Pneumonia – CXR – 3 AM SCAN

Pneumonia - CXR

What’s normal ?

For further details on the normal CXR please see specific section here.

For quick revision, in the normal CXR where both lungs are fully expanded, (because of the difference in tissue density) you should be able to see all sides border of the lungs, including the normal cardiomediastinal contour.

The table below described the relationship of each portion of the cardiomediastinal contour / silhouette with their respective part of the lungs.

SILHOUETTE	LUNG
SVC / Asc Aorta	Anterior segment of RUL
RA	Medial segment of RML
Aortic Arch	Apical-posterior segment of LUL
LV	Inferior segment of Lingula
Rt Hemidiaphragm	RLL
Lt Hemidiaphragm	LLL

What’s not normal ?

Consolidation vs. infiltrates

Consolidation and infiltrates often be used interchangeably to describe the increased lung opacification on CXR.

Normal lung with normal alveoli is spongy and appear dark on CXR. When alveoli become filled with fluid or other substances such as pus, blood, or aspirated material, their density increases, resulting in greater opacity on CXR.

The outcome of this “hardening” process is typically known as consolidation. In contrast, infiltrates refer to substances that abnormally appears within the alveoli, where they are not typically present.

Below is summary of the key differences between “consolidation” and “infiltrates”.

Infiltrates

Consolidation

· Refers to substances abnormally present within the alveoli, such as fluid, pus, or blood

· Appears less dense with an indistinct border

· You may still see pulmonary vascular markings underneath the “haziness”

· Initial or mild phase of the inflammatory or infectious process

· Refers to the “hardened” state of the lung

· Appears homogenously dense with clear border

· No visible pulmonary vascular markings

· Advanced stage of the inflammatory / infection process

Sometimes, evidence of infiltrates or consolidation does not conform to typical lobar anatomical boundaries; this is frequently described according to the affected zone of opacification (see image 1).

Image 1: Right middle zone consolidation

As with many clinical cases, it is common to observe that patients with pneumonia exhibit a combination of both sets of features.

Air bronchogram

In healthy lungs, both the small airways and alveoli possess thin walls and demonstrate similarly low density property, resulting in a darker appearance on x-ray.

In cases of pneumonia, the alveoli are infiltrated by pus, resulting in increased density and enhanced opacity on radiographic imaging.

In an area where the air-filled airways overlapped with the pus-filled alveoli, you will see this distinctive pattern known as “air bronchogram”.

Image 2: Right upper lobe consolidation with feature of air bronchogram (red marking)

The loss of silhouette sign

As previously mentioned, the difference in tissue density between the heart and the lungs allows for clear visualization of the heart border on an x-ray. The same principle applies to the diaphragm.

This phenomenon is frequently referred to as the “Silhouette Sign.”

When pneumonia involves a lobe of the lung, increased tissue density may obscure the borders of the heart or diaphragm on the CXR.

The absence of the silhouette sign along the cardiac margins or diaphragmatic surfaces may offer valuable diagnostic information concerning the location of an infection.

Image 3: lingular consolidation resulting in obscuration of the left lower heart border (green line), while the left hemidiaphragm border (red line) remains clearly visible.

Image 4: Right lower lobe consolidation resulting obscuration of the right hemidiaphragm border (yellow arrow), while the right heart border (red line) remains clearly visible.

Image 5: Right upper lobe consolidation (green line) and left lower lobe consolidation resulting obscuration of the left hemidiaphragm border. There is also fluid level (red line) indicating pleural effusion (likely empyema in this context).

Typical vs. atypical pattern of opacification

Bacterial pneumonia typically presents as lobar or segmental consolidation (see to Image 5).

In contrast, atypical pneumonia is often characterized by patchy or diffuse haziness that may involve both lungs (see Image 6), a pattern also commonly observed in viral pneumonia.

While these descriptions provide general guidance, they are not universally applicable. Definitive diagnosis requires microbiological confirmation through methods such as sputum culture or viral respiratory PCR testing.

Image 6: bilateral patchy consolidation with worsening consolidation on the left base

Complications: empyema and cavitation

Empyema or parapneumonic effusion frequently manifests in cases of severe pneumonia as a unilateral fluid accumulation (see Image 5). Differentiation between simple and complex collections cannot be achieved using chest radiography alone.

For a comprehensive discussion of effusion as observed on chest X-rays, please refer to this link.

Cavitation may indicate severe pneumonia. It is typically characterized by a thick-walled, round or oval lesion featuring a central hollow area. Pulmonary TB and fungal pneumonia are among the most common conditions associated with this type of lesion.

The tricks & the traps

Consolidation is caused by increased alveolar density, which may result from factors other than infection, such as cellular infiltration in cancer patients or blood from pulmonary hemorrhage or contusion. Accurate interpretation depends on clinical context.

Air bronchograms are not always seen with pneumonia. In later stages, smaller airways fill with pus, blending in with affected alveoli and hiding the typical air bronchogram appearance.

Consolidation or infiltrates and complications can indicate disease severity, but not always. Radiologic changes often lag behind symptoms; for example, initial chest X-rays in pneumonia may be normal, and symptom improvement usually occurs before radiographic resolution. A full discussion of these details is beyond this post’s scope.

References:

Broder JS. Diagnostic Imaging for Emergency Physician. 1^st North Carolina: Elsevier 2011
Walker CM, Abbott GF, Greene RE, Shepard JAO, Vummidi D, Digumarthy SR. Imaging Pulmonary Infection: Classic Signs and Patterns. American Journal of Roentgenology. 2014 Mar;202(3):479–92. Available at: https://www.ajronline.org/doi/10.2214/AJR.13.11463
Smithuis R. Chest X-ray – Lung disease. The Radiology Assistant [Internet]. 2014 Feb 1 [cited 2024 May 21]. Available from: https://radiologyassistant.nl/chest/chest-x-ray/lung-disease
Bell DJ, Glick S, et al. Pneumonia. Radiopaedia [Internet]. 2025 Apr 22 [cited 2024 May 21]. Available from: https://radiopaedia.org/articles/pneumonia
Knipe H, Gaillard F, et al. Lobar pneumonia. Radiopaedia [Internet]. 2024 Jan 7 [cited 2024 May 21]. Available from: https://radiopaedia.org/articles/lobar-pneumonia
Bell DJ, Jones J, et al. Bronchopneumonia. Radiopaedia [Internet]. 2024 Jul 13 [cited 2024 May 21]. Available from: https://radiopaedia.org/articles/bronchopneumonia
Shah SN, Bachur RG, Simel DL, Neuman MI. Does this child have pneumonia? The rational clinical examination systematic review. 2017;318(5):462-471. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC6717952/
Patterson HS, Sponaugle DN. Is infiltrate a useful term in the interpretation of chest radiographs? Physician survey results. 2005 Apr;235(1):5-8. doi: 10.1148/radiol.2351020759. PMID: 15798161.
Marrie TJ. Community-acquired pneumonia. Clin Infect Dis. 1994 Apr;18(4):501-13; quiz 514-5. doi: 10.1093/clinids/18.4.501. PMID: 8038304.
Jartti A, Rauvala E, Kauma H, Renko M, Kunnari M, Syrjälä H. Chest imaging findings in hospitalized patients with H1N1 influenza. Acta Radiol. 2011 Apr 1;52(3):297-304. doi: 10.1258/ar.2010.100379. Epub 2011 Mar 3. PMID: 21498366.
Franquet T, Chung JH. Imaging of Pulmonary Infection. 2019 Feb 20. In: Hodler J, Kubik-Huch RA, von Schulthess GK, editors. Diseases of the Chest, Breast, Heart and Vessels 2019-2022: Diagnostic and Interventional Imaging [Internet]. Cham (CH): Springer; 2019. Chapter 7.Available from: https://www.ncbi.nlm.nih.gov/books/NBK553877/ doi: 10.1007/978-3-030-11149-6_7
Ishiguro T, Yoshii Y, Kanauchi T, Hoshi T, Takaku Y, Kagiyama N, Kurashima K, Takayanagi N. Re-evaluation of the etiology and clinical and radiological features of community-acquired lobar pneumonia in adults. J Infect Chemother. 2018 Jun;24(6):463-469. doi: 10.1016/j.jiac.2018.02.001. Epub 2018 Mar 28. PMID: 29605556.
Franquet T. Imaging of Community-acquired Pneumonia. J Thorac Imaging. 2018 Sep;33(5):282-294. doi: 10.1097/RTI.0000000000000347. PMID: 30036297.