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BASICS OF HRCT INTERPRETATION-PART 3- NODULAR PATTERN

Nodular pattern

The distribution of nodules shown on HRCT is the most important factor in making an accurate diagnosis in the nodular pattern.
In most cases small nodules can be placed into one of three categories: perilymphatic, centrilobular or random distribution.
Random refers to no preference for a specific location in the secondary lobule.
Perilymphatic distribution
In patients with a perilymphatic distribution, nodules are seen in relation to pleural surfaces, interlobular septa and the peribronchovascular interstitium.
Nodules are almost always visible in a subpleural location, particularly in relation to the fissures.
Centrilobular distribution
In certain diseases, nodules are limited to the centrilobular region.
Unlike perilymphatic and random nodules, centrilobular nodules spare the pleural surfaces. 
The most peripheral nodules are centered 5-10mm from fissures or the pleural surface.
Random distribution
Nodules are randomly distributed relative to structures of the lung and secondary lobule. 
Nodules can usually be seen to involve the pleural surfaces and fissures, but lack the subpleural predominance often seen in patients with a perilymphatic distribution.
Algorithm for nodular pattern

The algorithm to distinguish perilymphatic, random and centrilobular nodules is the following:
  • Look for the presence of pleural nodules.
    These are often easiest to see along the fissures.
    If pleural nodules are absent or few in number, the distribution is likely centrilobular.
  • If pleural nodules are visible, the pattern is either random (miliary) or perilymphatic.
  • If there are pleural nodules and also nodules along the central bronchovascular interstitium and along interlobular septa, you are dealing with a periplymphatic distribution.
  • If the nodules are diffuse and uniformly distributed, it is likely a random distribution.
Perilymphatic distribution

 Perilymphatic nodules are most commonly seen in sarcoidosis.
They also occur in silicosis, coal-worker's pneumoconiosis and lymphangitic spread of carcinoma.
Notice the overlap in differential diagnosis of perilymphatic nodules and the nodular septal thickening in the reticular pattern.
Sometimes the term reticulonodular is used.
Here a typical case of perilymphatic distribution of nodules in a patient with sarcoidosis.

Notice the nodules along the fissures indicating a perilymphatic distribution (red arrows).
Always look carefully for these nodules in the subpleural region and along the fissures, because this finding is very specific for sarcoidosis.
Typically in sarcoidosis is an upper lobe and perihilar predominance and in this case we see the majority of nodules located along the bronchovascular bundle (yellow arrow).
Another typical case of sarcoidosis.

In addition to the perilymphatic nodules, there are multiple enlarged lymph nodes, which is also typical for sarcoidosis.
In end stage sarcoidosis we will see fibrosis, which is also predominantly located in the upper lobes and perihilar.

Centrilobular distribution

 Centrilobular nodules are seen in:
  • Hypersensitivity pneumonitis
  • Respiratory bronchiolitis in smokers
  • infectious airways diseases (endobronchial spread of tuberculosis or nontuberculous mycobacteria, bronchopneumonia)
  • Uncommon in bronchioloalveolar carcinoma, pulmonary edema, vasculitis


In many cases centrilobular nodules are of ground glass density and ill defined (figure).
They are called acinair nodules.

Tree-in-bud

In centrilobular nodules the recognition of 'tree-in-bud' is of value for narrowing the differential diagnosis.
Tree-in-bud describes the appearance of an irregular and often nodular branching structure, most easily identified in the lung periphery. 
It represents dilated and impacted (mucus or pus-filled) centrilobular bronchioles.
Tree-in-bud almost always indicates the presence of:

  • Endobronchial spread of infection (TB, MAC, any bacterial bronchopneumonia)
  • Airway disease associated with infection (cystic fibrosis, bronchiectasis)
  • less often, an airway disease associated primarily with mucus retention (allergic bronchopulmonary aspergillosis, asthma).
Here a tree-in-bud is seen.

In the proper clinical setting suspect active endobronchial spread of TB.
In most patients with active tuberculosis, the HRCT shows evidence of bronchogenic spread of disease even before bacteriologic results are available (6).
Random distribution

HRCT of a patient with random nodules as a result of miliary TB.
The random distribution is a result of the hematogenous spread of the infection.
Small random nodules are seen in:
  • Hematogenous metastases
  • Miliary tuberculosis
  • Miliary fungal infections
  • Sarcoidosis may mimick this pattern, when very extensive
  • Langerhans cell histiocytosis (early nodular stage)
Sarcoidosis usually has a perilymphatic distribution, but when it is very extensive, it spreads along the bronchovascular bundle to the periphery of the lung and may reach the centrilobular area.

Langerhans cell histiocytosis is an uncommon disease characterised by multiple cysts in patients with nicotine abuse.
In a very early stage, these patients show only nodules, that later on cavitate and become cysts (figure).

As in all smoking related diseases, there is an upper lobe predominance.




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