A Systematic Approach to Chest X-Ray Analysis J.S. Klein

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Introduction

This article reviews the approach to commonly encountered chest radiographic abnormalities, with a focus on the use of ancillary imaging studies for specific characterization of ra- diographic findings and to facilitate differential diagnosis.

Parenchymal Lung Diseases Associated with Increased Lung Density

Interstitial Lung Disease

The radiographic detection of diffuse interstitial lung dis- ease can be difficult, as the findings may be subtle despite the presence of symptoms. The earliest indications of inter- stitial disease are loss of vascular sharpness and an increase in lung density. The most common classification of inter- stitial abnormalities is to divide the observed changes into reticular, nodular, and linear interstitial disease. Reticular lung disease is usually further subclassified as fine, medi- um, or coarse, depending upon the size of the intervening lucent areas. Fine reticular interstitial disease is also termed ground-glass opacity. It generally reflects a process that re- sults in incomplete alveolar filling or lining, and therefore points to either interstitial or alveolar disease processes.

Since there are innumerable processes that can pro- duce diffuse interstitial opacities, the differential diagno- sis of diffuse interstitial disease depends primarily on the associated clinical and ancillary radiographic findings, as well as on more specific characterization of the findings as seen on high-resolution CT scan. In patients with acute interstitial abnormalities, the differential diagnosis is brief and primarily includes cardiac and infectious con- ditions. Table 1 lists the radiographic findings that, when identified in association with interstitial opacities, help limit the differential considerations.

Air-Space Disease

The findings of air-space disease are opacities that tend to obscure the underlying parenchyma. They are often ill-

defined, tend to coalesce over time, and can produce lo- bar or segmental opacification with air bronchograms that obscure the silhouettes of adjacent mediastinal struc- tures and the diaphragm. The approach to air-space dis- ease is best considered by determining the distribution of opacities, as the finding is very nonspecific and simply reflects the presence of material (blood, edema fluid, in- flammatory cells, tumor cells, or proteinaceous material) within the alveoli. The differential diagnosis of air-space disease is shown in Table 2.

Solitary Pulmonary Nodule

The primary goal of the radiologist when identifying a focal opacity on chest radiography is to determine whether the finding truly reflects a solitary pulmonary nodule (SPN). Identification of the opacity on orthogo- nal radiographic projections and methods of excluding abnormalities that may produce a nodular opacity on a single radiographic view, such as nipple shadows, skins lesions, and bone islands or healing rib fractures, are ex- cluded by comparison with prior chest radiographs. To

IDKD 2007

A Systematic Approach to Chest X-Ray Analysis

J.S. Klein

Department of Radiology, Fletcher Allen Health Care, Burlington, VT, USA

Table 1.Ancillary findings in patients with interstitial lung disease and differential considerations

Finding(s) Disease

Hilar lymph node enlargement Sarcoidosis, ,lymphangitic carci- nomatosis, viral pneumonia Clavicular/bony erosions Rheumatoid associated UIP Pleural effusions Infection, edema

Pleural plaques Asbestosis

Hyperinflation LCH, stage IV sarcoidosis, LAM/TS, emphysema with UIP Esophageal dilatation Scleroderma associated UIP, re-

current aspiration

Conglomerate masses Silicosis, sarcoidosis, talcosis Basilar sparing LCH, sarcoidosis

Basilar predominance UIP, aspiration

LAM/TS, lymphangioleiomyomatosis/tuberous sclerosis; LCH, Langerhans cell histiocytosis; UIP, usual interstitial pneumonia 090_092_Klein 28-02-2007 07:34 Pagina 90

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A Systematic Approach to Chest X-Ray Analysis 91

this end, additional radiographic views and techniques, such as nipple markers, oblique or apical lordotic pro- jections, and in some cases chest fluoroscopy, may be necessary. Once a new or enlarging SPN has been iden- tified, thin-section CT will almost invariably be needed for further evaluation. The differential diagnosis of a SPN is shown in Table 3.

Parenchymal Lung Diseases Associated with Decreased Lung Density

Emphysema

The ability to discern an abnormal decrease in lung den- sity is more difficult than the detection of increased den-

sity, owing to the intrinsic low attenuation of normal lung parenchyma. Furthermore, technical factors, including overpenetration, can produce a false appearance of lung hyperlucency. The radiologist must exclude technical is- sues and abnormalities of the chest wall before deter- mining that the lungs are indeed hyperlucent.

Diseases that cause an abnormal decrease in lung at- tenuation may be localized, unilateral, multifocal, or dif- fuse (Table 4). Emphysema is the most common cause of diffuse hyperlucency, and is usually recognized by asso- ciated findings, including low and flattened diaphragms, an increase in the anteroposterior diameter with enlarge- ment of the retrosternal air space, and attenuation of vas- cular markings (particularly in the upper lobes) with bul- lae containing thin walls.

Evaluation of a Mediastinal Mass

Mediastinal masses may be asymptomatic (e.g., bron- chogenic cyst), symptomatic, e.g., as a result of com- pression or invasion of adjacent mediastinal structures (intrathoracic thyroid goiter), or come to attention in patients examined for possible malignancy (e.g., thymoma in a patient with myasthenia gravis).

Radiographically, a mediastinal mass is characterized by sharp demargination, obtuse borders with the adja- cent lung, and an incomplete border sign where it aris- es from the mediastinum. While, in the past, precise lo- calization guided the specif ic imaging evaluation, presently virtually all mediastinal abnormalities are as- sessed by multidetector CT evaluation for localization and characterization of the mass. Common causes of mediastinal masses, based upon the primary site of ori- gin, are detailed in Table 5.

Table 5.Differential diagnosis of mediastinal masses

Superior Anterior Middle Posterior

Thyroid goiter Lymphoma Bronchogenic carcinoma Schwannoma

Lymphangioma Thymic neoplasm Lymph-node enlargement/mass Ganglion cell tumor

Lymphadenopathy Germ-cell neoplasm Foregut cyst Hiatal hernia

Aortic aneurysm Table 3.Differential diagnosis of the solitary pulmonary nodule

Granuloma

Bronchogenic carcinoma (adenoacarcinoma most common) Hamartoma

Solitary metastasis

Focal organizing pneumonia Hematoma

Table 4. Lung processes associated with decreased lung attenuation Peripheral ASO Eosinophilic pneumonia, ARDS,

contusion

Localized lucency (lucencies) Bullae, cyst, pneumatocele, cavity Unilateral hyperlucency Swyer-James syndrome, bronchial obstruction (e.g., carcinoid tumor) Multifocal lucency Asthma, constrictive bronchiolitis Diffuse hyperlucency Emphysema, asthma, constric-

tive bronchiolitis Table 2.Differential diagnosis of air-space opacification (ASO)

Finding(s) Disease

HFocal/segmental ASO Pneumonia, contusion, infarct, lung cancer (BAC)

Lobar disease Pneumonia, endogenous lipoid pneumonia, BAC

Patchy ASO Infection, cryptogenic organizing pneumonia, BAC, metas- tases, emboli

Diffuse ASO Edema, hemorrhage, pneumonia

Perihilar ASO Edema, hemorrhage

Peripheral ASO Eosinophilic pneumonia, ARDS, contusion

Rapidly changing/resolving ASO Edema, eosinophilic pneumonia, hemorrhage

ARDS, acute respiratory distress syndrome; BAC, bronchioloalve- olar carcinoma; COPD, chronic obstructive pulmonary disease 090_092_Klein 28-02-2007 07:34 Pagina 91

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92 J.S. Klein

Hilar Enlargement

Hilar disease can represent a number of conditions (Table 6). Radiographically, there is an increased hilar density, a lobulated and enlarged hilar shadow, or a discrete hilar mass. CT with contrast enhancement is necessary for fur- ther evaluation of suspected hilar disease.

Cardiac and Pericardial Disease

Although echocardiography has become the primary tech- nique to evaluate cardiac and pericardial disease, chest ra- diography can provide important information regarding the presence of cardiac or pericardial processes. These can cause symptoms that overlap with those of lung disease, including dyspnea and chest pain. Direct radiographic findings of cardiac disease are primarily cardiac enlarge- ment or an abnormal cardiac contour. Pericardial disease may manifest as an enlarged cardiomediastinal silhouette, pericardial calcification, or a positive stripe sign indicat- ing the presence of pericardial effusion as seen on lateral chest radiography. The causes of an enlarged cardiomedi- astinal silhouette are listed in Table 7.

Pleural Disease

Pleural disease may take on of several forms radiograph- ically: pleural mass, focal or diffuse smooth or nodular

thickening or calcification, or penumothorax. The char- acteristics of a pleural mass are similar to those of a me- diastinal mass: a sharply-marginated peripheral opacity with obtuse margins and an incomplete border. The com- mon etiologies of a pleural mass are listed in Table 8. The appearance of pleural air depends upon the amount, pa- tient position (upright vs. supine), and the presence or ab- sence of pleural adhesions.

Chest-Wall Disease

Lesions of the chest wall are recognized radiographical- ly when they protrude from the skin surface to be out- lined by air, or when associated with rib, vertebral, ster- nal, clavicular, or scapular involvement. Most patients will present with chest-wall pain. Common causes of chest-wall masses with rib involvement are listed in Table 9.