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 (T_SS) as well as the number of slice interleaves.

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 T_SS 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 (T_AS) 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 T_SS 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 T_SS = 1500 ms. The same acquisition was then repeated with an increased number of interleaves as determined in the previous experiments.

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 T_SS and 2 interleaves, which would not satisfy the required minimum δ. When 20 slices are acquired, 3 interleaves allows 98% for the two longest T_SS. When 40 slices are acquired (whole brain coverage), 3 or 4 interleaves allow 98% recovery for most T_ss, 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).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].