Arcot J. Chandrasekhar, M.D.

Educational Objective

1. Define the following terms:

Diffuse Interstitial Fibrosis

2. Understand the role of chest x-ray as a diagnostic tool and its limitations.

3. Ability to identify normal structure in chest x-ray

4. Ability to recognize types of densities in chest x-ray

5. Know the surface anatomy of lobes of the lungs

6. Know the radiological criteria for each pathological process (consolidation, Atelectasis) for now.

7. Understand the steps involved in interpreting chest x-rays.

8. Understand principle behind radiological signs like air bronchogram and silhouette sign.



In the United States, chest x-rays are routinely obtained for hospitalized adults. Pulmonary specialists will almost never provide a consultation without having seen a chest x-ray. In under developed countries chest x-rays are obtained very selectively and physicians rely mostly on physical exam and history for diagnosis. Physical examination of the chest has inherent limitations. Lesions located in the mediastinum, interstitium, and in the center of the lung are rarely picked up by physical exam. Ease of availability of chest x-ray has made many physicians avoid time consuming physical exam which in most cases fails to reveal all of the problems. As a result, physicians have lost the skill of physical exam. Just as physical examination has limitations, chest x-ray also has limitations, and it should be recognized that a normal chest x-ray does not rule out pulmonary problems. Interstitial, airway and pulmonary vascular disease in certain cases cannot be recognized by chest x-ray while it is easily evident on physical exam, e.g. asthmatics can have normal chest x-rays. Physical exam and chest x-ray provide a compliment of any information and they are not mutually exclusive. Physical exam in general is good for acute illness, while chest x-ray is better for chronic illness.


You should systematically evaluate mediastinum, lung fields, pleural space, diaphragm, and chest wall.

Also evaluate neck, axilla, and abdomen. Important clues can be recognized by looking at the entire chest x-ray.

Basically, you can recognize air, water and bone density on chest x-ray.

Lung fields appear dark because of air. 99% of the lung is air.

The pulmonary vasculature, interstitium constitute 1% and give the lacy lung pattern.

Most of the disease states replace air with a pathological process which usually is a liquid density and appears white.

Having a proper understanding of each of the pathological process or lung injury patterns is essential. Then we can develop roentgen signs that help us identify the nature of the pathological process.

Pathological Process:

Each tissue reacts to injury in a predictable fashion. Multiple etiology can evoke a similar pathological reaction. Let us just exam the pathological process that can occur in the lung. Lung injury or pathological states can be either a generalized or localized process.

Liquid Density:

Liquid density

Increased air density




Diffuse alveolar

Diffuse interstitial









Localized airway obstruction

Diffuse airway obstruction



In order for us to recognize these pathological process in chest x-ray first we need to have good understanding of pathology. Let us go through some common examples.

Consolidation: In the lobar consolidation, a lobe is involved. The alveolar space is filled with inflammatory exudate made up of WBC, bacteria, plasma, and debris. In Pneumococcal pneumonia, the most common cause for lobar consolidation, the lobe goes through red hepatization and gray hepatization stage. In the stage of resolution, some secretions can be in the airway. The interstitium and architecture of the lung remain intact and complete recovery occurs. The lobe swells up initially and may shrink slightly later if there is significant secretions in the airway causing some obstruction. The airway is patent.

Radiologically this transcribes to:

1. a density corresponding to a segment or lobe

2. airbronchogram, and

3. no significant loss of lung volume.

Atelectasis: Atelectasis means loss of air. In absorptive Atelectasis there is an obstructive lesion on the bronchus. There is no ventilation to the lobe beyond the obstruction. Gradually the air gets absorbed by pulmonary circulation. The involved lobe eventually is devoid of air and becomes atelectatic.

Radiologic criteria for absorptive Atelectasis is

1. a density corresponding to a segment or lobe,

2. significant signs of loss of volume, and

3. compensatory hyperinflation of normal lungs.

We have to develop similar understanding of cardiac, pleural, mediastinal and airway pathological processes.


In reading chest x-rays, I recommend that you do it in 4 steps:

Step:    1. Identify the abnormal shadows

2. Anatomically localize the lesion

3. Identify pathological process

4. Identify the etiology

Step 1: Identification of abnormal shadows

You have to know what is normal before you recognize abnormalities. Let us identify the normal structures in the thorax (heart, aorta, pulmonary artery, lung fields,  costophrenic angles, diaphragm,  trachea, etc.).

Which lung is larger?

Which diaphragm is higher and why?

What is the normal size of the heart?

What is the normal size and shape of the aorta?

You need to know normal and variations before you can detect and recognize abnormal shadows on chest x-ray.

Step 2: Localization of lesion

You have to have a good knowledge of anatomy.

Surface anatomy for each lobe is essential.

One should know the architecture of the lung acinus, respiratory bronchiole, etc.,

Localization of lesion is also facilitated by taking chest x-ray on PA, lateral and oblique views.

Certain roentgen signs also help, e.g. silhouette sign, Extra pleural sign, Air bronchogram.

Step 3: Identification of pathological process

We have already discussed this by examples of consolidation and Atelectasis. There are radiological signs for each of the pathological process. Most of the pathological processes cast a liquid density, i.e. a white shadow. We should be able to guess the pathological process by roentgen criteria established for each process. Let me show you an example of lobar consolidation and Atelectasis.

Step 4: Identification of etiology

A given pathological process can be produced by multiple etiological agents, e.g., pneumonia can be caused by Pneumococcus, TB, virus, chemical inhalation, aspiration, etc. In most instances cxr cannot identify the etiology. History and clinical setting usually helps us with this process. Let us take an example of a RUL Atelectasis  noted on chest x-ray. If it is observed on a patient with a 60 pack/years smoking history, with a history of cough, and hemoptysis, one would think it points to bronchogenic carcinoma. If it occurred in a healthy 3 year old child while eating peanuts, one would suspect an aspiration of peanuts.

Sometimes we do see evidence in chest x-ray, for example: a mass on the hilum suggesting malignancy, or a coin in the bronchus suggesting aspiration of foreign body.

Step 5: Confirmation of clinical suspicion

This involves doing appropriate tests. If Atelectasis is noted, appropriate tests would be sputum cytology, bronchoscopy, etc. If a consolidation is noted appropriate tests would be CBC with differential, sputum gram stain, blood cultures, etc.


Sometimes a problem cannot be resolved by plain x-ray. We can seek further help from:1. Introduction of contrast material: barium swallow, angiograms, bronchograms2. CT chest: Three dimensional, computerized evaluation3. MRI scan