X-ray pelvimetry--which is the best technique?

To provide definitive data on the radiation doses given to both mother and fetus using different pelvimetry techniques at a number of centres. This data was combined with an assessment of image quality, to determine the optimum technique.

Multiple thermoluminescent dosimeters (TLDs) were used to measure patient and fetal doses inside an anthropomorphic phantom of a pregnant women at full term. Measurements were made at twenty hospitals that routinely perform pelvimetry using techniques including computed tomography (CT), conventional radiography, conventional radiography using an air-gap technique, and digital fluorography. To assess image quality the images obtained by the different techniques were reviewed by a panel of two radiologists and a senior radiographer.

A wide range of absorbed doses and relative risks to mother and fetus was observed. The largest and smallest doses encountered in the study differed by a factor of approximately forty. CT pelvimetry was seen to give an average of 25% of the dose of conventional pelvimetry, and at some centres dose savings of over 90% would be possible by performing lateral CT scanograms in preference to plain film pelvimetry. However, a surprisingly wide variation in doses was observed between centres performing CT pelvimetry, and in some cases the potential dose saving would be small. An air-gap technique practised by one of the centres in the survey was found to give absorbed doses and relative risks comparable to average CT scanograms. A digital fluorography technique also gave a very low dose at one centre. However, a similar technique performed at another of the centres conferred little benefit over conventional pelvimetry. There was no observable correlation between the dose administered to the patient and image quality for any of the techniques, and CT and conventional pelvimetry both gave similar image quality.

While CT deserves its reputation as a low dose technique for pelvimetry, in some cases there may be little dose saving over conventional techniques. The total mAs from the CT examination should be kept as low as possible. Low absorbed doses with good image quality may also be achieved without specialized equipment using an air gap technique. This method is highly recommended as an alternative to CT, for example if no CT scanner is available.