Comparison of inter- and intraobserver agreement and reliability between three different types of placental volume measurement technique (XI VOCAL, VOCAL and multiplanar) and validity in the in-vitro setting

Abstract

Objectives: To compare XI VOCAL (eXtended Imaging Virtual Organ Computer-aided AnaLysis) for three-dimensional (3D) ultrasound volumetry of the placenta and of phantom objects with a rotational method using VOCAL and with the multiplanar method.

Methods: We acquired 3D volume datasets from 32 fetuses at 11-14 weeks' gestation. Placental volume was calculated twice by each of two observers using XI VOCAL (with 5, 10, 15 and 20 slices), multiplanar (1-mm interval) and VOCAL (with 12 degrees, 18 degrees and 30 degrees rotation) methods. In addition, validity was assessed using the in-vitro setting with three phantom objects of known volume.

Results: Both inter- and intraobserver reliabilities were very high for all three methods. There was no systematic bias between any two methods except between XI VOCAL (10 slices) and the multiplanar (1-mm interval) method, with a smaller volume using the former method. The limits of agreement were wide between any two of the three methods. In the in-vitro setting, there was a trend towards less valid measurements with the XI VOCAL technique and fewer slices. With the same number of steps, measurements made with VOCAL (12 degrees and 18 degrees) were more valid than were those made with XI VOCAL (15 and 10 slices, respectively).

Conclusion: XI VOCAL cannot be used interchangeably with VOCAL or multiplanar techniques in measuring placental volume at 11-14 weeks' gestation.