Automated Analysis of Fetal Heart Rate from VSI-Based Ultrasound Using Segmentation-Guided Optical Flow

Volume Sweep Imaging (VSI) enables standardized obstetric ultrasound acquisition by non-experts in low-resource settings; helping to the assessment of diverse fetal variables. While Doppler is used to detect fetal heart changes; due to the limited constraints is not possible to apply this method, and VSI has not yet been applied for fetal heart rate (FHR) measurement without Doppler capability. This study proposes a segmentation-guided optical flow method to estimate FHR from VSI-based B-mode cine-loops. Third-trimester VSI sweeps where fetal heart was visible were manually segmented frame-by-frame by a physician. Dense optical flow was computed within the heart mask, and the mean motion angle over time was bandpass-filtered (90-200 BPM) with variable Butterworth order. A confidence score (1-5) was proposed based on the filter order that was correlated to the quantity of frames with visible heart. Out of 114 cine-loops analyzed, all clips with confidence score ≤ 3 (n=49) were non-measurable by both the algorithm and physicians. Among high-confidence clips (≥ 4; n=65), 18 were excluded due to incomplete heart visualization, leaving 47 for Bland-Altman analysis. The mean bias was -8.63 BPM with 95% limits of agreement [-52.26,69.52] BPM and no statistically significant difference (p > 0.05). These findings demonstrate the feasibility of integrating automated FHR estimation into existing VSI obstetric protocols without Doppler, enabling functional cardiac assessment in settings with limited access to specialized sonography.