is the most frequent symptom even with diffuse pulmonary hemorrhage [30]. Other presenting symptoms include cough, dyspnea, chest pain, hemothorax, and pneumothorax, or pneumomediastinum.

associated with α1-antitrypsin deficiency, predominates in the lower lobes. Centrilobular emphysema affects the lobules around the central respiratory bronchioles, whereas panlobular emphysema uniformly affects the entire secondary lobule.

CT Findings

Pulmonary metastases of angiosarcoma commonly take the form of solid nodules with or without a GGO halo, but relatively few cases appear as cysts. A cystic pulmonary metastasis is a thin-walled, bulla-like lesion with or without accompanying nodules (Fig. 23.6). Pneumothorax or pneumomediastinum due to the rupture of cysts may occur [31].

CT–Pathology Comparisons

Cystic metastases may be caused by excavation of a nodular tumor through discharge of the necrotic material inside, or by infiltration of malignant cells into the walls of a preexisting benign pulmonary bulla, or by infiltration of malignant cells into the walls of air sacs formed by cystic distension of small airways through the ball-valve effect of the tumor [31].

Patient Prognosis

The prognosis of pulmonary metastasis of angiosarcoma is poor, with a median survival of 9 months after diagnosis.

Emphysema

Definition

Emphysema is characterized by permanently enlarged airspaces distal to the terminal bronchiole with destruction of alveolar walls [32]. The CT appearance of emphysema consists of focal areas or regions of low attenuation, usually without visible walls [16, 33] (Fig. 23.7).

Emphysema is usually classified in terms of the part of the acinus predominantly affected: proximal (centrilobular emphysema) (Fig. 23.7), distal (paraseptal emphysema) (Fig. 23.8), or whole acinus (panacinar or panlobular emphysema) (Fig. 23.9). The term bullous emphysema is bullous destruction of the lung parenchyma, usually on a background of paraseptal or panacinar emphysema. Paracicatricial emphysema is an irregular airspace enlargement occurring in patients with pulmonary fibrosis.

Distribution

Centrilobular and paraseptal emphysema usually predominate in the upper lobes, whereas panlobular emphysema,

Clinical Considerations

The term chronic obstructive pulmonary disease (COPD) is often used to describe patients who have chronic and irreversible airways obstruction, most commonly associated with combination of emphysema and chronic bronchitis. Centrilobular emphysema is the most common form of emphysema in cigarette smokers. Panlobular emphysema is the form of emphysema associated with α1-antitrypsin deficiency. Paraseptal emphysema can be an isolated phenomenon in young adults, often associated with spontaneous pneumothorax, or can be seen in older patients with centrilobular emphysema.

Key Points for Differential Diagnosis

1.Differentiation of paraseptal emphysema and honeycombing

Areas of paraseptal emphysema usually occur in a single layer at the pleural surface, predominantly in the upper lobes, and may be associated with other findings of emphysema such as large bullae, but are typically unassociated with significant fibrosis. However, honeycombing cysts are usually smaller, occur in several layers in the subpleural lung, tend to predominate at the lung bases, and are associated with disruption of lobular architecture and other findings of fibrosis such as traction bronchiectasis.

2.Differentiation of centrilobular emphysema and lung cysts

In patients with centrilobular emphysema, the focal areas of lucency lack visible walls; lung cysts, on the other hand, have walls recognizable on HRCT. However, in some patients with centrilobular emphysema, areas of lung destruction show very thin and less defined walls on HRCT, which reflect the presence of minimal lung fibrosis or compressed adjacent lung parenchyma. In patients with centrilobular emphysema, lucency can be seen involving only on part of a secondary pulmonary lobule. Lung cysts often appear larger than areas of centrilobular emphysema, which usually range from several millimeters to 10 mm.

3.The extent of emphysema is generally assessed by using CT densitometry parameters such as the relative low-attenuation area and the percentile of the frequency-attenuation distribution. Although several threshold values have been reported, a threshold