Interstitial vs Alveolar Lung PatternsThis is a featured page

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This page considers all aspects of the appearances of interstitial and alveolar opacity demonstrated on chest plain film imaging. Radiographers who are able to differentiate alveolar from interstitial lung patterns are operating at a very high level and will find a whole new appreciation of chest radiography. An appreciation of the features of these patterns will prevent you from describing appearances as cloudy, fuzzy etc. You are more likely to gain the respect of your fellow health professionals and impart meaningful information if you can describe an appearance using correct descriptors- e.g "there is widespread coarse interstitial opacity" rather than "the lungs look a bit fuzzy".

This is not easy- in fact, this is the difficult end of plain film image interpretation. The interpretation difficulties are as follows
  • The appearances can be subtle
  • Interstitial patterns can be mixed (linear, reticular, or nodular)
  • There can be mixed alveolar and interstitial patterns
  • There can be other patterns present (tumour, pleural effusion, atelectasis etc)
  • There can be acute on chronic changes.
  • The causes of interstitial lung opacity are numerous

Interstitial Opacity on CXR- anatomy, causes and appearances.
The interstitium of the lung consists of the supporting structures such as pulmonary vessels, bronchi, and connective tissue. On the normal CXR, the visible interstitium consists primarily of the pulmonary vessels, which are most pronounced at the hila, and decrease in prominence towards the periphery (in fact, under normal circumstances, the pulmonary vessels are not visible in the periphery on a CXR because they are too small to resolve at this point). Most lung diseases cause an increase in the radiodensity of the lung, and if this is due to a relative thickening of the interstitium- this will be manifest as increased prominance of the interstitial markings on the CXR. If generalized or diffuse, this will likely appear as a linear or reticular pattern, whereas if localized, it may appear as multiple tiny nodules.

Note that in diseases confined to the interstitium (i.e. sparing the alveoli) the lung will still appear aerated—the basic appearance is of an aerated lung that has too many “markings.” Common causes of this pattern include pulmonary edema, inflammation, fibrosis, and tumour.

The interstitium surrounds bronchi, vessels, and groups of alveoli. When there is disease in the interstitium it manifests itself by reticulonodular shadowing (criss cross lines or tiny nodules or both). The main two processes affecting the interstitium are accumulation of fluid (occurring in pulmonary oedema or in lymphangitis carcinomatosa) and inflammation leading to fibrosis (occurring in industrial lung disease, inflammatory arthritides such as rheumatoid arthritis, inflammation of unknown cause such as cryptogenic fibrosing alveolitis and sarcoidosis).

Quoted from: source unknown

Causes of Interstitial lung opacity

- Common identifiable causes:
a) infectious causes, e.g.: non-tuberculous mycobacteria & certain fungal infections

b) occupational causes, e.g.: asbestos & silica

c) drug reactions, e.g.: methotrexate & amiodarone

d) neoplastic causes, e.g.: metastatic cancer, bronchoalveolar cell carcinoma (a form of lung cancer)

e) radiation pneumonitis

f) hypersensitivity pneumonitis

g) rheumatologic diseases
(1) systemic lupus erythematosis

(2) rheumatoid arthritis

(3) scleroderma

(4) mixed connective tissue disease

(5) polymyositis

- Diseases of unknown cause, e.g.: sarcoidosis, Langerhan's cell granulomatosis (eosinophilic granuloma; histiocytosis X), lymphangioleiomyomatosis

- Idiopathic pulmonary fibrosis

Source: unknown

Causes of Alveolar Opacity
  • pneumonitis
  • pulmonary contusion
  • pulmonary oedema
  • Aspiration

The Meaning of the Term 'Consolidation'
One of the unfortunate aspects of the term consolidation is that its meaning can be different depending on who is using the term. When a clinician uses the term consolidation, he/she is usually referring to a consolidation associated with acute pneumonia. Thus, the term consolidation and pneumonia have very similar meaning and are almost used interchangeably. Strictly speaking, the term consolidation does not imply any particular aetiology or pathology. Acute pneumonia is the commonest cause but not the only cause of consolidation. (other causes include chronic pneumonia, pulmonary oedema and neoplasm). Thus when a radiologist has reported a chest X-ray examination and notes the presence of consolidation he/she is simply stating that some of the lung airspace has been replaced by a fluid. Sutton, Textbook of Radiology, 2nd ed.,1975,

This is a basic video that explains consolidation in simple terms

  • Consolidation may be complete or incomplete
  • The distribution of the consolidation can vary widely.
  • A consolidation could be described as “patchy”, “homogenous”, or generalised”.
  • A consolidation may be described as focal or by the lobe or segment of lobe affected
Sutton, Textbook of Radiology, 2nd ed.,1975, p325

Interstitial vs Alveolar Patterns

Recognition of a plain film interstitial pattern is important because it identifies the abnormal density as being within the supporting tissues of the lung rather than within the air spaces, pleural space, mediastinum, or outside the thoracic cavity- it indicates the infiltrate's cellular rather than fluid nature.

Alveolar Opacity
Interstitial Opacity
Alveolar Opacity- Definition
Alveolar opacity refers to opacity on chest X-ray associated with fluid filling of the airspaces.
Interstitial Opacity- Definition
The interstitium refers to the supporting structures of the lung. An interstitial opacity on chest X-ray refers to an abnormal appearance of the lung interstitium
Alveolar Opacity- Anatomy
Interstitial Opacity- Anatomy

interlobular septa
Well demonstrated normal interlobular septa in patient with pneumothorax
(pneumothorax causes alveolar compression and so, the intersitium forms a larger component of the lung volume).
Alveolar Opacity- Plain Film Pattern
Opacity is often described as fluffy, cotton wool-like, or cloud-like
Interstitial Opacity- Plain Film Pattern
interstitial patern
Interstitial opacity can be linear, reticular, nodular or reticulonodular.

Alveolar Lung Patterns on CXR

Alveolar pattern results from flooding of the end air spaces (acini) with fluid (pus, blood, edema) only rarely with cellular material. As individual acini become filled the fluid spreads to adjacent ones through the interalveolar pores. This results in the typical radiographic pattern of a poorly margined ("fluffy") density. The densities may spread and their borders coalesce. This may progress until all acini within a lung lobe are filled. There may be a sharp border at the edge of a lung lobe due to the pleura blocking further spread of the fluid into the adjacent lung lobe. As the number of fluid filled adjacent acini increases, the air filled, large and medium sized bronchi become evident as linear radiolucent branching structures (air bronchogram). The air-filled bronchi are surrounded by a fluid density and the bronchial wall and adjacent vessel are not seen. When a bronchus branches perpendicular to the x-ray beam it will be seen as a round radiolucent dot.

alveolar opacityThis patient aspirated IV contrast medium. The post-contrast image was taken within a few minutes of aspiration. The dense contrast media has filled the alveoli as well as coating some of the larger airways. The whispy/fluffy/cloudy pattern is characteristic of alveolar airspace filling.

Interstitial Lung Patterns on CXR

interstitial lung patterns
Pracical Approach to Interstitial Lung Diseases, A. Nour-Eldin
There are 4 basic interstitial lung patterns
  1. linear- septal lines (Kerley lines)
  2. reticular- mesh like appearance, lines in all directions
  3. nodular- discrete opacities
  4. reticulonodular- combination of 2 and 3

Linear Interstitial PatternLINEAR LUNG PATTERN
This is a fine linear interstitial pattern
Reticular Interstitial PatternNodular Interstitial PatternNODULAR PATTERNThis is a nodular pattern in a patient with lymphangitis carcinomatosis.Reticulonodular Interstitial PatternRETICULONODULARReticulonodular pattern in a patient with COPD

The linear pattern on chest radiography consists of thin linear opacities which are either 2 to 6 cm long within the lungs oriented radially toward the hila or 1 to 2 cm long at right angles to, and in contact with, the lateral pleural surfaces. These linear opacities have been referred to as Kerley A and Kerley B lines, respectively, although the descriptors “septal thickening” or “septal lines” are now preferred for the latter.

Histologically, this linear pattern represents thickening of either the bronchovascular/axial interstitium (Kerley A) or the peripheral interstitium (Kerley B).The linear opacities may be single or multiple, regional or diffuse, and short or long, depending on the etiology and severity of disease.

The most common cause of the linear pattern is hydrostatic pulmonary edema, but other etiologies include lymphangitic carcinomatosis, and atypical interstitial pneumonias such as those caused bymycoplasma, chlamydia,
cytomegalovirus (CMV), and respiratory syncytial virus (RSV). Interstitial pulmonary edema tends to be symmetric in distribution while atypical infections and lymphangitic carcinomatosis may be asymmetrical.

Scott D. Perrin, MD, Adam Ulano, MD, and Todd R. Hazelton, MD
Revisiting the pattern approach to interstitial lung disease on chest radiography
Aplied Radiology
Volume 38, Number 12, December 2009

The reticular pattern as seen on chest radiography and computed tomography (CT or HRCT) is depicted by numerous, small, linear opacities which, by summation, have been described as a lace-like or net-like in appearance.4,5 The reticular pattern can be divided into 3 distinct groups, each of which suggests different diagnoses: peripheral reticular pattern with small lung volumes, diffuse reticular/cystic pattern with normal or increased lung volumes, and airway/central reticular pattern.

Scott D. Perrin, MD, Adam Ulano, MD, and Todd R. Hazelton, MD
Revisiting the pattern approach to interstitial lung disease on chest radiography
Aplied Radiology
Volume 38, Number 12, December 2009

The nodular pattern on chest radiography is characterized by multiple small, discrete, rounded opacities that range in diameter from 2 to 10 mm.The differential diagnosis for the nodular pattern can be separated into 3 main categories based on etiology: nodular metastases, nodular pneumoconioses and the granulomatous diseases. The most common malignancies resulting in this pattern are thyroid, breast and renal-cell carcinoma, with the nodules measuring up to 10 mm in diameter.

Scott D. Perrin, MD, Adam Ulano, MD, and Todd R. Hazelton, MD
Revisiting the pattern approach to interstitial lung disease on chest radiography
Aplied Radiology
Volume 38, Number 12, December 2009

1. Linear
Kerley Lines
A linear pattern is seen when there is thickening of the interlobular septa, producing Kerley lines

The most common cause of interlobular septal thickening, producing Kerley A and B lines, is pulmonary edema

other causes
  • Mitral stenosis
  • Lymphangitic carcinomatosis
  • Malignant lymphoma
  • Idiopathic pulmonary fibrosis
  • Pneumoconiosis
  • Sarcoidosis
linear interstitial opacityNote the obscuration of the normal lung markings in the patient with the linear interstitial pattern

2. Reticular

A reticular pattern results from the summation or superimposition of irregular linear opacities. The term reticular is defined as meshed, or in the form of a network. Reticular opacities can be
described as fine, medium, or coarse, as the width of the opacities increases. A classic reticular pattern is seen with pulmonary fibrosis, in which multiple curvilinear opacities form small cystic spaces along the pleural margins and lung bases (honeycomb lung)

3. Nodular
nodular interstitial pattern
Source: Chest Radiology, Pretest USMLE, Part 2, Juzar Ali and Warren R. Summer
žA nodular pattern consists of multiple round opacities, generally ranging in diameter from 1 mm to 1 cm
žNodular opacities may be described as miliary (1 to 2 mm, the size of millet seeds), small, medium, or large, as the diameter of the opacities increases
ž A nodular pattern, especially with predominant distribution, suggests a specific differential diagnosis

This x-ray shows a bilateral diffuse miliary nodular pattern involving both lung fields with no loss of volume.

4. Reticulonodular
reticulonodular patternThis patient has a primary lung cancer with lymphatic spread (lymphangitic carcinomatosis). The lower zone of the left lung shows a coarse reticulonodular pattern.

Compare the appearance of the lower zone of the left lung with an earlier CXR on the same patient (below)

A reticulonodular pattern results from a combination of reticular and nodular opacities.
A reticulonodular pattern is often difficult to distinguish from a purely reticular or nodular pattern, and in such a case a differential diagnosis should be developed based on the predominant pattern. If there is no predominant pattern, causes of both nodular and reticular patterns should be considered.
PA chestCXR on same patient 2 years earlier.

Distinguishing Acinar nodules from from interstitial nodules

  • Varying in size
  • Indistinct edges
  • Larger than interstitial nodules

  • Same size
  • Sharp edges
  • smaller

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