O-CCR: Oriented Cervical Canal Region Detection Framework for Biomechanical Cervical Assessment in TVUS

Background: The cervix undergoes biomechanical changes during pregnancy in preparation for delivery. Assessing the progression of these changes using transvaginal ultrasound (TVUS) is crucial for preterm birth prediction. However, existing methods such as cervical length have limitations in capturing subtle tissue changes. Although tissue analysis using TVUS has been explored to address these limitations, achieving consistent and reproducible results in quantitative analysis remains challenging due to high inter-observer variability and a lack of standardized region of interest (ROI) definitions. This study proposes an oriented cervical canal region (O-CCR) framework that identifies a standardized CCR with relevant sonographic features for evaluating biomechanical change. Methods: We utilized 1436 TVUS images for training, validation, and testing, with 189 additional images from a different hospital for external validation. CCR was defined to include the cervical canal and its surrounding region after aligning the IO and EO parallel to ensure anatomical consistency in the cervix. To validate the effectiveness of O-CCR in handling various orientations, we applied five oriented object detection models (Oriented R-CNN, ReDet, S\(^{2}\)A-Net, R\(^{3}\)Det, and Oriented RepPoints) and evaluated their CCR localization performance. Results: We compared the performance of five models implemented within O-CCR framework. Among them, Oriented RepPoints achieved the highest average precision (AP) of 0.981 at the intersection over union (IoU) threshold of 0.5, compared to Oriented R-CNN (0.968), S\(^{2}\)A-Net (0.962), ReDet (0.964), and R\(^{3}\)Det (0.980) on the test dataset. Notably, Oriented RepPoints demonstrated superior performance even at higher thresholds of 0.6 (0.931) and 0.7 (0.743) and the lowest average orientation error (AOE) of 9.1980 in CCR localization. Conclusions: O-CCR showed reliable performance to localize CCR despite various orientations and shapes, providing standardized regions for assessing biomechanical changes of the cervix. The consistent CCR could be applied to quantitative analysis of cervical tissue properties in future research. Ultimately, this approach could support the development of automated cervical change assessment for prenatal care.