Recognizing Airspace Versus Interstitial Lung Disease

Chapter 3 Recognizing Airspace Versus Interstitial Lung Disease



Classifying Parenchymal Lung Disease



image Diseases that affect the lung can be arbitrarily divided into two main categories based in part on their pathology and in part on the pattern they typically produce on a chest imaging study.
• Airspace (alveolar) disease

• Interstitial (infiltrative) disease

image Why learn the difference?
• While many diseases produce abnormalities that display both patterns, recognition of these patterns frequently helps narrow the disease possibilities so that you can form a reasonable differential diagnosis (Box 3-1).


Box 3-1 Classification of Parenchymal Lung Diseases



Airspace Diseases



Acute



Pneumonia

Pulmonary alveolar edema

Hemorrhage

Aspiration

Near-drowning


Chronic



Bronchoalveolar cell carcinoma

Alveolar cell proteinosis

Sarcoidosis

Lymphoma


Interstitial Diseases



Reticular



Idiopathic pulmonary fibrosis

Pulmonary interstitial edema

Rheumatoid lung

Scleroderma

Sarcoid


Nodular



Bronchogenic carcinoma

Metastases

Silicosis

Miliary tuberculosis

Sarcoid


Characteristics of Airspace Disease


imageAirspace disease characteristically produces opacities in the lung that can be described as fluffy, cloudlike, or hazy.



image These fluffy opacities tend to be confluent, meaning they blend into one another with imperceptible margins.

image The margins of airspace disease are indistinct, meaning it is frequently difficult to identify a clear demarcation point between the disease and the adjacent normal lung.

image Airspace disease may be distributed throughout the lungs, as in pulmonary edema (Fig. 3-1), or it may appear to be more localized as in a segmental or lobar pneumonia (Fig. 3-2).

image Airspace disease may contain air bronchograms.
• The visibility of air in the bronchus because of surrounding airspace disease is called an air bronchogram.

• An air bronchogram is a sign of airspace disease.

image Bronchi are normally not visible because their walls are very thin, they contain air, and they are surrounded by air. When something like fluid or soft tissue replaces the air normally surrounding the bronchus, then the air inside of the bronchus becomes visible as a series of black, branching tubular structures—this is the air bronchogram (Fig. 3-3).

image What can fill the airspaces besides air?
• Fluid, such as occurs in pulmonary edema

• Blood, e.g., pulmonary hemorrhage

• Gastric juices, e.g., aspiration

• Inflammatory exudate, e.g., pneumonia

• Water, e.g., near-drowning
image

Figure 3-1 Diffuse airspace disease of pulmonary alveolar edema.


Opacities throughout both lungs primarily involve the upper lobes, which can be described as fluffy, hazy, or cloudlike and are confluent and poorly marginated, all pointing to airspace disease. This is a typical example of pulmonary alveolar edema (due to a heroin overdose in this patient).


image

Figure 3-2 Right lower lobe pneumonia.


An area of increased opacification is in the right midlung field (solid black arrow) that has indistinct margins (solid white arrow) characteristic of airspace disease. The minor fissure (dotted black arrow) appears to bisect the disease, locating this pneumonia in the superior segment of the right lower lobe. The right heart border and the right hemidiaphragm are still visible because the disease is not in anatomical contact with either of those structures.


image

Figure 3-3 Air bronchograms demonstrated on CT scan.


Numerous black, branching structures (solid black arrows) represent air that is now visible inside the bronchi because the surrounding airspaces are filled with inflammatory exudate in this patient with an obstructive pneumonia from a bronchogenic carcinoma. Normally, on conventional radiographs, air inside bronchi is not visible because the bronchial walls are very thin, they contain air, and they are surrounded by air.


imageAirspace disease may demonstrate the silhouette sign (Fig. 3-4).


image The silhouette sign occurs when two objects of the same radiographic density (fat, water, etc.) touch each other so that the edge or margin between them disappears. It will be impossible to tell where one object begins and the other ends. The silhouette sign is valuable not only in the chest but as an aid in the analysis of imaging studies throughout the body.

image The characteristics of airspace disease are summarized in Box 3-2.

image

Figure 3-4 Silhouette sign, right middle lobe pneumonia.


A, Fluffy, indistinctly marginated airspace disease is seen to the right of the heart. It obscures the right heart border (solid black arrow) but not the right hemidiaphragm (dotted black arrow). This is called the silhouette sign and establishes that the disease (1) is in contact with the right heart border (which lies anteriorly in the chest) and (2) is the same radiographic density as the heart (fluid or soft tissue). Pneumonia fills the airspaces with an inflammatory exudate of fluid density. B, The area of the consolidation is indeed anterior, located in the right middle lobe, which is bound by the major fissure below (dotted white arrow) and the minor fissure above (solid white arrow).



Box 3-2 Characteristics of Airspace Disease



Produces opacities in the lung that can be described as fluffy, cloudlike, and hazy.

The opacities tend to be confluent, merging into one another.

The margins of airspace disease are fuzzy and indistinct.

Air bronchograms or the silhouette sign may be present.


Some Causes of Airspace Disease



image Three of the many causes of airspace disease are highlighted here and will be described in greater detail later in the text.

image Pneumonia (see also Chapter 7)
• About 90% of the time, community-acquired lobar or segmental pneumonia is caused by Streptococcus pneumoniae (formerly known as Diplococcus pneumoniae) (Fig. 3-5).

• Pneumonia usually manifests as patchy, segmental, or lobar airspace disease.

• Pneumonias may contain air bronchograms.

• Clearing usually occurs in less than 10 days (pneumococcal pneumonia may clear within 48 hours).

image Pulmonary alveolar edema (see also Chapter 9)
• Acute alveolar pulmonary edema classically produces bilateral, perihilar airspace disease sometimes described as having a bat-wing or angel-wing configuration (Fig. 3-6).

• It may be asymmetrical but is usually not unilateral.

• Pulmonary edema, which is cardiac in origin, is frequently associated with pleural effusions and fluid that thickens the major and minor fissures.

• Because fluid fills not only the airspaces but also the bronchi themselves, usually no air bronchograms are seen in pulmonary alveolar edema.

• Classically, pulmonary edema clears rapidly after treatment (<48 hours).

image Aspiration (see also Chapter 7)
• Aspiration tends to affect whatever part of the lung is most dependent at the time the patient aspirates, and its manifestations depend on the substance(s) aspirated (Fig. 3-7).

• For most bedridden patients, aspiration usually occurs in either the lower lobes or the posterior portions of the upper lobes.

Related posts:

Recognizing Pneumonia Recognizing a Pleural Effusion Recognizing the Normal Abdomen Computed Tomography Recognizing Some Common Causes of Neck and Back Pain Ultrasonography

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Mar 2, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Recognizing Airspace Versus Interstitial Lung Disease

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