Lung HRCT Patterns: Nodules & Septal Thickening | Dr Anagha Joshi

High-resolution CT (HRCT) of the chest uses slice thickness ≤1.5mm with collimation between 0.625-1.5mm. Multi-detector CT employs contiguous slices. Reconstruction uses high spatial frequency/bone algorithms for edge detail versus standard algorithms for soft tissue. Standard algorithm shows smoother soft tissues; bone algorithm enhances parenchymal detail. Thin sections (≤1.5mm) prevent volume averaging that obscures pathology. Dedicated chest CTs use 1.25mm contiguous sections. Expiratory imaging identifies air trapping by showing posterior tracheal wall bowing forward. Prone imaging detects subtle posterior basal abnormalities in early IPF and asbestosis.

CT density can be measured in Hounsfield Units (HU) to identify tissue types: normal lung tissue ranges from -700 to -900 HU, blood appears at 50-80 HU, and bone appears at approximately 1000 HU. High-Resolution CT (HRCT) has much higher resolution than standard CT, with slices approximately 1mm thick versus 1cm for standard CT. HRCT is indicated for suspected interstitial lung disease, cystic lung disease, and small airway disease including bronchiectasis. HRCT does not require contrast injection but requires patient cooperation for breath-holding. The secondary pulmonary lobule is the smallest structural unit, consisting of approximately 12 alveoli, terminal bronchioles, blood vessels, and interlobular septa containing the lymphatic system.

HRCT (High-Resolution Computed Tomography) of the chest is a specialized CT scan technique that uses specific parameters including 1mm slice thickness with 5mm interval, axial scanning mode, and higher mA settings (150-180) compared to routine CT scans, enabling detailed visualization of interstitial lung diseases and parenchymal abnormalities that are not visible on standard CT imaging.

High-resolution CT (HRCT) requires specific technical parameters: thin slices (0.5-1.5 mm), lung-specific reconstruction algorithm (not standard soft tissue), high kilovoltage and milliamperage for optimal spatial resolution and contrast, 512x512 matrix, and viewing parameters of window level -500 and width 1500. Using standard algorithms with thick slices produces opaque images with poor resolution. Low-dose protocols can reduce radiation by half while maintaining diagnostic quality, which is crucial for follow-up studies in chronic interstitial lung diseases.

Regular CT scans use volume averaging to detect lung nodules by combining data from multiple slices, while High-Resolution CT (HRCT) provides detailed images of lung architecture without volume averaging, making it ideal for evaluating conditions like fibrosis, emphysema, and bronchiectasis but potentially missing small nodules between slices; therefore, regular CT is ordered when detecting or monitoring lung nodules, whereas HRCT is used for assessing lung tissue architecture and specific patterns such as ground-glass opacification or micro-nodular changes.

ILDs encompass over 200 diseases classified by etiology: known-cause ILDs include pneumoconiosis (asbestosis, silicosis), drug-induced ILD (amiodarone, bleomycin), and connective tissue disease-associated ILD (rheumatoid arthritis, scleroderma). Idiopathic ILDs include IPF (most common and dangerous), non-specific interstitial pneumonia, and smoking-related variants. Specialized subgroups include granulomatous ILD (sarcoidosis) and lymphoid interstitial pneumonia (Langerhans cell histiocytosis). Progressive pulmonary fibrosis (PPF) is a phenotype occurring across multiple ILD types, identifiable by worsening symptoms, declining lung function, and increasing fibrosis on HRCT. Pathophysiology involves restrictive lung disease through intrapulmonary (lung tissue stiffness) or extrapulmonary (external compression) mechanisms, leading to hypoventilation, hypoxemia, reduced diffusion capacity, potential pulmonary hypertension, and cor pulmonale. Asbestos exposure additionally increases risks of bronchogenic carcinoma and mesothelioma.

Interstitial lung diseases (ILD) are a heterogeneous group of conditions predominantly affecting the pulmonary interstitium—the space between epithelial and endothelial basement membranes containing fibroblasts and connective tissue. The distinction between parenchymal and interstitial diseases is somewhat artificial since the interstitium is constitutive of pulmonary parenchyma. The key pathophysiological change is transformation of elastic interstitial tissue into fibrotic, rigid tissue that limits lung expansion. This creates characteristic bilateral inspiratory crackles with a 'velcro' sound. The disease course can be acute or chronic, and understanding this fundamental pathology is essential for clinical recognition and management.

Interstitial lung disease (ILD) is a restrictive lung disorder characterized by scarring (fibrosis) of the lung interstitium, which involves the alveolar epithelium, basement membrane, interstitial space, and capillary endothelium. The pathophysiology begins with micro-injury to alveolar epithelium from factors like dust or smoking, leading to an anti-fibrinolytic environment with multiple clot formation. Growth factors (TNF-alpha, TGF-beta) are released, causing fibroblast migration and proliferation. This is followed by neovascularization due to imbalance between metalloproteinase enzymes and tissue inhibitors, and the formation of myofibroblasts that create permanent scar tissue. Clinical features include progressive dyspnea, non-productive cough, hemoptysis, and inspiratory crackles. Pulmonary function tests show decreased FVC with normal or elevated FEV1/FVC ratio. ILD is classified into granulomatous types (e.g., sarcoidosis, hypersensitivity pneumonitis) and fibrotic types (e.g., idiopathic pulmonary fibrosis, pneumoconiosis), with glucocorticoids being the mainstay of treatment.

This comprehensive section covers the fundamental concepts of interstitial lung disease. The pulmonary interstitium consists of epithelial cells, endothelium, basement membrane, and perivascular/lymphatic tissue organized into axial, parenchymal, and pleural zones. Interstitial thickening impairs gas exchange, causing hypoxemia, reduced compliance, and restrictive physiology. ILDs are classified by etiology: known causes (environmental, drug-related, connective tissue diseases), primary disorders (sarcoidosis, alveolar proteinosis), and idiopathic forms. IPF specifically is defined as chronic fibrosing interstitial pneumonia of unknown cause with progressive deterioration over six+ months, limited to the lungs without systemic involvement, and associated with UIP pattern. Pathogenesis involves continuous epithelial injury in genetically susceptible individuals over 50 years old triggering maladaptive repair responses with genomic instability, telomere dysfunction, and excessive extracellular matrix deposition.

Interstitial lung diseases (ILDs) are characterized by inflammation and fibrosis of the alveolar walls and interstitium, resulting in reduced lung compliance, restrictive lung function, impaired gas exchange, and potential pulmonary hypertension. ILDs are classified into known etiologies (inorganic exposures, drug-induced, hypersensitivity pneumonitis, connective tissue disease-associated, and smoking-related) and idiopathic forms (IPF, NSIP, COP, LIP, AIP). The pathogenesis involves alveolar epithelial damage, inflammatory cell activation, cytokine release (particularly TGF-β), fibroblast activation, and extracellular matrix deposition, with the balance between inflammation and fibrosis determining disease progression and treatment response.

Chest radiograph interpretation requires understanding five fundamental radiological densities: air (radiolucent/black), fat (dark gray), water/soft tissues (light gray), calcium/bone (white/radiopaque), and metal (very white/radiopaque). A systematic approach involves three steps: verifying patient identification, assessing radiographic quality, and performing ordered reading from bony structures to pulmonary parenchyma. Quality assessment includes evaluating exposure (visualization of fourth intervertebral space, pulmonary vasculature in outer third, and vessels through cardiac silhouette), projection (PA vs AP), rotation (clavicular ends equidistant from spinous processes), and inspiration (9-10 posterior intercostal spaces). The mediastinum is divided by the angle of Louis (inferior border of T4) into superior and inferior compartments. The cardiac index is calculated as (A+B)/C × 100, with normal values less than 0.50. The lungs contain 18 segments across 5 lobes. Alveolar opacities appear with the air bronchogram sign. Interstitial opacities include reticular, nodular, and mixed patterns. Atelectasis is classified into five types with direct and indirect signs. Pleural effusions are visible when exceeding 175 ml. Pneumothorax is identified by the deep sulcus sign, double line sign, and continuous diaphragm sign.

Chest X-ray interpretation requires understanding five tissue densities: air (black), fat, soft tissue (gray), bone (white), and metal. A systematic approach involves examining lungs first (right then left), evaluating from upper to lower zones, comparing sides for symmetry, assessing costophrenic angles, and examining diaphragm contours. Cardiac evaluation includes identifying chambers and measuring the cardiothoracic ratio (normal <0.5). Technical quality assessment includes evaluating exposure, rotation, and positioning. The lateral view is essential for evaluating retrosternal and retrocardiac spaces, lung fissures, and posterior structures. Lobar pathology localization uses fissure positions: major fissure (vertical, separates upper from lower lobe), minor fissure (horizontal, separates upper lobe from lingula). Pneumonia appears as lobar opacity with air bronchograms. Bronchopneumonia appears as patchy, peripheral infiltrates with ground-glass appearance. Tuberculosis typically affects upper lobes due to higher oxygen tension and may cause cavitation, bronchial spread, and fibrosis.

Chest X-ray interpretation requires systematic evaluation of key parameters including inspiration (counting 5-6 anterior and 8-10 posterior ribs), exposure quality (vertebral bodies visible through heart shadow), and rotation (clavicles equidistant from spinous processes). Different projections (PA vs AP) affect image quality, with PA views providing superior visualization of pathology. Normal anatomical landmarks include the cardiac axis (72 degrees), diaphragm position (right higher than left by 2.5 cm), and hilar structures. Pathology identification involves recognizing patterns such as consolidation (airspace disease), interstitial changes (Kerley lines), pleural effusion (meniscus sign), and pneumothorax (visceral pleural line). Nodule assessment considers size (>2cm suggests malignancy), calcification patterns (central/laminated suggests benign), and growth rate (>30% increase indicates malignancy).

Chest X-ray interpretation relies on recognizing specific radiographic patterns of increased and decreased density: alveolar patterns (cotton-like appearance from air space occupation by fluid, pus, or blood), interstitial patterns (reticular, nodular, or granular from interstitial thickening), atelectasis (lung collapse with mediastinal shift), masses/nodules (focal densities >2mm), hyperclarity patterns (emphysema with diaphragm flattening), destructive patterns (cavities from infection or neoplasms), bronchiectasis (ring and manguito signs), and pleural lesions (effusion, pneumothorax, thickening). Understanding these patterns, combined with systematic reading from outside to inside, enables accurate diagnosis of respiratory pathologies.

This section establishes the foundational principles for interpreting chest radiographs. Normal lung parenchyma exhibits specific characteristics: upper fields are more radiolucent than lower fields, costophrenic angles are sharp and closed, and cardio-phrenic angles are obtuse. Pathological density increases occur through three mechanisms: airspace replacement by fluid/cells (consolidation), air absorption without replacement (atelectasis), or interstitial thickening. The section introduces the concept that understanding these basic patterns is essential for identifying pathological processes in subsequent sections.