Three-dimensional ultrasound fetal urinary bladder volume measurement: reliability of rotational (VOCAL^TM) technique using different steps of rotation

The aim of the study was to investigate the inter- and intraobserver reliability in measuring the fetal bladder volume by 3d ultrasound imaging by VOCAL^TM. In addition a comparison of 15° and 30° rotation steps calculation had been performed.

Measurements (n = 240) of FB volume were performed in 30 low-risk patients with gestation times ranging from 22 to 39 weeks by two independent observers blind to each other. All fetuses were examined and no ultrasound detectable malformation was found. The volume for each FB was measured twice by the observers who used the Virtual Organ Computed-aided Analysis (VOCAL^TM) technique. Distinct sets of 12 and 6 planes were obtained after sequential rotations of 15° and 30°, respectively. The internal contour of fetal bladder was determined manually and carefully as to exclude adjacent structures from volume calculation. Wilcoxon signed-rank test was used for the comparison of paired samples in the cases of replication within and between observers. Spearman’s rank correlation was used to study the relationship among angles. Bland and Altman’s graphical approach was used to investigate the agreement between observers.

Both techniques were shown to be highly reliable. No significant difference was found between the measurements of FB volume with the VOCAL^TM technique by varying either the steps of rotation or the observers.

Excellent correlations were found for both observers in the use of rotation angles of 15° and 30°. Since a significantly faster evaluation was obtained by using a 30° rotation step it must be preferred to assess the FB volume. Our results show that the VOCAL^TM technique, with a plane rotation of 30°, can be used in a simple way to estimate fetal urine production.