Validation of diameter measurements by ultrasound: intraobserver and interobserver variations assessed in vitro and in fetal sheep

Abstract

Objectives Blood flow evaluation in the fetus is hampered by inaccuracy in the measurement of vessel diameter. The impact of biological variation on reproducibility studies is unknown. The present study evaluated diameter measurements carried out with modern ultrasound equipment by assessing inter‐ and intraobserver variations. Design Repeated measurements analyzed by a general factorial model of analysis of variance. Methods Three observers measured repeatedly (six or more times for each tube and session) silicone tube implants (0.6, 2.2 and 6 mm), applying ultrasound imaging in eight fetal sheep, and the same silicone tubes. Analysis of variance was carried out on 199 observations made and 537 in vivo. Results The upper 95% confidence limit for the intra‐ and interobserver variations was higher for measurements made than. It was highest for the largest diameter and varied between 0.10 and 0.38 mm. When the diameter was determined as an average of repeated measurements, the error was reduced: six measurements in vivo had upper 95% confidence limits for intraobserver variation of 0.04 and 0.09 mm for diameters of 0.6 and 6 mm, respectively. Conclusions The results show that ultrasound diameter measurements have a high reproducibility even for vessels of small dimensions when repeated measurements are taken with high‐frequency ultrasound under favorable conditions. The low variation described in the present investigation of silicone tubes, compared to previous studies, suggests that diameter changes of living vessels represent a separate source of measurement variation. Copyright © 1999 International Society of Ultrasound in Obstetrics and Gynecology