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RADIOLOGY OF THE PLEURAL FISSURES R.M. SOFRANIK, MD, B.H. GROSS, AND D.L. SPIZARNY, MD
The pleural fissures, formed by a double layer of visceral pleura, represent extensions of the pleural space between lobes of the lungs. The fissures are frequently an anatomic barrier to the spread of inflammatory or neoplastic disease. Identification of the fissures is important for the accurate localization of lung parenchymal or pleural pathology. The plain radiographic and computed tomographic (CT) imaging of normal and variantfissural anatomy, as well as of some abnormalities that may affect the fissures will be reviewed and illustrated. MAJOR FISSURES
The major fissures course obliquely forward in the chest from T3-T5 posteriorly and superiorly to the anterior lower hemithorax, usually paralleling the sixth rib (1). The suprahilar major fissures separate the posterior segments of the upper lobes from the superior segments of the lower lobes. The infrahilar major fissures separate the middle lobe or lingula from the anterior segments of the lower lobe. On both the right and the left, the suprahilar major fissure is generally concave and “faces” laterally (2-4). Proto and Speckman (3) observed medial “facing” of the infrahilar major fissure on both sides, giving rise to a propeller-like appearance for the entire fissure. Raasch et al. (4), however, found lateral “facing” of the right infrahilar major fissure in 43 out of 50 fixed and inflated specimens.
From the Department of Diagnostic Radiology, Henry Ford Hosnital, Detroit, Michigan 48202. Address reprint requests to: Dr. D.L. Spizarny, Department of Diamostic Radiolow. Henrv Ford Hosoital. 2799 W. Grand Boulevar& Detroit, MI 4EkJ2. _ Received July 1991; revised October 1991; accepted November 25, 1991. 0 1992 by Elsevier Science Publishing Co., ~nc. 655 Avenue of the Americas, 0899-707t192l$5.00
New York, NY 10010
MD,
The lateral chest radiograph allows tangential imaging of the major fissures, which are seen as thin, obliquely oriented linear opacities. Portions of the major fissures are almost always evident on the lateral projection, but the entire fissure is visualized in only 2% of studies (3). The left major fissure originates more superiorly and courses more vertically than the right (4). Visualization of contact with the minor fissure or right hemidiaphragm may help differentiate the right major fissure from the left major fissure on the lateral radiograph. Occasionally the major fissure can be partially visualized on the frontal radiograph. Seen as a curving edge or line along the upper lateral chest and made visible by extrapleural fat, the superolateral major fissure (Figure 1) can be appreciated on 14% of normal frontal radiographs (5). The superomedial major fissure (Figure 2) is identified on 8% of normal frontal radiographs, and is visible on the right near the azygous arch, and on the left near the aortic arch (6). Visualization of the major fissure also occurs with lower-lobe volume loss, less often with upper-lobe volume loss, or with adjacent parenchymal consolidation or pleural thickening (7). FIGURE 1. Superolateral major fissure. Arrows delineate superolateral major fissure in normal chest.
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FIGURE 2. (A) Thickening of superomedial major fissure (arrows). (B) Superomedial major fissure (arrows) marginates upper border of collapsed superior segment of RLL.
The major fissures are identified on CT scans with a frequency approaching 100% (1,2). More correctly, it is the parafissural lung that allows localization of the fissures on routine CT. In this subpleural region, secondary pulmonary lobules contain a sparse and very peripheral small caliber vasculature (8). This relatively avascular region may be imaged as a lucent band, a common appearance for the obliquely oriented major fissures on CT. However, when the major fissure is relatively perpendicular to the scanning plane, it is more likely to appear as a thin line. Visualization as a thin line occurs more commonly on the left, where the superior aspect of the major fissure is more vertical. A thin line is also more frequently seen on thin-section CT scans, where there is a lesser degree of volume averaging (Figure 3). Less commonly, the fissure may appear as a dense band, probably because of a partial volume effect or respiratory motion (2). A “double fissure” (Figure 4) may be seen on thin-section CT scans, due to motion artifact, particularly at the left base where there is cardiac motion (6).
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MINOR FISSURE The minor fissure separates the right upper and right middle lobes, and is usually horizontal at the fourth anterior interspace (10). This fissure is at least partially visualized in 56% of frontal and 44% of lateral radiographs (10). When there is a pneumothorax with extension of air into the minor fissure (Figure 5), the visceral pleural surfaces of the middle lobe and upper lobe can be seen (11). The CT appearance of the minor fissure was first described by Goodman as a “right mid-lung window,” an area of focally diminished vascularity lateral to the bronchus intermedius, at a level about 3-4 cm below its origin (12). The paucity of vasculature results from a lack of major branches from the descending interlobar artery after it has supplied the middle lobe. More detailed analysis of the CT appearance of the minor fissure has since been reported (1, 2,13,14). Given the parallel orientation of the minor fissure with respect to the axial scanning plane, it is seldom a line on routine scans with lo-mm collimation. The minor fissure is often appreciated as a triangular avascular area, with its apex at the hilum (1,2). Because of the convexity of the superior aspect of the right middle lobe, the highest portion of the minor fissure may appear as a round or oval avascular zone (2). On such scans, lung anterior to the fissural zone
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‘IGLJRE 4. Double fissure (arrows) produced by req )iraory or cardiac motion on a scan with 1.5mm collimat ion. in minor fissure. Lateral PI neunothorax (straight arrows) extends into minor fis sure curved arrows) in patient with pulmonary edema.
I XGURE 5. Pneumothorax
B FIGURE 3. Thin section CT in evaluating fissures. (A) Right major fissure (arrows) seen as an avascular zone on l-cm thick section. (B) Right major fissure (arrows) seen as a thin line on s-mm thick section.
is the anterior segment of the right upper lobe, and lung posterior to the fissural zone is the superior segment of the lower lobe. Detection of the minor fissure on routine CT scans is variable, ranging from 50%-100~%~ (1, 2). Using thin-section CT, Berkmen et al. (14) identified a minor fissure in 32 (80%] of 40 cases and categorized the appearance of the minor fissure into one of two configurations, based on whether the highest portion of the middle lobe upper surface was relatively medial (Type I) or lateral (Type II). Other variations on thin-section CT, including a large ill-defined opacity
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B
FIGURE 6. (A, B) Superior accessory fissure. Localized thickening of superior accessory fissure (open arrow) on
PA radiograph, with extension posterior to major fissure (arrows) on lateral radiograph. and a circular opacity with a lucent center, have also been described (15). Attention to bronchopulmonary segmental anatomy should also aid in the differentiation of the right upper and middle lobes on CT. At times, the minor fissure may be absent or distorted by significant parenchymal or pleural pathology. It has been observed that the upper-lobe arteries course medial to their respective bronchi, whereas those of the middle lobe course lateral to their bronchi (16). This relationship is constant even in the presence of volume loss. ACCESSORY FISSURES AND NORMAL VARIANTS
Although accessory fissures are more commonly visualized in anatomic specimens than on radiologic examinations, they can be demonstrated on plain radiographs or CT. Most common are the superior and inferior accessory fissures; other variants include the azygous, hemiazygous, and left minor fissures. The superior accessory fissure lies between the superior segment of the lower lobe and the remaining basilar segments. It is more common on the right, and is found in 5%30% of autopsy specimens (17).
Because it is usually horizontal, the superior accessory fissure resembles the minor fissure on frontal radiographs, except that it is located more inferiorly. On the lateral projection, this fissure extends posteriorly from the major fissure (Figure 6). It was recognized by Proto on 6% of lateral radiographs (3). On CT, the superior accessory fissure is more caudal and posterior than the minor fissure. This appearance should be distinguished from that of a major fissure which is horizontally oriented secondary to lowerlobe volume loss (17). When it is slightly oblique, the superior accessory fissure may resemble the major fissure. The inferior accessory fissure separates the medial basal segment from the remainder of the lower lobe and occurs more commonly on the right. Although this fissure is present in SO%--50% of autopsy specimens, it is radiographically demonstrable in only 5%10% of patients (I 7). On frontal and lateral radiographs, the inferior accessory fissure appears as a thin vertical linear opacity that originates along the medial aspect of the diaphragm. It lies posterior to the major fissure and courses superiorly and obliquely toward the hilum. The CT appearance is determined by whether routine or thin sections are obtained; when the fissure is visualized, it extends from the inferior pulmonary ligament toward the major fissure (17).
A left minor fissure, found in 8%-18% of anatomic specimens, separates the lingula from the remaining
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left upper lobe (18).Radiographs demonstrate a left minor fissure (Figure 7) in less than 2% of patients (18). The CT appearance is analogous to that of a right minor fissure, although the left minor fissure may be situated at a more cephalad level (18). An azygous fissure is distinct from all other fissures, in that it is formed by four rather than two layers of pleura. It invaginates into the lung apex and envelops a laterally displaced azygous vein. It occurs in 1% of anatomic specimens and is demonstrated radiographically in 0.5% of patients (19). Almost all cases occur on the right, although there are a few reports of a left azygous fissure (Figure 8) connecting with the left superior intercostal vein (20). Given its vertical orientation, an azygous fissure is often recognized on CT as an arcuate linear opacity extending from the posterolateral aspect of an upper thoracic vertebral body to the superior vena cava or right brachiocephalic vein (19,20). The posterior aspect of an azygous fissure may appear nodular secondary to intraparenchymal location of the azygous vein.
FIGURE 8. Left azygous fissure (straight arrows) leads to left superior intercostal vein (curved arrow). A
FIGURE 7. (A, B) Left minor fissure. PA chest radiograph reveals left minor fissure (open arrows), seen cephalad to right minor fissure (curved arrows) on lateral view.
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FIGURE 9. Sagittal minor fissure. (A) PA chest radiograph.
Thickened sagittal minor fissure (arrows) in paient with pulmonary edema. (B) CT photographed at lung windows. Sagittal minor fissure (black arrows), and major fissure (white arrows). (C) PA chest radiograph. Air space disease in anterior segment of RUL marginated inferiorly by sagittal minor fissure [curved arrows). (From Sagittal orientation of the anterior minor fissure: Radiography and CT. Gross et al., Radiology 1988;166:717-719). Reprinted by permission
C A sagittal anterior minor fissure (Figure 9) is a normal variant that’may give rise to confusion regarding the appearance of right upper- and right middlelobe disease (21). Appearing as a vertical and somewhat oblique linear opacity in the inferomedial right lung, the sagittal minor fissure allows inferomedial extension of the right upper lobe. As a result, the anterior segment of the upper lobe occupies a paracardiac location and can reach the diaphragm (21). Right upper-lobe disease may thus silhouette the heart, although pathologic processes within the medial segment of the right middle lobe tend to spare the cardiac border; in either case, there may be erroneous localization of disease.
PATHOLOGY
AFFECTING
THE FISSURES
The double layer of visceral pleura within fissures is usually an effective barrier to the spread of inflammation or neoplasms. This is especially true when
lobes are completely separated by fissures. However, anatomic studies have demonstrated fusion across a portion of the right major fissure in approximately 70% of specimens, across the left major fissure in approximately 40%~50% of specimens, and across the minor fissure in greater than 99% of specimens (4). Lobar fusion allows spread of disease that would otherwise be confined to one lobe or segment. When nonneoplastic disease crosses an intact fissure, it is usually an atypical infection, actinomycosis or, less commonly, nocardiosis, blastomycosis, or other fungal infections (10). Neoplastic processes do not usually transgress intact fissures, but lymphoma does have a predilection of crossing anatomic barriers and may present as a pleural mass (22). In lung-cancer patients who are candidates for lobectomy, extension of neoplasm anywhere across the left major fissure or across the right major fissure cephalad to the minor fissure necessitates pneumonectomy. On routine scans with lo-mm collimation,
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FIGURE 10.
CT evaluation for possible transfissural extension of tumor. RUL adenocarcinoma (curved arrow) abuts, but does not cross, right major fissure [open arrows).
the CT assessment of transfissural extension IO), as correlated with pathologic findings, 100% specific but only 50% sensitive (23). pected that thin-section CT scanning would sensitivity.
(Figure is up to It is eximprove
FIGURE 11. Parenchymal versus fissural mass. (A, B) PA and lateral chest radiographs. Elliptical mass (large arrows) oriented along right major fissure, thought to represent intrafissural fluid or mass. (C) CT scan. Mass (large arrow) is not sharply marginated anteriorly, suggesting it is of parenchymal origin. Biopsy proven adenocarcinoma of lung.
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Intrathoracic masses can usually be differentiated as parenchymal or extraparenchymal lesions on the basis of their radiographic appearances. A pleural mass usually has obtuse margins and is incompletely surrounded by lung; it is usually sharply marginated with smooth displacement of the adjacent bronchovascular structures. This distinction becomes more difficult, however, when a lesion is intrafissural in location or when a parenchymal mass abuts the fissure (Figure 11). Spizarny et al. (24) have described intrafissural fibrous mesotheliomas (Figure 12) with characteristics of parenchymal masses. In general, an elliptical shape and a progressively more anterior location on caudal CT images should suggest an intrafissural lesion. Because fissures are more clearly delineated on thin-section CT, use of narrow collimation scans can also help localize a lesion to a fissure (Figure 12B). Such a distinction is important because the differential diagnosis, prognosis, and surgical
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found along the lower portions (25).
of the major fissures
CONCLUSION As compared with plain chest radiography, CT increases the frequency of visualizing the fissures and provides a better demonstration of anatomic detail, particularly when thin-section scans are employed. CT allows more precise localization of benign or malignant parenchymal disease with respect to the fissures and can help identify pleural disease arising within the fissures. Thin-section CT scans are not routinely indicated. However, they should be obtained when patient management would be affected by such factors as whether a lesion is pleural or parenchymal or whether there is transfissural extension of disease. REFERENCES 1. Frija J, Schmit P, Katz M, et al. Computed tomography of the pulmonary fissures: Normal anatomy. J Comput Assist Tomogr 1982;6:1069-1074. 2. Proto AV, Ball JB. Computed tomography minor fissures. AJR 1983;140:439-448.
of the major and
3. Proto AV, Speckman JM. The left lateral radiograph chest. Med Radiogr Photogr 1979;55:30-74.
B FIGURE 12. Parenchymal versus fissural mass. (A) Standard CT demonstrates avascular zone [small arrows) surrounding intrathoracic nodule (large arrow). (B) Thin-section CT (1.5-mm collimation) localizes nodule (large arrow), subsequently proven to be benign fibrous mesothelioma, within right major fissure [small arrows).
of the
4. Raasch BN, Carsky EW, Lane EJ. Radiographic anatomy of the interlobar fissures: A study of 100 specimens. AJR 1982; 138:1043-1049. 5. Proto AV, Ball JB. The superolateral 1983;140:431-437. 6. Proto AV. The chest radiograph: Clin Chest Med 1984;5:213-246.
major fissures.
Anatomic
AJR
considerations.
7. Fisher MS. Significance of a visible major fissure on the frontal chest radiograph. AJR 1981;137:577-580. 8. Fraser RG, Pare JAP. Diagnosis of Diseases of the Chest. Philadelphia: W B Saunders, 1979.
management of a localized pleural mass differs from that of a parenchymal mass. Similar confusion may arise with intrafissural loculation of pleural fluid, commonly called a “psuedotumor.” Orientation along the fissural plane, smooth margins, and tapering edges should aid in diagnosing a loculated effusion, both on plain radiographs and on CT. Parietal pleural diseases can sometimes also affect the visceral pleura. Fissural thickening may indicate asbestos exposure and/or asbestosis (25). Rockoff et al. (25) and Solomon et al. (26) demonstrated a 25%55% incidence of fissural thickening following asbestos exposure, with the greatest incidence occurring in association with pulmonary fibrosis and parietal plaques. These studies suggest that there is a stronger correlation between asbestos exposure and/ or asbestosis with fissural thickening than with pleural plaques. Fissural pleural plaques are usually
9. Mayo JR, Muller NL, Henkelman RM. The double-fissure sign: A motion artifact on thin-section CT scans. Radiology 1987;165:580-581. 10. Felson B. Chest Roentgenology. 1973,~~. 74-81,379.
Philadelphia:
W B Saunders,
11. Spizarny DL, Goodman LR. Air in the minor fissure: A sign of right-sided pneumothorax. Radiology 1986;160:329-331. 12. Goodman LR, Golkow RS, Steiner RM. The right mid-lung window: A potential source of error in computed tomography of the lung. Radiology 1982;143:135-138. 13. Chaser-r MH, McCarthy MJ, Gilliland JD, et al. Concepts in computed tomography of the thorax. RadioGraphics 1986; 6:793-832. 14. Be&men YM, Auh YH, Davis SD, et al. Anatomy of the minor fissure: Evaluation with thin-section CT. Radiology 1989; 170:647-651. 15. Frija J, Yana C, Laval-Jeantet M. Anatomy of the minor fissure: Evaluation with thin-section CT. Radiology 1989; 173:571-572. 16. Otsuji H, Hatakeyama M, Kitamura I. Right upper lobe versus right middle lobe. Differentiation with thin-section, high-resolution CT. Radiology 1989;172:653-656.
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17. Godwin JD, Tarver RD. Accessory 1985;144:39-47. 18. Austin JH: 161:433-436.
The
left
minor
fissures of the lung. AJR fissure.
Radiology
1986;
19. Speckman JM, Gamsu G, Webb WR. Alterations in CT mediastinal anatomy produced by an azygous lobe. AJR 1981; 137:47-50. 20. Takasugi JE, Godwin 1989;171:133-134.
JD. Left azygous
lobe.
Radiology
21. Gross BH, Spizarny DL, Granke DS. Sagittal orientation of the anterior minor fissure: Radiography and CT. Radiology 1988;166:717-179.
22. Shuman LS, Libshitz HI. Solid pleural lymphoma. AJR 1984;142:269-273.
manifestations
229
of
23. Quint LE, Glazer GM, Orringer MB. Central lung masses: Prediction with CT of need for pneumonectomy versus lobectomy. Radiology 1987;165:735-738. 24. Spizarny DL, Gross BH, Shepard JO. CT findings in localized fibrous mesothelioma of the pleural fissure. J Comput Assist Tomogr 1986;10:942-944. 25. Rockoff SD, Kagan E, Schwartz A, et al. Visceral pleural thickening in asbestos exposure. J Thorac Imaging 1987;2:58-60. 26. Solomon A, h-wig LM, Sluid-Cremer GK, et al. Thickening of pulmonary interlobar fissures. Br J Ind Med 1979;36:195-198.
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RADIOLOGY OF THE PLEURAL FISSURES R.M. SOFRANIK, MD, B.H. GROSS, AND D.L. SPIZARNY, MD
The pleural fissures, formed by a double layer of visceral pleura, represent extensions of the pleural space between lobes of the lungs. The fissures are frequently an anatomic barrier to the spread of inflammatory or neoplastic disease. Identification of the fissures is important for the accurate localization of lung parenchymal or pleural pathology. The plain radiographic and computed tomographic (CT) imaging of normal and variantfissural anatomy, as well as of some abnormalities that may affect the fissures will be reviewed and illustrated. MAJOR FISSURES
The major fissures course obliquely forward in the chest from T3-T5 posteriorly and superiorly to the anterior lower hemithorax, usually paralleling the sixth rib (1). The suprahilar major fissures separate the posterior segments of the upper lobes from the superior segments of the lower lobes. The infrahilar major fissures separate the middle lobe or lingula from the anterior segments of the lower lobe. On both the right and the left, the suprahilar major fissure is generally concave and “faces” laterally (2-4). Proto and Speckman (3) observed medial “facing” of the infrahilar major fissure on both sides, giving rise to a propeller-like appearance for the entire fissure. Raasch et al. (4), however, found lateral “facing” of the right infrahilar major fissure in 43 out of 50 fixed and inflated specimens.
From the Department of Diagnostic Radiology, Henry Ford Hosnital, Detroit, Michigan 48202. Address reprint requests to: Dr. D.L. Spizarny, Department of Diamostic Radiolow. Henrv Ford Hosoital. 2799 W. Grand Boulevar& Detroit, MI 4EkJ2. _ Received July 1991; revised October 1991; accepted November 25, 1991. 0 1992 by Elsevier Science Publishing Co., ~nc. 655 Avenue of the Americas, 0899-707t192l$5.00
New York, NY 10010
MD,
The lateral chest radiograph allows tangential imaging of the major fissures, which are seen as thin, obliquely oriented linear opacities. Portions of the major fissures are almost always evident on the lateral projection, but the entire fissure is visualized in only 2% of studies (3). The left major fissure originates more superiorly and courses more vertically than the right (4). Visualization of contact with the minor fissure or right hemidiaphragm may help differentiate the right major fissure from the left major fissure on the lateral radiograph. Occasionally the major fissure can be partially visualized on the frontal radiograph. Seen as a curving edge or line along the upper lateral chest and made visible by extrapleural fat, the superolateral major fissure (Figure 1) can be appreciated on 14% of normal frontal radiographs (5). The superomedial major fissure (Figure 2) is identified on 8% of normal frontal radiographs, and is visible on the right near the azygous arch, and on the left near the aortic arch (6). Visualization of the major fissure also occurs with lower-lobe volume loss, less often with upper-lobe volume loss, or with adjacent parenchymal consolidation or pleural thickening (7). FIGURE 1. Superolateral major fissure. Arrows delineate superolateral major fissure in normal chest.
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FIGURE 2. (A) Thickening of superomedial major fissure (arrows). (B) Superomedial major fissure (arrows) marginates upper border of collapsed superior segment of RLL.
The major fissures are identified on CT scans with a frequency approaching 100% (1,2). More correctly, it is the parafissural lung that allows localization of the fissures on routine CT. In this subpleural region, secondary pulmonary lobules contain a sparse and very peripheral small caliber vasculature (8). This relatively avascular region may be imaged as a lucent band, a common appearance for the obliquely oriented major fissures on CT. However, when the major fissure is relatively perpendicular to the scanning plane, it is more likely to appear as a thin line. Visualization as a thin line occurs more commonly on the left, where the superior aspect of the major fissure is more vertical. A thin line is also more frequently seen on thin-section CT scans, where there is a lesser degree of volume averaging (Figure 3). Less commonly, the fissure may appear as a dense band, probably because of a partial volume effect or respiratory motion (2). A “double fissure” (Figure 4) may be seen on thin-section CT scans, due to motion artifact, particularly at the left base where there is cardiac motion (6).
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MINOR FISSURE The minor fissure separates the right upper and right middle lobes, and is usually horizontal at the fourth anterior interspace (10). This fissure is at least partially visualized in 56% of frontal and 44% of lateral radiographs (10). When there is a pneumothorax with extension of air into the minor fissure (Figure 5), the visceral pleural surfaces of the middle lobe and upper lobe can be seen (11). The CT appearance of the minor fissure was first described by Goodman as a “right mid-lung window,” an area of focally diminished vascularity lateral to the bronchus intermedius, at a level about 3-4 cm below its origin (12). The paucity of vasculature results from a lack of major branches from the descending interlobar artery after it has supplied the middle lobe. More detailed analysis of the CT appearance of the minor fissure has since been reported (1, 2,13,14). Given the parallel orientation of the minor fissure with respect to the axial scanning plane, it is seldom a line on routine scans with lo-mm collimation. The minor fissure is often appreciated as a triangular avascular area, with its apex at the hilum (1,2). Because of the convexity of the superior aspect of the right middle lobe, the highest portion of the minor fissure may appear as a round or oval avascular zone (2). On such scans, lung anterior to the fissural zone
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‘IGLJRE 4. Double fissure (arrows) produced by req )iraory or cardiac motion on a scan with 1.5mm collimat ion. in minor fissure. Lateral PI neunothorax (straight arrows) extends into minor fis sure curved arrows) in patient with pulmonary edema.
I XGURE 5. Pneumothorax
B FIGURE 3. Thin section CT in evaluating fissures. (A) Right major fissure (arrows) seen as an avascular zone on l-cm thick section. (B) Right major fissure (arrows) seen as a thin line on s-mm thick section.
is the anterior segment of the right upper lobe, and lung posterior to the fissural zone is the superior segment of the lower lobe. Detection of the minor fissure on routine CT scans is variable, ranging from 50%-100~%~ (1, 2). Using thin-section CT, Berkmen et al. (14) identified a minor fissure in 32 (80%] of 40 cases and categorized the appearance of the minor fissure into one of two configurations, based on whether the highest portion of the middle lobe upper surface was relatively medial (Type I) or lateral (Type II). Other variations on thin-section CT, including a large ill-defined opacity
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B
FIGURE 6. (A, B) Superior accessory fissure. Localized thickening of superior accessory fissure (open arrow) on
PA radiograph, with extension posterior to major fissure (arrows) on lateral radiograph. and a circular opacity with a lucent center, have also been described (15). Attention to bronchopulmonary segmental anatomy should also aid in the differentiation of the right upper and middle lobes on CT. At times, the minor fissure may be absent or distorted by significant parenchymal or pleural pathology. It has been observed that the upper-lobe arteries course medial to their respective bronchi, whereas those of the middle lobe course lateral to their bronchi (16). This relationship is constant even in the presence of volume loss. ACCESSORY FISSURES AND NORMAL VARIANTS
Although accessory fissures are more commonly visualized in anatomic specimens than on radiologic examinations, they can be demonstrated on plain radiographs or CT. Most common are the superior and inferior accessory fissures; other variants include the azygous, hemiazygous, and left minor fissures. The superior accessory fissure lies between the superior segment of the lower lobe and the remaining basilar segments. It is more common on the right, and is found in 5%30% of autopsy specimens (17).
Because it is usually horizontal, the superior accessory fissure resembles the minor fissure on frontal radiographs, except that it is located more inferiorly. On the lateral projection, this fissure extends posteriorly from the major fissure (Figure 6). It was recognized by Proto on 6% of lateral radiographs (3). On CT, the superior accessory fissure is more caudal and posterior than the minor fissure. This appearance should be distinguished from that of a major fissure which is horizontally oriented secondary to lowerlobe volume loss (17). When it is slightly oblique, the superior accessory fissure may resemble the major fissure. The inferior accessory fissure separates the medial basal segment from the remainder of the lower lobe and occurs more commonly on the right. Although this fissure is present in SO%--50% of autopsy specimens, it is radiographically demonstrable in only 5%10% of patients (I 7). On frontal and lateral radiographs, the inferior accessory fissure appears as a thin vertical linear opacity that originates along the medial aspect of the diaphragm. It lies posterior to the major fissure and courses superiorly and obliquely toward the hilum. The CT appearance is determined by whether routine or thin sections are obtained; when the fissure is visualized, it extends from the inferior pulmonary ligament toward the major fissure (17).
A left minor fissure, found in 8%-18% of anatomic specimens, separates the lingula from the remaining
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left upper lobe (18).Radiographs demonstrate a left minor fissure (Figure 7) in less than 2% of patients (18). The CT appearance is analogous to that of a right minor fissure, although the left minor fissure may be situated at a more cephalad level (18). An azygous fissure is distinct from all other fissures, in that it is formed by four rather than two layers of pleura. It invaginates into the lung apex and envelops a laterally displaced azygous vein. It occurs in 1% of anatomic specimens and is demonstrated radiographically in 0.5% of patients (19). Almost all cases occur on the right, although there are a few reports of a left azygous fissure (Figure 8) connecting with the left superior intercostal vein (20). Given its vertical orientation, an azygous fissure is often recognized on CT as an arcuate linear opacity extending from the posterolateral aspect of an upper thoracic vertebral body to the superior vena cava or right brachiocephalic vein (19,20). The posterior aspect of an azygous fissure may appear nodular secondary to intraparenchymal location of the azygous vein.
FIGURE 8. Left azygous fissure (straight arrows) leads to left superior intercostal vein (curved arrow). A
FIGURE 7. (A, B) Left minor fissure. PA chest radiograph reveals left minor fissure (open arrows), seen cephalad to right minor fissure (curved arrows) on lateral view.
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FIGURE 9. Sagittal minor fissure. (A) PA chest radiograph.
Thickened sagittal minor fissure (arrows) in paient with pulmonary edema. (B) CT photographed at lung windows. Sagittal minor fissure (black arrows), and major fissure (white arrows). (C) PA chest radiograph. Air space disease in anterior segment of RUL marginated inferiorly by sagittal minor fissure [curved arrows). (From Sagittal orientation of the anterior minor fissure: Radiography and CT. Gross et al., Radiology 1988;166:717-719). Reprinted by permission
C A sagittal anterior minor fissure (Figure 9) is a normal variant that’may give rise to confusion regarding the appearance of right upper- and right middlelobe disease (21). Appearing as a vertical and somewhat oblique linear opacity in the inferomedial right lung, the sagittal minor fissure allows inferomedial extension of the right upper lobe. As a result, the anterior segment of the upper lobe occupies a paracardiac location and can reach the diaphragm (21). Right upper-lobe disease may thus silhouette the heart, although pathologic processes within the medial segment of the right middle lobe tend to spare the cardiac border; in either case, there may be erroneous localization of disease.
PATHOLOGY
AFFECTING
THE FISSURES
The double layer of visceral pleura within fissures is usually an effective barrier to the spread of inflammation or neoplasms. This is especially true when
lobes are completely separated by fissures. However, anatomic studies have demonstrated fusion across a portion of the right major fissure in approximately 70% of specimens, across the left major fissure in approximately 40%~50% of specimens, and across the minor fissure in greater than 99% of specimens (4). Lobar fusion allows spread of disease that would otherwise be confined to one lobe or segment. When nonneoplastic disease crosses an intact fissure, it is usually an atypical infection, actinomycosis or, less commonly, nocardiosis, blastomycosis, or other fungal infections (10). Neoplastic processes do not usually transgress intact fissures, but lymphoma does have a predilection of crossing anatomic barriers and may present as a pleural mass (22). In lung-cancer patients who are candidates for lobectomy, extension of neoplasm anywhere across the left major fissure or across the right major fissure cephalad to the minor fissure necessitates pneumonectomy. On routine scans with lo-mm collimation,
OCTOBER-DECEMBER
RADIOLOGY
1992
FIGURE 10.
CT evaluation for possible transfissural extension of tumor. RUL adenocarcinoma (curved arrow) abuts, but does not cross, right major fissure [open arrows).
the CT assessment of transfissural extension IO), as correlated with pathologic findings, 100% specific but only 50% sensitive (23). pected that thin-section CT scanning would sensitivity.
(Figure is up to It is eximprove
FIGURE 11. Parenchymal versus fissural mass. (A, B) PA and lateral chest radiographs. Elliptical mass (large arrows) oriented along right major fissure, thought to represent intrafissural fluid or mass. (C) CT scan. Mass (large arrow) is not sharply marginated anteriorly, suggesting it is of parenchymal origin. Biopsy proven adenocarcinoma of lung.
OF THE PLEURAL
FISSURES
227
Intrathoracic masses can usually be differentiated as parenchymal or extraparenchymal lesions on the basis of their radiographic appearances. A pleural mass usually has obtuse margins and is incompletely surrounded by lung; it is usually sharply marginated with smooth displacement of the adjacent bronchovascular structures. This distinction becomes more difficult, however, when a lesion is intrafissural in location or when a parenchymal mass abuts the fissure (Figure 11). Spizarny et al. (24) have described intrafissural fibrous mesotheliomas (Figure 12) with characteristics of parenchymal masses. In general, an elliptical shape and a progressively more anterior location on caudal CT images should suggest an intrafissural lesion. Because fissures are more clearly delineated on thin-section CT, use of narrow collimation scans can also help localize a lesion to a fissure (Figure 12B). Such a distinction is important because the differential diagnosis, prognosis, and surgical
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found along the lower portions (25).
of the major fissures
CONCLUSION As compared with plain chest radiography, CT increases the frequency of visualizing the fissures and provides a better demonstration of anatomic detail, particularly when thin-section scans are employed. CT allows more precise localization of benign or malignant parenchymal disease with respect to the fissures and can help identify pleural disease arising within the fissures. Thin-section CT scans are not routinely indicated. However, they should be obtained when patient management would be affected by such factors as whether a lesion is pleural or parenchymal or whether there is transfissural extension of disease. REFERENCES 1. Frija J, Schmit P, Katz M, et al. Computed tomography of the pulmonary fissures: Normal anatomy. J Comput Assist Tomogr 1982;6:1069-1074. 2. Proto AV, Ball JB. Computed tomography minor fissures. AJR 1983;140:439-448.
of the major and
3. Proto AV, Speckman JM. The left lateral radiograph chest. Med Radiogr Photogr 1979;55:30-74.
B FIGURE 12. Parenchymal versus fissural mass. (A) Standard CT demonstrates avascular zone [small arrows) surrounding intrathoracic nodule (large arrow). (B) Thin-section CT (1.5-mm collimation) localizes nodule (large arrow), subsequently proven to be benign fibrous mesothelioma, within right major fissure [small arrows).
of the
4. Raasch BN, Carsky EW, Lane EJ. Radiographic anatomy of the interlobar fissures: A study of 100 specimens. AJR 1982; 138:1043-1049. 5. Proto AV, Ball JB. The superolateral 1983;140:431-437. 6. Proto AV. The chest radiograph: Clin Chest Med 1984;5:213-246.
major fissures.
Anatomic
AJR
considerations.
7. Fisher MS. Significance of a visible major fissure on the frontal chest radiograph. AJR 1981;137:577-580. 8. Fraser RG, Pare JAP. Diagnosis of Diseases of the Chest. Philadelphia: W B Saunders, 1979.
management of a localized pleural mass differs from that of a parenchymal mass. Similar confusion may arise with intrafissural loculation of pleural fluid, commonly called a “psuedotumor.” Orientation along the fissural plane, smooth margins, and tapering edges should aid in diagnosing a loculated effusion, both on plain radiographs and on CT. Parietal pleural diseases can sometimes also affect the visceral pleura. Fissural thickening may indicate asbestos exposure and/or asbestosis (25). Rockoff et al. (25) and Solomon et al. (26) demonstrated a 25%55% incidence of fissural thickening following asbestos exposure, with the greatest incidence occurring in association with pulmonary fibrosis and parietal plaques. These studies suggest that there is a stronger correlation between asbestos exposure and/ or asbestosis with fissural thickening than with pleural plaques. Fissural pleural plaques are usually
9. Mayo JR, Muller NL, Henkelman RM. The double-fissure sign: A motion artifact on thin-section CT scans. Radiology 1987;165:580-581. 10. Felson B. Chest Roentgenology. 1973,~~. 74-81,379.
Philadelphia:
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11. Spizarny DL, Goodman LR. Air in the minor fissure: A sign of right-sided pneumothorax. Radiology 1986;160:329-331. 12. Goodman LR, Golkow RS, Steiner RM. The right mid-lung window: A potential source of error in computed tomography of the lung. Radiology 1982;143:135-138. 13. Chaser-r MH, McCarthy MJ, Gilliland JD, et al. Concepts in computed tomography of the thorax. RadioGraphics 1986; 6:793-832. 14. Be&men YM, Auh YH, Davis SD, et al. Anatomy of the minor fissure: Evaluation with thin-section CT. Radiology 1989; 170:647-651. 15. Frija J, Yana C, Laval-Jeantet M. Anatomy of the minor fissure: Evaluation with thin-section CT. Radiology 1989; 173:571-572. 16. Otsuji H, Hatakeyama M, Kitamura I. Right upper lobe versus right middle lobe. Differentiation with thin-section, high-resolution CT. Radiology 1989;172:653-656.
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RADIOLOGY OF THE PLEURAL FISSURES
17. Godwin JD, Tarver RD. Accessory 1985;144:39-47. 18. Austin JH: 161:433-436.
The
left
minor
fissures of the lung. AJR fissure.
Radiology
1986;
19. Speckman JM, Gamsu G, Webb WR. Alterations in CT mediastinal anatomy produced by an azygous lobe. AJR 1981; 137:47-50. 20. Takasugi JE, Godwin 1989;171:133-134.
JD. Left azygous
lobe.
Radiology
21. Gross BH, Spizarny DL, Granke DS. Sagittal orientation of the anterior minor fissure: Radiography and CT. Radiology 1988;166:717-179.
22. Shuman LS, Libshitz HI. Solid pleural lymphoma. AJR 1984;142:269-273.
manifestations
229
of
23. Quint LE, Glazer GM, Orringer MB. Central lung masses: Prediction with CT of need for pneumonectomy versus lobectomy. Radiology 1987;165:735-738. 24. Spizarny DL, Gross BH, Shepard JO. CT findings in localized fibrous mesothelioma of the pleural fissure. J Comput Assist Tomogr 1986;10:942-944. 25. Rockoff SD, Kagan E, Schwartz A, et al. Visceral pleural thickening in asbestos exposure. J Thorac Imaging 1987;2:58-60. 26. Solomon A, h-wig LM, Sluid-Cremer GK, et al. Thickening of pulmonary interlobar fissures. Br J Ind Med 1979;36:195-198.
