Fleischner Society: Glossary of Terms for Thoracic Imaging1 Ⅲ PCA REVIEW SPECIAL

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Note: This copy is for your personal, non-commercial use only. To order presentation-ready copies for distribution to your colleagues or clients, use the Radiology Reprints form at the end of this article. EIW N COMMENTARY AND REVIEWS Fleischner Society: Glossary of Terms for Thoracic Imaging1 Ⅲ PCA REVIEW SPECIAL

David M. Hansell, MD, FRCP, FRCR Members of the Fleischner Society compiled a glossary of Alexander A. Bankier, MD terms for thoracic imaging that replaces previous glossa- Heber MacMahon, MB, BCh, BAO ries published in 1984 and 1996 for thoracic radiography Theresa C. McLoud, MD and computed tomography (CT), respectively. The need to Nestor L. Mu¨ller, MD, PhD update the previous versions came from the recognition Jacques Remy, MD that new words have emerged, others have become obsolete, and the meaning of some terms has changed. Brief descriptions of some diseases are included, and pictorial examples (chest radiographs and CT scans) are provided for the majority of terms.

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1 From the Department of Radiology, Royal Brompton Hospital, Sydney Street, London SW3 6NP, United King- dom (D.M.H.); Department of Radiology, Beth Israel Dea- coness Medical Center, Boston, Mass (A.A.B.); Depart- ment of Radiology, University of Chicago Hospital, Chi- cago, Ill (H.M.); Department of Radiology, Massachusetts General Hospital, Boston, Mass (T.C.M.); Department of Radiology, Vancouver General Hospital, Vancouver, British Columbia, Canada (N.L.M.); and Department of Radiology, CHRU de Lille, Hoˆpital Calmette, Lille, France (J.R.). Re- ceived April 21, 2007; revision requested May 29; revision received June 6; accepted August 7; ﬁnal version accepted September 19. Address correspondence to: D.M.H. (e-mail: [email protected]).

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he present glossary is the third chiole and is supplied by first-order re- ducing a geographic appearance; dense prepared by members of the Fleis- spiratory bronchioles; it contains alveo- opacification is seen in the dependent Tchner Society and replaces the lar ducts and alveoli. It is the largest unit lung (Fig 1). In the organizing phase, glossaries of terms for thoracic radiol- in which all airways participate in gas architectural distortion, traction bron- ogy (1) and CT (2), respectively. The exchange and is approximately 6–10 chiectasis, cysts, and reticular opacities impetus to combine and update the pre- mm in diameter. One secondary pulmo- are seen (7). vious versions came from the recogni- nary lobule contains between three and tion that with the recent developments 25 acini (4). in imaging new words have arrived, oth- Radiographs and CT scans.—Indi- Figure 2 ers have become obsolete, and the vidual normal acini are not visible, but meaning of some terms has changed. acinar arteries can occasionally be iden- The intention of this latest glossary is tified on thin-section CT scans. Accumu- not to be exhaustive but to concentrate lation of pathologic material in acini on those terms whose meaning may be may be seen as poorly defined nodular problematic. Terms and techniques not opacities on chest radiographs and thin- used exclusively in thoracic imaging are section CT images. (See also nodules.) not included. Two new features are the inclusion Figure 1 of brief descriptions of the idiopathic interstitial pneumonias (IIPs) and pictorial examples (chest radiographs and computed tomographic [CT] scans) for the majority of terms. The decision to include vignettes of the IIPs (but not other pathologic entities) was based on the perception that, despite the recent Figure 2: Transverse CT scan shows air bron- scrutiny and reclassification, the IIPs re- chogram as air-filled bronchi (arrows) against main a confusing group of diseases. We background of high-attenuation lung. trust that the illustrations enhance, but do not distract from, the definitions. In this context, the figures should be regarded as of less importance than the air bronchogram text—they are merely examples and Radiographs and CT scans.—An air should not be taken as representing the bronchogram is a pattern of air-filled full range of possible imaging appear- (low-attenuation) bronchi on a back- ances (which may be found in the refer- ground of opaque (high-attenuation) air- ences provided in this glossary or in less lung (Fig 2). The sign implies (a) pa- comprehensive textbooks). Figure 1: Transverse CT scan in a patient with tency of proximal airways and (b) evacua- We hope that this glossary of terms acute interstitial pneumonia. tion of alveolar air by means of absorption will be helpful, and it is presented in the (atelectasis) or replacement (eg, pneu- spirit of the sentiment of Edward J. Huth monia) or a combination of these pro- that “scientific writing calls for precision acute interstitial pneumonia, or AIP cesses. In rare cases, the displacement as much in naming things and concepts as Pathology.—The term acute interstitial of air is the result of marked interstitial in presenting data” (3). It is right to re- pneumonia is reserved for diffuse alveo- expansion (eg, lymphoma) (8). peat the request with which the last Fleis- lar damage of unknown cause. The chner Society glossary closed: “[U]se of acute phase is characterized by edema words is inherently controversial and we and hyaline membrane formation. The are pleased to invite readers to offer im- later phase is characterized by airspace provements to our definitions” (2). and/or interstitial organization (5). The histologic pattern is indistinguishable from that of acute respiratory distress Glossary syndrome. Published online before print Radiographs and CT scans.—In the 10.1148/radiol.2462070712 acinus acute phase, patchy bilateral ground- Radiology 2008; 246:697–722 Anatomy.—The acinus is a structural glass opacities are seen (6), often with unit of the lung distal to a terminal bron- some sparing of individual lobules, pro- Authors stated no financial relationship to disclose.

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Figure 3 CT scans.—Air trapping is seen on aortopulmonary window end-expiration CT scans as parenchy- Anatomy.—The aortopulmonary win- mal areas with less than normal in- dow is the mediastinal region bounded crease in attenuation and lack of volume anteriorly by the ascending aorta, pos- reduction. Comparison between in- teriorly by the descending aorta, crani- spiratory and expiratory CT scans can ally by the aortic arch, inferiorly by the be helpful when air trapping is subtle or left pulmonary artery, medially by the diffuse (11,12) (Fig 4). Differentiation ligamentum arteriosum, and laterally by from areas of decreased attenuation re- the pleura and left lung (15,16). sulting from hypoperfusion as a conse- Radiographs and CT scans.—Focal quence of an occlusive vascular disorder concavity in the left mediastinal border (eg, chronic thromboembolism) may be below the aorta and above the left pul- problematic (13), but other findings of monary artery can be seen on a frontal airways versus vascular disease are usu- radiograph (Fig 5). Its appearance may ally present. (See also mosaic attenua- be modified by tortuosity of the aorta. tion pattern.) The aortopulmonary window is a common site of lymphadenopathy in a vari- Figure 3: Magnified chest radiograph shows airspace air crescent (arrows) adjacent to mycetoma. ety of inflammatory and neoplastic dis- Anatomy.—An airspace is the gas-con- eases. taining part of the lung, including the respiratory bronchioles but excluding air crescent purely conducting airways, such as ter- Figure 6 Radiographs and CT scans.—An air minal bronchioles. crescent is a collection of air in a cres- Radiographs and CT scans.—This centic shape that separates the wall of a term is used in conjunction with consol- cavity from an inner mass (Fig 3). The idation, opacity, and nodules to desig- air crescent sign is often considered nate the filling of airspaces with the characteristic of either Aspergillus colo- products of disease (14). nization of preexisting cavities or retraction of infarcted lung in angioinva- sive aspergillosis (9,10). However, the Figure 5 air crescent sign has also been reported in other conditions, including tuberculosis, Wegener granulomatosis, intracavi- tary hemorrhage, and lung cancer. (See also mycetoma.) Figure 6: Magnified chest radiograph shows apical cap (arrow).

Figure 4 apical cap Pathology.—An apical cap is a caplike lesion at the lung apex, usually caused by intrapulmonary and pleural fibrosis pulling down extrapleural fat (17) or possibly by chronic ischemia resulting in hyaline plaque formation on the visceral pleura (18). The prevalence increases with age. It can also be seen in hema- Figure 4: Transverse CT scans at end inspira- toma resulting from aortic rupture or in tion and end expiration show air trapping. Figure 5: Magnified chest radiograph shows other fluid collection associated with in- aortopulmonary window. fection or tumor, either outside the parietal pleura or loculated within the air trapping pleural space (19). Pathophysiology.—Air trapping is re- Radiographs and CT scans.—The tention of air in the lung distal to an usual appearance is of homogeneous obstruction (usually partial). soft-tissue attenuation capping the ex-

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treme lung apex (uni- or bilaterally), atelectasis Radiographs and CT scans.—On a with a sharp or irregular lower border Pathophysiology.—Atelectasis is re- frontal chest radiograph, the recess is (Fig 6). Thickness is variable, ranging duced inflation of all or part of the lung seen as a vertically oriented interface up to about 30 mm (17). An apical cap (20). One of the commonest mechanisms between the right lower lobe and the occasionally mimics apical consolidation is resorption of air distal to airway ob- adjacent mediastinum (the medial limit on transverse CT scans. struction (eg, an endobronchial neo- of the recess). Superiorly, the interface plasm) (21). The synonym collapse is of- is seen as a smooth arc with convexity Figure 7 ten used interchangeably with atelectasis, to the left. Disappearance or distortion particularly when it is severe or accompa- of part of the interface suggests disease nied by obvious increase in lung opacity. (eg, subcarinal lymphadenopathy). On Radiographs and CT scans.—Re- CT scans, the recess (Fig 9) merits at- duced volume is seen, accompanied by tention because small lesions located in increased opacity (chest radiograph) or the recess will often be invisible on attenuation (CT scan) in the affected chest radiographs (23). part of the lung (Fig 8). Atelectasis is often associated with abnormal dis- azygos fissure placement of fissures, bronchi, vessels, See fissure. diaphragm, heart, or mediastinum (22). The distribution can be lobar, segmental, or subsegmental. Atelectasis is often Figure 10 Figure 7: Transverse CT scan shows architec- qualified by descriptors such as linear, tural distortion caused by pulmonary fibrosis. discoid, or platelike. (See also linear atelectasis, rounded atelectasis.) architectural distortion Pathology.—Architectural distortion is Figure 9 characterized by abnormal displacement of bronchi, vessels, fissures, or septa caused by diffuse or localized lung disease, particularly interstitial fibrosis. CT scans.—Lung anatomy has a distorted appearance and is usually associated with pulmonary fibrosis (Fig 7) and accompanied by volume loss. Figure 10: Transverse CT scan shows beaded septum sign (arrows). Figure 8 beaded septum sign CT scans.—This sign consists of irregular and nodular thickening of interlobu- Figure 9: Transverse CT scan shows azygo- lar septa reminiscent of a row of beads esophageal recess (arrows). (Fig 10). It is frequently seen in lymphangitic spread of cancer and less often in sarcoidosis (24).

azygoesophageal recess bleb Anatomy.—The azygoesophageal re- Anatomy.—A bleb is a small gas-con- cess is a right posterior mediastinal re- taining space within the visceral pleura Figure 8: Transverse CT scan shows atelecta- cess into which the edge of the right or in the subpleural lung, not larger than sis of right middle lobe as increased attenuation lower lobe extends. It is limited superi- 1 cm in diameter (25). (arrows) adjacent to right border of heart. orly by the azygos arch, posteriorly by CT scans.—A bleb appears as a the azygos vein and pleura anterior to thin-walled cystic air space contiguous the vertebral column, and medially by with the pleura. Because the arbitrary the esophagus and adjacent structures. (size) distinction between a bleb and

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bulla is of little clinical importance, the Radiographs and CT scans.—Bron- scans. (See also small-airways disease, use of this term by radiologists is dis- chioles are not identiﬁable in healthy tree-in-bud pattern.) couraged. individuals, because the bronchiolar walls are too thin (4). In inﬂammatory small-airways disease, however, thick- Figure 13 Figure 11 ened or plugged bronchioles may be seen as a nodular pattern on a chest radiograph or as a tree-in-bud pattern on CT scans.

Figure 12

Figure 11: Transverse CT scan shows varicose Figure 13: Coronal CT scan shows bron- bronchiectasis. chocele (arrow). bronchiectasis bronchocele Pathology.—Bronchiectasis is irrevers- Figure 12: Transverse CT scan shows bronchi- Pathology.—A bronchocele is bronchial ible localized or diffuse bronchial dilata- olectasis within fibrotic lung (arrow). dilatation due to retained secretions tion, usually resulting from chronic in- (mucoid impaction) usually caused by fection, proximal airway obstruction, or proximal obstruction, either congenital congenital bronchial abnormality (26). bronchiolectasis (eg, bronchial atresia) or acquired (eg, (See also traction bronchiectasis.) Pathology.—Bronchiolectasis is defined obstructing cancer) (34). Radiographs and CT scans.—Mor- as dilatation of bronchioles. It is caused Radiographs and CT scans.—A phologic criteria on thin-section CT by inflammatory airways disease (po- bronchocele is a tubular or branching Y- scans include bronchial dilatation with tentially reversible) or, more fre- or V-shaped structure that may resem- respect to the accompanying pulmonary quently, fibrosis. ble a gloved finger (Fig 13). The CT artery (signet ring sign), lack of tapering CT scans.—When dilated bronchi- attenuation of the mucus is generally of bronchi, and identification of bronchi oles are filled with exudate and are thick that of soft tissue but may be modified within 1 cm of the pleural surface (27) walled, they are visible as a tree-in-bud by its composition (eg, high-attenuation (Fig 11). Bronchiectasis may be classi- pattern or as centrilobular nodules material in allergic bronchopulmonary fied as cylindric, varicose, or cystic, de- (31,32). In traction bronchiolectasis, aspergillosis). In the case of bronchial pending on the appearance of the af- the dilated bronchioles are seen as atresia, the surrounding lung may be of fected bronchi. It is often accompanied small, cystic, tubular airspaces, associ- decreased attenuation because of re- by bronchial wall thickening, mucoid ated with CT findings of fibrosis (Fig duced ventilation and, thus, perfusion. impaction, and small-airways abnormal- 12). (See also traction bronchiectasis ities (27–29). (See also signet ring sign.) and traction bronchiolectasis, tree-in- bud pattern.) bronchiole Anatomy.—Bronchioles are non–carti- bronchiolitis lage-containing airways. Terminal bron- Pathology.—Bronchiolitis is bronchio- chioles are the most distal of the purely lar inflammation of various causes (33). conducting airways; they give rise to re- CT scans.—This direct sign of bron- spiratory bronchioles, from which the chiolar inflammation (eg, infectious alveoli arise and permit gas exchange. cause) is most often seen as the tree-in- Respiratory bronchioles branch into bud pattern, centrilobular nodules, and multiple alveolar ducts (30). bronchiolar wall thickening on CT

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Figure 14 broncholith bullous emphysema Pathology.—A broncholith, a calcified Pathology.—Bullous emphysema is bul- peribronchial lymph node that erodes lous destruction of the lung paren- into an adjacent bronchus, is most often chyma, usually on a background of para- the consequence of Histoplasma or tu- septal or panacinar emphysema. (See berculous infection. also emphysema, bulla.) Radiographs and CT scans.—The imaging appearance is of a small calcific Figure 17 focus in or immediately adjacent to an airway (Fig 15), most frequently the right middle lobe bronchus. Broncho- liths are readily identified on CT scans (38). Distal obstructive changes may include atelectasis, mucoid impaction, and bronchiectasis.

Figure 16

Figure 14: Transverse CT scan shows consoli- Figure 17: Transverse CT scan shows cavitat- dation with bronchocentric distribution. ing mass in right upper lobe. bronchocentric cavity CT scans.—This descriptor is applied to Radiographs and CT scans.—A cavity is disease that is conspicuously centered a gas-ﬁlled space, seen as a lucency or on macroscopic bronchovascular bun- low-attenuation area, within pulmonary dles (Fig 14). Examples of diseases with consolidation, a mass, or a nodule (Fig a bronchocentric distribution include 17). In the case of cavitating consolida- sarcoidosis (35), Kaposi sarcoma (36), tion, the original consolidation may re- solve and leave only a thin wall. A cavity and organizing pneumonia (37). Figure 16: Coronal CT scan shows large bulla is usually produced by the expulsion or in left lower lung zone. Figure 15 drainage of a necrotic part of the lesion via the bronchial tree. It sometimes contains a ﬂuid level. Cavity is not a syn- bulla onym for abscess. Pathology.—An airspace measuring more than 1 cm—usually several centi- centrilobular meters—in diameter, sharply demar- Anatomy.—Centrilobular describes the cated by a thin wall that is no greater region of the bronchiolovascular core of than 1 mm in thickness. A bulla is usu- a secondary pulmonary lobule (4,39,40). ally accompanied by emphysematous This term is also used by pathologists to changes in the adjacent lung. (See also describe the location of lesions beyond bullous emphysema.) the terminal bronchiole that center on Radiographs and CT scans.—A bulla respiratory bronchioles or even alveolar appears as a rounded focal lucency or ducts. area of decreased attenuation, 1 cm or CT scans.–A small dotlike or linear more in diameter, bounded by a thin opacity in the center of a normal sec- wall (Fig 16). Multiple bullae are often ondary pulmonary lobule, most obvious present and are associated with other within 1 cm of a pleural surface, repre- signs of pulmonary emphysema (centri- sents the intralobular artery (approxi- lobular and paraseptal). mately 1 mm in diameter) (41). Centri- Figure 15: Transverse CT scan shows a bron- lobular abnormalities include (a) nod- cholith (arrows). ules, (b) a tree-in-bud pattern indicating small-airways disease, (c) increased vis-

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Figure 21 ibility of centrilobular structures due to consolidation thickening or inﬁltration of the adjacent Pathology.—Consolidation refers to an interstitium, or (d) abnormal areas of exudate or other product of disease that low attenuation caused by centrilobular replaces alveolar air, rendering the lung emphysema (4). (See also lobular core solid (as in infective pneumonia). structures.) Radiographs and CT scans.—Con- solidation appears as a homogeneous Figure 18 increase in pulmonary parenchymal attenuation that obscures the margins of vessels and airway walls (45) (Fig 19). An air bronchogram may be present. The attenuation characteristics of con- solidated lung are only rarely helpful in differential diagnosis (eg, decreased attenuation in lipoid pneumonia [46] and increased in amiodarone toxicity [47]).

Figure 18: Transverse CT scan shows centri- Figure 21: Coronal CT scan shows a cyst. lobular emphysema. Figure 20

cyst centrilobular emphysema Pathology.—A cyst is any round circum- Pathology.—Centrilobular emphysema scribed space that is surrounded by an is characterized by destroyed centri- epithelial or fibrous wall of variable lobular alveolar walls and enlargement thickness (51). of respiratory bronchioles and associ- Radiographs and CT scans.—A cyst ated alveoli (42,43). This is the com- appears as a round parenchymal lu- monest form of emphysema in cigarette cency or low-attenuating area with a smokers. well-defined interface with normal lung. CT scans.—CT findings are centri- Cysts have variable wall thickness but Figure 20: Transverse CT scan shows crazy- lobular areas of decreased attenuation, Ͻ paving pattern. are usually thin-walled ( 2 mm) and usually without visible walls, of nonuni- occur without associated pulmonary form distribution and predominantly lo- emphysema (Fig 21). Cysts in the lung cated in upper lung zones (44) (Fig 18). usually contain air but occasionally con- The term centriacinar emphysema is crazy-paving pattern tain fluid or solid material. The term is synonymous. (See also emphysema.) CT scans.—This pattern appears as often used to describe enlarged thin- thickened interlobular septa and in- walled airspaces in patients with lym- collapse tralobular lines superimposed on a phangioleiomyomatosis (52) or Langer- See atelectasis. background of ground-glass opacity (Fig hans cell histiocytosis (53); thicker- 20), resembling irregularly shaped pav- walled honeycomb cysts are seen in ing stones. The crazy-paving pattern is patients with end-stage fibrosis (54). Figure 19 often sharply demarcated from more (See also bleb, bulla, honeycombing, normal lung and may have a geographic pneumatocele.) outline. It was originally reported in patients with alveolar proteinosis (48) and is also encountered in other diffuse lung diseases (49) that affect both the interstitial and airspace compartments, such as lipoid pneumonia (50).

cryptogenic organizing pneumonia, or COP See organizing pneumonia. Figure 19: Transverse CT scan shows multifo- cal consolidation.

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Figure 22 Figure 23 tal (paraseptal emphysema), or whole acinus (panacinar or, less commonly, panlobular emphysema). CT scans.—The CT appearance of emphysema consists of focal areas or regions of low attenuation, usually without visible walls (58). In the case of panacinar emphysema, decreased attenuation is more diffuse. (See also bullous emphysema, centrilobular emphysema, panacinar emphysema, paraseptal emphysema.) Figure 22: Transverse CT scan in a patient with Figure 23: Transverse CT scan shows ground- desquamative interstitial pneumonia. fissure glass opacity. Anatomy.—A fissure is the infolding of visceral pleura that separates one lobe desquamative interstitial pneumonia, or DIP or part of a lobe from another; thus, the ground-glass opacity Pathology.—Histologically, DIP is char- interlobar fissures are produced by two Radiographs and CT scans.—On chest acterized by the widespread accumula- layers of visceral pleura. Supernumer- radiographs, ground-glass opacity ap- tion of an excess of macrophages in the ary fissures usually separate segments pears as an area of hazy increased lung distal airspaces. The macrophages are rather than lobes. The azygos fissure, opacity, usually extensive, within which uniformly distributed, unlike in respira- unlike the other fissures, is formed by margins of pulmonary vessels may be tory bronchiolitis–interstitial lung dis- two layers each of visceral and parietal indistinct. On CT scans, it appears as ease, in which the disease is conspicu- pleura. All fissures (apart from the azy- hazy increased opacity of lung, with ously bronchiolocentric. Interstitial in- gos fissure) may be incomplete. preservation of bronchial and vascular volvement is minimal. Most cases of DIP Radiographs and CT scans.—Fis- margins (Fig 23). It is caused by partial are related to cigarette smoking, but a sures appear as linear opacities, nor- filling of airspaces, interstitial thicken- few are idiopathic or associated with mally 1 mm or less in thickness, that ing (due to fluid, cells, and/or fibrosis), rare inborn errors of metabolism (5). correspond in position and extent to the partial collapse of alveoli, increased Radiographs and CT scans.— anatomic fissural separation of pulmo- capillary blood volume, or a combina- Ground-glass opacity is the dominant nary lobes or segments. Qualifiers in- tion of these, the common factor being abnormality and tends to have a basal clude minor, major, horizontal, oblique, the partial displacement of air (59,60). and peripheral distribution (Fig 22). Mi- accessory, anomalous, azygos, and infe- Ground-glass opacity is less opaque crocystic or honeycomb changes in the rior accessory. than consolidation, in which broncho- area of ground-glass opacity are seen in vascular margins are obscured. (See some cases (55). folded lung also consolidation.) See rounded atelectasis. diffuse alveolar damage, or DAD See acute interstitial pneumonia. fungus ball See mycetoma. emphysema Pathology.—Emphysema is character- gas trapping ized by permanently enlarged airspaces See air trapping. distal to the terminal bronchiole with destruction of alveolar walls (42,43). ground-glass nodule Absence of “obvious fibrosis” was his- See nodule. torically regarded as an additional criterion (42), but the validity of that criterion has been questioned because some interstitial fibrosis may be present in emphysema secondary to cigarette smoking (56,57). Emphysema is usually classified in terms of the part of the acinus predominantly affected: proximal (centriacinar, more commonly termed centrilobular, emphysema), dis-

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Figure 24 Figure 25 Figure 26

Figure 25: Transverse CT scan shows Figure 26: Coronal CT scan shows reticular honeycombing. opacities and honeycombing in the lower zones, typical of idiopathic pulmonary ﬁbrosis.

honeycombing Pathology.—Honeycombing represents idiopathic pulmonary fibrosis destroyed and fibrotic lung tissue con- Pathology.—Idiopathic pulmonary fi- taining numerous cystic airspaces with brosis is a specific form of chronic fibro- Figure 24: Transverse CT scan shows several thick fibrous walls, representing the late sing interstitial pneumonia of unknown nodules exhibiting the halo sign (arrows). stage of various lung diseases, with cause and is characterized by a histo- complete loss of acinar architecture. logic pattern of usual interstitial pneu- The cysts range in size from a few milli- monia (5,64). halo sign meters to several centimeters in diame- Radiographs and CT scans.—The CT scans.—The halo sign is a CT finding ter, have variable wall thickness, and typical imaging findings are reticular of ground-glass opacity surrounding a are lined by metaplastic bronchiolar ep- opacities and honeycombing, with a nodule or mass (Fig 24). It was first ithelium (51). predominantly peripheral and basal dis- described as a sign of hemorrhage Radiographs and CT scans.—On tribution (Fig 26). Ground-glass opac- around foci of invasive aspergillosis chest radiographs, honeycombing ap- ity, if present, is less extensive than re- (61). The halo sign is nonspecific and pears as closely approximated ring ticular and honeycombing patterns. The may also be caused by hemorrhage as- shadows, typically 3–10 mm in diameter typical radiologic findings (65,66) are sociated with other types of nodules with walls 1–3 mm in thickness, that also encountered in usual interstitial (62) or by local pulmonary infiltration resemble a honeycomb; the finding im- pneumonia secondary to specific causes, by neoplasm (eg, adenocarcinoma). plies end-stage lung disease. On CT such as asbestos-induced pulmonary fi- (See also reversed halo sign.) scans, the appearance is of clustered brosis (asbestosis), and the diagnosis is cystic air spaces, typically of compara- usually one of exclusion. (See also usual hilum ble diameters on the order of 3–10 mm interstitial pneumonia.) Anatomy.—Hilum is a generic term that but occasionally as large as 2.5 cm (Fig describes the indentation in the surface 25). Honeycombing is usually subpleu- of an organ, where vessels and nerves ral and is characterized by well-defined connect with the organ. It is the site on walls (54). It is a CT feature of estab- the medial aspect of the lung where the lished pulmonary fibrosis (5). Because vessels and bronchi enter and leave the honeycombing is often considered spe- lung. cific for pulmonary fibrosis and is an Radiographs and CT scans.—A hi- important criterion in the diagnosis of lum appears as a composite opacity at usual interstitial pneumonia (63), the the root of each lung produced by bron- term should be used with care, as it may chi, arteries, veins, lymph nodes, directly impact patient care. nerves, and other tissue. The terms hilum (singular) and hila (plural) are preferred to hilus and hili respectively; the adjectival form is hilar.

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Figure 27 Figure 28 Figure 29

Figure 28: Transverse CT scan shows interlobular septal thickening and pleural effusions. Figure 27: Transverse CT scan shows pulmonary infarction. interlobular septal thickening Radiographs and CT scans.—This find- Figure 29: Transverse CT scan shows interlob- infarction ing is seen on chest radiographs as thin ular septum (arrow) in a healthy individual. Pathology.—Infarction is a process that linear opacities at right angles to and in may result in ischemic necrosis, usually contact with the lateral pleural surfaces the consequence of vascular compro- near the lung bases (Kerley B lines); it is interlobular septum mise such as occlusion of a feeding pul- seen most frequently in lymphangitic Anatomy.—Interlobular septa are sheet- monary artery by an embolus (venous spread of cancer or pulmonary edema. like structures 10–20-mm long that infarction is rare but recognized). Ne- Kerley A lines are predominantly situ- form the borders of lobules; they are crosis is relatively uncommon because ated in the upper lobes, are 2–6 cm more or less perpendicular to the pleura tissue viability is maintained by the long, and can be seen as fine lines radi- in the periphery. Interlobular septa are bronchial arterial blood supply. Pulmo- ally oriented toward the hila. In recent composed of connective tissue and con- nary infarction may be secondary to a years, the anatomically descriptive tain lymphatic vessels and pulmonary vasculitis (eg, Wegener granulomato- terms septal lines and septal thickening venules. sis). have gained favor over Kerley lines. On Radiographs and CT scans.—Inter- Radiographs and CT scans.—A pul- CT scans, disease affecting one of the lobular septa appear as thin linear opac- monary infarct is typically triangular or components of the septa (see interlobu- ities between lobules (Fig 29); these dome-shaped, with the base abutting lar septum) may be responsible for septa are to be distinguished from cen- the pleura and the apex directed toward thickening and so render septa visible. trilobular structures. They are not usu- the hilum (Fig 27). The opacity repre- On thin-section CT scans, septal thick- ally seen in the healthy lung (normal sents local hemorrhage with or without ening may be smooth or nodular (70) septa are approximately 0.1 mm thick) central tissue necrosis (67,68). (Fig 28), which may help refine the dif- but are clearly visible when thickened ferential diagnosis. (See also interlobu- (eg, by pulmonary edema). (See also infiltrate lar septum, beaded septum.) interlobular septal thickening, lobule.) Radiographs and CT scans.—Formerly used as a term to describe a region of pulmonary opacification caused by airspace or interstitial disease seen on radiographs and CT scans. Infiltrate remains controversial because it means different things to different people (69). The term is no longer recommended, and has been largely replaced by other descriptors. The term opacity, with rel- evant qualifiers, is preferred.

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Figure 30 Figure 31 juxtaphrenic peak Radiographs and CT scans.—A juxtaphrenic peak is a small triangular opacity based at the apex of the dome of a hemidiaphragm, associated with upper lobe volume loss of any cause (eg, po- stirradiation ﬁbrosis or upper lobec- tomy) (74). It is most readily appreci- ated on a frontal chest radiograph (Fig 32). The peak is caused by upward retraction of the inferior accessory ﬁssure (75) or an intrapulmonary septum associated with the pulmonary ligament (76).

Figure 30: Transverse CT scan shows interstitial emphysema (arrow). Figure 33 interstitial emphysema Figure 31: Transverse CT scan shows ln- tralobular lines. Pathology.—Interstitial emphysema is characterized by air dissecting within the interstitium of the lung, typically in the peribronchovascular sheaths, inter- intralobular lines lobular septa, and visceral pleura. It is CT scans.—Intralobular lines are visible most commonly seen in neonates re- as fine linear opacities in a lobule when ceiving mechanical ventilation. the intralobular interstitial tissue is ab- Radiographs and CT scans.—Inter- normally thickened (Fig 31). When nu- stitial emphysema is rarely recognized merous, they may appear as a fine retic- radiographically in adults and is infre- ular pattern. Intralobular lines may be quently seen on CT scans (Fig 30). It seen in various conditions, including in- Figure 33: Chest radiograph shows basal lin- appears as perivascular lucent or low- terstitial fibrosis and alveolar proteino- ear atelectasis. attenuating halos and small cysts sis (41). (71,72). interstitium Figure 32 linear atelectasis Anatomy.—The interstitium consists of Radiographs and CT scans.—Linear at- a continuum of connective tissue electasis is a focal area of subsegmental throughout the lung comprising three atelectasis with a linear configuration, subdivisions: (a) the bronchovascular almost always extending to the pleura (axial) interstitium, surrounding and (74). It is commonly horizontal but supporting the bronchi, arteries, and sometimes oblique or vertical. The veins from the hilum to the level of the thickness of the atelectasis may range respiratory bronchiole; (b) the paren- from a few millimeters to more than 1 chymal (acinar) interstitium, situated cm (Fig 33). Linear atelectasis is also between alveolar and capillary base- referred to as discoid or platelike atel- ment membranes; and (c) the subpleu- ectasis. (See also atelectasis.) ral connective tissue contiguous with lobe the interlobular septa (73). Figure 32: Chest radiograph shows juxta- Anatomy.—The lobe is the principal di- phrenic peak of the right hemidiaphragm. vision of the lungs (normally, three lobes on the right and two on the left); each lobe is enveloped by visceral pleura, except at the lung root (hilum) and when an interlobar fissure is incomplete.

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Figure 34 lobule lymphadenopathy Anatomy.—The lobule is the smallest Pathology.—By common usage, the unit of lung surrounded by connective- term lymphadenopathy is usually re- tissue septa, as deﬁned by Miller (78) stricted to enlargement, due to any and Heitzman et al (40). The lobule is cause, of the lymph nodes. Synonyms also referred to as the secondary pul- include lymph node enlargement (pre- monary lobule; it contains a variable ferred) and adenopathy. number of acini, is irregularly polyhe- CT scans.—There is a wide range in dral in shape, and varies in size from 1.0 the size of normal lymph nodes. Medias- to 2.5 cm in diameter. The centrilobular tinal and hilar lymph nodes range in size structures, or core structures, include from sub-CT resolution to 12 mm. bronchioles and their accompanying Somewhat arbitrary thresholds for the pulmonary arterioles and lymphatic ves- upper limit of normal of 1 cm in short- sels. The connective-tissue septa sur- axis diameter for mediastinal nodes rounding the pulmonary lobule—the in- (79) and 3 mm for most hilar nodes (80) Figure 34: Transverse CT scan shows lobular terlobular septa, which contain veins have been reported, but size criteria do core structure (arrow). and lymphatic vessels—are best devel- not allow reliable differentiation be- oped in the periphery in the anterior, tween healthy and diseased lymph lateral, and juxtamediastinal regions of nodes (Fig 36). lobular core structures the upper and middle lobes. Anatomy.—Lobular core structures are CT scans.—On thin-section CT the central structures in secondary pul- scans, the three basic components of Figure 37 monary lobules and consist of a centri- the lobule—the interlobular septa and lobular artery and bronchiole (40). septal structures, the central lobular re- CT scans.—The pulmonary artery gion (centrilobular structures), and the and its immediate branches are visible lobular parenchyma—can be identiﬁed, in the center of a secondary lobule on particularly in disease states. Peripheral thin-section CT scans, particularly if lobules are more uniform in appearance thickened (eg, by pulmonary edema) and pyramidal in shape than are central (Fig 34). These arteries measure ap- lobules (4) (Fig 35). (See also interlobu- proximately 0.5–1.0 mm in diameter. lar septa, lobular core structures.) However, the normal bronchiole in the center of the secondary pulmonary lobule cannot be seen on thin-section CT Figure 36 scans because of the thinness of its wall (approximately 0.15 mm) (4,41). (See also centrilobular, lobule.)

Figure 35

Figure 37: Transverse CT scan shows ground- glass opacities and perivascular cysts in a patient with lymphoid interstitial pneumonia.

lymphoid interstitial pneumonia, or LIP Pathology.—LIP is a rare disease characterized by diffuse pulmonary lymphoid proliferation with predominant interstitial involvement. It is included in Figure 36: Transverse CT scan shows lymph- the spectrum of interstitial pneumonias adenopathy (enlarged mediastinal lymph nodes). and is distinct from diffuse lymphomas of the lung. Features include diffuse hy- Figure 35: CT scan of resected lung shows perplasia of bronchus-associated lym- lobules. phoid tissue and diffuse polyclonal lym-

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Figure 39 phoid cell infiltrates surrounding the compartment models are the most com- airways and expanding the lung intersti- monly used. tium. LIP is usually associated with au- toimmune diseases or human immuno- micronodule deficiency virus infection (5,81). CT scans.—A micronodule is a discrete, CT scans.—Ground-glass opacity is small, round, focal opacity. A variety of the dominant abnormality, and thin- diameters have been used in the past to walled perivascular cysts may be define a micronodule; for example, a present (Fig 37). Lung nodules, a retic- diameter of no greater than 7 mm (86). ular pattern, interlobular septal and Use of the term is most often limited to bronchovascular thickening, and wide- nodules with a diameter of less than 5 spread consolidation may also occur mm (87) or less than 3 mm (88). It is Figure 39: Transverse CT scan shows mosaic (82,83). recommended that the term be re- attenuation pattern caused by obliterative small- served for opacities less than 3 mm in airways disease. mass diameter. (See also nodule, miliary pat- Radiographs and CT scans.—A mass is tern.) any pulmonary, pleural, or mediastinal mosaic attenuation pattern lesion seen on chest radiographs as an CT scans.—This pattern appears as opacity greater than 3 cm in diameter Figure 38 patchwork of regions of differing atten- (without regard to contour, border, or uation that may represent (a) patchy density characteristics). Mass usually interstitial disease, (b) obliterative small- implies a solid or partly solid opacity. airways disease (Fig 39), or (c) occlu- CT allows more exact evaluation of size, sive vascular disease (89). Mosaic at- location, attenuation, and other fea- tenuation pattern is a more inclusive tures. (See also nodule.) term than the original terms mosaic oligemia and perfusion (90). Air trapping mediastinal compartments secondary to bronchial or bronchiolar Anatomy.—Nominal anatomic com- obstruction may produce focal zones of partments of the mediastinum include decreased attenuation, an appearance the anterior, middle, posterior, and (in that can be enhanced by using expira- some schemes) superior compart- tory CT (91,92). The mosaic attenua- ments. The anterior compartment is tion pattern can also be produced by bounded anteriorly by the sternum and interstitial lung disease characterized by posteriorly by the anterior surface of ground-glass opacity; in this situation, the pericardium, the ascending aorta, areas of higher attenuation represent and the brachiocephalic vessels. The Figure 38: Magnified chest radiograph shows the interstitial process and areas of middle compartment is bounded by the miliary pattern. lower attenuation represent the normal posterior margin of the anterior division lung. and the anterior margin of the posterior division. The posterior compartment is mosaic oligemia, perfusion bounded anteriorly by the posterior miliary pattern See mosaic attenuation pattern. margins of the pericardium and great Radiographs and CT scans.—On chest vessels and posteriorly by the thoracic radiographs, the miliary pattern con- vertebral bodies. In the four-compart- sists of profuse tiny, discrete, rounded ment model, the superior compartment pulmonary opacities (Յ3 mm in diame- is defined as the compartment above ter) that are generally uniform in size the plane between the sternal angle to and diffusely distributed throughout the the T4-5 intervertebral disk or, more lungs (Fig 38). This pattern is a manifes- simply, above the aortic arch (84,85). tation of hematogenous spread of tuber- Exact anatomic boundaries between the culosis and metastatic disease. Thin- compartments do not exist, and there section CT scans show widespread, ran- are no barriers (other than the pericar- domly distributed micronodules. dium) to prevent the spread of disease between compartments. Other classifi- cations exist, but the three- and four-

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Figure 40 nodular pattern On CT scans, a nodule appears as a Radiographs and CT scans.—A nodular rounded or irregular opacity, well or pattern is characterized on chest radio- poorly defined, measuring up to 3 cm in graphs by the presence of innumerable diameter (Fig 42). (a) Centrilobular small rounded opacities that are dis- nodules appear separated by several crete and range in diameter from 2 to millimeters from the pleural surfaces, 10 mm (Fig 41). The distribution is fissures, and interlobular septa. They widespread but not necessarily uni- may be of soft-tissue or ground-glass form. On CT scans, the pattern may be attenuation. Ranging in size from a few classified as one of three anatomic dis- millimeters to a centimeter, centrilobu- tributions: centrilobular, lymphatic, or lar nodules are usually ill-defined (4). random. (See also nodule.) (b) A micronodule is less than 3 mm in diameter (see also micronodule). (c) A ground-glass nodule (synonym, non- solid nodule) manifests as hazy in- Figure 40: Coronal CT scan shows mycetomas Figure 42 creased attenuation in the lung that in both lungs. does not obliterate the bronchial and vascular margins. (d) A solid nodule has homogenous soft-tissue attenuation. mycetoma (e) A part-solid nodule (synonym, semi- Pathology.—A mycetoma is a discrete solid nodule) consists of both ground- mass of intertwined hyphae, usually of glass and solid soft-tissue attenuation an Aspergillus species, matted together components. (See also mass.) by mucus, fibrin, and cellular debris col- onizing a cavity, usually from prior fi- brocavitary disease (eg, tuberculosis or Figure 43 sarcoidosis). Radiographs and CT scans.—A mycetoma may move to a dependent location when the patient changes position and may show an air crescent sign (Fig 40). CT scans may show a spongelike pattern and foci of calcification in the mycetoma (93). A synonym is fungus Figure 42: Transverse CT scan shows irregular ball. (See also air crescent.) nodule in left lower lobe.

Figure 41 nodule Radiographs and CT scans.—The chest Figure 43: Transverse CT scan shows nonspe- radiographic appearance of a nodule is cific interstitial pneumonia. a rounded opacity, well or poorly defined, measuring up to 3 cm in diameter. (a) Acinar nodules are round or nonspecific interstitial pneumonia, or NSIP ovoid poorly defined pulmonary opaci- Pathology.—NSIP is characterized by a ties approximately 5–8 mm in diame- histologic pattern of uniform interstitial ter, presumed to represent an anatomic involvement by varying degrees of acinus rendered opaque by consolida- chronic inflammation or fibrosis. NSIP tion. This classification is used only in may be idiopathic or seen in other set- the presence of numerous such opaci- tings, including collagen vascular dis- ties. (b) A pseudonodule mimics a pul- ease, hypersensitivity pneumonitis, monary nodule; it represents, for exam- drug-induced lung disease, infection, ple, a rib fracture, a skin lesion, a device and immunodeficiency (including hu- on the surface of the patient, anatomic man immunodeficiency virus infection) variants, or composite areas of in- (5). Figure 41: Magnified chest radiograph shows creased opacity (94). CT scans.—NSIP has variable thin- a nodular pattern.

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Figure 45 Figure 46 section CT appearances: The most fre- quent is ground-glass opacities with reticulation, traction bronchiectasis or bronchiolectasis, and little or no honeycombing (Fig 43). The distribution is usually basal and subpleural (95).

Figure 44

Figure 46: Transverse CT scan shows panacinar emphysema.

panacinar emphysema Figure 45: Transverse CT scan shows crypto- Pathology.—Panacinar emphysema in- genic organizing pneumonia with subpleural and volves all portions of the acinus and sec- basal distribution. ondary pulmonary lobule more or less uniformly (42). It predominates in the Figure 44: Transverse CT scan shows oligemia lower lobes and is the form of emphy- (arrows) in left lung. organizing pneumonia sema associated with ␣1-antitrypsin de- Pathology.—Organizing pneumonia man- ficiency. ifests as a histologic pattern characterized CT scans.—Panacinar emphysema oligemia by loose plugs of connective tissue in the manifests as a generalized decrease ofthe Pathophysiology.—Oligemia is a reduc- airspaces and distal airways. Interstitial lung parenchyma with a decrease in the tion in pulmonary blood volume. Most inflammation and fibrosis are minimal or caliber of blood vessels in the affected frequently, this reduction is regional, absent. Cryptogenic organizing pneumo- lung (97,98) (Fig 46). Severe panacinar but occasionally it is generalized. Re- nia, or COP, is a distinctive clinical disor- emphysema may coexist and merge with gional oligemia is usually associated with der among the idiopathic interstitial severe centrilobular emphysema. The ap- reduced blood flow in the oligemic area. pneumonias (5), but the histologic pat- pearance of featureless decreased attenu- Radiographs and CT scans.—Olige- tern of organizing pneumonia is encoun- ation may be indistinguishable from se- mia appears as a regional or widespread tered in many different situations, includ- vere constrictive obliterative bronchiolitis decrease in the size and number of identi- ing pulmonary infection, hypersensitivity (99). The term panlobular emphysema is fiable pulmonary vessels (Fig 44), which pneumonitis, and collagen vascular dis- synonymous. (See also emphysema.) is indicative of less than normal blood eases. flow. (See also mosaic attenuation pattern, Radiographs and CT scans.—Air- pulmonary blood flow redistribution.) space consolidation is the cardinal feature of organizing pneumonia on chest opacity radiographs and CT scans. In COP, the Radiographs and CT scans.—Opacity distribution is typically subpleural and refers to any area that preferentially at- basal (Fig 45) and sometimes bron- tenuates the x-ray beam and therefore chocentric (96). Other manifestations of appears more opaque than the sur- organizing pneumonia include ground- rounding area. It is a nonspecific term glass opacity, tree-in-bud pattern, and that does not indicate the size or patho- nodular opacities (37). logic nature of the abnormality. (See also parenchymal opacification, ground- glass opacity.)

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Figure 47 Figure 48 peribronchovascular interstitium Anatomy.—The peribronchovascular interstitium is a connective-tissue sheath that encloses the bronchi, pulmonary arteries, and lymphatic vessels. It extends from the hila to the lung periphery.

Figure 49

Figure 47: Transverse CT scan shows paraseptal emphysema.

Figure 48: Transverse CT scan shows parenchymal bands (arrows). paraseptal emphysema Pathology.—Paraseptal emphysema is characterized by predominant involvement of the distal alveoli and their ducts parenchymal band and sacs. It is characteristically bounded Radiographs and CT scans.—A paren- Figure 49: Transverse CT scan shows organiz- by any pleural surface and the interlobu- chymal band is a linear opacity, usually ing pneumonia in a perilobular distribution lar septa (42,43). 1–3 mm thick and up to 5 cm long that (arrows). CT scans.—This emphysema is usually extends to the visceral pleura characterized by subpleural and peri- (which is often thickened and may be bronchovascular regions of low attenua- retracted at the site of contact) (Fig 48). perilobular distribution tion separated by intact interlobular It reflects pleuroparenchymal fibrosis Anatomy.—The perilobular region com- septa (Fig 47), sometimes associated and is usually associated with distortion prises the structures bordering the pe- with bullae. The term distal acinar em- of the lung architecture. Parenchymal riphery of the secondary pulmonary lob- physema is synonymous. (See also em- bands are most frequently encountered ule. physema.) in individuals who have been exposed to CT scans.—This pattern is charac- asbestos (100,101). terized by distribution along the struc- parenchyma tures that border the pulmonary lobules Anatomy.—Parenchyma refers to the parenchymal opacification (ie, interlobular septa, visceral pleura, gas-exchanging part of the lung, consist- Radiographs and CT scans.—Parenchy- and vessels) (102). The term is most ing of the alveoli and their capillaries. mal opacification of the lungs may or frequently used in the context of dis- Radiographs and CT scans.—The may not obscure the margins of vessels eases (eg, perilobular organizing pneu- portion of the lung exclusive of visible and airway walls (45). Consolidation in- monia) that are distributed mainly pulmonary vessels and airways. dicates that definition of these margins around the inner surface of the second- (excepting air bronchograms) is lost ary pulmonary lobule (103) (Fig 49). within the dense opacification, whereas This may resemble indistinct thickening ground-glass opacity indicates a smaller of the interlobular septa. increase in attenuation, in which the definition of underlying structures is preserved (59). The more specific terms consolidation and ground-glass opacity are preferred. (See also consolidation, ground-glass opacity.)

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Figure 50 Figure 51 Figure 52

Figure 52: Chest radiograph shows a pneumatocele (arrows).

pneumatocele Pathology.—A pneumatocele is a thin- walled, gas-filled space in the lung. It is Figure 50: Transverse CT scan shows sarcoid- Figure 51: Transverse CT scan shows pleural most frequently caused by acute pneu- osis with a perilymphatic distribution. plaque (arrow) anteriorly in right hemithorax. monia, trauma, or aspiration of hydro- carbon fluid and is usually transient. The mechanism is believed to be a combination of parenchymal necrosis and perilymphatic distribution pleural plaque check-valve airway obstruction (107). Anatomy.—This pattern is character- Pathology.—A pleural plaque is a fibro- Radiographs and CT scans.—A ized by distribution along or adjacent to hyaline, relatively acellular lesion aris- pneumatocele appears as an approxi- the lymphatic vessels in the lung. The ing predominantly on the parietal pleu- mately round, thin-walled airspace in routes of lymphatics are found along ral surface, particularly on the dia- the lung (Fig 52). bronchovascular bundles, in the inter- phragm and underneath ribs (105). lobular septa, around larger pulmonary Pleural plaques are almost invariably veins, and in the pleura; alveoli do not the consequence of previous (at least 15 Figure 53 have lymphatics. years earlier) asbestos exposure. CT scans.—Abnormalities along the Radiographs and CT scans.—Pleu- pathway of the pulmonary lymphatics— ral plaques are well-demarcated areas that is, in the perihilar, peribronchovas- of pleural thickening, seen as elevated cular, and centrilobular interstitium, as flat or nodular lesions that often contain well as in the interlobular septa and sub- calcification (Fig 51). Plaques are of pleural locations—have a perilymphatic variable thickness, range from less than distribution (104). A perilymphatic dis- 1 to approximately 5 cm in diameter, tribution typically seen in sarcoidosis and are more easily identified on CT (Fig 50) and lymphangitic spread of can- scans than on chest radiographs (106). cer. An en face plaque may simulate a pulmonary nodule on chest radiographs. platelike atelectasis (See also pseudoplaque.) See linear atelectasis. Figure 53: Magnified chest radiograph shows pneumomediastinum.

pneumomediastinum Pathology.—Pneumomediastinum is the presence of gas in mediastinal tissue outside the esophagus and tracheo- bronchial tree. It may be caused by

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Figure 56 spontaneous alveolar rupture, with sub- able from pneumomediastinum because sequent tracking of air along the bron- the lucency (low attenuation) caused by chovascular interstitium into the medi- air does not extend outside the pericar- astinum. Pneumomediastinum is partic- dial sac (Fig 54). (See also pneumome- ularly associated with a history of diastinum.) asthma, severe coughing, or assisted ventilation. Figure 55 Radiographs and CT scans.— Pneu- momediastinum appears as lucent streaks on chest radiographs, mostly vertically oriented (Fig 53). Some of these streaks may outline vessels and main bronchi. (See also pneumopericardium.) pneumonia Figure 56: Chest radiograph shows bilateral Pathology.—Pneumonia is inflamma- progressive massive fibrosis. tion of the airspaces and/or interstitium (eg, due to infection, as in bacterial pneumonia). Infective pneumonia is progressive massive fibrosis characterized by exudate resulting in Pathology.—This condition is caused by consolidation. The term is also used to slow-growing conglomeration of dust refer to a number of noninfectious dis- particles and collagen deposition in indi- orders of the lung parenchyma charac- Figure 55: Chest radiograph shows viduals (mostly coal workers) heavily terized by varying degrees of inflamma- pneumothorax. exposed to inorganic dust (108). tion and fibrosis (eg, idiopathic intersti- Radiographs and CT scans.—Pro- tial pneumonias) (5). gressive massive fibrosis manifests as pneumothorax and tension pneumothorax masslike lesions, usually bilateral and in Pathophysiology.—Pneumothorax re- the upper lobes (Fig 56). Background fers to the presence of gas in the pleural nodular opacities reflect accompanying Figure 54 space. Qualifiers include spontaneous, pneumoconiosis, with or without em- traumatic, diagnostic, and tension. Ten- physematous destruction adjacent to sion pneumothorax is the accumulation the massive fibrosis (109). Lesions similar of intrapleural gas under pressure. In to progressive massive fibrosis some- this situation, the ipsilateral lung will, if times occur in other conditions, such as normal, collapse completely; however, sarcoidosis and talcosis (109,110). a less than normally compliant lung may remain partially inflated. Radiographs and CT scans.—On chest radiographs, a visceral pleural edge is visible (Fig 55) unless the pneumothorax is very small or the pleural edge is not tangential to the x-ray beam. Tension pneumothorax may be associated with considerable shift of the me- Figure 54: Chest radiograph shows diastinum and/or depression of the pneumopericardium. hemidiaphragm. Some shift can occur without tension because the pleural pressure in the presence of pneumo- pneumopericardium thorax becomes atmospheric, while Pathology.—Pneumopericardium is the the pleural pressure in the contralat- presence of gas in the pericardial space. eral hemithorax remains negative. It usually has an iatrogenic, often surgical, origin in adults. Radiographs and CT scans.—Pneu- mopericardium is usually distinguish-

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Figure 57 Figure 60 pseudoplaque CT scans.—A pseudoplaque is a pulmonary opacity contiguous with the visceral pleura formed by coalescent small nodules. It simulates the appearance of a pleural plaque. This entity is encountered most commonly in sarcoidosis (Fig 58), silicosis, and coal-worker’s pneumoconiosis (86).

Figure 60: Transverse CT scan shows centri- Figure 59 lobular micronodules and patchy ground-glass opacity typical of respiratory bronchiolitis–interstitial lung disease.

Figure 57: Transverse CT scan shows pseudo- respiratory bronchiolitis–interstitial lung cavitation in an adenocarcinoma. disease, or RB-ILD Pathology.—RB-ILD is a smoking-related disease characterized by inflam- pseudocavity mation (predominantly by macro- CT scans.—A pseudocavity appears as phages) of the respiratory bronchioles an oval or round area of low attenuation and peribronchiolar alveoli (5), some- in lung nodules, masses, or areas of con- times with elements of or overlap with solidation that represent spared paren- nonspecific and desquamative intersti- chyma, normal or ectatic bronchi, or tial pneumonias (114). focal emphysema rather than cavita- CT scans.—RB-ILD typically mani- tion. These pseudocavities usually mea- fests as extensive centrilobular micronod- Figure 59: Chest radiograph shows redistribu- sure less than 1 cm in diameter. They ules and patchy ground-glass opacity cor- tion of blood flow to upper lung zones. have been described in patients with responding to macrophage-rich alveolitis adenocarcinoma (Fig 57), bronchioloal- (Fig 60), with or without fine fibrosis veolar carcinoma (111), and benign (115,116). It is often accompanied by conditions such as infectious pneumo- pulmonary blood flow redistribution bronchial wall thickening and minor cen- nia. Pathophysiology.—Pulmonary blood flow trilobular emphysema. Areas of air trap- redistribution refers to any departure ping reflect a bronchiolitic component. Figure 58 from the normal distribution of blood flow in the lungs that is caused by an increase in pulmonary vascular resistance else- where in the pulmonary vascular bed. Radiographs and CT scans.—Pul- monary blood flow redistribution is indi- cated by a decrease in the size and/or number of visible pulmonary vessels in one or more lung regions (Fig 59), with a corresponding increase in number and/or size of pulmonary vessels in other parts of the lung. Upper lobe blood diversion in patients with mitral valve disease is the archetypal example of redistribution (112,113).

Figure 58: Transverse CT scan shows pseudo- plaques (arrows) in a patient with sarcoidosis.

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Figure 61 Figure 62 reversed halo sign CT scans.—The reversed halo sign is a focal rounded area of ground-glass opacity surrounded by a more or less complete ring of consolidation (Fig 63). A rare sign, it was initially reported to be speciﬁc for cryptogenic organizing pneumonia (117,118) but was subsequently described in patients with para- coccidioidomycosis (119). Similar to the halo sign, this sign will probably lose its speciﬁcity as it is recognized in other conditions. (See also halo sign.)

Figure 64

Figure 61: Chest radiograph shows reticular Figure 62: Chest radiograph shows reticu- pattern. lonodular pattern. reticular pattern reticulonodular pattern Radiographs and CT scans.—On chest Radiographs and CT scans.—A com- radiographs, a reticular pattern is a col- bined reticular and nodular pattern, the lection of innumerable small linear reticulonodular pattern is usually the re- opacities that, by summation, produce sult of the summation of points of inter- an appearance resembling a net (syn- section of innumerable lines, creating onym: reticulation) (Fig 61). This ﬁnd- the effect on chest radiographs of super- ing usually represents interstitial lung imposed micronodules (Fig 62). The di- disease. The constituents of a reticular mension of the nodules depends on the pattern are more clearly seen at thin- size and number of linear or curvilinear section CT, whether they are interlobu- elements. (See also reticular pattern.) lar septal thickening, intralobular lines, On CT scans, the pattern appears as a Figure 64: Magniﬁed chest radiograph shows or the cyst walls of honeycombing. (Re- concurrence of reticular and mi- paratracheal stripe (arrows). ticular pattern and honeycombing cronodular patterns. The micronodules should not be considered synonymous. can be situated in the center of the retic- See also honeycombing.) ular elements (eg, centrilobular mi- right paratracheal stripe cronodules) or superimposed on the lin- Anatomy and radiographs.—The right ear opacities (eg, septal micronodules). paratracheal stripe is a vertical, linear, soft-tissue opacity less than 4 mm wide. It corresponds to the right tracheal wall, Figure 63 contiguous mediastinal tissues, and adjacent pleura (Fig 64). The stripe is 3–4 cm in height and extends from approximately the level of the medial ends of the clavicles to the right tracheobron- chial angle on a frontal chest radiograph (120). It is seen in up to 94% of adults but is widened or absent in individuals with abundant mediastinal fat. The commonest pathologic cause of widening, de- formity, or obliteration of this stripe is paratracheal lymph node enlargement. Figure 63: Transverse CT scan shows reversed halo sign (arrow).

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Figure 65 segment terized by abnormal reduced pulmonary Anatomy.—A segment is the unit of a arterial flow (eg, proximal interruption lobe ventilated by a segmental bron- of pulmonary artery [126] or chronic chus, perfused by a segmental pulmo- thromboembolism [127]). Occasionally, nary artery, and drained by an interseg- the tiny vascular opacity abutting a mental pulmonary vein. There are two bronchus is a bronchial, rather than a to five segments per lobe. pulmonary, artery. Radiographs and CT scans.—Indi- vidual segments cannot be precisely de- lineated on chest radiographs and CT Figure 67 scans, and their identification is made inferentially on the basis of the position of the supplying segmental bronchus and artery. When occasionally present, intersegmental fissures help to identify segments.

Figure 65: Sagittal CT scan shows rounded septal line atelectasis in left lower lobe. See interlobular septum.

septal thickening See interlobular septal thickening. rounded atelectasis Pathology.—Rounded atelectasis is rounded collapsed lung associated with Figure 66 Figure 67: Chest radiograph shows silhouette invaginated fibrotic pleura and thick- sign, with obscuration of right border of heart ened and fibrotic interlobular septa. (arrows). Most frequently, it is the consequence of an asbestos-induced exudative pleural effusion with resultant pleural scar- ring (121), but it may occur with any silhouette sign cause of pleural fibrosis. Radiographs.—The silhouette sign is Radiographs and CT scans.—On the absence of depiction of an anatomic chest radiographs, rounded atelectasis soft-tissue border. It is caused by con- appears as a mass abutting a pleural solidation and/or atelectasis of the adja- surface, usually in the posterior part of cent lung (Fig 67), by a large mass, or by a lower lobe. Distorted vessels have a contiguous pleural fluid (128,129). The curvilinear disposition as they converge silhouette sign results from the juxtapo- on the mass (the comet tail sign). The sition of structures of similar radio- degree of lobar retraction depends on graphic attenuation. The sign actually the volume of atelectatic lung. It is al- refers to the absence of a silhouette. It is most invariably associated with other Figure 66: Transverse CT scan shows signet not always indicative of disease (eg, un- signs of pleural fibrosis (eg, blunting of ring sign (arrow). explained absence of right border of costophrenic angle). CT is more sensi- heart is seen in association with pectus tive for the detection and display of the excavatum and sometimes in healthy in- characteristic features of rounded atel- signet ring sign dividuals). ectasis (122,123) (Fig 65). An addi- CT scans.—This finding is composed of tional sign is homogeneous uptake of a ring-shaped opacity representing a di- small-airways disease contrast medium in the atelectatic lung. lated bronchus in cross section and a Pathology.—This phrase is an arbitrary Synonyms include folded lung syndrome, smaller adjacent opacity representing term used more frequently in thin-sec- helical atelectasis, Blesovsky syndrome, its pulmonary artery, with the combination CT descriptions than in the patho- pleural pseudotumor, and pleuroma. tion resembling a signet (or pearl) ring physiology literature, where it was first (124) (Fig 66). It is the basic CT sign of coined (130). Small-airways disease secondary pulmonary lobule bronchiectasis (27,125). The signet ring now generally refers to any condition See lobule. sign can also be seen in diseases charac- affecting the bronchioles, whereas

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Figure 69 Figure 70 bronchiolitis more speciﬁcally describes inﬂammation of the bronchioles (131). CT scans.—Small airways are considered to be those with an internal diameter smaller than or equal to 2 mm and a normal wall thickness of less than 0.5 mm (32). Small-airways disease is manifest on CT scans as one or more of the following patterns: mosaic attenuation, air trapping, centrilobular micronodules, tree-in- bud pattern, or bronchiolectasis.

Figure 68

Figure 70: Transverse CT scan shows tree-in- bud pattern (arrows). Figure 68: Transverse CT scan shows a subpleural curvilinear line. tree-in-bud pattern CT scans.—The tree-in-bud pattern rep- subpleural curvilinear line resents centrilobular branching struc- CT scans.—This finding is a thin curvi- Figure 69: Transverse CT scan shows traction tures that resemble a budding tree. The linear opacity, 1–3 mm in thickness, ly- bronchiectasis (arrows) cased by retractile pulmo- pattern reflects a spectrum of endo- and ing less than 1 cm from and parallel to nary fibrosis. peribronchiolar disorders, including mu- the pleural surface (Fig 68). It corre- coid impaction, inflammation, and/or fi- sponds to atelectasis of normal lung if brosis (134,135) (Fig 70). This pattern is seen in the dependent posteroinferior traction bronchiectasis and traction most pronounced in the lung periphery portion of lung of a patient in the supine bronchiolectasis and is usually associated with abnormali- position and is subsequently shown to CT scans.—Traction bronchiectasis and ties of the larger airways. It is particularly disappear on CT sections acquired with traction bronchiolectasis respectively common in diffuse panbronchiolitis (136), the patient prone. It may also be en- represent irregular bronchial and bron- endobronchial spread of mycobacterial countered in patients with pulmonary chiolar dilatation caused by surrounding infection (137), and cystic fibrosis. A sim- edema (132) or fibrosis (other signs are retractile pulmonary fibrosis (130). Di- ilar pattern is a rare manifestation of arte- usually present). Though described in lated airways are usually identifiable as riolar (microangiopathic) disease (138). the context of asbestosis, this finding is such (Fig 69) but may be seen as cysts not specific for asbestosis. (bronchi) or microcysts (bronchioles in the lung periphery). The juxtaposition of numerous cystic airways may make the distinction from “pure” fibrotic honeycombing difficult.

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Figure 71 Glossary of terms for CT of the lungs: rec- 14. Murata K, Khan A, Herman PG. Pulmonary ommendations of the Nomenclature Com- parenchymal disease: evaluation with high- mittee of the Fleischner Society. Radiology resolution CT. Radiology 1989;170:629– 1996;200:327–331. 635.

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American Thoracic Society/European Re- tuberculosis: high-resolution CT findings. and subpleural honeycombing, indicative of usual spiratory Society International Multidisci- Radiology 1991;178:727–731. interstitial pneumonia. plinary Consensus Classification of the Idio- pathic Interstitial Pneumonias. This joint 18. Yousem SA. Pulmonary apical cap: a dis- statement of the American Thoracic Soci- tinctive but poorly recognized lesion in pul- ety (ATS), and the European Respiratory monary surgical pathology. Am J Surg usual interstitial pneumonia, or UIP Society (ERS) was adopted by the ATS Pathol 2001;25:679–683. Pathology.—UIP is a histologic pattern board of directors, June 2001 and by the 19. Dail DH. Pulmonary apical cap. Am J Surg ERS executive committee, June 2001. Am J Pathol 2001;25:1344. of pulmonary fibrosis characterized by Respir Crit Care Med 2002;165:277–304. temporal and spatial heterogeneity, [Published correction appears in Am J Re- 20. Woodring JH, Reed JC. 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