Question: How accurate is HRCT in distinguishing usual interstitial pneumonia and idiopathic pulmonary fibrosis (UIP/IPF) from fibrotic nonspecific interstitial pneumonia (NSIP)?

Answer: Classic examples of UIP/IPF show findings of reticulation, traction bronchiectasis, and honeycombing with little, if any, ground-glass opacity, with a basal and subpleural distribution. Classic examples of fibrotic NSIP show a preponderance of reticulation and traction bronchiectasis with a basal predominance, and with a peripheral or diffuse distribution. Honeycombing is uncommon and, when present, is not as extensive as seen with UIP/IPF. A finding highly suggestive of NSIP is sparing of the immediate subpleural lung, seen in 20-50% of cases. Ground-glass opacity is more common than with UIP/IPF. Such classic or advanced cases can be distinguished with a high degree of accuracy. However, there is considerable overlap between the appearances of early UIP/IPF and that of fibrotic NSIP, with reticulation and traction bronchiectasis being the predominant findings in both. Unless the subpleural sparing or considerable ground-glass opacity is present, thus suggesting the diagnosis of NSIP, an accurate distinction between these diseases using HRCT may be impossible. Considering all cases of suspected UIP/IPF or fibrotic NSIP, the diagnostic accuracy of HRCT is about 70%. It is important to emphasize that a distinction between these diseases is difficult in patients with early disease.

Travis WD, Hunninghake G, King TE, Jr, et al. Idiopathic nonspecific interstitial pneumonia: report of an American thoracic society project. Am J Respir Crit Care Med. 2008;177:1338-1347.

Question: I have a patient with progressive shortness of breath and no obvious exposures or predisposing diseases. His HRCT shows honeycombing. What is the differential diagnosis? Does the honeycombing mean he has IPF?

Answer: A basal, subpleural, and posterior predominance of honeycombing, with less severe abnormalities in the upper lobes, is typical of patients with usual interstitial pneumonia (UIP) on pathology. In the absence of any exposures or clinical findings of associated diseases, this appearance strongly suggests the diagnosis of IPF. However, the differential diagnosis of honeycombing on HRCT includes fibrotic nonspecific interstitial pneumonia (NSIP), collagen-vascular diseases, chronic hypersensitivity pneumonitis, and even sarcoidosis. Asbestosis and fibrosis associated with drug treatment can also result in this HRCT appearance, but an appropriate history is often available. It should be kept in mind that although fibrotic NSIP can result in honeycombing, other findings of fibrosis (eg, reticulation and traction bronchiectasis) and ground-glass opacity usually predominate in patients with NSIP. Honeycombing is less common in NSIP than in UIP/IPF and tends to be less extensive.

Question: What is the best HRCT technique to use in a patient with possible idiopathic pulmonary fibrosis?

Answer: Proper HRCT technique requires the use of thin slices (1 mm) and reconstruction of the scan data using a high-resolution algorithm, with scans obtained at full inspiration. However, there are many different ways of imaging the patient that can provide scans adequate for diagnosis of IPF or the other interstitial pneumonias.

Scans can be obtained with a multi-detector CT scanner using thin detectors (eg, 1.25 mm or 0.625 mm) and spiral technique. This allows volumetric acquisition and reconstruction in any plane. It has not been shown however, that multiplanar reconstructions add much to diagnostic accuracy.

Scans can be obtained with single axial slices being obtained at spaced intervals, either 1 or 2 cm. This technique has the advantage of low radiation dose, when compared to spiral scanning, and the accuracy of spaced scanning has been validated in numerous papers over the years.

Regardless of which scanning technique is chosen, it is prudent to obtain scans in both the supine and prone positions. In the supine position, dependent atelectasis can mimic early interstitial lung disease. Scans in the prone position obviate this problem. Since early IPF and nonspecific interstitial pneumonia predominate in the posterior and subpleural lung, diagnosis may be difficult if only supine scans are available.

Volpe J, Storto ML, Lee K, Webb WR. High-resolution CT of the lung: determination of the usefulness of CT scans obtained with the patient prone based on plain radiographic findings. AJR Am J Roentgenol. 1997;169:369-374.