Direct Signal Control of the steady-state response of 3D-FSE sequences

Purpose: Parallel transmission (PTx) offers spatial control of radiofrequency (RF) fields that can be used to mitigate nonuniformity effects in high-field MRI. In practice, the ability to achieve uniform RF fields by static shimming is limited by the typically small number of channels. Thus, tailored RF pulses that mix gradient with RF encoding have been proposed. A complementary approach termed "Direct Signal Control" (DSC) is to dynamically update RF shims throughout a sequence, exploiting interactions between each pulse and the spin system to achieve uniform signal properties from potentially nonuniform fields. This work applied DSC to T2-weighted driven-equilibrium three-dimensional fast spin echo (3D-FSE) brain imaging at 3T. Theory and Methods: The DSC concept requires an accurate signal model, provided by extending the spatially resolved extended phase graph framework to include the steady-state response of driven-equilibrium sequences. An 8-channel PTx body coil was used for experiments. Results: Phantom experiments showed the model to be accurate to within 0.3% (root mean square error). In vivo imaging showed over two-fold improvement in signal homogeneity compared with quadrature excitation. Although the nonlinear optimization cannot guarantee a global optimum, significantly improved local solutions were found. Conclusion: DSC has been demonstrated for 3D-FSE brain imaging at 3T. The concept is generally applicable to higher field strengths and other anatomies.