Terry DuBose,
MS, RDMS, Chairman, OBGYN.net
Ultrasound Editorial Advisory Board
Citation:
DuBoseT: "3D BPD Correction". July 2000
Introduction
The fetal head is a three
dimensional organ (1).
However, most sonographic approaches to the fetal head are
as if it exists in Flat Land; but it is really in Sphere
Land, or more appropriately, Flat and Curved Space-Time.
At this time the primary measurements of the fetal cranium
are the biparietal diameter (BPD) and frontooccipital
diameter (FOD), or the transverse head circumference.
(2,3,4)
Few
consider the vertical cranial diameter (VCD), even though
it is the most dynamic in the presence of molding.
This article offers the hypothesis that there is no significant difference between the mean biparietal diameter (BPD) and the average of the three diameters. It is further postulated that the VCD exhibits the greatest variability of the three diameters because of the location and arrangement of the skull sutures and fontanels. In the presence of fetal head molding this provides a method of correcting the BPD (3D BPD Correction). The 3D BPD can be used in any estimate of age or weight for more accuracy.
(Click on the figures and tables to full size)
Eleven trained sonographers (ARDMS) measured BPDs in 2123 pregnancies. Of this population three diameters were measured in 1883 (88.7%) fetuses with beating hearts at the end of the embryonic period (> 10 weeks). The data were gathered from consecutive referrals, in outpatient offices in Austin, Texas, USA, during the calendar year 1993. Data were accumulated in computer databases at exam time.(5)
The methods for measuring the three cranial diameters have been described in detail elsewhere. (6,7,8,9,10) The VCD is the height of a triangle in the coronal plane, see figure 1. The triangle base ("+" in Fig. 2) is an imaginary line which is tangential to the bases of the bilateral circles around the hippocampal gyri, i.e. hippocampal base line (HBL). This coronal triangle is perpendicular to the plane of the standard head circumference and passes through the vertical axis of the thalamus, see figure 2. The BPD was measured leading edge to leading edge, while the frontooccipital diameter (FOD, aka OFD) was measured mid-frontal to mid-occipital bone to minimize inclusion of beam width artifacts(11). See Figure 3.
 Figure 1 |
 Figure 2 |
 Figure 3 |
Statistical analysis was carried out using commercial spreadsheet software (11). The BPD measurements and the three diameter’s averages (3D) were compared using a t-Test of paired two sample means, and no significant difference was found ( p = 0.497, Pearson Correlation = 0.996). Regression analysis found the relationship to be:
BPD = 3D(1.0) - 0.45 mm
(p < 0.001, r2 = 0.992)
Figure
4 (above) is a scatter plot of the three diameters and the
3D BPD. It can be observed that the BPD and 3D BPD follow
the same curve.
To study the variability of the three diameters the data were normalized by calculating ratios of each diameter divided by the average of the three (3D BPD). The normalized data were tested using analysis of variance (ANOVA). The results are shown in Table 1 and the indices’ histograms are shown in Figure 5. From this analysis it can be seen that the VCD has the greatest variance. Figure 5 shows the normal distribution and relative location of each index.
 Table 1 |
 Figure 5 |
The 3D is equal to the normal BPD, i.e. (FOD+BPD+VCD)/3 = BPD. Figure 6 is a scatter plot of the BPD against the 3D BPD Correction. The 3D has been called the 3D BPD CORRECTION. (2) The 3D BPD can be used in any regression or nomogram in place of the BPD to correct for head shape and molding. The 3D BPD will yield more accurate results than will the BPD or the transverse head circumference after 30 weeks.
Figure
6
Observation and additional research has found that in the coronal plane the normally shaped skull is an arc in a perfect circle, see Figure 2 (10). If the coronal view of the fetal cranium has a normal circular shape, then the BPD alone is a valid measurement. However, if there is molding, then two diameters or a circumference in a single plane are not sufficient for the most accurate estimates of age or size. Often in the presence of dolichocephaly, an exaggeration of the VCD is compensatory, see Figure 7. Conversely, if the VCD is compressed against the uterine wall the BPD compensates by increasing, see Figure 8.
 Figure 7 |
 Figure 8 |
References
