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

Abstract

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 anteropostenor 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 anteropostenor digital radiograph is an inaccurate landmark for the level of the ischial spines.

Measurement error in computed tomography pelvimetry

“…Observer variation is not the only source of error in CT pelvimetry. Interspinous distance may be overestimated if measurement on an axial slice at the level of the fovea, as the ischial spines are at this level in only 40–60% of women 12 . The bony dimensions of the maternal pelvis vary by up to 4 mm with change in posture 23,24 and also change during labour.…”

“…Magnetic resonance pelvimetry has been advocated because of its accuracy and safety 8 . The accuracy of both X‐ray pelvimetry and CT pelvimetry has been called into question 9–13 . Over the last 15 years, a number of studies have cast doubt on the effectiveness of pelvimetry in predicting pregnancy outcome 14–19 .…”

MR Obstetric Pelvimetry: Effect of Birthing Position on Pelvic Bony Dimensions

“…MR imaging has become widely accepted as the imaging modality of choice for obstetric pelvimetry [7,[11][12][13][14][15][16][17][18], although gynecologic reference values are based on radiographic examinations [26][27][28][29].…”

“…The obstetric conjugate and the sagittal outlet were both assessed in the midsagittal plane. The interspinous and intertuberous diameters were assessed in the axial plane [17,18] (Figs. 2 and 3).…”

Magnetic resonance imaging pelvimetry: a useful adjunct in the management of the obese patient

“…1985). Dose is less than with conventional x‐ray pelvimetry, although further axial slices may sometimes be necessary to localise the ischial spines, and this will increase the radiation dose significantly (Moore & Shearer 1989; Aronson & Kier 1991). Use of correction formulae or careful positioning of the patient with respect to the plane of the centre of rotation of the scanner is needed for accuracy of the transverse measurements (Weisen et al .…”