Advancements of 2D speckle tracking of arterial wall movements
Sammanfattning: Cardiovascular diseases are the leading cause of death worldwide. In order to improve the diagnostics and facilitate early interventions of cardiovascular diseases, knowledge about the physiology of the vascular system in both healthy subjects and in subjects with vascular disease is needed. In order to learn more about the physiology of the vascular system and possibly predict cardiovascular diseases, accurate motion estimations of the arterial wall is needed. It has been the aim of this thesis to develop more robust motion estimation methods for use on cine loops to investigate the entire thickness of the arterial wall.In this thesis, the concept of 2D speckle block matching was expanded with the use of an extra kernel for improved robustness and tracking accuracy. It was shown that the use of an extra kernel reduced the motion estimation errors when using a constant kernel size (in silico and on phantoms), or reduced the needed size of the kernel while maintaining the level of motion estimation errors (in vivo). Further, a sub-sample estimation method has been developed which combines two previously presented methods: parabolic and grid slope sub-sample interpolation. It was found that by combining the two methods with a threshold determining which method to use, the proposed method reduced the absolute sub-sample estimation errors in simulated and phantom cine loops. A limited in vivo evaluation of estimations of the longitudinal movement of the common carotid artery using parabolic and grid slope sub-sample interpolation and the proposed method were conducted showing that the method worked well in vivo.The two methods were combined to estimate the longitudinal wall movement of the right common carotid artery on 135 healthy volunteers for improved understanding of the wall movements. The results show that the pronounced variation in patterns of longitudinal movement of the common carotid artery previously shown in young healthy subjects is also present in middle-aged and older healthy subjects. However, the patterns of movement seen in middle-aged and older subjects are different from those commonly seen in young subjects, including the appearance of two additional distinct phases of movement, and thus new complex patterns of movement.The use of ultrasound sampled at a high frame rate has the potential to visualize previously unknown information of the longitudinal movement. An iterative scheme for Lagrangian motion estimations in cine loops collected at high frame rates was developed. A phantom evaluation using ultrasound cine loops showed a reduction by an average 54% in the estimated velocity errors compared to a standard method. It also showed a reduction by an average 73 % in the estimated displacement errors. A feasibility test of tracking in vivo indicated good agreement with motion estimations using a low frame rate cine loop.This thesis thus present and evaluate refined methods to measure vascular function through the estimation of longitudinal movement.
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