Applying spatial-temporal image correlation to the fetal kidney: Repeatability of 3D segmentation and volumetric impedance indices

Introduction: Spatiotemporal image correlation (STIC) can evaluate fetal renal impedance using four‐dimensional volumetric indices. We assessed repeatability of three‐dimensional kidney segmentation and the repeatability of the resultant indices. Methods: In each of 57 healthy pregnant women, three renal artery pulsed‐wave Doppler (PWD) traces and three STIC volumes were acquired from the same fetal kidney and segmented by two observers. Vascularisation‐flow index (VFI) and fractional moving blood volume (FMBV) were calculated for every STIC frame and used to determine the volumetric pulsatility index (vPI), volumetric resistance index (vRI) and volumetric systolic/diastolic ratio (vS/D). Segmentation performance was assessed using Dice similarity coefficients (DSCs), Hausdorff distances, coefficient of variation (CoV) and the intraclass correlation coefficient (ICC). Intra/Inter volumetric index repeatability was assessed using ICCs. Results: Forty‐eight cases (84%) provided full data. Mean intra‐ and interobserver DSCs were 0.90 and 0.81. Mean intra‐ and interobserver Hausdorff distances were 3.88 mm and 5.27 mm. Average kidney volumes for observers 1 and 2 were 9.88 mL and 8.54 mL (mean difference 16.1%). Mean intra‐observer volumetric CoVs were 5.3% and 8.1%. Intra‐ and interobserver ICCs for kidney volume (same STIC volume) were 0.97 and 0.85. When assessing volume variation between STIC volumes, intra‐observer ICC was 0.97. ICCs were 0.77–0.81 for VFI‐derived volumetric indices and 0.61–0.62 for FMBV‐derived indices; ICCs for all PWD indices were between 0.58 and 0.59. Conclusions: Periodical variation in vascularity was demonstrated in the fetal kidney, and three‐dimensional segmentation was highly repeatable. Derived volumetric impedance indices show moderate variability but outperform corresponding two‐dimensional PWD indices in terms of reproducibility.