123 iterations M . The final estimation of the pixel motion, vend , was then calculated as the sum of the initial motion and all residual motions: $ QRS =. $TUQV WTUQV?! Apart from applying this iterative refinement, the choice of the block size is also crucial for OF to obtain accurate tracking results. For small blocks, the tracking becomes sensitive to local motion and noise, while for large blocks, the hypothesis of constant flow may be violated. The optimization of the block size was carried out ex vivo using the dedicated experimental procedure proposed in [17]. The optimized block size resulted to be 41×41 pixels2 (around 2.6×2.6 mm2). C. Anatomical Strain Framework UP is often observed close to the endometrium, in the junctional zone, rather than in the myometrium [23]. TMs were therefore selected along both the anterior and posterior walls of the endometrial cavity at the first frame of each TVUS recording. A semiautomatic approach was employed to maintain the same distance between each pair of neighbouring TMs both in the radial and longitudinal directions (see Figure 5.2). In this study, the distance was chosen corresponding to the optimal block size. Out-of-plane (OOP) motion is a frequent phenomenon during in vivo 2-D TVUS acquisition. The occurrence of OOP motion is mainly caused by the movement of the imaged target in the 3-D perpendicular to the observation plane. Sometimes, it might also be caused by the influence of probe motion and patient movement. Once OOP motion occurs, the speckle pattern being tracked moves out of the observation plane. The resulting decorrelation of the speckle pattern will lead to speckle tracking errors. To mitigate the influence of OOP motion on the accuracy of 2-D speckle tracking with in vivo data, we proposed a two-step approach. In the first step, the middle lining of the endometrium was manually drawn and ten TMs (TMsmidline) with isotropic distance were generated along the middle lining (see Figure 5.3a); speckle tracking was applied to the TMsmidline through the entire recording. We estimated the global translation of the endometrium
RkJQdWJsaXNoZXIy MTk4NDMw