Fact: The broncho-pulmonary tree arises at the hilum and spread out to the periphery in a radiating manner.
Tip: As we follow the broncho-pulmonary markings, there is lung tissue. when the broncho-pulmonary markings disappear and there is a line of demarcation between lung tissue and clear zone, there is Pneumothorax.
See the images below.
(click on the image to enlarge)