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Basic Chest Imaging Principles

Chest X-Ray

Normal chest radiograph is Postero-Anterior in full inspiration. X-RAY tube is 6 feet from the film. Part closer to the film is less magnified and sharper (chest against the film).

Portable Chest X-Ray

Antero-Posterior (back is against the film): shorter tube-to-film distance → greater magnification (heart), less sharpness. Usually done on very sick patient who is unable to hold breath in full inspiration and result is expiratory view (crowded structures, whiter lungs, elevated diaphragm, large heart).

Inspiratory and Expiratory Views

Use inspiratory and expiratory views for air trapping behind foreign body FB (normal lung will collapse and become whiter with expiration, lung w/ FB or emphysema will stay the same black).

Lateral chest X-Ray

Left chest against the film (less magnified heart).

Right Anterior Oblique chest X-Ray

Right chest against the film.

Lateral decubitus chest X-Ray

Helps to determine if costophrenic angle (CPA) bluntness or elevated hemidiaphragm is a fluid, air space disease, scarring or atelectasis; also for unclear PMX or in pt who can’t stand or sit.

Lordotic chest X-Ray

PA film taken w/ beam angled upward to project clavicles above the apices.

Chest X-Ray Anatomy

How to differentiate major fissures on lateral views?

The Left Major fissure ends on the Left hemidiaphragm which is

  1. Lower

  2. Has a stomach bubble beneath it, and

  3. Is not visible anteriorly due to heart sitting on it (normal silhouette sign)


Alveoli are arranged into acini, several acini forms a Secondary pulmonary lobule separated by interlobular septa

Interstitial space

Supporting tissues: vessels, lymphatics, bronchi, connective tissue (in periphery "beyond the resolution of the X-ray or CT")

!! Chest X-RAY is two-dimensional summation image, CT is not superimposed

Silhouette sign

Two substances of the same density in direct contact w/ each other cannot be differentiated on X-RAY → the loss of normal contour = Silhouette sign

Silhouetting of the right heart border

Right middle lobe lie in contact with right heart → airspace disease in the right middle lobe → silhouetting of the right heart border

Silhouetting of the left heart border

Lingula lie in contact with left heart → airspace disease in the lingula → silhouetting of the left heart border

Silhouetting of the right or left hemidiaphragm

Right or Left lower lobe airspace disease

Silhouetting of the ascending aorta, azygos

Right upper lobe airspace disease

Silhouetting of the aortic arch

LUL airspace disease

!! If spine cannot be visualized through the heart, the film is underpenetrated

Air bronchogram sign

Visible bronchi in a consolidated lung (pneumonia, edema, atelectasis, infarction, hemorrhage, chronic lung lesions). It is always sign of parenchymal disease (not pleural or mediastinal). Means open airways --> strong evidence that the lung disease is not due to an obstructing tumor in a smoker (there are no air bronchograms if bronchus is obstructed and filled w/ fluid, tissue or w/o air). Crowded air bronchograms are suggestive of atelectasis


Loss of volume, traction of an adjacent structures toward the collapsed lung (tracheal and mediastinal shift, elevation of hemidiaphragm), shift of the fissures (most reliable sign). Crowded bronchi and vessels. Hilar shift up or down w/ upper or lower lobe collapse. Right middle lobe and lingular collapse doesn’t move hila significantly. Expansive process like pneumonia will cause opposite characteristics.

Example: On lateral view, major fissure is displaced anteriorly, on the frontal view there is a opacification around the left hilum and elevation of left hemidiaphragm --> Left upper lobe collapse (including lingula)

Lingula and left upper lobe share same bronchus. On the right, bronchus intermedius supplies Right lower and middle lobes (often collapse together).

Postobstructive (resorption) atelectasis

Due to a resorption of the air distal to the obstruction:

Cicatricial atelectasis

Pulmonary fibrosis is causing loss of volume and shift of hilum, fissures, trachea

Adhesive atelectasis

Lack of surfactant --> hypoventilation atelectasis: lung bases after anesthesia

Relaxation (passive) atelectasis

with hemothorax, pneumothorax

Subsegmental atelectasis

involves segmental level, linear

Interstitial disease

Reticular interstitial thickening (fine spiderweb), seen more peripherally close to the chest wall, nodular interstitial thickening (multiple nodules).

Most diffuse interstitial disease is chronic due to fibrosis. Acute diffuse interstitial lung disease is usually due to pulmonary edema or viral/mycoplasma pneumonia. Calcified granulomas are most often due to healed TB or histoplasmosis