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Radiographic Identification of Interstitial Lung Disease
Answers to FAQs from Diagnosis and Classification of Interstitial Lung Disease: Considerations for Radiologists

Released: October 26, 2023

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Key Takeaways
  • When there is shallow inspiration or obesity, it is particularly important to perform prone imaging to determine whether a true interstitial abnormality is present.
  • When fibrosis becomes more severe, the typical pattern of fibrotic hypersensitivity pneumonitis (HP) may be difficult to identify
  • There are 3 subtypes of ground glass abnormality:
    • Pure ground glass
    • Ground glass superimposed on septal thickening and intralobular lines (crazy paving)
    • Ground glass abnormality superimposed on fine reticular abnormality

Interstitial lung disease (ILD) is an expansive umbrella term for a complex, heterogeneous set of diseases, including idiopathic pulmonary fibrosis (IPF) and non-IPF fibrosing ILD. Because of the complexity and heterogeneity of ILDs, diagnosis of IPF and other fibrosis ILDs is often delayed. Radiologists have a vital role in earlier identification and classification of ILD. This role spans from the identification and documentation of incidental findings of interstitial lung abnormality on high-resolution CT (HRCT) to the participation on multidisciplinary discussion panels. Patients with interstitial lung abnormalities are at risk for adverse outcomes, including potential fibrosing ILDs and death. Identification of an interstitial lung abnormality on HRCT is imperative to ensure patients with potentially clinically significant ILD are evaluated further and that those without current evidence of ILD are followed appropriately. 

This expert commentary, provided by David A. Lynch, MB, BCh, expands on answers to questions that learners submitted during a webcast on the diagnosis and classification of ILD.

Is it always important to do prone imaging on your HRCT?

Prone imaging is not always necessary on HRCT. At our institution, our technologists are trained to turn the patient prone only when there is significant dependent density on supine imaging. However, if this is not feasible, it is good practice to routinely do prone imaging.

How often do you use prone imaging to make a diagnosis?

On approximately 10% of HRCT scans that I read, there is dependent lung density because of either shallow inspiration or obesity. In these cases, it is particularly important to perform prone imaging to determine whether or not a true interstitial abnormality is present. If this is not done, I sometimes have to ask for the patient to come back for prone imaging, which is annoying for everybody but better than providing an equivocal report. Remember that the posterior lung bases are the most common site for early lung fibrosis, either idiopathic or in connective tissue disease. In addition, prone imaging is sometimes helpful in confirming the presence of honeycombing and may be helpful in evaluating the posterior lungs when pleural fluid is present.

Is it easier to diagnose hypersensitivity pneumonitis (HP) in nonfibrotic disease or fibrotic disease?

The diagnosis of HP is usually more straightforward with nonfibrotic disease because the signs are often more typical. Although the HRCT findings of fibrotic HP may be diagnostic (eg, with the 3-density sign), this is less common. In addition, when fibrosis becomes more severe, the typical pattern of fibrotic HP may be difficult to identify.

If you have ground glass opacity superimposed on reticulation, does this imply an alternative diagnosis?

This part of the IPF diagnostic algorithm is commonly misinterpreted. There are really 3 types of ground glass abnormality:

  • Pure ground glass opacity without any background texture. This is usually inflammatory and does indeed often imply an alternative diagnosis such as HP, drug toxicity, etc. A further diagnostic consideration would be infection or acute exacerbation superimposed on preexisting fibrotic lung disease. 
  • Crazy paving pattern. This is ground glass abnormality superimposed on septal thickening and intralobular lines, without any evidence of fibrosis. When present in relatively healthy outpatients, this is usually a result of pulmonary alveolar proteinosis with differential including lipoid pneumonia.
  • Ground glass opacity superimposed on fine reticulation. This most commonly represents microscopic fibrosis and one must look carefully for signs of fibrosis (architectural distortion, traction bronchiectasis, or traction bronchiolectasis). This pattern is found in fibrotic lung diseases including nonspecific interstitial pneumonia and usual interstitial pneumonia and would not support an alternative diagnosis.

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