The incidence of disease of the cardiovascular system is very high and increasing worldwide, especially in the developing world. The radial and ulnar arteries are implicated in some important ailments where blood flow related parameters such as flow rate (FR), wall shear rate (WSR), arterial wall motion (AWM) and pressure, all of which can be measured using ultrasound techniques, are useful in diagnosis and patient management. However these measurements are prone to error due to the manner of image formation and the complex flow conditions within the vessels. In this thesis, the errors in ultrasound-measured parameters in the radial and ulnar arteries are investigated using experimental phantoms, computer simulation and on volunteers. Using the Womersley theory, FR and WSR were estimated using a clinical ultrasound scanner with the pulsed wave (PW) mode and B mode. Experimental flow phantoms were designed to evaluate those measurements under different circumstances. A simulation technique which combined image-based computational fluid dynamics and ultrasound simulation was also used to evaluate ultrasound estimation of these parameters. A case study was then conducted on healthy volunteers to evaluate the method of measuring FR and WSR in-vivo. For the AWM in the radial artery, an auto-correlation method was used based on the radio-frequency (RF) data and validations were done by a flow phantom, simulation, and in-vivo trial. The blood pressure waveform in a volunteer’s radial artery was derived from the ultrasound measured AWM and compared with the waveform from a tonometry. FR and WSR were both found to be overestimated by up to 50%, mainly due to the beam-vessel angle in the PW Doppler ultrasound. Measurement of the vessel diameter and assumption of the blood flow direction can also influence the estimations. Other factors, such as flow amplitude, vessel size, imaging depth and flow waveforms, do not seem to affect the estimation of these two parameters. Results taken from the flow phantoms agree with those from simulation and the estimations from the in-vivo case study also agree with the published data. The auto-correlation method for the AWM was validated from the phantom and simulation. It is able to detect motion amplitude of about tens of micrometres. The trial on volunteers proved the feasibility of this motion detection method. Blood pressure waveforms at the radial artery of a volunteer, derived from this ultrasound-measured wall motion and from the tonometry, were very similar. The Womersley-based method is able to estimate the FR and WSR in the radial and ulnar arteries with high accuracy. Sources of the error and their magnitudes in estimation of the two parameters by ultrasound pointed out in this thesis are beam-vessel angle, vessel diameter measurement and flow direction assumption. Researchers and clinicians using these measurements in practice and research should be aware. The capability of ultrasound imaging to measure arterial AWM in the radial artery is demonstrated and it is found that the blood pressure waveform can also be derived from the arterial AWM.

Cardiovascular disease begins to develop in childhood. Physical activity positively impacts cardiovascular health and lowers cardiovascular disease risk in school-aged children and adults; however, there is insufficient evidence to determine the effects of physical activity on cardiovascular health during early childhood. The specific dose of physical activity required for favourable cardiovascular health in this age group is also unknown. The purpose of this thesis was to explore the relationships between physical activity and cardiovascular health indicators during early childhood. In the first study, we found that physical activity engagement has beneficial effects on cardiovascular fitness, blood pressure, autonomic function, and arterial stiffness during early childhood. We then determined in the second study that to avoid unfavourable cardiovascular health, preschool-aged children should engage in at least 240 minutes of activity at any intensity, 80 minutes of moderate-to-vigorous physical activity, or 8700 steps per day. Finally, in the third study we evaluated a novel technique for examining arterial wall properties, carotid artery longitudinal wall motion, and determined that it was weakly associated with an established indicator of arterial stiffness (pulse wave velocity) in early childhood. These findings highlight that the benefits of physical activity on cardiovascular health begin in early childhood and the minimal amount of activity to avoid unfavourable cardiovascular health is higher than current recommendations. For the most part, physical activity benefited boys and girls similarly, although it is unclear if the physical activity targets apply equally to boys. We also determined that carotid artery longitudinal wall motion may not be an indicator of arterial stiffness in young children. Future studies should determine if the favourable effects of physical activity on cardiovascular health during early childhood carry over to later years and if achieving the physical activity targets determined in this thesis are associated with additional health benefits. / Thesis / Doctor of Philosophy (PhD) / Heart disease develops slowly over time, starting in childhood. This thesis explored if physical activity can begin to prevent heart disease even in preschoolers, and how much physical activity preschoolers need in order to keep their hearts and blood vessels healthy. We found that preschool-aged children who were more active had better heart health, including better blood vessel health and higher fitness. While all activity was good, more intense, moderate-to-vigorous physical activity (sometimes called energetic play) was more beneficial and slowed down the stiffening of their blood vessels as the children grew. We determined that preschool-aged children should engage in at least 4 hours of physical activity at any intensity or 80 minutes of moderate-to-vigorous physical activity every day to avoid poor heart health. The findings in this thesis highlight the importance of regular physical activity participation to promote heart health even in preschool-aged children.