The pulmonary arteries carry deoxygenated blood at low pressure, supplying 99% of blood flow to the lungs. In health, the bronchial arteries account for less than 1% of cardiac output, carrying oxygenated blood at a pressure six times that of the pulmonary arteries.
In various respiratory conditions, pulmonary vascular remodelling occurs, and the contribution of bronchial arteries and other systemic vessels increases significantly. This circulation, often overlooked in the literature, becomes apparent in several clinical conditions as vessel hypertrophy and proliferation, observable on cross-sectional imaging. This recruitment process redirects more systemic blood flow to the lungs, increasing the shunt fraction (Qp/Qs ratio), which compares pulmonary arterial blood flow (Qp) to systemic (bronchial) vessel blood flow (Qs).
Studies suggest that the neurohumoral regulation of bronchial arteries is similar to other systemic vascular territories. In diseases causing systemic hypoxemia, alveolar hypoxia, pulmonary infarction, and chronic inflammation, bronchial artery hypertrophy and dilation deliver more oxygenated blood to ischemic lung tissue. Though studied in animal models and through invasive procedures, recent advancements in imaging technology have introduced non-invasive methods to assess this collateral circulation in humans.
Pulmonary vascular remodelling can cause various clinical manifestations, including haemoptysis. Most patients with haemoptysis and hypertrophied bronchial arteries at Royal Brompton Hospital have chronic bronchopulmonary inflammation secondary to cystic fibrosis (CF). Research on quantitative imaging measures in pulmonary vascular disease due to chronic respiratory conditions is limited. Systematic assessment of the shunt fraction in CF patients with bronchial hypertrophy has not been conducted.
MRI techniques have determined the Qp/Qs ratio in congenital heart disease patients, where differences in pulmonary and systemic circulation can be significant. In chronic respiratory conditions, this difference is likely small. This thesis evaluates MRI's precision and accuracy in calculating the shunt fraction, anticipating small shunts, marking the first systematic evaluation in respiratory patients.