  Bronchiectasis is the permanent dilation of airways. Patients with the
disease can suffer recurrent exacerbations, reducing their quality of life. The
gold standard to diagnose and monitor bronchiectasis is accomplished by
inspection of chest computed tomography (CT) scans. A clinician examines the
broncho-arterial ratio to determine if an airway is brochiectatic. The visual
analysis assumes the blood vessel diameter remains constant, although this
assumption is disputed in the literature. We propose a simple measurement of
tapering along the airways to diagnose and monitor bronchiectasis. To this end,
we constructed a pipeline to measure the cross-sectional area along the airways
at contiguous intervals, starting from the carina to the most distal point
observable. Using a phantom with calibrated 3D printed structures, the
precision and accuracy of our algorithm extends to the sub voxel level. The
tapering measurement is robust to bifurcations along the airway and was applied
to chest CT images acquired in clinical practice. The result is a statistical
difference in tapering rate between airways with bronchiectasis and controls.
Our code is available at https://github.com/quan14/AirwayTaperingInCT.
