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

Kettinger, Ádám O. and Kannengiesser, Stephan A. R. and Breuer, Felix A. and Vidnyánszky, Zoltán and Blaimer, Martin (2017) Controlling the object phase for g factor reduction in phase-constrained parallel MRI using spatially selective RF pulses. Magnetic Resonance in Medicine. pp. 1-32. ISSN 0740-3194, ESSN: 1522-2594

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Abstract

Parallel imaging generally entails a reduction in the signal-to-noise ratio (SNR) of the final image. Phase-constrained methods aim to improve reconstruction quality by employing symmetry properties of k space. Noise amplification in phase-constrained reconstruction heavily depends on the object background phase. The purpose of this work is to present a new approach of using tailored RF pulses to optimize the object phase distribution in order to maximize the benefit of phase-constrained reconstruction, and minimize the noise amplification. Intrinsic object phase and coil sensitivity profiles are measured in a prescan. Optimal phase distribution is computed to maximize SNR in the given setup. Tailored RF 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 multi-slice (3x) acquisitions. Mean g-factors are reduced by up to a factor of 2 compared to conventional techniques when an appropriate phase-constrained reconstruction is applied to phase-optimized acquisitions, enhancing the SNR of the final images and the visibility of small details. Combining phase-constrained reconstruction and phase optimization by tailored RF pulses can provide notable improvement in the SNR and reconstruction quality of accelerated MRI.

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