Magnetic Flux Density Measurement with Balanced Steady State Free Precession Pulse Sequence for MREIT: a Simulation Study

摘要

Magnetic Resonance Electrical Impedance Tomography (MREIT) utilizes the magnetic flux density B_z, generated due to current injection, to find conductivity distribution inside an object. This B_z can be measured from MR phase images using spin echo pulse sequence. The SNR of B_z and the sensitivity of phase produced by B_z in MR phase image are critical in deciding the resolution of MREIT conductivity images. The conventional spin echo based data acquisition has poor phase sensitivity to current injection. Longer scan time is needed to acquire data with higher SNR. We propose a balanced steady state free precession (b-SSFP) based pulse sequence which is highly sensitive to small off-resonance phase changes. A procedure to reconstruct B_z from MR signal obtained with b-SSFP sequence is described. Phases for b-SSFP signals for two conductivity phantoms of TX 151 and Gelatin are simulated from the mathematical models of b-SSFP signal. It was observed that the phase changes obtained from b-SSFP pulse sequence are highly sensitive to current injection and hence would produce higher magnetic flux density. However, the b-SSFP signal is dependent on magnetic field inhomogeneity and the signal deteriorated highly for small offset from resonance frequency. The simulation results show that the b-SSFP sequence can be utilized for conductivity imaging of a local region where magnetic field inhomogeneity is small. A proper shimming of magnet is recommended before using the b-SSFP sequence.