Apparent diffusion coefficient measurements of the fetal brain during the third trimester of pregnancy: how reliable are they in clinical practice?

OBJECTIVE: The objective of this study was to determine the reproducibility, the inter-hemispheric difference and the reference apparent diffusion coefficient (ADC) values of the fetal brain according to gestational age.
METHOD: One hundred and one normal fetal brain (29.4-38.4 weeks) were analysed with diffusion-weighted MR imaging. ADC was measured in frontal white matter (FWM), occipital white matter (OWM), centrum semi-ovale (CSO), basal ganglia (BG), cerebellar hemisphere (CBM) and pons. ADC ratios (fronto-occipital, fronto-cerebellar and occipito-cerebellar) were calculated. Inter-observer reproducibility was assessed on 27 studies, using intra-class correlation coefficient and Bland-Altman plot. Inter-hemispheric difference was evaluated with Bland-Altman plot. Gestation-specific reference intervals were estimated for each brain region.
RESULTS: Inter-observer bias was near zero. Limits of agreement (LOA) were clinically acceptable (-0.17; 0.20 to -0.38; 0.31 × 10(-3)  mm(2) /s) for all brain regions except for CSO and pons. Inter-hemispheric bias was near zero. Smallest LOA were for FWM (±0.09 mm(2) /s) and BG (±0.019 mm(2) /s). ADC values decreased, whereas ADC ratio increased with gestational age, reflecting normal maturation. Fronto-occipital, fronto-cerebellar and occipito-cerebellar ratios were consistently above 0.8, 1 and 1, respectively.
CONCLUSION: The fetal brain regions with the highest reproducibility and smallest inter-hemispheric differences are the frontal, occipital, cerebellar white matter and BG. ADC ratio could be useful to assess differential temporo-spatial maturation.