139 Discussion A. Validation in Healthy Volunteers 1) Estimation of UP Velocity, Direction, and Coordination: The trend of the UP velocity changes across the selected menstrual phases is well aligned with the observation of Ijland et al. [10]. In the LL phase, which is the most quiescent phase during the entire menstrual cycle, the visualization of UP is challenging. Thus, from manual observation, no UP is reported. However, with our proposed method, the velocities of the UP in both directions could still be assessed. Besides, significant differences in the velocity of UP propagation were found between LF and LL phases and between EL and LL phases in both C2F and F2C directions. These findings are in agreement with previous literature reporting different behaviour of the uterus in the different phases of a natural menstrual cycle [9], [25]. Our findings on UP direction are in agreement with our clinical expectations and the findings from other qualitative studies [25], [30]. For instance, during Menses, intrauterine streams were expected to be generated mainly in the F2C direction for menstrual emptying, and in the LF phase, pure onedirectional propagation was anticipated. No significant difference was found based on the coordination features among the four selected phases during the menstrual cycle. However, a trend of higher UP coordination was found during the LF phase, which may correspond to the generation of coordinated contractions favouring sperm transport from the vagina to the orifices of the fallopian tubes, where fertilisation occurs [25], [27]. 2) Intra-observer and Inter-observer Study: According to the ICC results from the intra-observer reproducibility study (see Table I), excellent agreements were found for most of the features, indicating good reproducibility and robustness of our proposed method. We also observed excellent agreement for the velocity features, and good agreement for the coordination features from the intra-observer and interobserver variability studies. Since the coordination features required a good spatial alignment of the TMs positioned at both sides of the endometrium, posing extra challenge on the new users, the interobserver variability was slightly
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