High-resolution computed tomography (HRCT)

HRCT PATTERNS OF DIFFUSE LUNG DISEASE

RETICULAR PATTERN

Definition

This is caused by thickened interlobular or intralobular septa or honeycomb (fibrotic) destruction it almost always represents significant interstitial lung disease (ILD)

• Causes include infiltration by fibrosis (interstitial fibrosis), abnormal cells (lymphangitis carcinomatosa), or fluid (pulmonary oedema)

• Smooth interlobular septal thickening: pulmonary oedema or alveolar proteinosis

• Irregular interlobular septal thickening: lymphangitic spread of tumour or the nodular septal thickening seen in sarcoidosis

HRCT

• A fine reticular pattern (most commonly seen with idiopathic pulmonary fibrosis)

• A coarse reticular pattern: this occurs with severe fibrosis and is characterized by interlacing irregular linear opacities an end-stage fibrotic (honeycomb) lung is characterized by cystic airspaces surrounded by irregular walls

Traction bronchiectasis/bronchiolectasis: extensive fibrosis can distort the lung morphology resulting in irregular segmental or subsegmental airway dilatation

• Ground-glass opacification (if the septal thickening is very fine)

NODULAR PATTERN

GROUND-GLASS PATTERN

Definition

A non-specific feature representing a generalized increase in lung parenchymal opacification that does not obscure the pulmonary vessels it represents a combination of partial airspace filling, interstitial thickening and displacement of air from the lung

• Causes: subacute hypersensitivity pneumonitis acute respiratory distress syndrome (ARDS) acute interstitial pneumonia (AIP) non-specific interstitial pneumonia (NSIP) diffuse pneumonias (particularly Pneumocystis jiroveci (P. carinii) pneumonia in AIDS patients)

HRCT

Identification of dilated airways within areas of ground-glass opacification is usually an indication of fine fibrosis (and usually irreversible disease)

MOSAIC ATTENUATION PATTERN

Definition

A non-specific sign representing regional lung parenchymal attenuation differences

• This is the dominant abnormality in small airways disease, occlusive vascular disease and infiltrative lung disease

In small airways and occlusive vascular disease the ‘black’ lung demonstrating decreased attenuation is abnormal

In infiltrative lung disease the ‘grey’ lung demonstrating normal or increased attenuation is abnormal

HRCT

Bronchial abnormalities and the presence of air trapping on expiratory CT are the most useful discriminatory features in identifying small airways disease as the cause of mosaic attenuation

IDIOPATHIC INTERSTITIAL PNEUMONIAS

IDIOPATHIC PULMONARY FIBROSIS (IPF)

DEFINITION

• Progressive fibrosis and end-stage lung destruction of unknown cause

• It is also known as cryptogenic fibrosing alveolitis (CFA) or usual interstitial pneumonia (UIP) UIP specifically refers to the histopathological pattern seen in patients with the clinical presentation of CFA or IPF

Clinical presentation

Cough dyspnoea weight loss clubbing it commonly affects patients who are 40–70 years old (M>F)

RADIOLOGICAL FEATURES

CXR

Bilateral asymmetric peripheral reticular opacities these are most profuse at the lung bases although there is associated volume loss, the lung volumes may be preserved or increased if there is coexisting emphysema

HRCT

With disease progression the lung changes ‘creep’ around the lung periphery to involve the anterior aspects of the upper lobes (this is an important discriminator between UIP and other conditions with a similar clinical presentation) mediastinal adenopathy is frequently seen (up to 2cm and unrelated to infection or malignancy) pleural effusions are uncommon pulmonary hypertension can be seen with severe disease

• Early stage: ground-glass change

• Late stage: a predominantly subpleural bibasal reticular pattern

• End stage: areas of honeycomb destruction (end stage) with associated traction bronchiectasis

PEARL

• Other causes of a UIP-type histological pattern: chronic hypersensitivity pneumonitis asbestosis connective tissue disease rarely drugs

NON-SPECIFIC INTERSTITIAL PNEUMONIA (NSIP)

DEFINITION

• Varying degrees of interstitial inflammation and fibrosis without any specific features to allow a diagnosis of UIP or DIP to be made

Clinical presentation

As for UIP it commonly affects patients who are aged between 40 and 50 years (M = F)

RADIOLOGICAL FEATURES

HRCT

A predominant pattern of ground-glass opacification (with a basal and subpleural distribution) ± associated airway distortion a reticular pattern is common there may be significant fibrosis (with a uniform temporality in comparison with UIP) but honeycombing is sparse

• NSIP may be distinguished from UIP by more prominent ground-glass attenuation, a finer reticular pattern, and an absence of honeycombing

PEARLS

• The heterogeneity of pathological processes encompassed by NSIP makes a confident CT diagnosis less likely than with UIP

• NSIP has a better prognosis than UIP

CRYPTOGENIC ORGANIZING PNEUMONIA (COP)

DEFINITION

• A clinicopathological entity of an isolated organizing pneumonia seen in patients without an identifiable associated disease (e.g. infection, malignancy or connective tissue disease)

• COP was previously known as bronchiolitis obliterans organizing pneumonia (BOOP)

Clinical presentation

A non-productive cough dyspnoea malaise weight loss it commonly affects patients during the 6^th decade (M = F)

RADIOLOGICAL FEATURES

CXR

Areas of patchy consolidation which is often subpleural and basal (with a propensity to progress and change location over time) there is lung volume preservation

HRCT

Consolidation corresponding with areas of organizing pneumonia is commonly seen within the lower zones with either a subpleural or a peribronchial distribution (the peribronchial distribution is typically seen in patients with polymyositis or dermatomyositis) ground-glass opacification, subpleural linear opacities and a distinctive perilobular pattern is commonly seen the lung architecture is generally well preserved with cavitation rarely seen

• ‘Reverse halo’ sign: multifocal areas of ground-glass opacification with a surrounding rim of consolidation

PEARL

• There is usually a complete response to a long (2–3 months) course of high-dose steroids

IDIOPATHIC INTERSTITIAL PNEUMONIAS

RESPIRATORY BRONCHIOLITIS–INTERSTITIAL LUNG DISEASE (RB–ILD) AND DESQUAMATIVE INTERSTITIAL PNEUMONIA (DIP)

DEFINITION

• This is characterized by alveolar space filling with macrophages and a strong association with cigarette smoking

• RB–ILD and DIP are part of the same disease spectrum, with DIP the more severe form

Clinical presentation

Insidious dyspnoea cough

RADIOLOGICAL FEATURES

CXR

This is relatively insensitive

HRCT

• RB–ILD: areas of patchy ground-glass opacification (due to macrophage accumulation within the alveolar spaces and ducts) poorly defined low attenuation centrilobular nodules upper lobe centrilobular emphysema and areas of air trapping (usually to a limited extent and reflecting the bronchiolitic element)

Thickening of the interlobular septa and features of interstitial fibrosis is unusual

• DIP: ground-glass opacification is also the dominant feature this typically affects the peripheral lower zones and may be patchy occasionally there are features of established fibrosis (which is usually to a limited extent)

PEARLS

• It demonstrates a relatively stable clinical course

• Smoking cessation is an important part of the management

LYMPHOID INTERSTITIAL PNEUMONIA

DEFINITION

• This is due to a widespread interstitial lymphoid lung infiltrate it resembles lymphoma but its clinical course is more akin to a chronic interstitial pneumonia

Clinical presentation

Progressive cough and dyspnoea (2F:1M)

RADIOLOGICAL FEATURES

HRCT

Nodules of varying sizes (which may be ill-defined) ground-glass opacification thickened bronchovascular bundles and interlobular septal thickening discrete thin-walled cysts lying deep within the lung parenchyma (measuring up to 3cm)

• Airspace disease, large nodules and pleural effusions are rare

PEARLS

• Evolution to a frank lymphoproliferative disease is rare

• It occurs in association with autoimmune diseases (e.g. Sjögren’s syndrome), dysproteinaemias, autologous bone marrow transplantation, infection (viral, mycobacterial, HIV) and Castleman’s disease

ACUTE INTERSTITIAL PNEUMONIA (FORMERLY HAMMAN–RICH SYNDROME)

DEFINITION

• This can be regarded as an idiopathic form of adult respiratory distress syndrome (ARDS)

• It consists of diffuse alveolar damage – there is an acute exudative phase with subsequent organizing and fibrotic phases

Clinical presentation

An acute onset with a similar presentation to ARDS (M = F)

RADIOLOGICAL FEATURES

CXR

Bilateral patchy airspace opacification

HRCT

Ground-glass opacification and consolidation (all phases) bronchial dilatation and architectural distortion (fibrotic phase)

PEARLS

• With resolution there is clearing of any ground-glass attenuation leaving residual fibrosis – fibrosis is more common than that seen in ARDS

• It has a poor prognosis

Clinico–radiological–pathological criteria	Histological pattern	HRCT features*
*Idiopathic pulmonary fibrosis*	Usual interstitial pneumonia	Peripheral (subpleural) and basal reticular opacities honeycombing areas of ground-glass opacity (associated with traction bronchiectasis)
*Non-specific interstitial pneumonia*	Non-specific interstitial pneumonia	Areas of ground-glass opacity ± traction bronchiectasis minimal honeycombing
*Cryptogenic organizing pneumonia*	Organizing pneumonia	Peripheral or peribronchial consolidation areas of ground-glass opacity a perilobular pattern is increasingly recognized
*Acute interstitial pneumonia*	Diffuse alveolar damage	Consolidation (within the dependent lung) areas of ground-glass opacity traction bronchiectasis (organizing phase)
*Respiratory bronchiolitis–interstitial lung (RB–ILD)*	RB–ILD	Poorly defined centrilobular nodules areas of ground-glass opacity bronchial wall thickening limited emphysema
*Desquamative interstitial pneumonia (DIP)*	DIP	Areas of ground-glass opacity features of interstitial fibrosis
*Lymphoid interstitial pneumonia (LIP)*	LIP	Areas of ground-glass opacity poorly defined centrilobular nodules thickened interlobular septa thin-walled discrete cysts air trapping