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High resolution elastography for determining local airway wall deformation during bronchoconstriction

Excessive shortening of airway smooth muscle (ASM) is responsible for airway hyperresponsiveness (AHR) in asthma but its cause is poorly understood. The forces of breathing have been implicated as preventing AHR in vivo by straining the ASM. However, it is unclear the extent of ASM strain within the airway wall and recent evidence from intact airway studies suggest tidal breathing does not modulate AHR. The ASM is embedded inside the airway wall with heterogeneous mechanical properties which affect the strain felt by the ASM as well as the mechanical load opposing ASM shortening. We captured two ultrasound images of a cross section of an intact airway and applied an image registration technique to calculate the displacement field resulting from a small deformation. The displacement field was then used as an input to solve an inverse problem to estimate the shear modulus distribution. The estimates were compared to the strain of the airway wall and Young's modulus calculated using a thin-walled cylinder approximation.