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High-Resolution Computed Tomography (HRCT)


HRCT Patterns of Diffuse Lung Disease

HRCT Technique

  • Thin collimation (1-2 mm) with a high spatial frequency algorithm reconstruction ▸ interspaced versus volumetric acquisition (dose considerations)

  • Reducing slice thickness < 1 mm will not increase spatial resolution but increase image noise

  • A sharp reconstruction algorithm reduces images smoothing making structures visibly sharper (at the cost of increased image noise)

  • IV contrast should be avoided if possible as it can spuriously increase parenchymal opacification

  • Images usually acquired in the supine position from the apices to lung bases at full inspiration (at 1-2 cm intervals if non-volumetric)

    • ▪

      If early interstitial fibrosis is suspected, HRCT can be performed in the prone position to distinguish from potential dependent change that can be seen in the posterobasal segments when supine

    • ▪

      The need for expiratory CT is controversial – although end-expiration images can reveal subtle areas of air trapping, if clinically significant this is usually evident on inspiratory images

Mosaic attenuation in a patient with bronchiectasis in the lower lobes (not shown). HRCT image taken in inspiration (A) shows subtle mosaicism, emphasized in the section acquired at end-expiration (B), indicating small airways disease. **

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)

Widespread ground-glass opacification in a patient with desquamative interstitial pneumonia. The pulmonary vasculature is not obscured by this degree of opacification and the air-filled bronchi not unduly dilated. ©2

Mosaic Attenuation Pattern

Definition

A non-specific sign representing regional lung parenchymal attenuation differences ▸ lung attenuation depends on the amount of blood, parenchymal tissue and air and is therefore a non-specific sign

  • 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

Mosaic attenuation in the upper lobes of a patient with sickle cell disease and pulmonary hypertension. Note the increased calibre of the vessels within the increased attenuation (lighter) lung compared with the decreased attenuation (darker) parts of the lung, suggesting a vascular cause of the mosaic attenuation pattern in this case. ©2

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)

Reticular pattern: HRCT showing a coarse reticular pattern with extensive subpleural and basal honeycombing (arrows) in a patient with usual interstitial pneumonia.

Widespread smooth thickening of the interlobular and intralobular septa in areas of ground-glass opacification in a patient with pulmonary alveolar proteinosis. ©2

Nodular Pattern

Definition

This is a feature of both interstitial and airspace disease

Nodules within the lung interstitium

This is seen within the interlobular septa and the subpleural and peribronchovascular regions (especially those related to lymphatic vessels) ▸ nodules can be < 5 mm from the pleural surface

  • Causes: sarcoidosis ▸ lymphangitis carcinomatosa

Centrilobular nodules

These are related to endobronchial and small airway disease ▸ most peripheral nodules are > 5 mm from the pleural surface ▸ a ‘tree-in-bud’ appearance suggests endobronchial disease

  • Causes: subacute hypersensitivity pneumonitis ▸ respiratory bronchiolitis–interstitial lung disease (RB–ILD) ▸ diffuse panbroncholitis ▸ endobronchial spread of TB ▸ cryptogenic organizing pneumonia

Random distribution

The distribution is not related to the secondary pulmonary lobule and can involve the pleural surface

  • Causes: haematogenous spread of TB ▸ pulmonary metastases ▸ pneumoconiosis ▸ sarcoidosis (rare)

Nodular pattern: HRCT showing scattered nodules within the lung interstitium in a peribronchovascular distribution, with beading along the fissures (arrows) in a patient with pulmonary sarcoidosis.

Marked thickening of the interlobular septa with randomly distributed small nodules in an individual with silicosis. ©2

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

Ground-glass change is not predominant ▸ 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 2 cm 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

  • A confident HRCT diagnosis of UIP is difficult without honeycombing

  • Extent of fibrosis is predictive of survival and mortality

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 (temporally uniform in comparison with UIP) but honeycombing is sparse ▸ abnormalities are usually peribronchovascular or peripheral, and occasionally spare the subpleural lung

  • 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

Cryptogenic-organizing pneumonia. HRCT through the upper lobes demonstrates areas of consolidation in a subpleural and peribronchial distribution in association with areas of ground-glass opacification (left upper lobe). *

Biopsy-proven-organizing pneumonia. There are poorly defined arcade-like and polygonal opacities (the perilobular pattern) in the subpleural and posterior regions of both lungs. The opacities resemble ill-defined thickened interlobular septa. *

Usual interstitial pneumonia. HRCT abnormalities predominate in the posterior, and subpleural regions of the lower lobes and comprise honeycombing and traction bronchiectasis within the abnormal lung. *

Usual interstitial pneumonia. In the upper lobes anteriorly there are peripheral irregular lines with areas of honeycombing. *

There is generalized ground-glass opacification and a few thickened interlobular septa. There is marked dilatation of the bronchi (arrows) reflecting the presence of fine interstitial fibrosis. Biopsy-proven non-specific interstitial pneumonia. ©2

Non-specific interstitial pneumonia. The predominant abnormality is patchy, bilateral ground-glass opacification, mild reticulation and traction bronchiectasis. There is no frank honeycombing destruction. *

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 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

  • Because of significant overlap, ‘smoking-related interstitial lung disease’ (SR-ILD) has been proposed to encompass DIP, RB-ILD, LCH and interstitial fibrosis

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 3 cm)

  • 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/Diffuse Alveolar Damage (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

RB–ILD. HRCT shows (A) subtle areas of ground-glass opacification and (B) ill-defined centrilobular nodules. *

DIP. There is diffuse ground-glass opacification throughout the lungs due to inflammatory infiltrate. †

Lymphocytic interstitial pneumonitis. There is a background of ground-glass opacification and a few thin-walled cystic airspaces (the pathogenesis of these cysts is unclear). *

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

Sarcoidosis

Sarcoidosis

Definition

  • A multisystem non-caseating granulomatous disorder of unknown aetiology

  • The lungs, hilar and mediastinal nodes are the most commonly affected organ system

    • ▪

      Other affected organs: skin > peripheral lymph nodes > eyes > spleen > CNS > parotid glands > bones

Clinical Presentation

  • Fatigue ▸ malaise ▸ weight loss ▸ fever and night sweats ▸ dyspnoea ▸ erythema nodosum ▸ arthralgia ▸ 30% of patients are asymptomatic

    • ▪

      Respiratory symptoms are most commonly seen in the black female population

  • Onset is usually during the 2 nd to 4 th decades (F>M)

Radiological Features

  • Lung granulomas have a characteristic distribution along the lymphatics within the bronchovascular sheath, interlobular septa and subpleural regions

CXR (lymphadenopathy)

  • Lymph nodes appear lobulated with a well-demarcated outline (they can be massive) ▸ they can calcify in a characteristic ‘eggshell’ fashion ▸ airway or vascular compression is unusual

  • Garland's triad: bilateral symmetrical hilar and paratracheal lymphadenopathy

    • ▪

      The lymphadenopathy can occasionally (1–5%) be asymmetrical or unilateral – marked asymmetry should bring the diagnosis into question ▸ unilateral paratracheal lymphadenopathy is usually right-sided (left-sided lymphadenopathy causes enlargement of the aortopulmonary window nodes)

  • 40% of patients with nodal enlargement will develop parenchymal opacities within 1 year – of these ▸ will develop persistent fibrotic shadowing (± traction bronchiectasis)

    • ▪

      Nodal enlargement does not develop after parenchymal opacification

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