An improved method to measure transfer functions using MRI

A previously published method for MRI-based transfer function assessment makes use of the so-called transceive phase assumption (TPA). This limits its applicability to shorter leads and/or lower field strengths. A new method is presented where the background electric field is determined from both $B_1^{+}$ - and $B_1^{-}$ -field distributions, avoiding the TPA and making it more generally applicable.

These $B_1$ -distributions are determined from a spoiled gradient echo multiflip angle acquisition. From the separated $B_1$ -components the background electrical field and the induced current are computed. Further improvement is achieved by recasting the $B_1$ -field model as a "magnitude squared least squares" problem. The proposed reconstruction method is used to determine transfer functions of various copper wire lengths up to 40 cm inside an elliptical ASTM phantom. The method is first tested on EM-simulated data and subsequently phantom and bench measurements are used to determine transfer functions experimentally.

In silica reconstructions demonstrate the validity of the proposed $B_1$ -field model resulting in highly accurate reconstructed $B_1$ -fields, currents, incident electric fields and transfer functions. The experimental results show slight deviations in the field model, however, resulting transfer functions are accurately determined with high similarity to simulations and comparable to bench measurements.

A more generally applicable method for MRI-based transfer function assessment is presented. The proposed method circumvents phase assumptions making it applicable for longer objects and/or higher field strengths. Additional improvements are implemented in the $B_1$ -mapping method and the solution algorithm.