Three-dimensional ultrasound: a novel technique for investigating the urethral sphincter in the third trimester of pregnancy.

OBJECTIVES

To measure urethral sphincter volume by three-dimensional (3D) ultrasound. To assess the reproducibility of this measurement technique and to compare volumes obtained using 3D ultrasound with volumes calculated from a formula based on 2D ultrasound measurements.

METHODS

Women were recruited as part of an ongoing study of changes to the pelvis resulting from pregnancy and childbirth. One hundred and eleven women in the third trimester of pregnancy (between 32 and 41 completed weeks' gestation) underwent a 3D transvaginal ultrasound scan of the urethra. In 10 cases the scan was analyzed twice by different observers to assess the reproducibility of the measurements from the scans and the results were analyzed using limits of agreement.

RESULTS

The interobserver error was consistent between all the linear, 2D and 3D measurements obtained from the area scanned. There was a significant difference between volumes calculated directly by 3D ultrasound and the approximated volumes from conventional 2D measurements.

CONCLUSIONS

Three-dimensional ultrasound appears to be a useful tool in measuring urethral sphincter volume. The error is consistent with that of linear and 2D imaging. However, the increase in normal range generated by biological variation in all three planes makes 3D ultrasound a more sensitive method of evaluating change to the urethral sphincter.