Fetal echocardiography is essential for detecting congenital heart disease (CHD), facilitating pregnancy management, optimized delivery planning, and timely postnatal interventions. Among standard imaging planes, the four-chamber (4CH) view provides comprehensive information for CHD diagnosis, where clinicians carefully inspect the end-diastolic (ED) and end-systolic (ES) phases to evaluate cardiac structure and motion. Automated detection of these cardiac phases is thus a critical component toward fully automated CHD analysis. Yet, in the absence of fetal electrocardiography (ECG), manual identification of ED and ES frames remains a labor-intensive bottleneck. We present ORBIT (Orientation-Robust Beat Inference from Trajectories), a self-supervised framework that identifies cardiac phases without manual annotations under various fetal heart orientation. ORBIT employs registration as self-supervision task and learns a latent motion trajectory of cardiac deformation, whose turning points capture transitions between cardiac relaxation and contraction, enabling accurate and orientation-robust localization of ED and ES frames across diverse fetal positions. Trained exclusively on normal fetal echocardiography videos, ORBIT achieves consistent performance on both normal (MAE = 1.9 frames for ED and 1.6 for ES) and CHD cases (MAE = 2.4 frames for ED and 2.1 for ES), outperforming existing annotation-free approaches constrained by fixed orientation assumptions. These results highlight the potential of ORBIT to facilitate robust cardiac phase detection directly from 4CH fetal echocardiography.

翻译：胎儿超声心动图对于检测先天性心脏病（CHD）至关重要，有助于妊娠管理、优化分娩计划以及及时的产后干预。在标准成像平面中，四腔心（4CH）切面为CHD诊断提供全面信息，临床医生通过仔细检查舒张末期（ED）和收缩末期（ES）相位来评估心脏结构与运动。因此，这些心脏相位的自动检测是实现全自动CHD分析的关键环节。然而，在缺乏胎儿心电图（ECG）的情况下，手动识别ED与ES帧仍是一项劳动密集型的瓶颈任务。本文提出ORBIT（基于轨迹的方向鲁棒心跳推断），这是一种自监督框架，能够在不同胎儿心脏方位下无需人工标注即可识别心脏相位。ORBIT以图像配准作为自监督任务，学习心脏形变的潜在运动轨迹，其轨迹转折点捕捉了心脏舒张与收缩之间的转换，从而实现了跨不同胎儿体位的ED与ES帧的精确且方向鲁棒的定位。仅使用正常胎儿超声心动图视频进行训练，ORBIT在正常病例（ED的MAE = 1.9帧，ES的MAE = 1.6帧）和CHD病例（ED的MAE = 2.4帧，ES的MAE = 2.1帧）上均取得了一致的性能表现，优于现有受固定方位假设限制的无标注方法。这些结果凸显了ORBIT直接从4CH胎儿超声心动图中实现鲁棒心脏相位检测的潜力。