How true is a 'true' midsagittal section?

To study the deviation of presumed midsagittal sections of the fetal head from the true image plane, to compare whether certification (by The Fetal Medicine Foundation (FMF), UK) has an effect on the degree of this deviation, and to investigate the impact of imperfect midsagittal sections on nuchal translucency thickness (NT) measurement.

Eight sonographers, four of them FMF-certified (Group A) and the other four not (Group B), were asked to obtain image sections which they believed to represent true midsagittal sections of the fetal head and neck in five patients each. Three-dimensional volume datasets were acquired for subsequent analysis. The angle between the falx cerebri and the vertical axis was measured and the effect of this angle on NT measurement was assessed.

The mean angle of deviation was significantly lower among those who were FMF-certified (5.7 degrees vs. 14.7 degrees, P < 0.001). The 1.8% underestimation of NT by Group A was not statistically significant (P = 0.76), while in Group B the NT could not be measured when the angle of deviation was 14.7 degrees.

FMF-certified operators obtain a better midsagittal section compared with non-certified operators. The minor deviation from the true midsagittal section by certified operators has no significant effect on the value of NT measurement. This study could provide a model for operator training and auditing.