Multimodal assessment of fetal head circumference and biparietal diameter: systematic differences between MRI and US measurements and development of an US-equivalent prediction regression model.

OBJECTIVE: To investigate the differences between fetal magnetic resonance imaging (MRI) and ultrasonography (US) in measuring head circumference (HC) and biparietal diameter (BPD), and to develop and validate a regression model for predicting US-equivalent values based on MRI measurements.

METHODS: Imaging data of 124 mid-to-late trimester fetuses were retrospectively included. The differences in HC and BPD measurements between the two modalities were quantitatively analyzed. A regression model was constructed using MRI measurements, gestational age (GA), and the time interval between examinations as independent variables to predict the US-equivalent values (USpred).

RESULTS: Compared with US, MRI demonstrated significant systematic overestimation for both HC (mean difference: 14.0 mm, p < 0.001) and BPD (median difference: 3.18 mm, p < 0.001). The established USpred model showed high goodness-of-fit (adjusted R 2 > 0.95). When classifying based on US standards, the USpred values exhibited significantly improved classification efficacy compared to direct MRI measurements: overall accuracy (HC: 0.782 vs. 0.629; BPD: 0.863 vs. 0.782) and Matthews correlation coefficient (HC: 0.512 vs. 0.322; BPD: 0.639 vs. 0.528).

CONCLUSION: Fetal MRI measurements of HC and BPD show a systematic overestimation compared to US. The USpred model can effectively correct for this systematic bias in MRI measurements and the influence of examination time intervals, significantly improving agreement with commonly used clinical US standards. It provides a reliable tool for the precise assessment of prenatal MRI.