Comprehensive assessment of vascular haemodynamics by magnetic resonance imaging

Haemodynamics is concerned with the physiological and mechanical factors that determine pressure and flow in the circulation. Currently no diagnostic modality in clinical practice provides simultaneous pressure and flow data, and therefore our analysis of fundamental haemodynamic problems is limited. Cardiac catheterisation with conventional technology can measure pressure as a function of time but can only provide estimates of mean flow. In contrast cardiac magnetic resonance imaging (CMR) can measure flow as a function of time, but cannot directly measure pressure. It is desirable that both pressure and flow signals be acquired and integrated using non-invasive techniques to measure fundamental haemodynamic components. In this thesis it is proposed to develop methods for comprehensive haemodynamic assessment using non-invasive CMR data. The cross-sectional area of a vessel is related to its intra-luminal pressure. The first experiment of this thesis tested the hypothesis that central aortic systolic blood pressure (c-SBP) could be derived from the time-varying cross-sectional area of the ascending aorta, using models of the pressure area relationship. The method was validated using carotid tonometry as a surrogate of central aortic pressure in 20 volunteers. The second experiment tested the hypothesis that the principle components of central haemodynamics: systemic vascular resistance, central compliance, characteristic impedance and wave reflections could be measured non-invasively. The study utilised the previously developed techniques in a study of 50 patients with repaired coarctation of the aorta and 25 healthy controls. Pressure, area and flow data were further integrated to assess wave reflections in the aorta using a technique called wave intensity analysis. Using these methods it was demonstrated that patients with repaired coarctation have elevated c-SBP despite similar peripheral SBP to controls. Furthermore patients have increased vascular stiffness and abnormal wave reflections. These parameters were found to be important determinants of elevated LV mass in this population, and were superior to conventional biomarkers such as coarctation index and peripheral-SBP. The final experiment tested the hypothesis that wave reflections could be assessed in the pulmonary circulation. 20 patients with pulmonary hypertension and 10 controls were recruited. It was hypothesized that wave intensity analysis could detect differences in reflections in PH patients compared to healthy controls and could also differentiate certain PH subtypes. This experiment showed that the presence of a backwards compression wave, reflected from the lungs, identified patients with PH and its magnitude showed discriminatory capacity for the presence of proximal PA clot in patients with Chronic Thromboembolic Pulmonary Hypertension (CTEPH)