119 (OF). BM segments the image into blocks and seeks the best matches of these blocks in subsequent frames based on a chosen matching criterion. On the other hand, OF is a pixel-to-pixel gradient approach that estimates the velocity of the target object between two subsequent frames. In our study, OF is chosen over BM due to its higher sensitivity to subpixel motion [15]. Moreover, as introduced by Bouguet [16], the tracking accuracy of OF can be further improved by implementing iterative spatial warping. The adopted OF method was first optimized and validated in vitro using a dedicated setup with human ex vivo uteri [17]. For each TVUS recording, tracking markers (TMs) were manually placed along both the anterior and posterior walls of the endometrium (see Figure 5.2). The TMs were always positioned starting 5 mm from the fundal extremity of the endometrium and moving toward the cervix, spaced 2.5 mm from each other. Movements of these TMs were tracked over time. Radial strain rates (RSRs) were derived between each pair of TMs to generate a time–space representation of UP along the endometrium. Relevant features, such as UP velocity, direction, and coordination, were then extracted from the resulting time–space representation. Figure 5.2 Tracking marker grid (red dots) positioning along the anterior and posterior walls of the endometrium. TVUS recording acquired from a healthy volunteer during the LF phase.
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