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) u ltrasound. To assess the reproducibility of this measurement technique and to compare volumes obtained using 3D ultrasound with volumes calculated fro m 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 wome n in the third trimester of pregnancy (between 32 and 41 completed weeks ge station) underwent a 3D transvaginal ultrasound scan of the urethra. In 10 cases the scan was analyzed twice by different observers to assess the repr oducibility of the measurements from the scans and the results were analyze d using limits of agreement. Results The interobserver error was consistent between all the linear, 2D a nd 3D measurements obtained from the area scanned. There was a significant difference between volumes calculated directly by 3D ultrasound and the app roximated volumes from conventional 2D measurements. Conclusions Three-dimensional ultrasound appears to he a useful tool in mea suring urethral sphincter volume. The error is consistent with that of line ar and 2D imaging. However, the increase in normal range generated by biolo gical variation in all three planes makes 3D ultrasound a more sensitive me thod of evaluating change to the urethral sphincter.