Diagnostic Imaging in Asbestosis
Chest radiograph – The chest radiograph of asbestosis patients generally reveals small bilateral parenchymal opacities characterized by a multinodular or reticular pattern, usually with related pleural abnormalities. These findings may not be clearly evident. However, in one study, for instance, around 15-20% of individuals with histopathologic signs of pulmonary fibrosis did not have any evidence of interstitial abnormalities on their chest radiographs.
Typically, the interstitial process commences in the lower lung zones and is linked with bilateral mid-lung zone plaques found on the parietal pleura. During the early stages of asbestosis, the combined pleural and interstitial involvement may render an obscure, “ground glass” appearance to the chest radiograph. This may lead to blurring of the diaphragm and heart border, resulting in the "shaggy heart” sign. Involvement of the upper lobe and honeycombing are noticeable in advanced stages of the disease. Mediastinal lymphadenopathy and hilar are not found among asbestosis patients and the occurrence of these should be indicative of another process.
Pleural disease – Pleural involvement is characteristic of exposure to asbestos, whereas it is uncommon in other forms of interstitial lung disorders. Pleural plaques develop in around 50% of individuals exposed to asbestos. The plaques mostly involve the parietal pleura adjoining the ribs, are less widespread in the intercostal spaces, and occur only rarely on the visceral pleura. Generally, plaques can be found alongside the diaphragm and along the sixth through ninth ribs. Their absence is clearly evident at the lung apices and in the region of the costophrenic sulci. Calcifications are noticeable via chest radiography in 20% cases, through CT scanning in 50% cases, and by morphologic examination in 80% cases.
Pleural adhesions can lead to atelectasis of a specific part of the peripheral lung, and this generally renders a rounded appearance on the chest radiograph (referred to as “rounded atelectasis”). This can result from any form of pleural inflammation, but it is usually linked with exposure to asbestos.
Generally small and unilateral, benign asbestos pleural effusions (BAPEs) occur many years prior to the development of interstitial disease. Around 1/3rd of BAPEs have increased levels of pleural eosinophils, at times up to 50% of the total nucleated cell count. The pleural fluid may have a serous, serosanguinous, or overtly bloody appearance.
Thoracoscopic evaluation may be required to exclude pleural malignancy. BAPEs usually resolve spontaneously over a period of several weeks, but they may leave a conspicuous blunting of the costophrenic angle or noticeable thickening of the visceral pleura.
Computed tomographic scanning – When it comes to detecting parenchymal abnormalities among individuals exposed to asbestos, high resolution computed tomography (HRCT) turns out to be more sensitive in comparison to plain films. Up to 30% of individuals exposed to asbestos reveal an abnormal HRCT even after a normal chest radiograph. However, in histopathologically proven asbestosis cases, HRCT may still appear normal or near normal.
Below are some of the common HRCT findings of asbestosis:
- Subpleural linear densities of different length adjacent to the pleura
- Basilar and dorsal lung parenchymal fibrosis, characterized by intralobular, peribronchiolar, and interlobular septal fibrosis
- Coarse parenchymal bands (length in the range of 2-5 cm), usually contiguous with the pleura
- Coarse honeycombing occurring in advanced disease
- Pleural plaques, which, as discussed above, help distinguish between asbestos-induced parenchymal disease and other forms of interstitial lung diseases.
- The relevance of abnormal HRCT scans among asymptomatic asbestos-exposed individuals as well as the appropriate role of HRCT in the detection of asbestos-induced lung disease requires further study.
Gallium lung scanning – Asymptomatic asbestos-exposed individuals may demonstrate increased gallium uptake in the lung parenchyma, which is indicative of an active inflammatory or immune process. The extent of gallium uptake does not compare with findings of the chest radiograph, pulmonary function tests, or bronchoalveolar lavage (BAL) cell profile.
Hence, the use of gallium scan for the assessment of asbestosis patients remains experimental.