Clinical Radiology
Volume 65, Issue 10 , Pages 823-831, October 2010

The idiopathic interstitial pneumonias: understanding key radiological features

  • S. Dixon
    • ,
  • R. Benamore

      Affiliations

    • Corresponding Author InformationGuarantor and correspondent: R. Benamore, Department of Radiology, The Churchill Hospital, Old Road, Oxford OX3 7LJ, UK. Tel.: +44 1865 235746.

Department of Radiology, The Churchill Hospital, Old Road, Oxford OX3 7LJ, UK

Received 10 December 2009; received in revised form 18 March 2010; accepted 22 March 2010. published online 10 May 2010.

Article Outline

Many radiologists find it challenging to distinguish between the different interstitial idiopathic pneumonias (IIPs). The British Thoracic Society guidelines on interstitial lung disease (2008) recommend the formation of multidisciplinary meetings, with diagnoses made by combined radiological, pathological, and clinical findings. This review focuses on understanding typical and atypical radiological features on high-resolution computed tomography between the different IIPs, to help the radiologist determine when a confident diagnosis can be made and how to deal with uncertainty.

 

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Introduction 

The term idiopathic interstitial pneumonia (IIP) refers to a group of acute and chronic diffuse parenchymal lung disorders of unknown aetiology that often share similar features. The current accepted classification is based on histological criteria; each histological pattern is associated with characteristic high-resolution computed tomography (HRCT) imaging features1, 2, 3 (Table 1). IIPs are divided into idiopathic pulmonary fibrosis–usual interstitial pneumonia (IPF-UIP) or IIPs other than IPF-UIP: nonspecific interstitial pneumonia (NSIP); cryptogenic organising pneumonia (COP), acute interstitial pneumonia (AIP); respiratory bronchiolitis interstitial lung disease (RBILD); desquamative interstitial pneumonia (DIP); and lymphocytic interstitial pneumonia (LIP).

Table 1. Table adapted from American Thoracic Society/European Respiratory Society guidelines1 demonstrating the relationship between the histopathological and the corresponding clinical–radiological–pathological diagnoses for idiopathic interstitial pneumonias.
Histological featuresClinical–radiological–pathological diagnosis
Usual interstitial pneumoniaIdiopathic pulmonary fibrosis
Nonspecific interstitial pneumoniaNonspecific interstitial pneumonia
Organising pneumoniaCryptogenic organising pneumonia
Diffuse alveolar damageAcute interstitial pneumonia
Respiratory bronchiolitisRespiratory bronchiolitis interstitial lung disease
Desquamative interstitial pneumoniaDesquamative interstitial pneumonia
Lymphoid interstitial pneumoniaLymphoid interstitial pneumonia

The British Thoracic Society (BTS) classification [adapted from the American Thoracic Society/European Respiratory Society (ATS/ERS) guidelines, 20021] delineates the roles of the pathologist, radiologist and clinician in diagnosing these conditions. It stresses the importance of distinguishing UIP from other IIPs, as UIP is associated with a poorer prognosis. HRCT is now the recommended imaging mode of choice. To make an accurate radiological diagnosis, it is important to use standardised terms with a pattern-based approach.4 There is considerable overlap among the IIPs and although classification is based on histological criteria, there is considerable interobserver disagreement between pathologists in the diagnosis of interstitial pneumonias. Therefore, histopathology should not necessarily be considered the reference standard and the diagnosis, even though extremely challenging within a multidisciplinary team setting, can only be achieved by means of agreement with stringent correlation of the clinical, imaging, and pathological findings.1, 2 In this article, we illustrate the main radiological HRCT patterns of IIPs to help the radiologist to make a confident diagnosis or differential.

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Idiopathic pulmonary fibrosis-usual interstitial pneumonia 

IPF-UIP is the most common IIP. UIP is the pathological pattern of lung injury seen in IPF patients, characterised by temporal and spatial heterogeneity (the identification of interstitial inflammation and fibrosis of different stages by fibroblast foci, mature fibrosis, and honeycombing). This distinguishes it from other IIPs that are temporally uniform (i.e., changes appear to have occurred over a single, relatively narrow time span).5, 6 UIP is seen in patients with connective tissue diseases and asbestosis,7, 8, 9, 10 and it is important to exclude underlying causes of a UIP pattern as these patients often have a better prognosis than those with IPF.7

Typical HRCT features 

The main aim of HRCT is to separate patients with typical features of IPF-UIP from the other IIPs. If the typical features are present, radiologists can make a confident diagnosis of UIP in greater than 90% when compared to surgical lung biopsy as the reference standard, justifying non-invasive diagnosis.1, 11, 12, 13, 14, 15, 16, 17 The typical features are reticulation, honeycombing and traction bronchiectasis, with basal and subpleural predominance. In the early stages there may only be irregularity seen at the pleura–parenchymal border at the lung bases, whilst extensive honeycombing is characteristic in end-stage disease1, 2, 5, 12, 18 (Fig. 1).

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  • Figure 1 

    HRCT images demonstrating the typical features of UIP; reticulation (thin arrows), honeycombing (thick arrows), and traction bronchiectasis (arrowheads) in a basal, subpleural predominance. (a) In the early stages there may only be irregularity/reticulation seen at the pleura–parenchymal border, whilst (b) honeycombing is characteristic in end-stage disease. The lungs are generally of reduced volume.

Honeycombing is the strongest indicator of UIP on CT (with a 90% positive predictive value, PPV), occurring in 70–80% of cases. Pathologically, honeycombing represents destroyed and fibrotic lung tissue, with cystic air spaces (from a few millimetres to several centimetres) with thick fibrous walls, indicating end-stages of the disease. This correlates on HRCT with clustered cystic air spaces of varying sizes with well-defined walls in a subpleural, predominantly lower zone location.3 Its presence may infer a poorer survival. If UIP is seen with honeycombing the median survival is 2.1 years, whilst UIP without honeycombing has a median survival of 5.8 years.1, 11, 12, 13, 14, 15, 16

Atypical HRCT features 

Ground-glass opacification (GGO) or nodules are not typical features.1, 5, 12, 14, 15, 17, 19, 20 If present, they can increase in extent or progress to fibrosis despite treatment.21 If GGO is seen as a predominant feature, another diagnosis should be considered. However, it is necessary to recognise that GGO can be commonly seen in UIP, where it is usually associated with dilated airways in keeping with established fine fibrosis. Difficulty arises when a reticular pattern and GGO coexist; in this instance it is important to recognise which is the dominant feature. If a reticular pattern is the dominant pattern and the GGO occurs in the same area, then it is most likely UIP with micro-honeycombing. If there are no reticular changes or traction bronchiectasis then the diagnosis is less likely to be UIP.22 Although GGO is not a typical feature in UIP, it is characteristic of accelerated UIP, when consolidation may also be seen. This is an acute idiopathic deterioration of a patient with known IPF, which generally presents with a short history of worsening dyspnoea, only occasionally associated with systemic symptoms.23, 24 The GGO and/or consolidation may be diffuse or peripheral, and must be distinguished from infections that have similar appearances on CT (e.g., Pneumocystis jiroveci).

Rarely, IPF-UIP presents with upper zone predominance and/or a peribronchovascular distribution,25, 26, 27 but alternative diagnoses, such as chronic hypersensitivity pneumonitis (HP), should be strongly considered. Consolidation is a very unusual feature of UIP. Mediastinal adenopathy can be seen in two-thirds of patients.28 When atypical features are present, the pathological diagnosis is UIP in approximately 50% of cases22; however, a biopsy is often required to confirm the diagnosis.

Differential diagnosis 

The main differentials of UIP are NSIP, asbestosis, connective tissue diseases, and chronic HP. The cardinal feature distinguishing UIP from NSIP is GGO, which is a more predominant feature in NSIP.29 It is harder to quantify the exact proportion of GGO on CT, therefore, recognising the degree of honeycombing is now recommended to help distinguish UIP from NSIP.1, 30

The differentiation of UIP and asbestosis is extremely difficult. No one CT feature can distinguish between them. Biopsy is not advocated 31 as the presence of a UIP pattern of fibrosis and asbestos bodies histologically does not necessarily always indicate asbestosis.8, 32 Bronchoalveolar lavage may have a role, but occupational history is the main key to diagnosis, as well as the presence of pleural plaques. In the absence of pleural plaques, to aid differentiation, subpleural dot-like or branching opacites, curvilinear lines, and band-like opacities are significantly more common in patients with asbestosis, whereas honeycombing, visible bronchioles, and bronchiolectasis within consolidation are significantly more common in patients with IPF33 Bronchiolar obstruction in the subpleural region may be more prominent in asbestosis, whereas bronchiolar dilatation may be more prominent in IPF.33

IPF-UIP may co-exist with emphysema. There is usually upper zone paraseptal (>90%) emphysema and lower zone fibrosis. Emphysema is distinguished from honeycombing by the lack of well-defined walls and the presence of internal centrilobular structures.34, 35 The lung volumes are usually preserved and associated with pulmonary arterial hypertension in approximately 50%. If, however, the fibrosis and emphysema coexist in the same part of the lung, it can mimic cystic lung disease.32, 34, 35, 36

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Nonspecific interstitial pneumonia 

NSIP was first recognised in the late 1990s,5 and is now accepted as a distinct clinical entity.37 Pathologically, there is temporal homogeneity of inflammation and fibrosis with the absence of honeycombing and fibrotic foci.38 There are two recognised types of NSIP: fibrotic NSIP and cellular NSIP. Fibrotic NSIP is a combination of fibrosis and inflammation and is much more common than pure cellular NSIP. Cellular NSIP, although rare, has a better prognosis than fibrotic NSIP.10

In contrast to UIP, which is usually idiopathic,39 NSIP is often associated with collagen vascular disease (with known major involvement in scleroderma,40 polymyositis, dermatomyositis, or Sjogrens syndrome) and is a frequent pattern in drug-induced pneumonitis. Therefore, a histopathological diagnosis of NSIP should prompt a search for an underlying cause. Immunosuppression with oral corticosteroids and immunosuppressive agents is the treatment of choice.

Typical HRCT features 

NSIP is often difficult to diagnose with confidence on HRCT, as the features are more variable than UIP.41, 42 If the typical features are present and diagnosed correctly, the PPV for the diagnosis of NSIP is approximately 67%.29

GGO is found in 75–100% of cases. It is usually bilateral, symmetrical and subpleural in two-thirds of patients and has lower zone predominance in over 50%. Reticulation and irregular linear opacities are present in 50–85%, of which up to 50% are peribronchovascular in nature. The appearance of traction bronchiectasis is variable, ranging from 35–95% (Fig. 2). There may be associated volume loss, especially in fibrotic NSIP. Honeycombing is not a major feature of NSIP but occurs in 25–30%; it is associated with finer fibrosis that is “temporally homogeneous” whereas fibrosis in UIP is “temporally heterogeneous”.29, 43

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  • Figure 2 

    Histologically proven NSIP. HRCT images demonstrating (a) GGO in a typical bilateral, symmetrical, predominantly subpleural, and lower zone predominance (arrowheads). GGO may be the only finding in one-third of cases. (b) Reticulation/irregular linear opacities (thin arrows) occur in 50–85%. Traction bronchiectasis occurs in 35–95% (thick arrows). (c) CT with lung windows showing GGO can be peribronchovascular in up to 50% (dotted arrows) and can be indistinguishable from organising pneumonia.

Atypical features and differential diagnosis 

Consolidation, centrilobular nodules and pleural effusions are rare.11, 41, 42 There is an overlap in the imaging between NSIP and UIP in up to 25%. In 32% of cases NSIP presents with less generalised GGO, a coarser pattern of fibrosis than expected and does not conform to its usual subpleural location, appearing indistinguishable from UIP. These cases require a biopsy to help differentiate NSIP and UIP. If the two diseases co-exist pathologically, HRCT and histology need to be reviewed together.13, 29, 42, 44, 45 However, even with an atypical distribution of GGO (i.e., apical or non-subpleural), in the absence of honeycombing, the diagnosis favours NSIP over UIP.11, 46

Subtle early NSIP can be mistaken for dependent change, with subpleural GGO only at the lung bases. If in doubt, a prone scan will help clarification. NSIP can co-exist with organising pneumonia (14%), and HP (20%),42 and in this circumstance, responds better to treatment with steroids. Other differentials are chronic HP and DIP.

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Cryptogenic organising pneumonia 

COP, previously known as bronchiolitis obliterans–organising pneumonia (BOOP) is a patchy organising pneumonia caused by bronchiolar obstruction by plugs of loose organising connective tissue that may wax and wane.1, 47, 48

Typical HRCT features 

The main finding of COP is consolidation, seen in 90% and is usually multifocal and bilateral. Unilateral COP can also occur. In 50%, consolidation is subpleural or peribronchovascular. It has a lower zone predominance, but can affect any lobe.49 Dilated airways are often seen on HRCT with air bronchograms mimicking acute pneumonia (Fig. 3). GGO is a prominent pattern in 60%. Linear opacities occur in isolation or in association with multifocal areas of consolidation in COP (Fig. 3b). Nodules, usually less than 10mm are seen in 30–50%. Lung volumes are usually preserved in 75%. If the typical features of COP are present the radiological accuracy for correct diagnosis is 60–79%.14, 50

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  • Figure 3 

    HRCT images demonstrating proven COP. Consolidation (short arrows) occurs in 90%. It is multifocal with a peribronchovascular (a) and subpleural (b)(dotted arrows) distribution in 50%. Within the consolidation dilated airways are often seen (arrowheads). Coarse linear opacities are often seen, predominantly within the lower zones (white arrows) (b).

Atypical HRCT features and differential diagnoses 

Pleural effusions are very rare. Pleural thickening occurs in 33% and parenchymal bands in 25%. In approximately 15% of cases COP presents as multiple masses, which need to be differentiated from infection and neoplasms, particularly lymphoma (Fig. 4a). COP rarely presents as the “reversed halo sign”,51 which is also seen in South American blastomycosis and lymphomatoid granulomatosis52, 53 (Fig. 4b). COP may demonstrate a perilobular distribution resembling interlobular septal thickening.51, 54, 55

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  • Figure 4 

    CT with lung windows demonstrating biopsy-proven COP. (a) COP presenting as multiple masses. Note the air bronchograms within the masses. (b) HRCT demonstrating the reverse halo sign (arrows), which is areas of GGO surrounded by crescent and ring-shaped consolidation.

The differential diagnosis for multifocal consolidation includes bronchoalveolar cell carcinoma, lymphoma, sarcoid, vasculitis, eosinophilic pneumonia, connective tissue diseases (poly/dermatomyositis), drug-induced diseases, and infection. If GGO is a predominant feature, NSIP must be excluded. If the disease is subpleural in distribution then eosinophilic pneumonia and vasculitis should be excluded. Mediastinal lymphadenopathy is less commonly seen in COP than the other IIPs, with an overall prevalence of 38% compared to 66% in IPF, 81% in NSIP, and 71% in RBILD/DIP.28 If clinical and radiographic features are uncertain, a lung biopsy is required.

COP presenting as consolidation or nodules has a better prognosis than if presenting predominantly as linear opacities, as these may not resolve with treatment.56

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Acute interstitial pneumonia 

AIP is a distinct histological entity characterised by diffuse alveolar damage of unknown aetiology.1

HRCT features 

The HRCT features of AIP depend on whether it presents in an early or late stage of the disease process. In both stages it has lower zone predominance in 40%. GGO is invariable and the extent usually correlates with disease duration. It is often in a geographic pattern, with lobular sparing. Consolidation occurs in 90% of patients with early AIP, but is not as common as GGO, and mostly occurs posteriorly. In 85% of cases there is a degree of bronchial dilatation with interlobular septal thickening (Fig. 5). There is associated parenchymal distortion but honeycombing is not a feature acutely, being more common in the later stages.48 Pleural effusions occur in 33%.50, 57 Most patients are ventilated in the early stages of the disease.

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  • Figure 5 

    CT with lung windows demonstrating histologically proven AIP in a young women post-partum. There is lower zone predominance with consolidation and GGO with lobular sparing (arrow).

In the later stages, consolidation is replaced by GGO, representing fibroblast proliferation histologically. There is cyst formation with increased areas of lucency. At this stage parenchymal distortion becomes more obvious, especially within the non-dependent regions of the lung. This is best explained by the “protective” effect of the consolidation within the dependent portions of the lungs against barotrauma of mechanical ventilation.58

Differential diagnoses 

The main differential is acute respiratory distress syndrome (ARDS). AIP and ARDS are commonly indistinguishable, although AIP is often more symmetrical and lower zone predominant.57, 59 Other considerations are infection [such as Pneumocystis pneumonia (PCP)], pulmonary oedema, and pulmonary haemorrhage dependent upon clinical features.

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Respiratory bronchiolitis interstitial lung disease 

Respiratory bronchiolitis (RB) is a histopathological diagnosis found in almost all smokers. The term RBILD is reserved for clinically symptomatic smokers with imaging/histological findings of RB.48, 60 It usually occurs in the fourth/fifth decade, with a male predominance (2:1).

Histologically, there is macrophage accumulation and chronic inflammation in the respiratory bronchioles, which manifests as centrilobular nodules on HRCT (Fig. 6). GGO is typically multifocal and upper zone predominant; it is due to the accumulation of macrophages within the air spaces. There is bronchial wall thickening with centrilobular emphysema and air trapping (Fig. 6).

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  • Figure 6 

    HRCT images demonstrating RBILD. There is subtle multifocal ground glass attenuation, some of which is centrilobular in distribution (arrows) within the upper zones on inspiration (a). There is accentuation of the mosaic pattern on expiration (b) confirming air-trapping.

Reticulation may occur, but is not a major feature. If present, there is no associated honeycombing or traction bronchial dilatation. The features seen on HRCT may regress or show improvement after the patient stops smoking with or without additional treatment with steroids.61

Differential diagnoses 

There are three main differentials: (1) HP, which is often associated with a history of allergen exposure, but there is a decreased incidence of HP in smokers compared with non-smokers62, 63; (2) DIP, where the GGO is more extensive but centrilobular nodules are uncommon. The differentiation between DIP and RBILD can be indistinguishable by CT60 and may be referred to as a spectrum of “smoking-related ILD”; (3) NSIP, which usually has a lower zone predominance when compared with the mid to upper zone predominance in RBILD.

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Desquamative interstitial pneumonia 

DIP is an extremely rare interstitial pneumonia. It is a diffuse process with intra-alveolar macrophage accumulation and alveolar septal thickening. It has a homogeneous pattern on CT with a basal peripheral predominance. Its main feature is GGO. Reticulation and cysts may be seen.38, 41 Like RBILD it is part of the “smoking-related ILD” spectrum. It primarily affects smokers in their fourth/fifth decade so centrilobular emphysema often co-exists. There is a male predominance (2:1).61, 64, 65, 66 In general the clinical prognosis is good with a 70% 10-year survival.1

The GGO has a lower zone predominance in 75%, with a peripheral distribution in over half of cases, a patchy distribution in a quarter, and a diffuse/uniform pattern in 18% (Fig. 7). Small cystic spaces may be seen within the GGO. The GGO may remain stable or improve with treatment.21, 67 Progression to reticulation and honeycombing with architectural distortion is uncommon (<20%), and usually improves with treatment.65

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  • Figure 7 

    HRCT image demonstrating histologically proven DIP. Image shows GGO, which is in a patchy and diffuse distribution (seen in 18% of cases), but in over 50% there is predominantly a peripheral distribution.

Differential diagnoses 

Pathologically, DIP generally shows diffuse rather than bronchocentric lung involvement.60 Acute or sub-acute HP can appear similar to DIP, but as in RBILD, smoking history and allergen exposure are important20; infections such as PCP may mimic DIP.

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Lymphocytic interstitial pneumonia 

LIP is a rare disorder, characterised pathologically by polyclonal lymphocyte aggregates that accumulate diffusely in the interstitium.68 It is usually associated with an underlying disease, mainly connective tissue diseases (especially Sjögren’s) or in patients with immunodeficiency. Therefore, it is important to exclude an underlying cause in all patients.1

Typical HRCT findings are GGO with a diffuse distribution in two-thirds of patients, thin-walled perivascular/subpleural cysts in two-thirds of patients, and reticulation in 50%. Nodules typically follow the lymphatics, therefore, are seen in a centrilobular (100%), subpleural (86%), and perichronchovascular distribution69 (Fig. 8). Unlike other IIPs, lymphadenopathy is common, being seen in two-thirds. Atypical features include perivascular honeycombing, larger nodules, and consolidation.41, 50

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  • Figure 8 

    HRCT images displaying the typical features of histologically proven LIP. (a) GGO is invariable (occurs in a diffuse distribution two thirds) with centrilobular nodules (arrows), although as nodules occur in the distribution of the lymphatics they can also occur in subpleural and peribronchovascular locations. (b) Thin-walled perivascular cysts occur in two-thirds of cases (arrowheads). (c) Diffuse nodular GGO with atypical features of nodular consolidation (white arrowheads) and traction bronchiectasis (white arrows).

Malignant transformation is unusual. HRCT may suggest the diagnosis of LIP, but it is extremely difficult to diagnose confidently, and almost always requires surgical lung biopsy. Other conditions that may mimic LIP are sarcoid, HP, and Langerhan’s cell histiocytosis (LCH).70 In LCH the nodules are ill-defined, rarely subpleural, and the cysts are characteristically bizarre shaped with thick walls, sparing the lung bases, whilst in LIP the cysts are thin-walled and randomly distributed.69

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Conclusion 

The IIPs pose a diagnostic challenge. HRCT is indicated in virtually all patients with suspected IIP, and in order to make a confident diagnosis, one should be aware of key radiological features. Specific entities, such as a UIP pattern of pulmonary fibrosis and typical COP, can often be diagnosed with confidence by the radiologist. The diagnoses of the other IIPs require a multidisciplinary approach. In general, honeycombing, reticulation, and traction bronchiectasis are characteristic of UIP, whilst GGO suggests NSIP or DIP. Consolidation is seen in COP and AIP, whilst cysts are a feature of DIP or LIP.

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PII: S0009-9260(10)00124-8

doi:10.1016/j.crad.2010.03.006

Clinical Radiology
Volume 65, Issue 10 , Pages 823-831, October 2010

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