Alternating steady state free precession for estimation of current-induced magnetic flux density: A feasibility study

Purpose To develop a novel, current-controlled alternating steady-state free precession (SSFP)-based conductivity imaging method and corresponding MR signal models to estimate current-induced magnetic flux density (B_z) and conductivity distribution. Methods In the proposed method, an SSFP pulse sequence, which is in sync with alternating current pulses, produces dual oscillating steady states while yielding nonlinear relation between signal phase and B_z. A ratiometric signal model between the states was analytically derived using the Bloch equation, wherein B_z was estimated by solving a nonlinear inverse problem for conductivity estimation. A theoretical analysis on the signal-to-noise ratio of B_z was given. Numerical and experimental studies were performed using SSFP-FID and SSFP-ECHO with current pulses positioned either before or after signal encoding to investigate the feasibility of the proposed method in conductivity estimation. Results Given all SSFP variants herein, SSFP-FID with alternating current pulses applied before signal encoding exhibits the highest B_z signal-to-noise ratio and conductivity contrast. Additionally, compared with conventional conductivity imaging, the proposed method benefits from rapid SSFP acquisition without apparent loss of conductivity contrast. Conclusion We successfully demonstrated the feasibility of the proposed method in estimating current-induced B_z and conductivity distribution. It can be a promising, rapid imaging strategy for quantitative conductivity imaging.