Objective
Sonography-based Automated Volume Count (SonoAVC) is an automated, operator-independent, 3-dimensional ultrasound technique. The study goal was to assess agreement between SonoAVC and Virtual Organ Computer-aided AnaLysis (VOCAL) for bladder volume measurements and fetal urine production (FUP).
Study Design
This was a prospective study of recipient bladder volumes in twin-to-twin transfusion syndrome acquired with SonoAVC and VOCAL every 2 minutes during 30 minutes before and 24 hours after laser coagulation. FUP formulas: (V 2 – V 1 ) × 60/time interval in minutes; and slope of linear regression of ≥3 consecutively increasing bladder volumes × 60 minutes. Bland-Altman plots were used to compare techniques.
Results
In 38 pregnancies, 560 bladder measurements were acquired. Mean FUP was 7.29 mL/h (95% confidence interval, 5.66–8.92) using SonoAVC and 6.79 mL/h (95% confidence interval, 5.16–8.42) using VOCAL ( P = .14). Limits of agreement were −0.20 to 0.22 cm 3 .
Conclusion
Fetal bladder volume and FUP measurements using SonoAVC were comparable to VOCAL. SonoAVC is operator independent, easy, and fast.
The sonographic assessment of fetal urine production (FUP) has been of interest to clinicians for more than 3 decades. Many pathologic conditions are known to be associated with abnormal FUP, such as fetal growth restriction, maternal diabetes, and monochorionic twins. Despite this, assessment of FUP is not a commonly used diagnostic tool in obstetrics. This is likely related to the limited accuracy and feasibility of measurement methods, until recently, based on 2-dimensional (2D) ultrasonography.
Three-dimensional (3D) ultrasound has been shown to be superior in measuring fetal organ volumes compared to 2D ultrasound, particularly for irregularly shaped organs. With the introduction of new 3D ultrasound techniques, such as Virtual Organ Computer-aided AnaLysis (VOCAL), volume measurements of fetal organs have become increasingly easy to obtain and to reproduce. Recently, 3 studies have used VOCAL to assess FUP in singleton and monochorionic pregnancies and demonstrated that FUP can be measured reliably with this new technique. VOCAL, however, is still operator dependent, requiring a sonographer to manually trace the contour of the bladder.
Sonography-based Automated Volume Count (SonoAVC) is a new and fully automated 3D volume measuring technique. The software uses a static 3D sweep of the target organ as a starting point. After acquiring the volume the SonoAVC option is selected and the region of interest is positioned around the target organ. The software can identify the center of hypoechoic structures within the region of interest. Progressively, all surrounding voxels are calculated toward the border of the selected target organ and the software extrapolates its volume and dimensions ( Figure 1 ). Thus far, this technique has been used to calculate fetal stomach volumes by Rizzo et al, but has not been applied to measure fetal bladder volumes.
The aim of this study was to assess agreement between SonoAVC and VOCAL for bladder volume measurements and FUP.
Materials and Methods
This was a prospective study of 38 monochorionic twin pregnancies with twin-to-twin transfusion syndrome (TTTS) collected at Leiden University Medical Center and Baylor College of Medicine. The study was approved by the institutional review board at each of the centers. A 4- to 8-MHz 3D transducer of a Voluson E8 ultrasound machine (GE Healthcare Technology, Milwaukee, WI) was used to make a 3D acquisition of the recipients’ bladder every 2 minutes during 30 minutes. Acquisition was performed with minimal fetal movement and without interference of shadowing of fetal iliac bone. Data presented are acquisitions made before and 24 hours after laser coagulation of vascular anastomoses. As the primary goal of this study was to compare a new technique to an existing one, this population was chosen because members already required extensive ultrasound examination and were motivated because they understood the importance of FUP measurements. Image settings (speckle reduction imaging 11, dynamic contrast 6, quality of 3D acquisition in sectional planes mode “high,” volume angle 20-45 degrees) and equipment used for fetal bladder volumes were identical in both centers. All patients gave oral and written informed consent. FUP (mL/h) was calculated using 2 methods. Method 1: (V 2 – V 1 ) × 60/time interval, with bladder volumes expressed in milliliters and time interval in minutes. Method 2: slope of linear regression of at least 3 consecutively increasing bladder volumes × 60 minutes. Each 3D acquisition was analyzed offline using software (4D View, version 7.0; GE Medical Systems, Zipf, Austria) with default settings for SonoAVC without any postprocessing. The software can be downloaded from www.volusonclub.net/emea/4dview/downloads and used with the appropriate USB dongle. FUP was only calculated when there were at least 3 consecutively increasing bladder volumes measured with SonoAVC (GE Medical Systems). This was done to confirm that volumes were acquired during the bladder filling phase and that no voiding had occurred between acquisitions. Only acquisitions fulfilling these criteria were also analyzed with VOCAL. If an acquisition was of inadequate quality but the following acquisition clearly showed an increased volume as compared to the previous adequate volume, the inadequate volume was discarded.
VOCAL was applied with a rotational angle of 30 degrees on the same acquisitions to calculate bladder volume. When several series of ≥3 consecutively increasing bladder volumes occurred within the 30 minutes of acquisition, the average FUP was used. Acquisitions and measurements were done by 1 observer in each center.
Bladder volumes measured with SonoAVC and VOCAL were compared using a Bland-Altman plot, to demonstrate bias and agreement between the 2 measurement techniques.
The limits of agreement were calculated by estimating the mean and SD of the difference between the 2 measurements where 95% of these differences lie within 1.96 SD of the mean. Repeated measurements within patients were accounted for by using a mixed model. Linear regression of at least 3 increasing bladder volumes was used to calculate the FUP of each individual fetus. A quadratic regression analysis was performed using software (Prism 5.0; Graphpad Software Inc, La Jolla, CA) to assess the relation between FUP and gestational age. To evaluate the difference in FUP using bladder volumes measured with SonoAVC and method 1 or 2 a paired t test was performed within a mixed model using software (SPSS 16.0; SPSS Inc, Chicago, IL). The same was done for VOCAL; a P value of < .05 was considered significant.
Acquisitions were analyzed in each center separately, no volume data set was analyzed by 2 observers. VOCAL is an established reliable and reproducible method for volume calculation, with good intraobserver and interobserver variability. With SonoAVC the observer has no role in the volume calculation, it is done by the software. As a result, only the intraobserver intraclass correlation coefficient (ICC) was reported. Intraobserver ICC was calculated in SPSS with reliability analysis for bladder volumes measured with SonoAVC and VOCAL. Patient characteristics are presented as median with corresponding range, FUP rates as mean and 95% confidence intervals (CIs).
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