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Approach to nodular lesions on HRCT Basic anatomy for HRCT interpretation Airways Arteries and veins Lymphatics Interstitium Primary pulmonary lobule • The primary pulmonary lobule consists of alveolar ducts, alveolar sacs and alveoli • Approximately 30–50 primary pulmonary lobules can be found in one secondary pulmonary lobule. ACINUS • Acinus is the portion of lung distal to a terminal bronchiole and supplied by a first-order respiratory bronchiole or bronchioles. • The reported number of acini in one secondary pulmonary lobule varies considerably in different studies and numbers are found between 3 and 12 The secondary pulmonary lobule • The smallest functional unit of lung structure • Marginated by connective tissue septa • Irregularly polyhedral in shape • 1 to 2.5 cm in diameter in most locations Components • Interlobular septa. • Centrilobular structures. • Lobular parenchyma and acini Interlobular septae and centrilobular central artery Venule visualisation is indirect evidence of visualisation of interlobular septae Pulmonary edema with interlobular septal thickening Centrilobular structures • Centrilobular artery • Centrilobular bronchiole and its divisions • Supporting connective tissue Normal measurements • Secondary lobular artery and bronchiole -1 mm in diameter. • Intralobular terminal bronchioles and arteries - 0.7 mm in diameter. • Acinar bronchioles and arteries range from 0.3 to 0.5 mm in diameter. Appearance of intralobular artery • A linear, branching, or dot like opacity. • Within the centre of a lobule or within 1 cm of the pleural surface. Definition – HRCT nodule. Rounded opacity, well defined or ill defined with size not more than 3 cm in diameter HRCT NODULES • Size • Appearance • Attenuation • Distribution Nodule size • Miliary nodule -(1-3 mm). • Micronodule. – (3 - 7 mm). Fleischer Society has recommended that “micronodule” be used to refer to nodules no larger than 7 mm in diameter, but generally this term should be reserved for nodules less than 3 mm in diameter. • Small nodule -Smaller than 1 cm. • Large nodule – 1- 3 cm. • Mass ( > Mass). NODULE APPEARANCE Predominantly interstitial or predominantly air space • Airspace nodules are usually caused by a replacement of alveolar air by fluid and/or cells. Air space nodules • Airspace nodules can have a soft tissue density or a ground-glass density and can be sharply defined but are usually ill defined. • Airspace nodules are mostly located near the centre of the secondary pulmonary lobule. • when disease progresses, involve the entire lobule. Ill defined airspace nodules - Hypersensitivity pneumonitis Well defined airspace nodules -Acute viral bronchilolitis Merging of airspace nodules can be responsible for the development of larger nodules and areas of ground-glass opacity and lung consolidation Differential diagnosis for airspace nodules Infectious • Bronchiolitis • Bronchopneumonia • Tuberculosis • Mycoplasma • Aspergillus • Bronchoinvasive aspergillosis Smoking related parenchymal lung disease • (RB-ILD) Others • Hypersensitivity pneumonitis • Organising pneumonia. • Bronchio alveolar carcinoma • Pulmonary oedema • Pulmonary haemoaspiration • Cystic fibrosis • Allergic bronchopulmonary aspergillosis TREE IN BUD • Reflects the presence of dilated centrilobular bronchioles • Lumina impacted with mucus, fluid, or pus. • Associated with peribronchiolar inflammation Pathogenesis Interstitial nodules • Interstitial nodules are usually caused by a nodular cellular proliferation in the interstitium. • Interstitial nodules are mostly sharply defined and have in most cases a soft-tissue attenuation. • Most interstitial nodules are related to diseases that show a vascular or a (peri)lymphatic distribution, although some of them are located in the interstitial tissue of bronchiolar walls. Nodule attenuation Differentials for high attenuation nodules on HRCT • Infections. • Lung metastases • Chronic pulmonary hemorrhage • Pneumoconiosis. • Metastatic calcifications • Pulmonary alveolar microlithiasis Axial CT scans shows multiple small calcific PNs due to old healed histoplasmosis Calcispheres within alveoli (consist of Pulmonary alveolar calcium phosphate) microlithiasis • Very rare • 20-50 years of age • Familial incidence in half • Often asymptomatic at diagnosis • Abnormalities progress slowly Radiographs • Dense lungs • Basal predominance • Individual calcified nodules visible • Black pleural line HRCT • Subpleural and perivascular predominance of calcified nodules • Black pleural line = emphysema Metastatic calcification Metastatic calcification in the lungs in a patient with chronic renal failure Metastatic calcifications Calcification due to abnormal calcium and phosphate metabolism Etiology. • Hypercalcemia • Renal failure • Secondary hyperparathyroidism • Chronic hemodialysis • Commonly affects lung • Apices most often involved (more alkaline) HRCT • Opacities often centrilobular • May be ground-glass opacity • Not all nodules are calcified. High attenuation nodules due to Talc embolism following the post pulmonary alveolar hemorrhage. percutaneous vertebral plasty Calcified metastasis • Osteosarcoma, chondrosarcoma. • Synovial sarcoma, mucin-producing carcinoma. • Adenocarcinoma, thyroid tumours . Distribution pattern of the nodules Perilymphatic nodules Centrilobular nodules Random nodules Perilymphatic distribution of nodules Centrilobular distribution of nodules Random distribution of nodules Small nodules – distribution perilymphatic random centrilobular Perilymphatic nodules • Pleural surfaces. • Interlobular septa • Peribronchovascular interstitium. • Fissures. PERILYMPHATIC DISTRIBUTION • Sarcoidosis • Silicosis and CWP • Lymphangitic spread of carcinoma • Lymphoma • Lymphoproliferative disease such as lymphoid interstitial pneumonia. Random nodules Random nodules. • Hematogenous metastases • Miliary tuberculosis • Miliary fungal infection • Disseminated viral infection • Silicosis or coal-worker’s • pneumoconiosis • Langerhans’ cell histiocytosis Miliary infections • Recognized tendency to predominate in the lung periphery and at the lung bases. Randomly distributed nodules in case of GB carcinoma Randomly distributed cavitating nodules in case langhans of histiocytosis Centrilobular nodules with tree in bud • Bacterial pneumonia with infectious • bronchiolitis • Typical and atypical mycobacterial infections • Aspiration • Allergic bronchopulmonary aspergillosis • Cystic fibrosis • Diffuse panbronchiolitis • Endobronchial neoplasms (particularly • bronchioloalveolar carcinoma) Centrilobular nodules without tree in bud • All causes of Centrilobular nodules with tree-in-bud opacity • Hypersensitivity pneumonitis • Respiratory bronchiolitis and respiratory • bronchiolitis-interstitial lung disease • Cryptogenic organizing pneumonia • Pneumoconiosis' (especially silicosis • and coal-worker’s pneumoconiosis) • Langerhans’ cell histiocytosis • Pulmonary edema • Vasculitis • Pulmonary hypertension Centrilobular distribution • Reflect the presence of either interstitial or air-space abnormalities. • May be well-defined or ill-defined. • Range from a few millimeters to about 1 cm in size. • A single centrilobular nodule or a centrilobular rosette of nodules may be visible. • Nodules are roughly evenly spaced from one another and approach Centrilobular nodules • Do not contact visceral pleural surfaces. • Usually positioned about 5 to 10 mm from the visceral pleural surface. Ill-defined centrilobular nodules in a patient with hypersensitivity pneumonitis Ill defined centrilobular nodules in patient with cryptogenic organizing pneumonia Cases Sarcoidosis Sarcoidosis. • Nodules ranging in size from several millimeters to 1 cm or more in diameter. • Sharply defined despite their small size. • Nodules are most frequently seen in relationship to the perihilar peribronchovascular interstitium. Silicosis There are pleural nodules ,lymphatic distribution and are in upper lobe predominantly posterior in distribution Silicosis • Well defined. • Soft tissue attenuation. • Calcification may be present. • Symmetric. • Posterior and upper lobe predominance. • Other features – fibrosis , pleural plaques .Egg shell calcification in lymphnodes. Lymphangatic carcinoma These nodules are predominantly located in the subpleural interstitium (also along the great fissures) and in the interlobular septa (beaded septa). Some nodules are centrilobular, whereas some interlobular septa show a more homogenous thickening (linear opacities) Lymphangatic carcinoma • Tumor growth in the lymphatic system of the lungs. • Usually results from haematogenous spread to the lung later interstitial and lymphatic invasion. HRCT • Small, well-defined nodules , Soft-tissue density • Septal lines, subpleural thickening, proximal Peribronchovascular thickening • Combination with nodules and beaded septa Lymphoid interstitial pneumonia Small, sharply defined nodules are visible, particularly along the right major fissure. Interlobular septal thickening and nodules are visible. Ground-glass opacity is also present on the left side, and a single cyst is visible in the left lower lobe (black arrow). Lymphoid interstitial pneumonia • It can have both centrilobular and perilymphatic nodules. • Cyst s are present. • Ground glassing will be seen. Miliary TB post primary. Hypersensitivity pneumonitis HRCT through the upper lobes shows patchy areas of ground-glass opacity and ill- defined nodular opacities with a centrilobular predominance, HRCT at the lung base shows patchy ground-glass opacities. Focal areas of relative lucency represent mosaic perfusion. The combination of patchy ground-glass opacity and areas of lucency is termed the headcheese sign and is typical of hypersensitivity pneumonitis.
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