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Obstetric imaging

Pelvimetry

Conventional standing lateral pelvimetry means a very high radiation dose with increased risk of subsequent malignancy in the fetus. However, it is still thought to be of value in cases where trial of labour for breech presentation is being considered or cephalopelvic disproportion is suspected.

Pelvimetry can be obtained with digital radiography or CT with a desirable dose reduction, or with MRI. With CT, both sagittal and AP views can be obtained (Fig. 6 a, b). Dose reductions are typically around 10% of conventional radiography with fast: film screen combinations. However, accuracy can only be achieved with protocols for the most ideal window and level settings and proper training in interpretation. This should be done by the radiologist. Obstetricians not being used to CT often misplace the calipers.

Pelvimetry using MRI should observe the heat related hazards mentioned previously, but when available MRI is now the method of choice. Fetal movement does not blur the landmarks and therefore there is no need for fetal sedation (Fig. 7).

a	Figure 6. CT pelvimetry a) Lateral view showing outlet measurement b) AP view for maximum transverse diameter of the inlet
b

In order to obtain the best visualization of pelvic landmarks and keep to safe limits the use of a proton density, sequences in the sagittal projection is recommended, with single echo or GE axial projections, if necessary. The specific absorption rate (head deposited) should be obtained and recorded in the obstetric notes.

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Figure 7.
MRl pelvimetry with AP inlet and outlet measurements.

Con Metreweli