2.4. Image acquisition

Following the pilot scan and B₀ and B₁ calibration scans, anatomical imaging using T₂‐weighted turbo‐spin‐echo sequences and a combined T2*‐diffusion scan were performed as depicted in Figure 1. For this combined T2*‐diffusion scan using a technique referred to as ZEBRA, ^{1 , 21} the diffusion‐weighted spin‐echo scan was extended to include four TEs after each diffusion‐weighting was performed, here at 78, 114, 150, and 186 ms. The repeatability of the T2*‐diffusion sequence has been demonstrated as a first step previously in MR phantom and adult brain studies. ²¹ To further investigate in vivo repeatability of the T2*‐diffusion sequence and image processing pipeline also in vivo, this sequence was repeated at the end of a scanning session for four participants in the longitudinal cohort. The details of this experiment and the results are described in the Supporting material (Table S1, Figure S1). The chosen diffusion preparations were maintained from a previous study optimizing them for the properties of the human placenta. ²² This resulted in three rotating diffusion gradient directions being used at b = [5, 10, 25, 50, 100, 200, 400, 600, 1200, 1600] s mm⁻², eight directions at b = 18 s mm⁻², seven at b = 36 s mm⁻², and 15 at b = 800 s mm⁻² (Table 1). The choice of the gradients was performed to avoid directional bias as described in Slator et al. ²²

Scan parameters for the considered functional sequences: multi‐echo gradient echo EPI and multi‐echo diffusion‐weighted EPI

Multi‐echo Gradient Echo dMRI Sequence Parameters

3 mm³ isotropic resolution, TE = (78, 114, 150, 186) ms, TR = 7.5 ms, coronal plane to maternal habitus.

b = (5, 10, 25, 50, 100, 200, 400, 600, 1200, 1600) s mm⁻²; 3 directions

b = 18 s mm⁻²; 8 directions

b = 36 s mm^−2; 7 directions

b = 800 s mm⁻²; 15 directions

Abbreviation: dMRI, diffusion MRI.