Objective
During labor, transperineal sonography is increasingly used to evaluate fetal head descent. The aim of this study was to compare the angle of progression assessed by open magnetic resonance imaging (MRI) vs transperineal ultrasound.
Study Design
A total of 31 pregnant women at term (>37 weeks), who were not in labor, underwent MRI in an open 1.0-T system. A midsagittal plane of the maternal pelvis was stored. Immediately after, without changing the supine position, a transperineal ultrasound was performed. The angle of progression was measured offline by transperineal ultrasound and MRI.
Results
The angles of progression measured by transperineal ultrasound (mean, 79.05 degrees; SD 11.44) and MRI (mean, 80.48 degrees; SD 11.06) correlated significantly ( P < .001). The intraclass correlation coefficient between the 2 methods was 0.89 (95% confidence interval, 0.78–0.94).
Conclusion
The angle of progression measurements obtained by transperineal ultrasound and open MRI showed very good agreement.
Various studies have shown that vaginal palpation of fetal head station is highly subjective, examiner dependent, and thus not accurately reliable, even though it is the standard practice used to determine labor progress in all delivery units around the world. Nevertheless, during labor, transperineal sonography is becoming an established method for evaluating labor progress and the success rate of vacuum extraction. Barbera et al were the first to use transperineal ultrasound to measure the angle between the maternal symphysis pubis and the leading part of the fetal skull (angle of progression). There is growing evidence suggesting the angle of progression may constitute a suitable, objective tool to evaluate labor progress.
The goal of this study was to study the agreement in assessing angle of progression between open magnetic resonance (MR) imaging (MRI) (the gold standard) and transperineal ultrasound in women at term, who were not in labor. To our knowledge, this is the first study describing MRI evaluation of the angle of progression.
Materials and Methods
From January 2009 through September 2009, we prospectively evaluated women at term from our antenatal care unit. Inclusion criteria were a live singleton pregnancy of >37 completed weeks with a fetus in an occiput anterior position. The fetal head position was diagnosed by transabdominal ultrasound as previously described by Akmal et al. Exclusion criteria were known fetal abnormalities, preterm ruptured membranes, active labor demonstrated by regular uterine contractions, and contraindications to the use of MRI. The study protocol was approved by the local medical ethics committee. Informed consent was obtained from all patients. All 31 healthy patients underwent MRI examination and, subsequently, a transperineal ultrasound.
MRI was performed in a 1.0-T open high-field MR scanner with a vertical magnetic field orientation (Panorama; Philips Healthcare, Best, The Netherlands) using a body coil. Pregnant patients were examined with an empty bladder, in a supine decubitus position, with appropriate padding. A T1-weighted fast spin-echo sequence was obtained using the following settings: time of echo, 19 milliseconds; time of repetition, 790 milliseconds; and thickness, 3 mm. The total individual study time was <30 minutes in all cases. MRIs were analyzed offline ( Figure 1 , A) ; on the basis of a midsagittal slice orientation, the angle of progression was measured by 1 radiologist, in the same way described for the transperineal approach (see below), who was blinded to the ultrasound results.
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