Purpose or Case Report: The purpose of this study is to determine the reliability of sonographic (US) lung measurements compared with Fetal MR in fetuses with congenital diaphragmatic hernia. The sonographic lung head ratio (LHR) is a widely used prognostic tool in CDH, but can be difficult to obtain due to shifted mediastinum and fetal position. To our knowledge, comparison of MRI measured LHR with US has not been previously reported.
Methods & Materials: A retrospective review was performed of 40 subjects with a prenatal diagnosis of CDH, who underwent MRI and US on the same day. On both US and MRI the LHR was obtained using both the longest perpendicular diameter method and the method of tracing the lung area on the 4-chamber heart view. Three radiologists with expertise in prenatal imaging performed the measurements. Interobserver agreement was calculated. A paired samples t test was used to detect differences between US LHR and MRI LHR.
Results: Eight subjects were excluded due to lack of head circumference measurement. There were 6 right and 26 left CDHs. There was a high degree of interobserver agreement with lung area tracing on both ultrasound and MRI (correlations of 0.86 and 0.96 respectively). Inter-observer correlation was lower with the longest diameter method (0.82). US consistently underestimated lung area as compared to MRI (p<0.0001), by a mean of 70 mm² (± 137) by longest diameter method and by a mean of 147 mm² (± 263) by tracing method. All methods of LHR measurement significantly correlated with total lung volume: US LHR by longest diameter p < 0.014, US LHR by area p < 0.047, MRI LHR by longest diameter and area both with p <0.0001. US LHR measurements by longest diameter correlate more strongly to total lung volume than do US LHR measurements by area (z = 0.34).
Conclusions: The difficulty of obtaining an accurate US lung area measurement in fetuses with congenital diaphragmatic hernia may result in underestimation of the LHR and influence the prognostic assessment. Although ultrasound measurements by longest diameter are not as reproducible, the measurement correlates better with MRI measured total lung volume. Increased agreement between observers with MRI over ultrasound suggests MRI as a better gold standard of measurement.