Bronchi
Bronchietasis is defined by a bronchus-to-artery ratio of >1.2 on chest radiographs1§.§ Based on the external diameters of both the pulmonary arteries and bronchi on chest radiographs.
Normal bronchi are not usually seen in the most peripheral 5 to 10 mm of the lung on CT2.
References:
1. Woodring JH. Pulmonary artery-bronchus ratios in patients with normal lungs, pulmonary vascular plethora, and congestive heart failure. Radiology 1991; 179:115-22.
2. Siegel MJ. Pediatric Body CT. Lippincott Williams & Wilkins: Philadelphia. 1999. p. 101.