Examining the relationship between the lung-to-head ratio measured on ultrasound and lung volumetry by magnetic resonance in fetuses with isolated congenital diaphragmatic hernia

Purpose: In fetuses with isolated congenital diaphragmatic hernia (CDH), lung development can be measured by the lung-to-head ratio (LHR) using ultrasound as well as by lung volumetry determined by fetal magnetic resonance imaging (MRI). We aimed to investigate their relationship as well as to analyze the factors that may have an impact on it.

Material and methods: In 153 consecutive fetuses with isolated CDH, both the LHR and total fetal lung volume (TFLV) were measured. The observed LHR was calculated by dividing the lung area by the head circumference. On MRI, planimetric measurements of ipsilateral, contralateral and TFLV were performed on T(2)-HASTE (half-Fourier acquisition single-shot turbo spin echo) sequences in transverse as well as coronal or sagittal planes. All values were expressed as a ratio of what was observed over what is expected in a gestational age-matched normal fetus. Secondary analyses were performed for right- versus left-sided hernia and for measurements made prior to 25 weeks' gestation. A multivariate linear regression approach was used to determine the influence of the independent variables such as observed/expected (O/E) LHR, gestational age, liver position and CDH side on the dependent variables O/E TFLV and O/E contralateral FLV, and to determine the optimal formulas for calculation of the O/E TFLV as well as contralateral FLV.

Results: In total, 200 pairs of measurements were obtained between 20 and 37 weeks' gestation (median 26+6). There was a significant association between the O/E contralateral FLV and O/E LHR (R(2) = 0.44; p < 0.001) as well as between the O/E TFLV and the O/E LHR (R(2) = 0.37; p < 0.001). After adding the independent variables that were first shown to be significant on univariate analysis, the multiple regression analysis demonstrated that gestational age (p = 0.017) and side of the defect (p < 0.001) were predictive of O/E LHR (p < 0.001) and strongly improved the estimation of O/E TFLV (R(2) = 0.43 instead of 0.37 when using O/E LHR only). In terms of estimating O/E contralateral FLV, only the O/E LHR was a significant (p < 0.001) independent predictor (R(2) = 0.44). These correlations also applied when considering only left-sided CDH cases. For measurements done prior to the third trimester, the O/E LHR (p = 0.034), gestational age (p = 0.035) as well as liver herniation (p = 0.029) were significantly correlated to the O/E TFLV (R(2) = 0.33). In terms of predicting the O/E contralateral FLV (R(2) = 0.25), only O/E LHR (p = 0.008) and gestational age (p = 0.037) were useful predictors.

Conclusion: Measurement of the O/E LHR on ultrasound allows a good estimation of the O/E contralateral FLV as well as TFLV as measured by MRI. Whereas the additional parameters such as gestational age, liver position and side of the defect did not improve the estimation of the contralateral FLV, they did so for estimating the TFLV.