MRI provides quantitative indices including relaxation time measurements, T1 and T2, which theoretically relate to placental composition and indices relating to perfusion, measured by Intra-Voxel Incoherent Motion (IVIM); related variables D and D* (coefficients of diffusion and pseudo-diffusion) and f% (the fraction of moving blood per voxel). We hypothesised these indices would be useful in the non-invasive assessment of the human placenta and are related to placental tissue morphology.
Methods 40 (30 normal, 10 fetal growth restriction (FGR)) women underwent MRI examination (1.5 T) between 20 and 41 weeks. Placental relaxation times and IVIM measurements were acquired from a mid-depth placental region. Fixed embedded sections of these placentas, collected at delivery were stained and subjected to stereological analysis of morphology.
Results Placental T1 and T2 show a significant negative correlation with gestation in normal pregnancy (p=0.01, 0.03, n=30), whereas IVIM parameters were unchanged (n=10). In the group as a whole, significant correlations were observed between f% and placental fibrin volume and capillary surface area (p=0.02, 0.03, n=6). In the group as a whole, there were no correlations between morphological features and relaxation times (n=19), D or D*(n=6). However, in a subset of pregnancies scanned within a week of delivery, a significant correlation was observed between placental fibrin volume and T2 (p=0.02, n=7).
Discussion The correlations between MRI indices and morphological variables support that these indices may be useful in non-invasive assessment of placental morphology. Ultimately, such indices might be useful in FGR, where alterations in placental morphology are observed.
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