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CHEST 2016 video sessions: HRCT of the chest
ACCP CHEST
Overview
This video session focuses on interpreting High-Resolution Computed Tomography (HRCT) scans of the chest, specifically differentiating between pattern-based and pathology-based terms. It systematically explores various interstitial lung diseases by categorizing findings into reticulations, nodules, cysts, and airspace disease. For each category, the presentation illustrates characteristic HRCT appearances with clinical cases, discusses differential diagnoses, and highlights key distinguishing features to aid in accurate diagnosis and understanding of these complex lung conditions.
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Chapters
- Distinguish between pathology-based terms (e.g., UIP, NSIP) and pattern-based terms (e.g., reticulations, nodules, cysts, airspace disease).
- Pattern-based terms are descriptive findings on CT that reflect underlying pathology.
- The session will cover common entities associated with reticulations, nodules, cysts, and airspace disease.
Understanding the distinction between descriptive CT patterns and specific pathological diagnoses is crucial for accurate interpretation and communication in radiology.
Reticulations, nodules, and cysts are pattern-based terms, while Idiopathic Pulmonary Fibrosis (IPF) is a pathology-based term.
- UIP (Usual Interstitial Pneumonia) is characterized by peripheral and basal reticulations, honeycombing, and traction bronchiectasis, often with spatial and temporal heterogeneity.
- NSIP (Nonspecific Interstitial Pneumonia) typically shows more homogeneous ground glass and reticulation, often with subpleural sparing and less architectural distortion or honeycombing.
- Ancillary findings like pleural plaques (asbestosis) or dilated esophagus (connective tissue disease) can help differentiate causes of reticular patterns.
- UIP associated with connective tissue diseases may present atypically with more ground glass and less honeycombing.
Differentiating UIP from NSIP is critical as they have different prognoses and treatment responses; UIP is often progressive, while NSIP can be steroid-responsive.
A classic UIP pattern shows basal and peripheral reticulations with honeycombing, while NSIP might show ground glass opacities and reticulations with sparing of the lung periphery.
- Nodular patterns are assessed based on the nodules' relationship to secondary pulmonary lobule structures (perilymphatic, random, centrilobular, small airway).
- Perilymphatic nodules are found along fissures, pleura, septa, and peribronchovascular lymphatics, characteristic of sarcoidosis, silicosis, and lymphangitic carcinomatosis.
- Random nodule distribution suggests hematogenous spread, seen in miliary tuberculosis or metastases.
- Centrilobular and small airway nodules are typically inhalational, seen in conditions like RB-ILD or hypersensitivity pneumonitis.
The distribution and morphology of nodules provide crucial clues to the underlying disease process, guiding the differential diagnosis towards specific conditions like granulomatous diseases or infections.
Stage II sarcoidosis presents with numerous micronodules preferentially lining lymphatic structures such as the pleura, fissures, and bronchovascular bundles.
- Airspace disease encompasses consolidation, ground glass opacities, and sometimes a reverse halo sign.
- Organizing pneumonia is characterized by patchy airspace disease, often peribronchovascular and subpleural, and can be responsive to treatment.
- Acute interstitial pneumonia (AIP) and diffuse alveolar damage (DAD) present with diffuse ground glass and consolidation, often rapidly progressive.
- Clinical history is paramount in evaluating airspace disease due to its broad differential diagnosis.
Recognizing airspace disease patterns is essential because it can represent a wide range of conditions, from inflammatory processes like organizing pneumonia to acute lung injury, requiring prompt diagnosis and management.
Patchy airspace consolidation, sometimes with a reverse halo sign, along with bronchial wall thickening, can be seen in organizing pneumonia.
- Cystic lung diseases are characterized by air-filled spaces, which can be thin-walled or thick-walled.
- Lymphangioleiomyomatosis (LAM) typically shows thin-walled cysts diffusely distributed, often with normal intervening lung, occurring in women of childbearing age.
- Langerhans Cell Histiocytosis (LCH) presents with cysts that may have thicker walls, often associated with nodules and upper lobe predominance, with abnormal intervening lung.
- Central centrilobular emphysema and Birt-Hogg-Dubé are other causes of cystic lung changes.
Distinguishing between different types of cystic lung disease is important for prognosis and management, as LAM and LCH have distinct clinical associations and progression patterns.
LAM is characterized by numerous thin-walled cysts with normal intervening lung, whereas LCH may show cysts with more irregular walls and nodularity in the surrounding lung parenchyma.
- Small airways disease involves the bronchioles and can manifest as centrilobular nodules, tree-and-bud opacities, or bronchial wall thickening.
- Mosaic attenuation, seen on inspiratory CT, is characterized by alternating areas of normal and abnormal lung, often due to air trapping or vascular issues.
- Expiratory CT is crucial to differentiate air trapping (small airways disease) from obliterative vascular processes, as air trapping causes increased heterogeneity on expiration.
- Conditions like respiratory bronchiolitis-interstitial lung disease (RB-ILD) and hypersensitivity pneumonitis can cause centrilobular nodules and mosaic attenuation.
Identifying small airways disease and mosaic attenuation requires specific imaging techniques like expiratory CT to accurately diagnose conditions that affect the peripheral airways and can lead to air trapping.
Mosaic attenuation, where areas of black lung (hypolucent) are juxtaposed with gray lung (hyperlucent), can become more pronounced on expiration if it's due to air trapping in small airways.
Key takeaways
- HRCT interpretation relies on systematically categorizing findings into patterns (reticulations, nodules, cysts, airspace disease) to narrow down potential pathologies.
- The distribution, morphology, and association of findings with anatomical structures (lymphatics, airways, lobules) are critical for differential diagnosis.
- Ancillary findings on CT, such as pleural changes, adenopathy, or specific organ involvement, provide vital clues to the underlying cause.
- Differentiating between diseases with similar patterns (e.g., UIP vs. NSIP, LAM vs. LCH) requires careful attention to subtle imaging features and clinical context.
- Expiratory CT imaging is essential for evaluating air trapping and differentiating small airways disease from vascular abnormalities.
- A comprehensive understanding of both pattern-based and pathology-based terminology is fundamental for accurate HRCT interpretation of interstitial lung diseases.
Key terms
High-Resolution Computed Tomography (HRCT)Pattern-based termsPathology-based termsReticulationsHoneycombingGround glass attenuationNodulesPerilymphatic distributionRandom distributionCentrilobular nodulesCystsAirspace diseaseConsolidationMosaic attenuationAir trappingUIP (Usual Interstitial Pneumonia)NSIP (Nonspecific Interstitial Pneumonia)LAM (Lymphangioleiomyomatosis)LCH (Langerhans Cell Histiocytosis)SarcoidosisMiliary Tuberculosis
Test your understanding
- How do pattern-based terms on HRCT differ from pathology-based terms, and why is this distinction important?
- What are the key distinguishing features between UIP and NSIP patterns on HRCT, and why does this differentiation matter clinically?
- Describe the concept of perilymphatic nodule distribution and list at least two conditions that commonly exhibit this pattern.
- Explain the role of expiratory CT imaging in evaluating mosaic attenuation and differentiating small airways disease from vascular processes.
- What are the characteristic HRCT findings that help differentiate Lymphangioleiomyomatosis (LAM) from Langerhans Cell Histiocytosis (LCH)?