Chest Imaging

Emphysema

condition of the lung characterized by permanent, abnormal enlargement of airspaces distal to the terminal bronchiole, accompanied by the destruction of their walls. Emphysema is usually classified into 3 main subtypes based on the anatomical distribution of the areas of lung destruction:

  • centrilobular emphysema;

  • panlobular emphysema; and

  • paraseptal emphysema.

    Emphysema is thought to result from destruction of elastic fibres caused by an imbalance between proteases and protease inhibitors in the lung and from the mechanical stresses of ventilation and coughing. Proteases are normally released in low concentration by phagocytes in the lung. Protease inhibitors mainly a1-protease inhibitor (a1-antitrypsin), prevent them from causing structural damage to the lung. Imbalance in the proteaseantiprotease activity may result from antiprotease deficiency (see alpha 1 antitrypsin deficiency) caused by excess release of protease stimulated by environmental agents, or defective repair of protease-induced damage. Tobacco smoke increases the number of pulmonary macrophages and neutrophils, reduces antiprotease activity, and may impair the synthesis of elastin. As emphysema develops, lung destruction progresses, air-spaces enlarge, and elastic recoil declines, reducing radial traction on bronchial walls and on blood vessels and allowing bronchi and vessels to collapse. Airflow obstruction reflects relatively fixed obstruction in small airways, and variable obstruction in large airways. Pulmonary function tests may demonstrate elevated lung volume, air trapping, reduced expiratory airflow and destruction of the capillary bed.

    The main radiographic manifestations of emphysema are overinflation and alterations in lung vessels. Signs of overinflation include height of the right lung being greater than 29.9 cm, location of the right hemidiaphragm at or below the anterior aspect of the seventh rib, flattening of the hemidiaphragm, enlargement of the retrosternal space, widening of the sternodiaphragmatic angle and narrowing of the transverse cardiac diameter (Fig.1). Alterations in lung vessels include arterial depletion, an increase in calibre and number in peripheral vessels, often associated with cor pulmonale, absence or displacement of vessels caused by bullae, widened branching angles with loss of side branches and vascular redistribution.

    • CT scans and particularly HRCT scans are the most accurate means of detecting emphysema in vivo. On HRCT scan, emphysema is characterized by the presence of areas of abnormally low attenuation which can be easily contrasted by surrounding normal lung parenchyma if sufficiently low window levels (-800 HU, to -1000 HU) are used. Focal areas of emphysema usually lack distinct walls as opposed to lung cysts. In many patients, it is possible to classify the type of emphysema on the basis of its HRCT appearance (see centrilobular emphysema, panlobular emphysema, paraseptal emphysema and irregular emphysema). The different types of emphysema may be present in association in the same patient as well as bullae. HRCT has a high specificity for diagnosing emphysema and is more sensitive than the chest radiograph in diagnosing emphysema and in determining its type and extent. It can demonstrate emphysema even before abnormalities are present on pulmonary function tests. The extent of emphysema may be assessed by using quantitative CT.

    CT is indicated for grading emphysema in patients with severe lung destruction who are potential candidates for bullectomy, volume lung reduction surgery or lung transplantation. Although there is good correlation between the CT visual scores and the pathological scores of emphysema extent, the extent of destruction may be underestimated and mild emphysema can be missed on HRCT scans.

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    Fig.1

    PA (a) and lateral (b) chest radiographs in a patient with severe obstructive pulmonary disease related to emphysema, showing low position of the diaphragm increasing the angle of the costophrenic sinuses and prominence of costophrenic muscle slips. There is enlargement of the retrosternal space, a decrease of the heart width on the PA chest film, and interposition of lung parenchyma between the heart and the diaphragm. All these features are characteristic of diffuse overinflation.
    Emphysema, Fig.1 (a)
    Emphysema, Fig.1 (b)