Controlling the object phase for g‐factor reduction in phase‐Constrained parallel MRI using spatially selective RF pulses

Purpose Parallel imaging generally entails a reduction in the signal‐to‐noise ratio of the final image. Phase‐constrained methods aim to improve reconstruction quality by using symmetry properties of k‐space. Noise amplification in phase‐constrained reconstruction depends heavily on the object background phase. The purpose of this work is to present a new approach of using tailored radiofrequency pulses to optimize the object phase distribution in order to maximize the benefit of phase‐constrained reconstruction, and to minimize the noise amplification. Methods Intrinsic object phase and coil sensitivity profiles are measured in a prescan. Optimal phase distribution is computed to maximize signal‐to‐noise ratio in the given setup. Tailored radiofrequency pulses are designed to introduce the optimal phase map in the following accelerated acquisitions, subsequently reconstructed by phase‐constrained methods. The potential of the method is demonstrated in vivo with in‐plane accelerated (8x) and simultaneous multislice (3x) acquisitions. Results Mean g‐factors are reduced by up to a factor of 2 compared with conventional techniques when an appropriate phase‐constrained reconstruction is applied to phase‐optimized acquisitions, enhancing the signal‐to‐noise ratio of the final images and the visibility of small details. Conclusions Combining phase‐constrained reconstruction and phase optimization by tailored radiofrequency pulses can provide notable improvement in the signal‐to‐noise ratio and reconstruction quality of accelerated MRI. Magn Reson Med 79:2113–2125, 2018. © 2017 International Society for Magnetic Resonance in Medicine.