Application and Interpretation of High-Resolution Computed Tomography for the Diagnosis of Interstitial Lung Disease (ILD): ILD Protocol

Abstract

Interstitial lung disease (ILD) encompasses a heterogeneous group of over 200 diffuse parenchymal lung diseases. Accurate diagnosis of ILD is essential for appropriate treatment but can be challenging to achieve. For patients with suspected ILD, high-resolution computed tomography (HRCT) is a minimally invasive approach that allows for convenient repeat imaging. Compared to a conventional computed tomography (CT) protocol, the HRCT protocol for ILD patients involves two additional series with different technical requirements: a prone inspiration scan, and a supine expiration scan. The prone scan is useful when dependent opacification is observed on a supine chest CT scan. It facilitates the diagnosis of honeycombing, reducing observer variation in diagnosing idiopathic pulmonary fibrosis. The expiratory scan is useful for identifying air trapping, which is essential for the diagnosis of hypersensitivity pneumonitis. It is used to assess the three-density sign, which is specific for hypersensitivity pneumonitis and characterized by patchy distribution of normal-appearing lobules, ground glass opacities, and lobules with reduced lung density and vessel size. The HRCT ILD protocol should be performed regularly to assess changes in the extent of fibrosis and to diagnose lung cancer or acute exacerbation when clinically indicated.