Radiography
The characteristic finding in asbestosis is the presence of small, irregular opacities, usually in the midlung and lower lung zones (see the images below).[2] According to the 1980 International Labour Organization classification,[3] "small irregular opacities" describe irregular, linear shadows that develop in the lung parenchyma and obscure normal bronchovascular markings. Asbestosis-related opacities are seen in the radiographs below.
Asbestosis. Posteroanterior chest radiograph reveals a few reticulonodular opacities at the lung bases consistent with mild asbestosis. 
Asbestosis. Asbestosis and asbestos-related pleural disease in a 70-year-old man. Posteroanterior chest radiograph reveals prominent linear opacities at both bases, with obscuring of the cardiac borders and diaphragm. The thick, linear band at the right lateral base likely represents the subpleural, curvilinear opacities observed on high-resolution CT scans. Stages of asbestosis progression
The progression of asbestosis is divided into 3 stages.
In the first stage, a fine, reticular pattern may be seen, usually at the lung bases, in association with a ground-glass appearance, which may represent a combination of alveolitis and interstitial fibrosis.
The second stage is characterized by progression of the small, irregular opacities into a prominent interstitial pattern. During this stage, a combination of parenchymal and pleural abnormalities may partially obscure the heart border (shaggy heart sign; see the image below) and diaphragm.
Asbestosis. Posteroanterior chest radiograph in a 54-year-old man with asbestosis demonstrates coarse, linear opacities at the bases more prominent on the left, obscuring the cardiac borders and diaphragm (shaggy heart border sign). In the last stage, progression of the coarse interstitial pattern and honeycombing to the upper lung zones occurs; in addition, the heart and diaphragm are further obscured.
The radiographic findings described above are rather nonspecific, which may lead to a high false-positive rate. However, the presence of pleural abnormalities and of a compatible clinical history increases specificity in the diagnosis of asbestosis.
Estimates of the sensitivity of chest radiography in the detection of asbestos-related interstitial fibrosis vary widely, from 40-90%.
As previously mentioned, conventional radiographs are relatively insensitive in the detection of early asbestosis and tend to underestimate the severity of disease.
Computed Tomography
Common HRCT scan findings in early asbestosis are intralobular, small, rounded or branching opacities; thickened interlobular septa; subpleural curvilinear lines; and parenchymal bands.[4, 5] The interlobular septal lines typically represent fibrosis. With progression of disease, honeycombing is seen.
Similar to the radiographic appearance, the findings described above predominantly involve the subpleural regions of the lower lung zones. As most of the abnormalities seen in mild asbestosis may be confined to the posterior basal lung zones, the CT scan study should include images with the patient in the prone position to differentiate normal dependent parenchymal opacity from mild subpleural fibrosis. CT scans of patients with asbestosis appear below.
Asbestosis. High-resolution CT scan through the midlung zone demonstrates a parenchymal band on the left (arrow). 
Asbestosis. High-resolution CT scan more inferiorly reveals subpleural, curvilinear opacities bilaterally (white arrows) and thickened interstitial lines (black arrows). 
Asbestosis. High-resolution CT scan through the lower lung zone nicely demonstrates thickened septal lines (white arrows) and small, rounded, subpleural, intralobular opacities (black arrow). Also note the calcified diaphragmatic pleural plaque on the left. On HRCT scans, the earliest lesions observed that indicate fibrosis are believed to be subpleural, intralobular, rounded or branching opacities that correspond to peribronchiolar fibrosis, the earliest microscopic finding. Theoretically, prior to fibrosis, a ground-glass appearance involving the air spaces may indicate alveolitis.
Subpleural lines, which also are present early on, are curvilinear opacities observed parallel to and within 1 cm of the pleura. They are usually 5-10 cm long and likely represent a combination of peribronchiolar fibrosis and atelectasis.
Parenchymal bands are thick, linear opacities that are 2-5 cm long; they course through the lung and contact the pleural surface. These correspond to fibrosis along a bronchovascular sheath or interlobular septa with architectural distortion.
Honeycombing is characterized by thick-walled, multiple, cystic spaces smaller than 1 cm in diameter.
Degree of confidence
With its ability to show lung abnormalities not appreciated on conventional radiographs, CT scanning is considerably more sensitive than conventional radiography in the detection of asbestosis, especially in early or mild disease.
The advent of HRCT scanning has further improved the sensitivity of this modality
As with radiography, CT scan findings in asbestosis are nonspecific, and the identification of a single abnormality is not diagnostic.
One study assessing the accuracy of HRCT in asbestos-exposed individuals demonstrated 100% specificity when at least 3 of the abnormalities described above were present on the scans.[6] However, the exceptionally high specificity came at the expense of a decreased sensitivity (56% when 3 findings were present). The converse holds true as well: diagnosis based on 1 or 2 abnormalities increased sensitivity but decreased specificity.
Magnetic Resonance Imaging
Currently, the role of MRI in the diagnosis of asbestosis is limited.
A 1998 study showed MRI to be more sensitive than chest radiography in the detection of subclinical asbestosis,[7] while a 2004 study found that MRI compared favorably with CT scanning in the detection of asbestos-related pleural disease.[8]
Nuclear Imaging
Before HRCT gained popularity, gallium-67 scans were often helpful in diagnosing asbestosis in patients with appropriate clinical presentations but normal or equivocal chest radiographs.
Gallium-67 scans are usually positive in patients with asbestosis and may even provide a measure of inflammatory activity, because gallium-67 is believed to be engulfed by alveolar macrophages.
National Institute for Occupational Safety and Health. Chest Radiography: B Reader Information for Medical Professionals. Centers for Disease Control and Prevention. Available at http://www.cdc.gov/niosh/topics/chestradiography/breader-info.html. Accessed Jan 26, 2011.
McLoud TC. Conventional radiography in the diagnosis of asbestos-related disease. Radiol Clin North Am. Nov 1992;30(6):1177-89. [Medline].
International Labour Office. Guidelines for the Use of the ILO International Classification of Radiographs of Pneumoconioses. Revised Edition 2000. Occupational Safety and Health Series. No.22. Geneva: International Labour Office; 2000.
Akira M, Yamamoto S, Yokoyama K. Asbestosis: high-resolution CT-pathologic correlation. Radiology. Aug 1990;176(2):389-94. [Medline].
Staples CA. Computed tomography in the evaluation of benign asbestos-related disorders. Radiol Clin North Am. Nov 1992;30(6):1191-207. [Medline].
Gamsu G, Salmon CJ, Warnock ML. CT quantification of interstitial fibrosis in patients with asbestosis: a comparison of two methods. AJR Am J Roentgenol. Jan 1995;164(1):63-8. [Medline].
Bekkelund SI, Aasebo U, Pierre-Jerome C. Magnetic resonance imaging of the thorax in the evaluation of asbestosis. Eur Respir J. Jan 1998;11(1):194-7. [Medline].
Weber MA, Bock M, Plathow C, Wasser K, Fink C, Zuna I, et al. Asbestos-related pleural disease: value of dedicated magnetic resonance imaging techniques. Invest Radiol. Sep 2004;39(9):554-64. [Medline].
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