CT pelvimetry: the foveae are not an accurate landmark for the level of the ischial spines.

The most important distance measured during CT pelvimetry is the interspinous diameter of the pelvis as seen on axial scans, because most cases of obstructed labor occur at the level of the ischial spines. As the spines are difficult to identify on the anteroposterior digital scout radiograph, the fovea of the femoral head has been used as a landmark to determine the level of the appropriate axial slice. We assessed the accuracy of this landmark on CT scans in 23 women undergoing CT pelvimetry and in 20 nonpregnant women having pelvic CT scans. In each case the relationship between the level of the foveae and the level of the ischial spines was determined. True interspinous distances were measured and compared with the apparent interspinous distances at the level of the foveae. In none of the women were the ischial spines above the foveae. In 15 (65%) of 23 pregnant women and in seven (35%) of 20 nonpregnant women, the ischial spines were below the foveae. An incorrect scanning level led to an overestimation of interspinous distance by an average of 1.0 cm. We conclude that for CT pelvimetry, the fovea as seen on the anteroposterior digital radiograph is an inaccurate landmark for the level of the ischial spines.