2.2\ Palpation of the Pelvis

The aim of palpation of the pelvis is to identify prominent bony structures that may obstruct labor. The examiner evaluates the angle of the pubic arch (>90°), the promontory (cannot be reached), the anterior surface of the sacrum (smooth), the coccyx (not prominent and elastic), and the ischial spines (not prominent).

Palpation has the disadvantage that the results cannot be standardized. The examination is extremely uncomfortable for the patient.

3\ MR Pelvimetry

Magnetic resonance (MR) pelvimetry was introduced in 1985 by Stark et al. (1985). MRI offers the benefit of accurate measurements of bony pelvic structures without exposure to ionizing radiation. The technique further allows imaging of soft-tissue structures, including the fetus, and has therefore replaced X-ray and computed tomography (CT) pelvimetry to become the modality of choice for obstetric pelvimetry (Stark et al. 1985; Pfammatter et al. 1990; Keller et al. 2003).

3.1\ Safety Issues

and Contraindications

Whereas prenatal X-ray exposure has been associated with an increased risk of childhood cancer (Stewart and Kneale 1968; Doll and Wakeford 1997), MRI does not use ionizing radiation. However, theoretically, safety issues could be related to possible adverse biologic effect associated with exposure to the static magnetic, gradient magnetic, and RF electromagnetic fields. Numerous studies of MRI in pregnant women have not revealed any experimental or clinical evidence of fetal harm. Thus, to our current knowledge, MRI is considered safe for both the mother and the developing fetus (Kanal et al. 1993; Baker e al. 1994; Masselli et al. 2013; DeWilde

et al. 2005; Shellock Frank and Crues John 2003; Ray Joel et al. 2016).

Nevertheless, there is at present still a general­ consensus that MRI should be performed in the first trimester of pregnancy only if there are clear medical indications, since rapid organogenesis takes place at this time and the fetus is thus most susceptible to any potentially hazardous external influences.

In our institution, MR pelvimetry is typically performed in the last trimester of pregnancy in women whose previous delivery was complicated by protracted labor with strong suspicion for cephalopelvic disproportion who wish to undergo a trial of labor. Alternatively, MR pelvimetry can be performed postpartum in women who plan to become pregnant again.

MR pelvimetry is a short examination, approximately 10 min examination time, without the need of intravenous contrast agents.

Due to the lower energy deposition in tissue, gradient-echo sequences might be preferred to spin-echo sequences for MR pelvimetry in pregnant women (Wright et al. 1992; Wentz et al. 1994; Urhahn et al. 1991; Michel et al. 2002; van Loon et al. 1990; Liselele et al. 2000; Pattinson and Farrell 1997; Van Loon et al. 1997).

On the other hand, T2-weighted spin-echo sequences allow for better assessment of soft tissue structures including the uterus. In our experience, most institutions have therefore now switched to T2-weighted spin-echo imaging.

Pregnant patients should be informed that, to date, there has been no indication that the use of clinical MR imaging during pregnancy has produced deleterious effects, and the MR pelvimetry may be performed with oral and written informed consent (Masselli et al. 2013).

A substantial contraindication to MRI, in general, is claustrophobia; other contraindications such as pacemakers and metallic splinters are comparatively rare in the obstetric population.

It should be kept in mind that many women referred for MR pelvimetry are unfamiliar with MRI and may be intimidated by the sheer bulk of the equipment. Despite current evidence that

MRI has no adverse fetal effects and of which, as discussed above, the women should be informed before MRI, the noise and claustrophobia of an MR exam may well induce fear for the fetus when imaging pregnant women, and they should thus be especially well cared for during the exam by the staff of the MRI suite.

In women with physical effects like vena cava compression syndrome that may occur in late pregnancy, imaging can be performed in the lateral decubitus position.

3.2\ MR Imaging Protocol

It has been shown in the literature that there are no significant differences in pelvimetric measurements between spin-echo and gradient-echo sequences (Keller et al. 2003; Wentz et al. 1994; Urhahn et al. 1991).

MR pelvimetry is usually performed in the supine position. Images of the maternal pelvis are

a

c

acquired with the body coil in axial, sagittal, and oblique (in a plane through the symphysis and sacral promontory) orientation as shown in Fig. 2.

In our institution, MR pelvimetry is always per- formedona1.5-Tscanner.WeareusingT2-weighted turbo spin-echo (TSE) sequences. T1-weighted fastspoiled gradient-echo sequences (FSPGR) might be used alternatively as discussed above. A large field- of-view (FOV), e.g., 360 mm, is used. Total imaging time is approximately 10 min.

3.3\ Image Analysis

After the MR examination, pelvimetric measurements are performed on a workstation using the exterior surface of the appropriate bony cortex as the measuring point (Figs. 2, 3, and 4). The following pelvic distances are measured:

•\ The obstetric conjugate from the sacral promontory to the top inner cortex of the pubic bone at the symphysis is assessed in the midsagittal plane.

b

d

Fig. 2  (a–g) Imaging protocol for MR pelvimetry in a 34-year-old woman with a history of secondary cesarean section and retroverted uterus. (a) Coronal localizing image for the axial plane (TRUFI, TR 6.0 ms, TE 2.53 ms, FOV 400 mm). (b) Axial T2-weighted TSE sequence at the level of the interspinous distance (TSE, TR 4500 ms, TE 102 ms, FOV 360 mm). (c) Axial T2-weighted TSE sequence at the level of the intertuberous distance (for parameters see (b)). (d) Localizing image for the midsag-

ittal plane (for parameters see (b, c)). (e) Sagittal T2-weighted TSE sequence (TSE, TR 3200 ms, TE 102 ms, FOV 350 mm): the obstetric conjugate and sagittal outlet are measured in the midsagittal plane. (f) Sagittal localizing image for transverse diameter (for parameters see (e)). (g) Coronal-oblique T2-weighted TSE sequence (TSE, TR 3200 ms, TE 102 ms, FOV 360 mm): the transverse diameter represents the widest transverse distance

Fig. 3  (a–d) Pelvimetric diameters (drawings by G. Roth). (a) Obstetric conjugate and sagittal outlet. (b)

Interspinous diameter. (c) Intertuberous diameter. (d) Transverse diameter (From Michel et al. 2002)

Fig. 4  (a–d) MR pelvimetry (T1-weighted gradient-echo imaging) in a 29-year-old pregnant woman in the last trimester with small pelvic dimensions. Vaginal delivery was attempted but failed and secondary cesarean section became necessary. The midsagittal section shows (a) the obstetric conjugate (10.7 cm) and sagittal outlet (9.8 cm).

Axial sections show (b) the interspinous distance (10.0 cm), measured at the level of the foveae of the femoral heads, and (c) the intertuberous distance (11.7 cm). The oblique section (d) shows the transverse diameter (11.8 cm)

•\ The intertuberous distance is the widest distance between the ischial tuberosities and is also measured in the axial plane.

•\ The transverse diameter represents the largest transverse distance (through the promontory and the symphysis) in the oblique axial plane (Keller et al. 2003; Michel et al. 2002).