A reasoned approach to explore single shot FSE acquisition for fetal MRI
Maelene Lohezic1,2, Joshua van Amerom1,2, Kelly Pegoretti2, Laura McCabe2, Christina Malamateniou2, Olivia Carney2, Matthew Fox2, Joanna Allsop2, Mary Rutherford2, and Joseph Hajnal1,2

1Department of Biomedical Engineering, King's College London, London, United Kingdom, 2Centre for the Developing Brain, King's College London, London, United Kingdom

### Synopsis

Fetal MRI is a growing field but it is still far from being a standard examination and setting up a protocol demands careful optimization. Here we explore some factors to consider when choosing an appropriate slice acquisition order pattern to mitigate the effect of crosstalk between temporally consecutive slices and the partial saturation from the excitation of the adjacent slice, while taking moderate range of fetal motion into account.

### Introduction

Fetal MRI is a growing field but it is still far from being a standard examination and setting up a protocol demands its own optimization. The use of rapid MR sequences is essential for successful fetal imaging, where both maternal and fetal motion are present. T2 weighted single shot Fast Spin Echo (ss-FSE) sequences are widely used as they provide good T2 contrast for anatomy examination while mostly freezing intrascan motion [1]. However, visualizing the small size of the fetal organs requires high in plane resolution, as well as large packs of thin contiguous slices and careless choice of imaging parameters may result in suboptimal image quality due to slice crosstalk, partial saturation or magnetization transfer from adjacent slices [2]. Here we explore some factors to consider when choosing an appropriate slice acquisition order pattern by adjusting the delay between slice samplings (TSS) as well as the number of slice interleaves.

### Methods

All imaging was performed on a 1.5T Siemens Aera system with the spine array removed and using two 18-element cardiac coils closely wrapped around the maternal abdomen.The slice interleave pattern shown in Figure 1A was explored by adjusting TSS and the number of slice interleaves. The minimum distance between 2 consecutive slices (δ) to avoid crosstalk between slices in the absence of fetal motion was determined for the sequence under test on a spectroscopy phantom provided by the vendor by acquiring ss-FSE stacks of 21 axial slices (Table 1), with slice separation of 1 to 6 slice thicknesses. Then, based on T1 values relevant to fetal MRI, simulations of the recovery of the longitudinal magnetization (Mz) after full and partial saturation (factor α) were performed to determine the minimum time delay between 2 adjacent slices (TAS) to allow signal recovery.

$$Mz=1-\alpha e^{-\frac{T_{AS}}{T_{1}}}$$ with $$T_{AS}=T_{SS}\frac{N_{Slices}}{N_{Interleaves}}$$

We considered stacks of 10 to 80 slices, 2 to 6 interleaves, and TSS of 600, 1000, 1500 and 2000 ms. Finally, one pregnant woman (gestational age: 32 weeks) was scanned at 1.5T. Two ss-FSE stacks of 40 slices each were acquired sagittally to the fetus brain using the imaging parameters in Table 1. The first stacks was acquired using 2 interleaves and TSS = 1500 ms. The same acquisition was then repeated with an increased number of interleaves as determined in the previous experiments.

### Results

The signal intensity in the central slice was plotted against the slice separation (Fig. 1B). The signal intensity in the phantom was within 2% of its maximum for a slice separation greater than 2.75, which is satisfied with 3 or more interleaves. Figure 1C shows the recovery of Mz in the amniotic fluid (T1 = 4 s) after 50% saturation. For 10 slices 98% recovery is only obtained for the longest TSS and 2 interleaves, which would not satisfy the required minimum δ. When 20 slices are acquired, 3 interleaves allows 98% for the two longest TSS. When 40 slices are acquired (whole brain coverage), 3 or 4 interleaves allow 98% recovery for most Tss, while 4 to 8 interleaves can be used for larger coverage (80 slices), thus limiting the risk of slice contamination due to fetal motion. Figure 2 compares the images obtained on the same volunteer with two different acquisition schemes. The second stack of ss-FSE images was acquired with 4 interleaves. An improvement in SNR as well as in contrast is clearly visible in the higher interleave factor acquisition. For instance, the higher interleave slice order allows differentiation of the corpus callosum and contrast between the deep grey matter and the white matter (arrows). Moreover, smaller slice separation makes the acquisition more susceptible to motion artefact (Fig. 2 Bottom left).

### Conclusion

We demonstrated that slice acquisition order has a great effect on image quality of ss-FSE in fetal brain MRI. Other parameters need to be taken into consideration when setting up a ss-FSE imaging protocol such as the choice of the refocussing flip angle, that would help control the SAR of the acquisitions to improve the scan efficiency while preserving the patient’s safety [3]. Another confounding factor is fetal motion. We have here only considered artefact caused by limited range of motion, which can be mitigated by spacing out successive slices. The effect of flowing amniotic fluid and more complex types of motion, as encountered at earlier gestational age will require further investigation and study of the full spin history [4].

### Acknowledgements

This work was supported by the iFind Project (Wellcome Trust IEH Award 102431) and BRC.

### References

[1] Y Yamashita et al. MR imaging of the fetus by a HASTE sequence. American Journal of Roentgenology. 1997;168: 513-519

[2] Melki PS, Mulkern RV. Magnetization transfer effects in multislice RARE sequences. Magnetic Resonance in Medicine. 1992; 24:189–195.

[3] J Mugler. Optimized Three-Dimensional Fast-Spin-Echo MRI. Journal of Magnetic Resonance Imaging. 2014; 39:745–767

[4] G. Ferrazzi et al. Resting State fMRI in the moving fetus: A robust framework for motion, bias field and spin history correction. NeuroImage. 2014; 101:555–568

### Figures

Table 1: Imaging parameters for ss-FSE

Figure 1: Optimization of ssT2wFSE slice acquisition order. A. Example of the slice acquisition pattern, with 12 slices and 4 interleaves. B. Determination of the minimum distance between 2 consecutive slices. C. Simulation of the recovery of the longitudinal magnetization in the amniotic fluid (T1 = 4 s) for varying number of slices, interleaves and TSS.

Figure 2: Adjacent slices acquired in a fetal brain with (Left) the standard 2-interleave and (Right) the higher interleave acquisition pattern. The 2-interleave pattern results in poor SNR and low contrast in the brain. The corpus callosum can only be differentiated in the 4-interleave acquisition (arrows), with good contrast between the deep grey matter (asterisk) and the white matter. Also slice contamination due to fetal motion is more important in the 2-interleave pattern (Bottom left).

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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