routine follow-up but returned 15 months later with worsening symptoms. A CT scan was repeated (right). The brosis consisting of reticulation and traction bronchiectasis has coarsened and increased in extent suggesting both maturation of brosis and disease progression. No honeycombing was present. The maturation of brosis is evidenced in the left lower lobe by structures in the lung such as airways and vessels being pulled closer together. An increase in brosis extent (disease progression) is demonstrated by previously normal-appearing lungs now containing ground glass density and reticulation (arrow).

The identi cation of disease progression on serial CT imaging of brosing lung diseases is going to become increasingly important in the years to come. As the use of anti brotic medication becomes more widespread, increasing numbers of patients will undergo declines in pulmonary function tests that lie within the range of measurement variation. Changes in CT variables, both measured visually and quantitively using computer tools may prove a more sensitive measure of disease worsening. Identifying changes in CT measures may allow arbitration of marginal lung function declines.

Introduction

Interstitial lung disease (ILD) encompasses a group of disorders which affect the connective tissue framework of the lung, termed the pulmonary interstitium [1]. There are many causes of ILD, and although not an exhaustive list, some of the most common aetiologies include connective tissue diseases (CTD), drug and antigen exposures, and idiopathic causes. ILD appearances can be challenging to interpret on CT imaging and individual ILDs may demonstrate different rates of disease progression over time. The formulation of a diagnosis requires the interpretation of radiological features alongside the patient’s clinical and exposure history, laboratory results and histopathological ndings. Consideration of these varied components is best achieved in the setting of a multidisciplinary team discussion where imaging interpretation plays a central part [2].

Diseases of the interstitium can be brosing and associated with permanent and progressive destruction of the connective tissue framework. Detailed descriptions of the role of imaging for the numerous brosing lung diseases (FLDs) and nonbrosing ILDs are beyond the scope of this chapter. Instead, we will focus on characterising the radiological fea-

tures of the main FLDs, with an emphasis on those conditions with formalised imaging classi cations and diagnostic guidelines.

The chapter will also outline the challenges associated with determining disease progression on CT imaging and why this has particular relevance for both clinical management and drug trials in the era of anti brotic therapies. We will also outline the role that computer analysis of CT imaging may play in the years to come as an alternative to visual CT interrogation.

Background

Chest Radiographs and High-Resolution

Computed Tomography

Imaging techniques are a core component of the diagnostic pathway for interstitial lung diseases which comprise over 200 distinct entities [3]. Whilst some are rare, most ILDs manifest a well-described combination of shortness of breath and changes on the chest radiograph [4]. In addition to identifying the presence of an ILD, a comparison of serial chest radiographs acquired at different time points can con rm the presence of disease progression. In clinical practice, chest radiographs are most commonly employed as the rst-line diagnostic test in a patient with shortness of breath with the aim of broadly excluding other pathology, for example, left heart failure, infection or lung cancer. However, radiographs may appear normal in patients with ILD [5]. Chest radiographs are also commonly utilised when a patient with a known ILD diagnosis presents with worsening breathlessness, as complications requiring urgent management such as a pneumothorax or infection can be rapidly diagnosed.

High-resolution computed tomography (HRCT) has been emphasised as being essential in the diagnosis of ILDs in the most recent American Thoracic Society/European Respiratory Society clinical practical guidelines [6]. HRCT is a more sensitive modality for ILD detection than chest radiography and conventional thoracic computed tomography. HRCT can also allow the identi cation of appropriate sites for bronchoalveolar lavage and lung biopsy and can guide treatment strategies [7]. As a large array of diseases can affect the pulmonary interstitium, separation of CT appearances into those exhibitingbrosis and those without obvious brosis allows the identi - cation of patients that are more likely to develop a progressive disease course. The following sections will rst describe the optimal HRCT acquisitions that should be used to imagebrosing lung diseases. Subsequently, we will outline the speci c HRCT features that are used to describe the appearances of the various brosing lung diseases.