Registration-based regional lung mechanical analysis

The main function of the respiratory system is gas exchange. Since many disease or injury conditions can cause biomechanical or material property changes that can alter lung function, there is a great interest in measuring regional lung ventilation and regional mechanical changes. We describe a technique that uses multiple respiratory-gated CT images of the lung acquired at different levels of inflation with both breath-hold static scans and retrospectively reconstructed dynamic scans, along with non-rigid 3D image registration, to make local estimates of lung tissue expansion. The degree of regional lung expansion is measured using the Jacobian (a function of local partial derivatives) of the registration displacement field. We compare the ventral-dorsal patterns of lung expansion estimated across seven phase changes and three pressure changes to a xenon CT based measure of specific ventilation in four anesthetized sheep studied in the supine orientation. Using 3D image registration to match images acquired at 50% and 75% phase points of the inspiratory portion of the respiratory cycle and 20 cm H2O and 25 cm H2O airway pressures gave the best match between the average Jacobian and the xenon CT specific ventilation respectively (linear regression, average r2=0.85 and r2=0.84). We validate the registration accuracy by 200 semi-automatically matched landmarks and both the dynamic and static scans show landmark error on the order of 2mm.