Technical aspect of hrct; normal lung anatomy & hrct findings of lung disease

HRCT- Technical aspect , normal lung anatomy and HRCT finding of lung disease

1. TECHNICAL ASPECT OF HRCT; NORMAL LUNG ANATOMY & HRCT FINDINGS OF LUNG DISEASE 1 Presenter : Dr. Sarbesh Tiwari PGT Radiodiagnosis Assam Medical College Dibrugarh

2. HRCT ------ MEANING It is often used for anything and everything to do with “high resolution”. Resolution : Means ability to resolve small object that are close together ,as separate form. Actual meaning A scan performed using high- spatial frequency algorithm to accentuate the contrast between tissue of widely differing densities, eg ., - air & vessels (lung) - air & bone (temporal & paranasal sinus) 2

3. INTRODUCTION HRCT -- Use of thin section CT images (0.625 to 2 mm slice thickness) often with a high-spatial-frequency reconstruction algorithm to detect and characterize disease affecting the pulmonary parenchyma and airways. Superior to chest radiography for detection of lung disease, points a specific diagnosis and helps in identification of reversible disease. 3

4. HISTORY 1982– The term HRCT was first used by TODO et. Al 1985 – Nakata et.al and Naidich et.al published first report on HRCT Since then has been an important tool in pulmonary medicine Recent development of MDCT scanner capable of volumetric high resolution scanning has improved the investigation 4

5. TECHNICAL ASPECT Parameters Slice thickness Kvp mAs Scan time FoV Interslice gap (collimation) Filming. 5

6. SLICE THICKNESS Thin sections 0.5 – 1.5 mm is essential for optimal spatial resolution Thicker slices are prone for volume averaging and reduces ability to resolve smaller structure Better for delineation of bronchi, wall thickness and diameter 6

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8. Reconstruction Algorithm Denotes the frequency at which the acquired scan data are recorded when creating the image. Using a high-resolution algorithm is critical element in performing HRCT. High spatial frequency or sharp algorithm -- bone algorithm is used which reduces image smoothing and better depicts normal and abnormal parenchymal interface. 8

9. Standard algorithm High resolution algorithm 9

10. Kilovolts (Peak), Milliamperes, and Scan Time In HRCT image, noise is more apparent than standard CT. Noise – 1/√ mAs X Kvp X scan time As increasing scan time is not feasible, mAs and Kvp are altered to reduce noise Noise decreases with increase in Kvp and mAs . 10

11. For routine technique – Kvp -- 120-140 mAs -- 200- 300 Increased patient and chest wall thickness are associated with increase image noise, may be reduced by increasing mA s and Kvp Scan Time : As low as possible (1-2 sec) to minimize motion artifact. 11

12. WINDOW SETTINGS Lung window Window level setting ranging from – 600 to – 700 HU and window widths of 1000 to 1500 HU are appropriate for a routine lung window. Soft tissue window Window level/width setting of 40-50/ 350-450 HU are best for evaluation of the mediastinum, hila, and pleura. 12

13. LOW DOSE HRCT Low dose HRCT uses Kvp of 120- 140 and mA of 30-20 at 2 sec scan time. Equivalent to conventional HRCT in 97 % of cases Disadvantage : Fails to identify GGO in few cases and have more prominent streak artifact. Not recommended for initial evaluation of patients with lung disease. Indicated in following up patients with a known lung abnormality or in screening large populations at risk for lung ds . 13

14. Matrix size, Field of View, and Target reconstruction Matrix size : Largest available matrix s/b used – 512 x 512 Field of view : smallest FOV that will encompass the patient is used as it will reduce the pixel size. (commonly 35 to 40) Retrospectively targeting image reconstruction to a single lung instead of the entire thorax significantly reduces the FOV and image pixel size, and thus increases spatial resolution. 14

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16. INTERSLICE GA P – varies from examination to examination, but is usually 10 - 20 mm INSPIRATORY LEVEL : Routine HRCT is obtained in suspended full inspiration, which optimizes contrast between normal structures, various abnormalities and normal aerated lung parenchyma; and reduces transient atelectasis , a finding that may mimic or obscure significant abnormalities. EXPIRATORY SCAN : valuable in obstructive lung disease or airway abnormality 16

17. Patient Position and the Use of Prone Scanning Supine adequate in most instances. Prone for diagnosing subtle lung abnormalities. e.g., asbestosis, suspected early lung fibrosis Prone scan is useful in differentiating dependent lung atelectasis from early lung fibrosis 17

18. Axial CT image shows opacity in the posterior part of the lung which could represent dependent opacity or pulmonary inflammation. The prone images shows complete resolution of the opacity suggesting dependent atelectasis. 18

19. Persistent opacity in the posterior lung in a patient with pulmonary fibrosis . 19

20. TECHNIQUE OF SCAN ACQUISITION : Spaced axial scans : O btained at 1cm intervals from lung apices to bases. In this manner, HRCT is intended to “sample ” lung anatomy It is assumed that the findings seen at the levels scanned will be representative of what is present throughout the lungs Results in low radiation dose as the individual scans are widely placed 20

21. 2. Volumetric HRCT - MDCT scanner are capable of rapid scanning and thin slice acquisition. Advantages : 1. Viewing of contagious slice for better delineation of lung abnormality 2. Complete imaging of lung and thorax 3. Reconstruction of scan data in any plane using MIPs or MinIPs . 4. diagnosis of other lung abnormalities Disadvantage : greater radiation dose. It delivers 3-5 times greater radiation. 21

22. Multidetector Helical HRCT Multidetector CT is equipped with a multiple row detector array Multiple images are acquired due to presence of multiple detectors Advantages : - shorter acquisition times and retrospective creation of both thinner and thicker sections from the same raw data  Acquisition time is so short that whole-lung HRCT can be performed in one breath-hold. 22

23. Which is better HRCT or MD- HRCT Various study shows the image quality of axial HRCT with multi-detector CT is equal to that with conventional single-detector CT. HRCT performed with spaced axial images results in low radiation dose as compared with MD-HRCT. Increased table speed may increase the volume-averaging artifact and may result in indistinctness of subtle pulmonary abnormalities.  MDCT provides for better reconstruction in Z axis 23

24. Radiation dose Annual background radiation ----- --- 2.5 mSv PA CHEST Radiograph ----- ----- ----- 0.05 mSv Spaced axial HRCT (10mm space) ----- 0.7 mSv ( 14 X ray) Spaced axial HRCT (20 mm space) ------ 0.35 mSv ( 7 X ray) Low Dose Spaced axial HRCT -------- 0.02 mSV MD-HRCT ---- ------- 4 - 7 msv ( 60-80 x ray) Combining HRCT scan at 20 mm interval with low mAs scan (40 mAs ) would result in radiation comparable to conventional X ray. 24

25. HRCT ARTIFACT Streak Artefacts : Fine, linear, or netlike opacities Radiate from the edges of sharply marginated , high-contrast structures such as bronchial walls, ribs, or vertebral bodies. More evident on low mA Mechanisms: beam hardening, photon starvation, and aliasing. 25

26. Motion-related artifacts Pulsation / Star artefacts Doubling artefacts. Stair-step artefacts in sag/ coro reconstruction. 26

27. MODIFICATION OF SCAN PROTOCOL Scan protocol can be modified in relation to disease or patients comfort. If a disease has basal predominance, it may be wise to begin scanning near the diaphragm and proceed cephalic . Caudad for disease with an upper-lobe predominance (e.g., sarcoidosis) An alternative approach - cephalad in all patients. 27

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29. Review of anatomy PART 2 29

30. LUNG ANATOMY Right lung is divided by major and minor fissure into 3 lobes and 10 broncho -pulmonary segments Left lung is divided by major fissure into 2 lobes with a lingular lobe and 8 bronchopulmonary segments 1.1 kg 30

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32. There are approximately 23 generation of dichotomous branching From trachea to the alveolar sac HRCT can identify upto 8 th order central bronchioles 32

33. TRACHEAL ANATOMY 10-12 cm in length, from C6 level to upper border of D5. Extrathoracic (2-4cm) and Intrathoracic (6-9 cm beyond manubrium ) In men , tracheal diameter – 25-27 mm women – 21- 23 mm The posterior portion of the tracheal wall is a thin fibromuscular membrane----- allows for oesophageal expansion. 33

34. BRONCHIAL ANATOMY Approximately 23 generations of branches from the trachea to the alveoli. Bronchi with a wall thickness of less than 300 um is not visible on CT or HRCT. As a consequence, normal bronchi less than 2 mm in diameter or closer than 2 cm from pleural surfaces equivalent to seventh to ninth order airways are generally below the resolution even of high-resolution CT 34

35. BRONCHUS BLOOD SUPPLY  Bronchial Arteries— 2 on left side i.e. superior and inferior 1 on right side Left arises from thoracic aorta Right from either thoracic aorta, sup. left bronchial or right 3 rd intercostal artery VENOUS DRAINAGE  on right- azygous vein on le ft- left superior intercostal or accessory hemiazygous vein NERVE SUPPLY  Pulmonary plexus at hilum ( vagus and sympathetic) 35

36. BRONCHOARTERIAL RATIO (B/A) Internal diameter of both bronchus and accompanying arterial diameter calculated and ratio measured. Normal ratio is 0.65-0.70 B/A ratio >1 indicates bronchiectasis . NB:: B/A ratio increases with age and may exceed 1 in normal patients > 40 years. 36

37. SECONDARY PULMONARY LOBULE Smallest lung unit that is surrounded by connective tissue septa (Miller) The basic anatomic unit Irregular polyhedral in shape. Measures 1 to 2.5 cm 37

38. Anatomy of the Secondary Lobule and Its Components Interlobular septa and contiguous subpleural interstitium, 2. Centrilobular structures, and 3. Lobular parenchyma and acini. 38

39. Interlobular septa and contiguous subpleural interstitium The secondary pulmonary lobule is marginated by septa which extends from the pleural surface. They measure 0.1 mm in thickness. They are less well defined in central lung Lobular core : The secondary lobule is supplied by arteries and bronchioles that measures approximately 1 mm in diameter. It consists of functioning lung parenchyma namely the alveoli, alveolar duct and vessels. The parenchyma is supported by network of central and peripheral fibers of interstitium. 39

40. PULMONARY ACINUS Portion of lung parenchyma supplied by a single respiratory Bronchiole. Size is 7 to 8 mm in adults 3 to 24 acini = Sec Pul . Lobule Primary Lobule : Lung parenchyma associated with a single Alveolar duct. 4-5 Primary Lobules  Acinus 40

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42. A group of terminal bronchioles 42

43. Accompanying pulmonary arterioles 43

44. Surrounded by lymph vessels 44

45. Pulmonary veins 45

46. Pulmonary lymphatics 46

47. 47 Connective Tissue Stroma

48. LUNG INTERSTITUM 48

49. The peribronchovascular interstitum invests the bronchi and pulmonary artery in the perihilar region. The centrilobular interstitium are associated with small centrilobular bronchioles and arteries The subpleural interstitium is located beneath the visceral pleura; envelops the lung into fibrous sac and sends connective tissue septa into lung parenchyma. Interlobular septa constitute the septas arising from the subpleural interstitium. LUNG INTERSTITUM 49

50. The normal pulmonary vein branches are seen marginating pulmonary lobules. The centrilobular artery branches are visible as a rounded dot 50

51. Anatomy of pleural surfaces and chest wall. 51

52. NORMAL LUNG ATTENUATION Normal lung attenuation : –700 to – 900 HU Attenuation gradient : densest at dependent region of lung as a result of regional difference in blood and gas density due to gravity Difference in attenuation of anterior and posterior lung ranges from 50 – 100 HU In children, lung attenuation is greater than adults. 52

53. NORMAL EXPIRATORY HRCT Performed to detect air trapping in small airway obstruction Attenuation increases with expiration (ranges from 100 to 130 HU) 60 % of normal individual shows air trapping in the superior segment of lower lobe and involving single lobule, normal variant. 53

54. PATTERN OF LUNG DISEASE IN HRCT PART 3 54

55. Q.1 . What is the dominant HR-pattern ? Q.2. Where is it located within the secondary lobule (centrilobular, Perilymphatic or random) ? Q.3. Is there an upper versus lower zone or a central versus peripheral predominance ? Q.4. Are there additional findings (pleural fluid, lymphadenopathy, traction bronchiectasis) ? STRUCTURED APPROACH 55

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57. LINEAR AND RETICULAR OPACITIES Represents thickening of interstitial fibers of lung by - fluid or - fibrous tissue or - infiltration by cells 57

58. Interface sign Irregular interfaces between the aerated lung parenchyma and bronchi, vessels, or visceral pleural surfaces. Represent thickened interlobular septa, intralobular lines, or irregular scars. Nonspecific. Common in patients with an interstitial abnormality, fibrotic lung disease . 58

59. Peribronchovascular Interstitial Thickening Venous, lymphatic or infiltrative disease lymphatic or infiltrative diseases 59

60. sarcoidosis UNILATERAL LYMPHANGITIC SPREAD OF CARCINOMA 60

61. INT ER LOBULAR SEPTAL THICKENING Normally, only a few septa seen On HRCT, if numerous interlobular septas are seen, it almost always indicate abnormality. Septal thickening d/t -interstitial fluid, cellular infiltration or fibrosis. The thickened interstitium outline the secondary pulmonary lobules and are perpendicular to the pleura. D/D are similar to that of PBIT. 61

62. Smooth Septal thickening Septal thickening and ground-glass opacity with a gravitational distribution in a patient with cardiogenic pulmonary edema. 62

63. Nodular Septal thickening Focal septal thickening in lymphangitic carcinomatosis Lymphangitic carcinomatosis : show diffuse smooth and nodular septal thickening. Sarcoidosis : right lung base shows interlobular septal thickening associated with several septal nodules giving beaded appearance 63

64. Int ra lobular interstitial thickening (Intralobular lines) Results in a fine reticular pattern on HRCT, with the visible lines separated by a few millimeters Fine lace- or netlike appearance Causes : Pulmonary fibrosis Asbestosis Chronic Eosinophilic pneumonitis. 64

65. PARENCHYMAL BANDS Non tapering , reticular opacity usually 1 to 3 mm in thickness and from 2 to 5 cm in length. Is often peripheral and generally contracts the pleural surface D/D : 1. Asbestosis 2. Sarcoidosis 3. Silicosis/ coal worker pneumoconiosis 4. Tuberculosis with associated scarring. 65

66. Subpleural Interstitial Thickening Mimic thickening of fissure. DD similar to that of interlobular septal thickening. more common than septal thickening in IPF or UIP of any cause. 66

67. HONEYCOMBING Defined as - small cystic spaces with irregularly thickened walls composed of fibrous tissue. Predominate in the peripheral and subpleural lung regions Subpleural honeycomb cysts typically occur in several contiguous layers. D/D- paraseptal emphysema in which subpleural cysts usually occur in a single layer. Indicates the presence of “END stage” disease regardless of the cause. 67

68. Causes Lower lobe predominance : 1 . UIP or interstitial fibrosis 2. Connective tissue disorders 3. Hypersensitivity pneumonitis 4. Asbestosis 5. NSIP (rare) Upper lobe predominance : 1. End stage sarcodosis 2. Radiation 3. Hypersensitivity Pneumonitis 4. End stage ARDS 68

69. Size, Distribution , Appearance Nodules and Nodular Opacities Size Small Nodules: <10 mm Miliary - <3 mm Large Nodules: >10 mm Masses - >3 cms Appearance Interstitial opacity : Well-defined, homogenous, Soft-tissue density Obscures the edges of vessels or adjacent structure Air space: Ill-defined , inhomogeneous. Less dense than adjacent vessel – GGO small nodule is difficult to identify 69

70. Interstitial nodules Air space opacity 70 Miliary tuberculosis sarcoidosis in a lung transplant patient with bronchopneumonia

71. RANDOM: no consistent relationship to any structures PERILYMPHATIC: corresponds to distribution of lymphatics CENTRILOBULAR: related to centrilobular structures Distribution 71

72. Perilymphatic distribution Nodules in relation to pulmonary lymphatics at # perihilar peribronchovascular interstitium, # interlobular septa, # subpleural regions, and # centrilobular interstitium. 72

73. Perilymphatic nodules: D/D Sarcoidosis Lymphangitic carcinomatosis Lymphocytic interstitial pneumonia (LIP) Lymphoproliferative disorders Amyloidosis 73

74. Centrilobular nodules Distributed primarily within the centre of the secondary pulmonary lobule Reflect the presence of either interstitial or airspace abnormalities Dense or ground-glass opacity Subpleural lung is typically spared- distinguishes from diffuse random nodules. 74

75. Tree-in-bud Centrilobular nodules m/b further characterized by presence or absence of ‘‘tree-in-bud.’’ Tree-in-bud -- Impaction of centrilobular bronchus with mucous, pus, or fluid, resulting in dilation of the bronchus, with associated peribronchiolar inflammation . Dilated, impacted bronchi produce Y- or V-shaped structures This finding is almost always seen with pulmonary infections. 75

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77. 77 Centrilobular nodules with or without tree-in-bud opacity: D/D : With tree-in-bud opacity Bacterial pneumonia Typical and atypical mycobacteria infections Bronchiolitis Diffuse panbronchiolitis Aspiration Allergic bronchopulmonary aspergillosis Cystic fibrosis Endobronchial- neoplasms (particularly Bronchioloalveolar carcinoma) Without tree-in-bud opacity All causes of centrilobular nodules with tree-in-bud opacity Hypersensitivity pneumonitis Respiratory bronchiolitis Cryptogenic organizing pneumonia Pneumoconioses Langerhans’ cell histiocytosis Pulmonary edema Vasculitis Pulmonary hypertension

78. 78 Random nodules Random nodules – N o definable distribution Are usually distributed uniformly throughout the lung parenchyma in a bilaterally symmetric distribution. Random nodules: Miliary tuberculosis. Axial HRCT image shows multiple nodules scattered uniformly throughout the lung parenchyma.

79. 79 Random nodules: D/D Haematogenous metastases Miliary tuberculosis Miliary fungal infection Disseminated viral infection Silicosis or coal-worker’s pneumoconiosis Langerhans’ cell histiocytosis

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81. Parenchymal Opacification Ground-glass opacity Consolidation Lung calcification & high attenuation opacities. 81

82. GROUND GLASS OPACITIES Hazy increased attenuation of lung, with preservation of bronchial and vascular margins Pathology : it is caused by # partial filling of air spaces, # interstitial thickening, # partial collapse of alveoli, # normal expiration, or # increased capillary blood volume D/t volume averaging of morphological abnormality too small to be resolved by HRCT 82

83. IMPORTANCE OF GGO Can represent - microscopic interstitial disease (alveolar interstitium) - microscopic alveolar space disease - combination of both In the absence of fibrosis, mostly indicates the presence of an ongoing, active, potentially treatable process NB :: Ground Glass opacity should be diagnosed only on scans obtained with thin sections : with thicker sections volume averaging is more - leading to spurious GGO, regardless of the nature of abnormality 83

84. DIFFERNTIAL DIAGNOSIS : GGO 84

85. The location of the abnormalities in ground glass pattern can be helpful : Upper zone predominance : Respiratory bronchiolitis PCP. Lower zone predominance : UIP, NSIP, DIP. Centrilobular distribution : Hypersensitivity pneumonitis, Respiratory bronchiolitis 85

86. GGO with few cystic and reticular lesion in HIV + ve patient -- PCP Combination of GGO with fibrosis and tractional bronchiectasis-- NSIP 86

87. CRAZY PAVING PATTERN It is scattered or diffuse ground-glass attenuation with superimposed interlobular septal thickening and intralobular lines. Causes: 87

88. Combination of ground glass opacity and septal thickening : Alveolar proteinosis. 88

89. 89 CONSOLIDATION Consolidation is defined as increased attenuation, which results in obscuration of the underlying vasculature, usually producing air bronchogram. The presence of consolidation implies that the air within affected alveoli has been replaced by another substance, such as blood, pus, oedema, or cells. When consolidation is evident on a chest radiograph, HRCT does not usually provide additional diagnostically useful information.

90. D/D on the basis of presentation Acute consolidation is seen in: - Pneumonias (bacterial, mycoplasma , PCP) - Pulmonary edema due to heart failure or ARDS - Hemorrhage - Acute eosinophilic pneumonia Chronic consolidation is seen in: - Organizing Pneumonia - Chronic eosinophilic pneumonia - Fibrosis in UIP and NSIP - Bronchoalveolar carcinoma or lymphoma 90

91. 91 Patchy ground-glass opacity, consolidation, and nodule mainly with peribronchovascular distribution with reversed halo signs (central ground-glass opacity and surrounding air-space consolidation) Peripheral consolidations with upper lobe predominance (photo negative of pulmonary edema)

92. Lung calcification & high attenuation opacities Multifocal lung calcification • Infectious granulomatous ds - TB, histoplasmosis, and varicella , pneumonia; • Sarcoidosis , silicosis, Amyloidosis Fat embolism associated with ARDS Diffuse & dense lung calcification • Metastatic calcification, • Disseminated pulmonary ossification, or • Alveolar microlithiasis 92

93. High attenuation opacity • Talcosis asso with fibrotic mass, • inhalation of metals (tin/barium) Small focal areas of increased attenuation • injection and embolized radiodense materials such as mercury or acrylic cement Diffuse, increased lung attn in absence of calcification • amiodarone lung toxicity or • embolization of iodinated oil after chemoembolization 93

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95. HRCT findings manifesting as decreased lung opacity Lung Cysts, Emphysema, and Bronchiectasis 95

96. Lung cysts Thin walled (less than 3mm) , well defined and circumscribed air containing lesions They are lined by cellular epithelium, usually fibrous or epithelial in nature. Common cause are : 1. Lymphangiomyomatosis 2. Langerhans Histiocytosis 3. Lymphoid interstitial pneumonia They need to be differentiated from emphysematous bullae , blebs and pneumatocele . 96

97. Axial HRCT image through the upper lobes shows multiple bilateral bizarre-shaped cysts and small centrilobular nodules in a smoker with Langerhans’ cell histiocytosis. Axial HRCT image through the upper lobes shows multiple bilateral uniform, thin-walled cysts. 97

98. BRONCHIEACTASIS Bronchiectasis is defined as localized, irreversible dilation of the bronchial tree. HRCT findings of the bronchiectasis include # Bronchial dilatation # Lack of bronchial tapering # Visualization of peripheral airways. 98

99. BRONCHIAL DILATATION # The broncho -arterial ratio (internal diameter of the bronchus /pulmonary artery) exceeds 1. # In cross section it appears as “Signet Ring appearance” LACK OF BRONCHIAL TAPERING # The earliest sign of cylindrical bronchiectasis # One indication is lack of change in the size of an airway over 2 cm after branching . VISUALIZATION OF PERIPHERAL AIRWAYS # Visualization of an airway within 1 cm of the costal pleura is abnormal and indicates potential bronchiectasis 99

100. Coned axial HRCT image shows bronchial dilation with lack of tapering . Bronchial morphology is consistent with varicose bronchiectasis. 100

101. A NUMBER OF ANCILLARY FINDINGS ARE ALSO RECOGNIZED : # Bronchial wall thickening : normally wall of bronchus should be less than half the width of the accompanying pulmonary artery branch. # Mucoid impaction # Air trapping and mosaic perfusion Extensive, bilateral mucoid impaction Mosaic perfusion caused by large and small airway obstruction. Small centrilobular nodules are visible in the right lower lobe 101

102. Types CYLINDRICAL BRONCHIECTASIS # mildest form of this disease, # thick-walled bronchi that extend into the lung periphery and fail to show normal tapering VARICOSE BRONCHIECTASIS # beaded appearance of bronchial walls - dilated bronchi with areas of relative narrowing # string of pearls. # Traction bronchiectasis often appears varicose. 102

103. 3. CYSTIC BRONCHIECTASIS : # Group or cluster of air-filled cysts, # cysts can also be fluid filled, giving the appearance of a cluster of grapes . 4.TRACTION BRONCHIECTASIS : # Defined as dilatation of intralobular bronchioles because of surrounding fibrosis # due to fibrotic lung diseases 103

104. Differential diagnosis 1. Infective causes : specially childhood pneumonia, pertusis, measles, tuberculosis 2. Non- infective causes : Bronchopulmonary aspergillosis, inhalation of toxic fumes 3. Connective tissue disorder : Ehlers- Danlos Synd , Marfan synd , tracheobronchomeglay 4. Ciliary diskinesia : Cystic fibrosis, Kartangener synd , agammaglobulinemia . 5. Tractional bronchiectasis in interstitial fibrosis. 104

105. EMPHYSEMA Permanent, abnormal enlargement of air spaces distal to the terminal bronchiole and accompanied by the destruction of the walls of the involved air spaces. 105

106. Centrilobular (proximal or centriacinar) emphysema Found most commonly in the upper lobes Manifests as multiple small areas of low attenuation without a perceptible wall, producing a punched-out appearance . Often the centrilobular artery is visible within the centre of these lucencies. 106

107. PANLOBULAR EMPHYSEMA Affects the entire secondary pulmonary lobule and is more pronounced in the lower zones Complete destruction of the entire pulmonary lobule. Results in an overall decrease in lung attenuation and a reduction in size of pulmonary vessels 107

108. Paraseptal (distal acinar) emphysema Affects the peripheral parts of the secondary pulmonary lobule Produces subpleural lucencies. 108

109. Cicatricial Emphysema/ irregular air space enlargement previously known as irregular or cicatricial emphysema • can be seen in association with fibrosis • with silicosis and progressive massive fibrosis or sarcoidosis BULLOUS EMPHYSEMA : D oes not represent a specific histological abnormality Emphysema characterized by large bullae Often associated with centrilobular and paraseptal emphysema 109

110. Paraseptal Emphysema vs Honeycombing Paraseptal emphysema Honeycomb cysts occur in a single layer at the pleural surface may occur in several layers in the subpleural lung predominate in the upper lobes predominate at the lung bases unassociated with significant fibrosis Asso with other findings of fibrosis. Associated with other findings of emphysema Absent 110

111. Bullae A sharply demarcated area of emphysema ≥ 1 cm in diameter a thin epithelialized wall ≤ 1 mm. uncommon as isolated findings, except in the lung apices Usually asso with evidence of extensive centrilobular or paraseptal emphysema When emphysema is associated with predominant bullae , it may be termed bullous emphysema 111

112. Pneumatocele Defined as a thin-walled, gas-filled space within the lung, Associated with acute pneumonia or hydrocarbon aspiration. • Often transient. • believed to arise from lung necrosis and bronchiolar obstruction. • Mimics a lung cyst or bulla on HRCT and cannot be distinguished on the basis of HRCT findings. 112

113. CAVITARY NODULE Thicker and more irregular walls than lung cysts • In diffuse lung diseases - LCH, TB, fungal infections, and sarcoidosis. Also seen in rheumatoid lung disease, septic embolism, pneumonia, metastatic tumor, tracheobronchial papillomatosis, and Wegener granulomatosis Cavitary nodules or cysts in tracheobronchial papillomatosis. fungal pneumonia 113

114. Mosaic attenuation & perfusion Lung density and attenuation depends partially on amount of blood in lung tissue. The term 'mosaic attenuation' is used to describe density differences between affected and non-affected lung areas. It is seen as inhomogeneous attenuation of lung parenchyma with focal region of lucency which show smaller size of vessels May be due to vascular obstruction, abnormal ventilation or airway disease/ 114

115. Mosaic attenuation due to small airway disease # Air trapping and bronchial dilatation commonly seen. # Areas of increased attenuation have relatively large vessels, while areas of decreased attenuation have small vessels. # Causes include: Bronchiectasis, cystic fibrosis and bronchiolitis obliterans. Mosaic attenuation due to vascular disease # common in patients with acute or chronic pulmonary embolism (CPE), and # decreased vessel size in less opaque regions is often visible 115

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117. Inhomogeneous lung opacity: mosaic perfusion in a patient with bronchiectasis. central bronchiectasis with multifocal, bilateral inhomogeneous lung opacity. The vessels within the areas of abnormally low attenuation are smaller than their counterparts in areas of normal lung attenuation. 117

118. Air trapping on expiration Most patients with air trapping seen on expiratory scans have inspiratory scan abnormalities, such as bronchiectasis, mosaic perfusion, airway thickening, or nodules suggest the proper differential diagnosis. Occasionally, air trapping may be the sole abnormal finding on an HRCT study. The differential diagnosis include --- bronchiolitis obliterans; asthma; chronic bronchitis; and hypersensitivity pneumonitis 118

119. Air trapping on expiratory imaging in the absence of inspiratory scan findings in a patient with bronchiolitis obliterans. Axial inspiratory image through the lower lobes shows no clear evidence of inhomogeneous lung opacity. Axial expiratory image shows abnormal low attenuation (arrows) caused by air trapping, representing failure of the expected increase in lung attenuation that should normally occur with expiratory imaging. 119

120. Head cheese sign It refers to mixed densities which includes presence of- # consolidation # ground glass opacities # normal lung # Mosaic perfusion Signifies mixed infiltrative and obstructive disease Common cause are : Hypersensitive pneumonitis Sarcoidosis DIP 120

121. Axial HRCT image in a patient with hypersensitivity pneumonitis shows a combination of ground-glass opacity, normal lung, and mosaic perfusion (arrow) on the same inspiratory image. 121

122. Distribution within the lung Upper lung zone preference is seen in: 1.Inhaled particles: pneumoconiosis (silica or coal) 2.Smoking related diseases (centrilobular emphysema 3. Respiratory bronchiolitis (RB-ILD) 4.Langerhans cell histiocytosis 5.Hypersensitivity pneumonitis 6.Sarcoidosis Lower zone preference is seen in: 1. UIP 2. Aspiration 3. Pulmonary edema 122

123. Central vs peripheral zone Central Zone Peripheral zone 1. Sarcoidosis 1. COP 2. Cardiogenic pulmonary 2. Ch Eosinophilic Pneumonia edema 3. UIP 3. Bronchitis 4. Hematogenous mets 123

124. Additional findings Pleural effusion  is seen in: Pulmonary edema Lymphangitic spread of carcinoma - often unilateral Tuberculosis Lymphangiomyomatosis (LAM) Asbestosis 124

125. Hilar and mediastinal lymphadenopathy # In sarcoidosis the common pattern is right paratracheal and bilateral hilar adenopathy ('1-2-3-sign'). # In lung carcinoma and lymphangitic carcinomatosis adenopathy is usually unilateral. #'Eggshell calcification' in lymph nodes occurs in ---- Silicosis and coal-worker's pneumoconiosis and is sometimes seen in sarcoidosis, post irradiation Hodgkin disease, blastomycosis and scleroderma . 125

126. Conclusion • A thorough knowledge of the basic anatomy is of utmost importance. When attempting to reach a diagnosis or differential diagnosis of lung disease using HRCT, the overall distribution of pulmonary abnormalities should be considered along with their morphology, HRCT appearance, and distribution relative to lobular structures. Correlation of the radiological findings with patients clinical and laboratory findings to reach a likely diagnosis 126

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