29  Imaging Approach to Interstitial Lung Disease	507

Image Acquisition

The 2018 Fleischner Society diagnostic criteria for idiopathic pulmonary brosis (IPF) emphasised in clear and precise terms how important good quality CT acquisitions are to allow de nitive interpretation of disease patterns [8]. These recommendations were echoed in the 2018 update to the IPF diagnostic guidelines [6]. In line with the 2011 IPF diagnostic guidelines [7], CTs evaluating suspected FLDs were recommended to be performed supine, without the administration of intravenous contrast (Table 29.1). However, refecting the general improvement in scanner technology in the intervening decade, the 2018 IPF guidelines stipulated that CTs be acquired volumetrically (without gaps between slices) and were required to have a maximum slice thickness of 1.5 mm. The 2011 guidelines accepted the acquisition of HRCTs with gaps of between 1 and 2 cm between image sections. The necessity for gaps between images was predominantly related to limitations in scanner acquisition abilities, but also allowed dose reductions in CT studies. Interspaced imaging essentially sampled the lung parenchyma to provide an overall impression of disease patterns. Yet subtle features could be missed when acquiring non-contiguous images, for example small nodules (<1 cm in size) representing early lung cancer might have developed in the gaps between CT slices. Similarly, the architectural distortion and deformation of lung parenchyma in patients with brosis could make the identi cation of disease progression on serial interspaced imaging much harder than when examining their volumetric equivalents.

The 2018 IPF diagnostic guidelines recommended the routine use of expiratory CT imaging in the work-up ofbrosing lung disease patients [6], whereas in 2011 it remained an optional acquisition [7]. Expiratory imaging which can be performed volumetrically or interspaced can accentuate the presence of air-trapping and thereby help identify the presence of small airways disease in conditions such as connective tissue disease-related ILD (CTD-ILD)

Table 29.1  Optimal acquisition techniques for computed tomography (CT) imaging in the evaluation of brosing lung diseases. mm millimetre, mSV millisievert, CTPA CT pulmonary angiogram

Optimal CT series	Optimal acquisition
Non contrast supine inspiratory CT	Volumetric, collimation
	<1 mm
Expiratory supine CT recommended	Interspaced or volumetric
Prone inspiratory CT to con rm solely
dependent abnormalities
CTPA with or without initial	Non contrast CT:
non-contrast CT	Interspaced or volumetric
Dose reduction techniques advised to	No CTs to be acquired at
achieve 1–3 mSv dose	<1 mSv dose

and hypersensitivity pneumonitis (HP) [9]. Guidelines related to the use of prone CT imaging to con rm the presence of subtle dependent lung changes that could be confused with atelectasis have not changed since 2011. However, the emergence of iterative reconstruction techniques that can reduce CT doses has been recommended for use in the evaluation of FLD. Speci cally, low-dose CT acquisitions at a dose range of 1–3 mSv are suggested. Ultra-low dose imaging (<1 mSv) which can be associated with an excess of imaging noise is not recommended for routine clinical evaluation.

Finally, the utility of performing a non-contrast CT prior to a CT pulmonary angiogram (CTPA) was highlighted in the 2018 iteration of the IPF guidelines [6]. CTPAs are often requested in patients with acute shortness of breath or more commonly in the case of patients with FLD when acute shortness of breath supervenes on chronic breathlessness. Lungbrosis can result in heterogenous vascular perfusion of the lung which following contrast administration can manifest with subtle foci of increased parenchymal attenuation related to altered vascular distributions. Hyper-­attenuation secondary to contrast material can be confused with increased parenchymal density from infection, pulmonary oedema, aspiration or an acute exacerbation of FLD, all of which constitute the differential diagnoses of acute ­worsening of breathlessness. To distinguish artefactual increased parenchymal density consequent to contrast administration from appearances resulting from genuine lung damage, a non-contrast CT should be acquired immediately prior to a CTPA. The parenchymal appearances on an interspaced non-­contrast CT can then be used as a reference to determine whether hyperdense parenchymal changes are genuine or artefactual.

Key Features of Fibrosis

Two key radiological features of lung brosis are traction bronchiectasis and volume loss. The Fleischner Society glossary of terms de nes traction bronchiectasis as “irregular bronchial and bronchiolar dilatation caused by surrounding retractile pulmonary brosis” [10]. Traction bronchiectasis is typically seen in the lung periphery and exists amidst other features of brosis such as ground glass opaci cation and/or reticulation [11]. It is important to differentiate traction bronchiectasis from other causes of abnormal airway dilatation unrelated to brosis. In the latter case, the surrounding lung will appear normal or hyperlucent as opposed to dense and brotic. Distinguishing between traction bronchiectasis and honeycombing on HRCT can prove challenging, particularly where conglomerate peripheral traction bronchiectasis at the lung bases may mimic honeycombing. A review of

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508	T. M. Jacob et al.
a	b

Fig. 29.1  Lobar volume loss can be identi ed by the position of the lung ssures at the level of the hemidiaphragms. In a patient with connective tissue disease-related lung brosis (a) the right oblique ssure (arrow) has only reached the mid-point of the chest when the right

hemidiaphragm rst comes into view (star). In a normal healthy patient, however (b), the right oblique ssure (arrow) is visible at the anterior aspect of the chest at the level of the right hemidiaphragm (star)

imaging on multiplanar reformats (MPR) and employment of post-processing reconstruction algorithms, for example, minimum intensity projection can help to distinguish traction bronchiectasis from honeycombing [12].

Parenchymal volume loss is useful in determining the distribution of brotic disease, particularly if honeycombing and traction are not obvious. Volume loss can be assessed by comparing the position of the oblique ssures relative to each other and whether they reach the anterior chest wall at the level of the hemidiaphragms as observed in normal lungs [11]. In idiopathic pulmonary brosis (IPF) for example, which is a lower zone predominant disease, preferential contraction of the lower lobes will retract the oblique ssures posteriorly, with the result that they only reach the midpoint of the chest wall at the level of the hemidiaphragms (Fig. 29.1).

When brosis is severe, honeycomb cysts can occur representing end-stage lung. The Fleischner Society de nes honeycombing as “destroyed and brotic lung tissue containing numerous cystic airspaces with thick brous walls” [10]. Honeycomb cysts are typically well-de ned cystic spaces 3–5 mm in diameter which can extend to 25 mm in size. In the setting of IPF, they often cluster in a peripheral, subpleural and basal distribution [13]. However, honeycomb cysts can occur in FLDs other than IPF, where their distribution can vary. Several stacked layers of cysts or even a single subpleural layer of two or three adjoining cysts are a key characteristic of the UIP pattern of brosis which is associated with a poor outcome in several FLDs.

Ancillary Features of Fibrosis

Reticulation and ground glass opaci cation are commonly seen in FLD, although both may also occur in nonbrosing conditions. On chest radiographs, reticulation refects innumerable small linear opacities which relate to interlobular or intralobular septal thickening on CT [10]. Ground glass opaci cation represents areas of increased density, which on CT preserves the margins of the bronchovascular structures.

The 2011 IPF guidelines [7] stipulated that a CT that contained ground glass opacities which were more extensive than the extent of reticulation would be inconsistent with a diagnosis of IPF. The 2018 guidelines [6] suggest that CTs with predominant ground glass opacities should necessitate consideration of an alternative diagnosis to IPF. The underlying rationale has been that IPF is generally not associated with infammation and therefore a CT demonstrating extensive infammation refected in ground glass opacities is more likely to refect an alternative diagnosis.

Yet a fundamental challenge exists with what CT patterns radiologists choose to de ne as a ground glass opacity. Figure 29.2 demonstrates two CTs that show distinct imaging patterns but which have been described using the same “ground glass opacity” terminology. In Fig. 29.2a, the increased lung density in the right lower lobe is associated with underlying reticular lines and traction bronchiectasis, whilst in Fig. 29.2b, the increased lung density in the medial left lower lobe is overlaid on normal background lung parenchyma. In Fig. 29.2a the co-existence of brotic features