Comparison of two- and three-dimensional ultrasonography in lung volume measurement of normal fetuses

Aim: The purpose of this study was to compare the two-and three-dimensional methods for measuring fetal lung volume of normal fetuses. Methods: A cross-sectional study was performed with 51 normal pregnant women between 20 and 35 weeks. The ellipsoid formula (X*Y*Z*0.52) was used for volume calculation with the two-dimensional (2D) method. With the VOCAL (TM) (Virtual Organ Computer-aided Analysis) method, a rotation angle of 30 degrees was used. Intraclass correlation coefficient (ICC), paired Student's t-test. and Bland-Altman plots were used to compare the techniques. To calculate the intraobserver variability we used the ICC and compared the means between the two measures using the paired Student's p-test. Results: VOCAL (TM) and 2D methods were highly correlated (ICC = 0.919, and 0.873 for the right and left lungs, respectively), however, there was a disagreement. The fetal lung volume means calculated by the 2D method were always overestimated in relation to the means obtained by the VOCAL (TM), for the right lung (24.02 mL x 19.15 mL; P < 0.001), as well as for the left (16.03 mL x 13.77 mL; P = 0.002). As for the intraobserver variability, a good reproducibility was observed for the volume measurement of the left lung by the 2D technique (mean=0.40 mL.; P=0.57) and by the VOCAL (TM) (mean= -0.22 mL; P=0.63). The 2D method, however, presented low reproducibility for the right lung (mean = 1.73 mL; P = 0.31). Conclusion: The two-dimensional method had low agreement and low reproducibility in relation to the threedimensional method for measurement of fetal lung volume in normal fetuses.