD (Finite Difference Time Domain) numerical simulations to demonstrate that B1- should not be ignored in loaded coils at either 1.5T or 3T.