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

Abstract

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.

Dual polarity encoded MRI using high bandwidth radiofrequency pulses for robust imaging with large field inhomogeneity

“…Fully sampled, dual polarity MP‐SSFP as implemented herein is approximately 3.17x faster than fully sampled, single polarity MAVRIC, provided spatial resolution is fixed. As demonstrated in Figure 3, tuning of the flip angle and number of pulses used in MP‐SSFP significantly changed the SNR efficiency 45 . With the experimental sequence parameters and phantom used herein, the SNR efficiency of MP‐SSFP is approximately 1.15x that of MAVRIC.…”

“…As demonstrated in Figure 3, tuning of the flip angle and number of pulses used in MP-SSFP significantly changed the SNR efficiency. 45 With the experimental sequence parameters and phantom used herein, the SNR efficiency of MP-SSFP is approximately 1.15x that of MAVRIC. However, the SNR efficiency of MP-SSFP exceeding that of MAVRIC may not always be the case, depending on MAVRIC sequence parameters (eg, bin interleaving strategy, 42 RF pulse train length, flip angles, etc.)…”

Real‐time B0 compensation during gantry rotation in a 0.35‐T MRI–Linac

“…Like center frequency offsets, field inhomogeneities can cause spatial shifts in the slice-selection or readout direction, and variations in image intensity due to signal dephasing during the 2D bSSFP acquisition. 37 Field inhomogeneities can also cause geometric distortion. Based on our previous measurements of field inhomogeneities, geometric distortion and spatial shifts should have a small effect on tracking (e.g., Dice coefficient).…”

Contemporary approaches to high-field magnetic resonance imaging with large field inhomogeneity

“…At the temporal position (n + 1)τ, there is a superposition of a large number of spin and stimulated echoes, as can be seen in the extended phase graph formalism [67][68][69]. Kobayashi et al explicitly calculated the extended phase graph for MPSSFP with n = 4 pulses [70,71], providing a graphical realization of the echo formation mechanism.…”

“…FLASH [78,79]). MPSSFP, with appropriate choice of RF phase cycling and flip angle, can offer both T1 and T2 weighting, along with diffusion weighting [71]. Unfortunately, there is not yet an option to achieve T2* weighting with large magnetic field inhomogeneity.…”