Estimating B₁⁺ in the breast at 7 T using a generic template

Dynamic contrast-enhanced MRI is the workhorse of breast MRI, where the diagnosis of lesions is largely based on the enhancement curve shape. However, this curve shape is biased by RF transmit (B₁⁺ ) field inhomogeneities. B₁⁺ field information is required in order to correct these. The use of a generic, coil-specific B₁⁺ template is proposed and tested. Finite-difference time-domain simulations for B₁⁺ were performed for healthy female volunteers with a wide range of breast anatomies. A generic B₁⁺ template was constructed by averaging simulations based on four volunteers. Three-dimensional B₁⁺ maps were acquired in 15 other volunteers. Root mean square error (RMSE) metrics were calculated between individual simulations and the template, and between individual measurements and the template. The agreement between the proposed template approach and a B₁⁺ mapping method was compared against the agreement between acquisition and reacquisition using the same mapping protocol. RMSE values (% of nominal flip angle) comparing individual simulations with the template were in the range 2.00-4.01%, with mean 2.68%. RMSE values comparing individual measurements with the template were in the range8.1-16%, with mean 11.7%. The agreement between the proposed template approach and a B₁⁺ mapping method was only slightly worse than the agreement between two consecutive acquisitions using the same mapping protocol in one volunteer: the range of agreement increased from ±16% of the nominal angle for repeated measurement to ±22% for the B₁⁺ template. With local RF transmit coils, intersubject differences in B₁⁺ fields of the breast are comparable to the accuracy of B₁⁺ mapping methods, even at 7 T. Consequently, a single generic B₁⁺ template suits subjects over a wide range of breast anatomies, eliminating the need for a time-consuming B₁⁺ mapping protocol.