Iliac crest angle: a novel sonographic parameter for the prediction of Down syndrome risk during the second trimester of pregnancy

Abstract

Objectives: To validate a new sonographic technique for the display and measurement of fetal iliac crest angle (ICA), and to determine the relative contribution of standardized fetal pelvic measurements for estimating risk of Down syndrome during the second trimester of pregnancy.

Methods: Three-dimensional ultrasonography of the fetal pelvis was performed during a second-trimester scan for genetic amniocentesis. A coronal view of the 'virtual pelvis' was obtained by applying a maximum intensity projection algorithm to visualize both iliac wings. Two different ICAs were measured from a coronal projection of the fetal pelvis (ICA-coronal 1 and ICA-coronal 2); the latter approach relied on using ischial tuberosities as reference landmarks. Next, a coronal projection of the fetal pelvis was rotated to demonstrate a rendered view of the axial fetal pelvis. Axial inner (ICA-inner), middle (ICA-middle) and outer (ICA-outer) ICAs were measured. The maximum anterior iliac distance measurement was defined as the widest separation between the most anterior points of the rendered iliac crests. The reproducibility of each ICA method was analyzed using Bland-Altman statistics. Simple and multiple logistic regression analysis determined the relative contribution of each parameter as a prenatal predictor of Down syndrome.

Results: Ninety-four normal fetuses and 19 fetuses with Down syndrome were examined. The ICA-middle and ICA-coronal 2 parameters were the most reproducible angle measurements. The mean +/- SD ICA-middle measurement for fetuses with Down syndrome was significantly greater than that for normal subjects (94.5 +/- 9.8 degrees vs. 83.1 +/- 6.5 degrees ; P < 0.001). The mean +/- SD ICA-coronal 2 angle measurement for fetuses with Down syndrome was slightly greater than that for normal subjects (57.9 +/- 5.5 degrees vs. 51.9 +/- 7.1 degrees ; P = 0.0014). A multiple logistic regression model including ICA-middle and ICA-coronal 2 provided a predictive ability of 88.1% based on the area under the receiver-operating characteristics curve. This combination had a sensitivity of 94.4% for a false-positive rate of 5% in the detection of Down syndrome.

Conclusions: Standardized iliac crest measurements of the fetal pelvis can be used to identify some fetuses at risk for trisomy 21 during the second trimester of pregnancy.

Figure 1. Three-dimensional multi-planar view of the axial fetal pelvis

Volume data of the axial fetal pelvis should be scanned from a “fetal prone” position with the spine at the 12 o'clock position. Standardized multi-planar views of the fetal pelvis are displayed so the axial slices, representing the original planes of image acquisition, are seen in the top left window (top left window). The cutting plane (green line) allows the examiner to view the volume-rendered spine from a posterior perspective (bottom right window). The volume-rendered spine is manually rotated 90 degrees counter-clockwise (white curved arrow).

Figure 2. Iliac crest angle measurement - coronal view of the fetal pelvis

Coronal views of the fetal pelvis are displayed for placement of two electronic lines along the lateral borders of both iliac crests (ICA-Coronal 1, left side). Ischial tuberosities (small white arrows) are near both measurement lines but, are not used to standardize their alignment to the iliac wings. In another fetus, the maximum intensity projection algorithm is again used to display a coronal pelvis (ICA-Coronal 2, right side). This approach requires that: 1) the proximal line be first anchored against the lateral aspect of each ischial tuberosity and 2) the angle line is swung medially against the lateral border of the iliac wing.

Figure 3. Rotation of superior iliac crests for standardized axial view of fetal pelvis

The left panel demonstrates a coronal projection of the posterior pelvis with both superior iliac crests (IC) and the more inferiorly positioned ischial tuberosities (arrowheads). After the maximum intensity projection filter is applied, the most echogenic voxels are displayed and the sacrum (S) becomes more apparent (center window). The examiner uses a mouse to rotate the pelvis around a horizontal x-axis for an axial view of the iliac wings (right panel). This maneuver aligns the superior iliac crests (white arrows) over the more inferior ischial tuberosities until they are no longer visible. This “birds-eye view” of the superior iliac crests is taken from the perspective of a camera pointed down toward the fetal legs. The maximum anterior iliac distance (MAID) was also measured in this plane (white line). Iliac distance was taken from this projection (right panel, not shown, reference 19).

Figure 4. Iliac crest angle measurements (axial view) - three approaches

ICA-Outer (left) requires that placement of the angle measurement lines along the outer surface of the superior iliac crests. ICA-Middle (center) requires that the angle measurement lines be placed through the center of the superior iliac crest bones. ICA-Inner (right) requires that the angle measurement lines run along the internal surfaces (outer one-third to one-half) of the superior iliac crests.

Figure 5. Receiver Operator Curve Summary for Prediction Parameters

Receiver operator curves summarize the relationship between sensitivity and specificity in relation to different cutoffs for ICA-Middle, ICA-Coronal parameters. The combined prediction model (red line) has the greatest area under the curve (c-statistic = 0.881).