Optimizing maternal fat suppression with constrained image-based shimming in fetal MR.

Andreia S Gaspar,Joseph V Hajnal,Anthony N Price,Emer J Hughes,Shaihan J Malik,Mary A Rutherford,Giulio Ferrazzi,Jana Hutter,Kelly P Baruteau,Laura Mccabe,Rita G Nunes

doi:10.1002/mrm.27375

Abstract

PurposeEcho planar imaging (EPI) is the primary sequence for functional and diffusion MRI. In fetal applications, the large field of view needed to encode the maternal abdomen leads to prolonged EPI readouts, which may be further extended due to safety considerations that limit gradient performance. The resulting images become very sensitive to water‐fat shift and susceptibility artefacts. The purpose of this study was to reduce artefacts and increase stability of EPI in fetal brain imaging, balancing local field homogeneity across the fetal brain with longer range variations to ensure compatibility with fat suppression of the maternal abdomen.MethodsSpectral Pre‐saturation with Inversion‐Recovery (SPIR) fat suppression was optimized by investigating SPIR pulse frequency offsets. Subsequently, fetal brain EPI data were acquired using image‐based (IB) shimming on 6 pregnant women by (1) minimizing B0 field variations within the fetal brain (localized IB shimming) and (2) with added constraint to limit B0 variation in maternal fat (fat constrained IB shimming).ResultsThe optimal offset for the SPIR pulse at 3 Tesla was 550 Hz. Both shimming approaches had similar performances in terms of B0 homogeneity within the brain, but constrained IB shimming enabled higher fat suppression efficiency.ConclusionOptimized SPIR in combination with constrained IB shimming can improve maternal fat suppression while minimizing EPI distortions in the fetal brain.

Highlights

GASPAR ET AL.Functional and diffusion MRI of the fetus provide a means to study emerging human brain connectivity during its development in utero
We focus on the use of a single static shim setting common for both fat suppression and echo planar imaging (EPI) readout to overcome both susceptibility and water-fat shift (WFS) artefacts, and seek an optimal trade-off between homogeneities of B0 over the fetal brain and maternal fat regions; the IB shimming method has the flexibility needed for this
We explore the feasibility of constrained IB shimming in combination with an optimized SPIR pulse for fetal imaging with EPI

Summary

Introduction

Functional and diffusion MRI of the fetus provide a means to study emerging human brain connectivity during its development in utero. Echo planar imaging (EPI) is the preferred acquisition method because of its high temporal resolution.[1,2,3] there are specific safety concerns when running EPI in the fetus, such as acoustic noise, peripheral nerve stimulation,[4,5] and specific absorption rate.[6] Part of these restrictions can be accommodated by lowering the bandwidth in the phase encoding direction at the expense of increased sensitivity to susceptibility-induced artefacts This problem is further increased by the necessity to use single-shot readouts to freeze fetal motion and the enlarged encoding matrix required to cover the surrounding maternal tissue

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Discussion

Conclusion