Measurement and validation in arterial mechanics for clinical diagnosis

Abstract

The theme of the thesis is 'arterial mechanics' incorporating blood flow, wall dynamics and wall stress. The underpinning aim has been the development and assessment of techniques for use in clinical diagnosis. These techniques include peak systolic velocity within arterial stenosis for estimation of the degree of stenosis; mean velocity for assessment of volumetric flow; wall stress for prediction of disease development and rupture; and elastic modulus for prediction of disease development and rupture. The thesis is divided into 3 themes; 'development of phantoms', 'velocity measurement' and 'wall dynamics and stress measurement'.The author developed 'phantom' based methods for simulation of flow and wall motion, principally the flow phantom, but also string and electronic injection phantoms. Simulation of physiological flow and wall distension, in a phantom with tissue-equivalence of soft tissue, vessel wall and blood, is now achievable allowing validation of imaging-based measurements related to blood velocity, wall distension and wall elasticity.The author evaluated the magnitude and origin of velocity measurement errors in clinical ultrasound systems. Techniques were developed for improved estimation of blood velocity, and for related quantities such as volumetric flow, using methods which could be easily implemented on clinical ultrasound systems.The author developed and was involved in clinical evaluation of simplified methods for estimation of wall shear stress and of arterial elasticity. Methods for estimation of flowfield data in complex 3D geometries were developed using MRI and using computational fluid dynamics.Concluding the thesis as a whole; for the measurement of quantities relevant to arterial mechanics for clinical diagnosis, care and attention must be paid in the development of measurement methods with high accuracy, properly validated using an appropriate tissuemimicking phantom. There is an ongoing clinical problem on velocity measurement for degree of stenosis with no apparent intention to resolve this on the part of manufacturers. Simplified imaging-based measurement methods, of volumetric flow, wall shear stress and arterial elasticity, may have clinical roles where the model assumptions can be justified, which is principally in healthy arteries and early disease. In advanced disease, where there is complex 3D geometry and anisotropy, estimation of 3D flow field data, wall stresses and elasticity should be performed using image guided modelling.