Synopsis

Measurement of blood flow in the fetal heart and the great vessels is challenging due to fetal motion and small vessel sizes. 2D methods for fetal flow imaging require significant slice piloting to locate the vessels, and small changes in fetal position can often necessitate reacquisition. In this work, we demonstrate the potential for motion-corrected whole-heart 4D flow imaging in the fetus using stacks of highly accelerated 2D bSSFP slices, which are inherently sensitive to velocity. Real-time acquired images were aligned in space and cardiac phase, and vectorially combined to yield time-resolved flow information.

Introduction

Measuring blood flow in the fetal heart and great vessels is challenging because the heart beats very rapidly, the vessels are small and the fetus is prone to spontaneous movement. Recently, we have developed a framework for retrospective motion-corrected 4D reconstruction of the fetal heart and great vessels from 2D real-time balanced steady state free precession (bSSFP) [1]. The framework was initially designed for cine imaging and disregarded the phase in the 2D bSSFP images, however, the bSSFP sequence is intrinsically velocity-sensitive because the first-moment is non-zero along the read and slice directions [2]. In this work, we use motion-corrected volumetric reconstruction of the velocity-sensitive phase of 2D bSSFP images to allow for 4D visualisation of blood flow in the fetal heart.

2D phase contrast gradient echo (PC-GRE) methods for measuring fetal blood flow [3] are susceptible to both in- and through-plane motion, often requiring repeated slice-planning and reacquisition. PC-GRE also uses a segmented acquisition that causes temporal mixing of acquired data. Mapping the blood velocity in the entire heart using motion-corrected methods may address these shortcomings.

Methods

All imaging was performed on a 1.5T Philips Ingenia scanner. To demonstrate proof of principle, stacks of parallel 2D bSSFP slices were acquired in a flow phantom (3 stacks orthogonal to scanner axes, 3 oblique), and in a single pregnant volunteer (5 stacks) using the following parrameters: TR/TE 3.8/1.9ms, flip angle 60°, 8x k-t SENSE [4] acceleration, field of view 400×304mm, voxel size 2.0×2.0x6.0mm, slice overlap 3-4mm, 96 images per slice in 7s, 37 slices. In the phantom study, images were acquired with voxel size 1.5x1.5x3.0mm, slice overlap 1.5mm and without acceleration.

4D Reconstruction Framework: 4D velocity-sensitive volumetric reconstructions were performed using a processing pipeline, shown in Fig1, modified from work described for image-domain volume reconstruction of the fetal brain [5] and fetal heart [1]. Magnitude and phase volumes were reconstructed separately, using motion-correction, cardiac synchronisation and outlier rejection parameters estimated from magnitude-valued data. Velocity-sensitive phase was separated from background phase in the the acquired bSSFP images by subtracting a third-order 3D polynomial fit to the phase in static amniotic fluid and tissue in each acquired stack isolated [6]. The velocity-sensitive directions in scanner space of each image were calculated from the first moments of gradients, the stack orientation and motion correction parameters, before being vectorially combined.

Flow Phantom: Water was passed through plastic tubing at a constant flow rate using a water pump. Two types of tubing with different internal diameters were formed into a continuous loop, which returned water to the pump. To minimise bSSFP artefacts, the tubing was housed inside a water-filled spherical flask (Fig2a). The phantom was imaged using PC-GRE for comparison with the volumetric PC-bSSFP reconstruction.

Results

Discussion

Acknowledgements

Thomas A. Roberts and Joshua F.P. van Amerom are joint first authors of this work.

This work was supported by the Wellcome EPSRC Centre for Medical Engineering at King’s College London (WT 203148/Z/16/Z) and by the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy’s and St Thomas’ NHS Foundation Trust and King’s College London. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.

References

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