29  Imaging Approach to Interstitial Lung Disease	509
a	b

Fig. 29.2  The two types of ground glass opacity are visible on computed tomography (CT) imaging in patients with brosing lung disease. Increased density parenchyma is visible in the right lower lobe (a) and occurs in the background of reticulation and traction bronchiectasis (arrow). The increased density is likely to refect ne brosis and thick-

ening of the interand intralobular septae beyond the resolution of CT imaging. Increased density (arrow) in the posterior left lower lobe (b) occurs on background normal lung which has no evidence of architectural distortion. The appearances are more in keeping with infammation supervening within the airspaces rather than ne brosis

(reticulation and traction bronchiectasis) amongst the increased density lung suggests that this pattern refects nebrosis and should be compatible with an IPF diagnosis. Increased density occurring on background normal lungs as in Fig. 29.2b has a greater likelihood of being a transitory abnormality and is more likely to represent infammation. Ground glass opacities are now understood to refect either the replacement of air within airspaces by fuid, blood, pus or cells [14, 15] or the summed increase in density that results from lung regions where the interstitium is thickened, the individual components of which are beyond the resolution of CT [16, 17]. The confation of two distinct patterns of abnormality as “ground glass opacities” on CTs of patients with FLD has long been a cause of confusion. Better appreciation of these two distinct CT appearances which happen to share a common descriptor should help resolve some of the earlier challenges in CT interpretation of the term ground glass opacity.

Other Imaging Findings in FLD

Small foci of calci cation (<3 mm in size) which may be linear or nodular are more common in CTs of patients with usual interstitial pneumonia (UIP) pattern of brosis than in patients with non-speci c interstitial pneumonia (NSIP) pattern of brosis (Fig. 29.3). The lesions, which are due to ossi cation [18], are predominantly located within areas ofbrosis and are more commonly seen in cases of IPF than HP

[19]. Though not speci ed as diagnostic criteria in current guidelines, the identi cation of nodular ossi cations may help re ne the characterisation of a UIP pattern of brosis.

A variation in the attenuation characteristics of lung parenchyma represents a mosaic attenuation pattern. In the context of FLD, the extensive presence of low attenuation pulmonary lobules (the low attenuation component of a mosaic attenuation pattern) refecting small airways disease has been strongly linked with a diagnosis of HP [20]. However, CTs of IPF patients not uncommonly contain low attenuation pulmonary lobules [20, 21] (Fig. 29.4), which are thought to occur secondary to smoking-related damage to the airways [22]. A more nuanced measure to distinguish HP from IPF outlined in the Fleischner IPF guidelines [8] involves identifying low attenuation lobules in spared/non-­ brotic regions of the lung, which are more commonly seen in HP as a consequence of the airway-cen- tred brotic process [23]. In cases where brosis is extensive and spared regions of the lung may be sparse, the examination of historic CTs, acquired when the disease was limited in extent may increase the likelihood of identifying low attenuation lobules within the normal lung parenchyma. More recent work has also highlighted the value of the “headcheese sign” (Fig. 29.5), also termed the three density pattern [24], where ground-glass attenuation, normal attenuation and low attenuation lobules co-exist in the same lobe and may represent a good differentiator between chronic HP (CHP) and IPF [25].

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510	T. M. Jacob et al.
a	b

Fig. 29.3  Nodular ossi cation in a patient with idiopathic pulmonarybrosis. The lung window imaging (a) demonstrates peripheral traction bronchiectasis and honeycombing most marked within the right lower

lobe. There are subtle nodules (<3 mm in size) of calci ed density within the brotic lung, which are more readily visible (arrowheads) on the mediastinal window image (b)

a

b

Fig. 29.4  Variable distributions of low attenuation lung (arrowheads) in two brosing lung diseases. In a patient with idiopathic pulmonary brosis (a), expanded low attenuation secondary pulmonary lobules are visible within or adjacent to areas of the brotic lung, but no low attenuation lob-

ules were visible within the normal lung. In a patient with hypersensitivity pneumonitis, however (b), low attenuation secondary pulmonary lobules are visible within the normal parenchyma (arrowhead), a feature that may increase the likelihood of a diagnosis of hypersensitivity pneumonitis

29  Imaging Approach to Interstitial Lung Disease	511
a	b

c

Fig. 29.5  Axial computed tomography images in the upper (a), middle (b), and lower (c) regions of the lungs in a patient with hypersensitivity pneumonitis. The combination of normal (black arrows) and low (arrow-

heads) attenuation secondary pulmonary lobules and increased (white arrows) density lobules represents the “headcheese” sign which improves the speci city for reaching a diagnosis of hypersensitivity pneumonitis

UIP in IPF

IPF is a chronic, progressive, brosing interstitial pneumonia of unknown cause, occurring predominantly in older adults, associated with the histopathological and/or radiological pattern of usual interstitial pneumonia (UIP) [6]. The 2018 ATS/ERS/ALAT/JRS diagnostic guidelines [6] and Fleischner Society guidelines [8] require that to make a diagnosis of IPF, there must be (a) exclusion of other known causes of ILD, (b) the presence of a de nite/probable UIPIPF pattern on HRCT and (c) where CT features are indeterminate, histopathological con rmation of UIP (Table 29.2). In cases where imaging is indeterminate for a UIP-IPF pattern and surgical biopsy is not possible, case discussion in a multidisciplinary meeting may allow assignation of a working diagnosis of IPF. The working diagno-

sis may be revisited/con rmed as the particular evolutionary trajectory of a patient’s disease becomes clearer over time. The following sections will describe the imaging criteria required to assign a UIP pattern appropriate for an IPF diagnosis (UIP-IPF).

Defnite UIP-IPF

In the appropriate clinical context, honeycombing with or without traction bronchiectasis, located in a subpleural and basal predominant distribution (Fig. 29.6a–c), without features of an alternative diagnosis, strongly correlates with histopathological UIP [26]. If honeycombing is present but in an atypical distribution (not lower zone predominant), an alternative diagnosis should be considered.

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512	T. M. Jacob et al.

Probable UIP-IPF

A probable UIP-IPF pattern (Table 29.3) is analogous to a de nite UIP-IPF pattern but where peripheral traction bronchiectasis or bronchiolectasis is distributed subpleurally and basally and no honeycombing is present (Fig. 29.6d–f). The derivation of the term “probable UIP pattern” emerged following histopathological studies that identi ed UIP features in patients labelled possible UIP using the 2011 guidelines [27, 28]. Similar rates of disease progression were also shown between patients with de nite and possible UIP patterns in drug trial cohorts emphasising the prognostic impor-

Table 29.2  Computed tomography (CT) features are required to be present in a basal peripheral lower zone distribution when classifying a usual interstitial pneumonia pattern in the context of idiopathic pulmonary brosis. CT features suggestive of an alternative diagnosis including predominant ground glass opacities and low attenuation lobules within the normal background lung parenchyma, cysts, nodules and consolidation should not be present. An indeterminate usual interstitial pneumonia pattern may also refect brosis where the distribution of disease does not specify a particular aetiology and where CT features do not suggest an alternative diagnosis

CT feature	De nite	Probable	Indeterminate
Honeycombing	+	−	−
Traction bronchiectasis	+/−	+	−
Reticulation	+/−	+/−	+

tance of traction bronchiectasis in idiopathic pulmonarybrosis [29].

Indeterminate

An Indeterminate UIP-IPF pattern may refect two scenarios. Subpleural and basal predominant reticulation may be identi-ed without honeycombing or traction bronchiectasis being present (Fig. 29.7a–c). Whilst this may refect the very earliest signs of UIP-IPF, a UIP pattern cannot be assigned on CT analysis until progression to traction bronchiectasis occurs. The second scenario associated with an indeterminate UIP-­ IPF pattern occurs when CT features and disease distribution are not typical for UIP-IPF (Fig. 29.7d–f) and no features suggestive of an alternative diagnosis are identi ed on the CT.

Table 29.3  The importance of traction bronchiectasis presence in de ning the likelihood of a usual interstitial pneumonia pattern on computer tomography (CT) imaging has been the primary change in the 2018 imaging classi cation of idiopathic pulmonary brosis

	Consensus Guideline	Consensus Guideline
CT feature	De nition: 2011	De nition: 2018
Honeycombing	De nite	De nite
Traction	Possible	Probable
bronchiectasis
Reticulation	Possible	Indeterminate

a

b

c

d

e

f

Fig. 29.6  Axial computed tomography images in the upper (a + d), middle (b + e), and lower (c + f) regions of the lungs in two idiopathic pulmonary brosis patients with a de nite and probable usual interstitial pneumonia pattern. A de nite usual interstitial pneumonia pattern (a–c) demonstrates honeycombing in a predominantly basal, peripheral

and subpleural distribution. The brosis is relatively asymmetrical being worse in the right lung. A probable usual interstitial pneumonia pattern (d–f) demonstrates traction bronchiectasis in a predominantly basal, peripheral and subpleural distribution with no evidence of honeycombing on the scan