Under normal circumstances the pulmonary parenchyma on HRCT has a density ranging from −700 (750) to −920 (950) Hounsfield Units (HU). A decrease in HRCT density on below −920, or −950 HU, is considered pathological. It is caused by pathologies leading to a decrease of density or by cystic formations. They arise from traction, pathological widening of bronchial walls, focal hyperinflation, or by increased endoluminal bronchial pressure. Pulmonary emphysema is divided into centrilobular, panlobular or paraseptal, according to the location of the pathology in relation to the anatomy of the secondary pulmonary lobule. Centrilobular pulmonary emphysema is most commonly associated with chronic obstructive pulmonary disease (COPD) (Fig. 5.1).