Development and validation of 3D MP-SSFP to enable MRI in inhomogeneous magnetic fields

Purpose: We demonstrate the feasibility of MRI with missing-pulse steady-state free precession (MP-SSFP) in a 4T magnet with artificially degraded homogeneity. Methods: T₁, T₂, and diffusion contrast of MP-SSFP was simulated with constant and alternate radiofrequency (RF) phase using an extended phase graph. To validate MP-SSFP performance in human brain imaging, MP-SSFP was tested with two types of artificially introduced inhomogeneous magnetic fields: (1) a pure linear gradient field, and (2) a pseudo-linear gradient field introduced by mounting a head-gradient set at 36 cm from the magnet isocenter. Image distortion induced by the nonlinear inhomogeneous field was corrected using B₀ mapping measured with MP-SSFP. Results: The maximum flip angle in MP-SSFP was limited to ≤10° because of the large range of resonance frequencies in the inhomogeneous magnetic fields tested in this study. Under this flip-angle limitation, MP-SSFP with constant RF phase provided advantages of higher signal-to-noise ratio and insensitivity to B₁⁺ field inhomogeneity as compared with an alternate RF phase. In diffusion simulation, the steady-state magnetization in constant RF phase MP-SSFP increased with an increase of static field gradient up to 8 to 21 mT/m depending on simulation parameters. Experimental results at 4T validated these findings. In human brain imaging, MP-SSFP preserved sufficient signal intensities, but images showed severe image distortion from the pseudo-linear inhomogeneous field. However, following distortion correction, good-quality brain images were achieved. Conclusion: MP-SSFP appears to be a feasible MRI technique for brain imaging in an inhomogeneous magnetic field.