Development and evaluation of non-invasive ultrasonic methods for arterial characterization
Sammanfattning: As cardiovascular diseases have been the most common cause of mortality in the western world for decades, the demand for appropriate diagnostic methods is obvious. To facilitate the examination non-invasive methods are preferable since they do not require sensors within the circulation system. Ultrasonic Doppler is a frequently used technique in non-invasive hemodynamic studies. To increase the accuracy in volume flow recordings, the pulsatile vessel diameter and flow velocity were measured simultaneously. The method was evaluated in vitro as well as in vivo. The in vitro results were compared with corresponding data produced by an MRI-scanner. The in vivo evaluation was performed through the reproducibility on normally grown fetuses in a longitudinal study. New diagnostic methods for the vascular system are frequently validated in vitro. To facilitate this work, an arbitrary flow generator was designed and evaluated. The flow generator produces constant and pulsatile flow. The generator is furthermore capable of generating typical physiological waveforms found in the human arterial circulation system. The achieved performance in terms of accuracy, bandwidth, ripple and load invariance makes the flow generator very suitable for its application. Vascular impedance is a quantity that characterizes the flow resistance in the arterial tree. Traditional methods for impedance determination require invasive measurements which limits their applicability. To estimate the impedance non-invasively, the relationship to phase velocity was utilized. The agreement between theoretical relationship and measured data was evaluated in vitro. The validation was concentrated on parameters used for clinical diagnosis. Pulse wave velocity is, in contrast to vascular impedance, a local measure of the arterial wall elasticity. A new method for non-invasive PWV estimation is described and evaluated. The PWV estimation was based on arterial wall movements which were detected by Tissue Doppler Imaging (TDI), a color Doppler optimized for low tissue velocities. The TDI modality allowed the artery to be measured at multiple positions simultaneously which improved the PWV estimation considerably. The method was initially evaluated and optimized on an in vitro set-up and thereafter validated with respect to repeatability and reproducibility in a clinical study.
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