(Radiographics. 2001;21:861-874.)
© RSNA, 2001
Radiographic and CT Appearances of the Major Fissures1
Kuniaki Hayashi, MD,
Aamer Aziz, MD,
Kazuto Ashizawa, MD,
Hideyuki Hayashi, MD,
Kenji Nagaoki, MD and
Hideaki Otsuji, MD
1 From the Department of Radiology, Nagasaki University School of Medicine, Sakamoto 1-7-1, Nagasaki 852-8501, Japan (K.H., A.A., K.A., H.H., K.N.); and the Department of Radiology, Saiseikai Suita Hospital, Osaka, Japan (H.O.). Recipient of an Excellence in Design award for a scientific exhibit at the 1999 RSNA scientific assembly. Received February 6, 2001; revision requested March 7 and received April 24; accepted April 25. Address correspondence to K.H. (e-mail: hayashik@net.nagasaki-u.ac.jp).
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Abstract
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The major fissure is an important anatomic landmark in the interpretation of chest radiographs and computed tomographic (CT) scans. At radiography, the major fissures normally appear as hairlines of soft-tissue density; at conventional CT, they typically appear as lucent, hypovascular bands; and at high-resolution CT, they most often appear as sharp lines. The superolateral major fissure usually manifests as a curving edge at the upper lateral lung field with lateral opacity and medial lucency. The vertical fissure line appears as a fine, linear shadow, commencing in or near the costophrenic angle and coursing upward. The superomedial major fissure manifests as a short, obliquely oriented straight line. Progressive widening of the major fissure inferiorly manifests as a triangular area of increased opacity and represents intrafissural fat. Various inflammatory, granulomatous, neoplastic, and abnormal hemodynamic conditions involving the major fissure can affect its imaging appearance. Oblique orientation of the major fissure may complicate radiographic interpretation. The fissure may be incomplete or absent, complicating identification of various diseases. An incomplete major fissure may lead to disease spread, collateral air drift, or the "incomplete fissure sign," a sign that may, however, also be present in cases of complete fissure. Knowledge of the anatomy and normal variants of the major fissures is essential for recognizing their variable imaging appearances as well as related abnormalities.
Index Terms: Lung, anatomy, 60.92 • Lung, collapse, 60.74 • Lung, CT, 60.1211 • Lung, diseases, 60.**
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Discuss the radiographic and CT findings in the major fissures and their normal variants.
- Describe the effects of various diseases on the imaging appearance of the major fissures.
- Discuss the CT appearances and clinical significance of incomplete major fissure.
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Introduction
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The major fissure (also known as the oblique or greater fissure) is an important anatomic landmark in the interpretation of chest radiographs and computed tomographic (CT) scans. Radiologic identification of a lesion in relation to the major fissure is important for precise localization of the lesion to the anatomic pulmonary lobes. In this article, we describe the radiographic and CT appearances of the normal major fissures and normal variants (including superolateral and superomedial major fissures, vertical fissure line, and intrafissural fat) and related abnormalities (adjacent infiltrate or atelectasis; intrafissural effusion, air, or tumor; distortion by tumor, granulomatous disease, or fibrosis). We also discuss and illustrate imaging pitfalls due to oblique orientation of the major fissure and the radiologic appearance and clinical implications of incomplete major fissure.
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Normal Anatomy and Normal Variants
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Knowledge of the anatomy of the major fissures is essential for understanding their variable appearances at chest radiography and CT and recognizing various abnormal findings (1–3).
The normal major fissures consist of double layers of infolded invaginations of the visceral pleura. The major fissures separate the lower pulmonary lobes from the upper lobe on the left and from the upper and middle lobes on the right. The right major fissure is shorter and wider than the left one and has a greater overall area, the left fissure starting a little cephalad and terminating slightly posterior to the right fissure. Both fissures face anteriorly and pass obliquely downward along the fifth rib, having an undulating course with a propeller-like configuration (Fig 1). The upper part of the fissure is concave anteriorly and faces laterally, whereas the lower part is convex anteriorly and faces medially. However, there is considerable variation in the appearances of the major fissures, and the fissures may not always have the classical propeller-like configuration, especially on the right side.
The major fissures are not oriented tangential to the x-ray beam; thus, they are not normally visualized at posteroanterior chest radiography and are visualized only up to a variable length at lateral chest radiography. However, they may be visualized at posteroanterior or lateral chest radiography when some portion becomes tangential to the x-ray beam. In such cases, the major fissures are normally very thin and appear as hairlines of soft-tissue density (1). In addition, the major fissure may be visualized when it contains fluid, fat, air, or tumor or becomes thickened due to various causes. It may also be visualized as an interface when it abuts intrapulmonary disease (4). It is rare to see the entire length of the major fissure because of its undulating course (5), although it can sometimes be seen as double lines. In addition, the more posterior location of the left major fissure may sometimes be appreciated at lateral radiography (Fig 2).
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Figure 2. Normal major fissures. Lateral chest radiograph demonstrates the two major fissures. Note that both fissures are seen as double lines and that the left fissure (arrows) is behind the right fissure (arrowheads).
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At conventional CT, the major fissures are visualized most frequently as lucent bands devoid of vascularity (Fig 3). Less commonly, they may appear as lines or dense bands (Fig 4) (6). At high-resolution CT, the major fissures most often appear as sharp lines (7,8): The upper parts are visualized with the lateral margin posterior to the medial margins; the opposite is true for the lower parts (Fig 5).
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Figure 5a. Normal major fissures. High-resolution CT scans (b obtained at a lower level than a) show the major fissures as lines (arrows). Note that their orientation changes as they pass from superior to inferior.
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Figure 5b. Normal major fissures. High-resolution CT scans (b obtained at a lower level than a) show the major fissures as lines (arrows). Note that their orientation changes as they pass from superior to inferior.
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Superolateral Major Fissure
On posteroanterior chest radiographs, the major fissure may be visualized as a curving contour, line, or edge at the upper lateral lung field in either hemithorax (Fig 6a). Most often, it appears as a curving edge with lateral opacity and medial lucency. This finding represents the major fissure coursing laterally along the superior segment of the lower lobe. The left superolateral major fissure almost always extends slightly higher than the right one (level of the fourth rib versus level of the fifth rib). Both contours approach the lateral chest wall in the vicinity of the sixth rib. This finding is attributed to extrapleural fat (9). The extrapleural costal fat is oriented along the long axis from the fourth to eighth ribs and parallels or hangs as a flap from the rib. The pliable fat is compressed between the upper lobe and the superior segment of the lower lobe; it is thereby drawn into the major fissure plane and enters the lips of the major fissure (Fig 6b, 6c) (9). The x-ray beam is tangential to the point of contact between the fat and the superior segment of the lower lobe, resulting in medial lucency and lateral opacity. In a study of more than 1,000 posteroanterior chest radiographs by Proto and Ball (9), the superolateral fissure could be seen in 14% of cases. It was noted on the right side only in 4% of cases, on the left side only in 6%, and bilaterally in 4%.
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Figure 6a. Superolateral major fissures. (a) Posteroanterior radiograph shows the superolateral major fissures as curving contours with lateral opacity and medial lucency bilaterally (arrows). Note that the left fissure extends higher than the right one. (b, c) High-resolution CT scans (c obtained at a lower level than b) show extrapleural fat entering the lips of the major fissures (arrow).
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Figure 6b. Superolateral major fissures. (a) Posteroanterior radiograph shows the superolateral major fissures as curving contours with lateral opacity and medial lucency bilaterally (arrows). Note that the left fissure extends higher than the right one. (b, c) High-resolution CT scans (c obtained at a lower level than b) show extrapleural fat entering the lips of the major fissures (arrow).
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Figure 6c. Superolateral major fissures. (a) Posteroanterior radiograph shows the superolateral major fissures as curving contours with lateral opacity and medial lucency bilaterally (arrows). Note that the left fissure extends higher than the right one. (b, c) High-resolution CT scans (c obtained at a lower level than b) show extrapleural fat entering the lips of the major fissures (arrow).
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Vertical Fissure Line
Vertical fissure line was first described by Davis in 1960 (10). It is seen as a fine (seldom > 1 mm in width), linear shadow, commencing in or somewhat medial to the costophrenic angle and coursing upward and inclining slightly toward the hilum to end at or below the minor fissure (Fig 7). Vertical fissure line is more often seen on the right side but can be seen on the left side as well (Fig 8). It has been reported in infants and children with cardiac disease and may occur with cardiomegaly or respiratory infection, which rotates or partially collapses the lower lobe. This finding may indicate partial volume loss or impaired ventilation in the lower lobe (11). It may also be seen in healthy adults (12). We have seen left-sided vertical fissure line in a patient with transient heart failure (Fig 8).
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Figure 7. Vertical fissure line in a child with a ventricular septal defect. Chest radiograph demonstrates the right major fissure as a curving line in the right lower lung field (arrows).
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Figure 8. Vertical fissure line in a woman with abruptio placentae and transient heart failure. Chest radiograph shows the left major fissure as a curving line in the left middle and lower lung fields (arrows).
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Superomedial Major Fissure
Superomedial major fissure manifests as a relatively short, obliquely oriented straight line (2). The right superomedial fissure is found in the vicinity of the right tracheobronchial angle near the azygos vein. The left superomedial fissure is found in the vicinity of the aortic knob and extends slightly higher than the right one. It represents a specific coronal level of the major fissure that is tangential to the posteroanterior x-ray beam (Fig 9). Increased opacity above or below this line indicates upper or lower lobe disease, respectively. However, effusion loculated in this part of the major fissure may mimic upper lobe disease because it manifests as increased opacity abutting the fissure line superiorly. In a study of 1,000 patients by Proto (2), the superomedial major fissure was seen in 8% of cases.
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Figure 9a. Superomedial major fissure. (a) Chest radiograph shows a superomedial right major fissure (arrows). The minor fissure is seen as double lines. (b) High-resolution CT scan helps confirm the direction of the medial portion of the right major fissure as tangential to the x-ray beam (arrow).
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Figure 9b. Superomedial major fissure. (a) Chest radiograph shows a superomedial right major fissure (arrows). The minor fissure is seen as double lines. (b) High-resolution CT scan helps confirm the direction of the medial portion of the right major fissure as tangential to the x-ray beam (arrow).
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Intrafissural Fat
A major fissure may demonstrate progressive widening inferiorly that appears as a triangular area of increased opacity at lateral chest radiography. This triangular area represents intrafissural fat and abuts the anterior diaphragmatic surface at the base and tapers into the major fissure at the apex (13). It can be confused with right middle lobe collapse (Fig 10).
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Figure 10a. Intrafissural fat. (a) Lateral chest radiograph shows a triangular area of increased opacity (arrows) mimicking right middle lobe collapse. (b) CT scan reveals that this triangular area represents extrapleural fat extending into the right major fissure (arrow).
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Figure 10b. Intrafissural fat. (a) Lateral chest radiograph shows a triangular area of increased opacity (arrows) mimicking right middle lobe collapse. (b) CT scan reveals that this triangular area represents extrapleural fat extending into the right major fissure (arrow).
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Abnormal Conditions
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A number of inflammatory, granulomatous, and neoplastic diseases as well as abnormal hemodynamic conditions involving the major fissure can affect the imaging appearance of the major fissure.
Adjacent Infiltrate or Atelectasis
A major fissure that is adjacent to a lobe may abut consolidation of all or part of the lobe. This creates an interface between the area of increased opacity due to consolidation on one side of the major fissure and the normally aerated adjacent lobe on the other. Conventional radiography, especially lateral views, may demonstrate the position of the major fissure forming the interface (Figs 11, 12).
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Figure 11a. Lobar pneumonia. Posteroanterior (a) and lateral (b) chest radiographs demonstrate lobar pneumonia of the left upper lobe. The left major fissure is seen as an interface on the lateral view (arrows).
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Figure 11b. Lobar pneumonia. Posteroanterior (a) and lateral (b) chest radiographs demonstrate lobar pneumonia of the left upper lobe. The left major fissure is seen as an interface on the lateral view (arrows).
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Figure 12a. Lobar pneumonia. Posteroanterior (a) and lateral (b) chest radiographs demonstrate lobar pneumonia of the left lower lobe. The left major fissure is seen as an interface on the lateral view (arrows) (cf Fig 11b).
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Figure 12b. Lobar pneumonia. Posteroanterior (a) and lateral (b) chest radiographs demonstrate lobar pneumonia of the left lower lobe. The left major fissure is seen as an interface on the lateral view (arrows) (cf Fig 11b).
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Volume loss of either the upper or lower lobe may shift and distort the major fissure, which separates the two lobes. The major fissure can be identified as the displaced edge of a high-density collapsed lobe.
Intrafissural Effusion, Air, or Tumor
Pleural effusion may extend into the major fissures and give them a characteristic appearance. Pleural fluid may loculate within the fissures, particularly in heart failure, and manifest as vanishing or phantom tumors (pseudotumors).
Interlobar pneumothorax may also occur in the major fissure, giving it a cystic appearance (14) or producing an air-fluid level. CT is sometimes necessary for the diagnosis (Fig 13).
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Figure 13a. Interlobar hydropneumothorax. (a) Posteroanterior chest radiograph shows a rounded area with an air-fluid level in the right lower lung field (arrow). (b) High-resolution CT scan shows a collection of air and fluid in the right major fissure (*), a finding that was confirmed on other views (not shown).
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Figure 13b. Interlobar hydropneumothorax. (a) Posteroanterior chest radiograph shows a rounded area with an air-fluid level in the right lower lung field (arrow). (b) High-resolution CT scan shows a collection of air and fluid in the right major fissure (*), a finding that was confirmed on other views (not shown).
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In addition, benign and malignant tumors of the pleura may originate in the major fissures (15,16). The relationship of the mass to the pleura may easily be appreciated at CT (Fig 14).
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Figure 14a. Localized fibrous tumor of the pleura. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show a lobulated mass in the left middle lung field. (c) High-resolution CT scan demonstrates the relationship of the lesion to the major fissure (arrows).
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Figure 14b. Localized fibrous tumor of the pleura. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show a lobulated mass in the left middle lung field. (c) High-resolution CT scan demonstrates the relationship of the lesion to the major fissure (arrows).
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Figure 14c. Localized fibrous tumor of the pleura. (a, b) Posteroanterior (a) and lateral (b) chest radiographs show a lobulated mass in the left middle lung field. (c) High-resolution CT scan demonstrates the relationship of the lesion to the major fissure (arrows).
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Distortion by Tumor, Granulomatous Disease, or Fibrosis
Any pulmonary diseases that cause anatomic change, fibrosis, or scarring may distort the major fissures. Adjacent lung carcinoma may cause indentation of the fissures. They can also invade and even cross over into the adjacent lobes through the fissures (Fig 15). Multiplanar reformatted spiral imaging has a very high sensitivity to lung cancer involvement of the major fissures (17). The perilymphatic manifestation of sarcoidosis as nodules along the major and minor fissures is sometimes well depicted at high-resolution CT (Fig 16).
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Imaging Pitfalls Due to Oblique Orientation
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On a lateral chest radiograph, the major fissure can be seen as a continuous oblique line. This actually represents continuous levels of the fissure from lateral to medial along its plane. Thus, the portion of the lung immediately behind the major fissure may not always represent the lower lobe. A small nodule in the upper lobe adjacent to the major fissure may be projected posteriorly on a lateral chest radiograph (2). Because of the undulating course of the major fissure, the minor fissure sometimes appears to be crossing it on a lateral view (Fig 17). In one case we encountered that involved complete right middle lobe collapse, the major fissure was projected more anteriorly than the minor fissure on the lateral view (Fig 18).
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Figure 17. Minor fissure "crossing" a major fissure. Lateral radiograph shows a minor fissure (arrowheads) that appears to cross a major fissure (arrows) and extends further posteriorly.
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Figure 18a. Right middle lobe collapse (middle lobe syndrome). On a lateral radiograph (b) as well as two CT scans (c, d), the major fissure (arrows) is anterior to the minor fissure (arrowheads). Incorrect localization of a pulmonary nodule may result from the same mechanism.
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Figure 18b. Right middle lobe collapse (middle lobe syndrome). On a lateral radiograph (b) as well as two CT scans (c, d), the major fissure (arrows) is anterior to the minor fissure (arrowheads). Incorrect localization of a pulmonary nodule may result from the same mechanism.
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Figure 18c. Right middle lobe collapse (middle lobe syndrome). On a lateral radiograph (b) as well as two CT scans (c, d), the major fissure (arrows) is anterior to the minor fissure (arrowheads). Incorrect localization of a pulmonary nodule may result from the same mechanism.
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Figure 18d. Right middle lobe collapse (middle lobe syndrome). On a lateral radiograph (b) as well as two CT scans (c, d), the major fissure (arrows) is anterior to the minor fissure (arrowheads). Incorrect localization of a pulmonary nodule may result from the same mechanism.
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Incomplete Major Fissure
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The major fissures as well as the minor fissures are often incomplete, and the lung parenchyma of the adjacent pulmonary lobes fuse. Studies differ as to how often the fissure is incomplete (Table) (3,7,18–21). When part of the fissure is absent, it is always toward the mediastinal side of the fissure. The major fissures are usually incomplete near the hilar area and in the upper parts. The degree of incompleteness may range from falling just short of the mediastinum to almost complete absence. Incomplete fissure may lead to the spread of disease and collateral air drift. Incomplete major fissure also affects the distribution pattern of pleural effusion. Familiarity with the appearance and implications of incomplete fissure is important for planning of lobar resection because there is a higher prevalence of air leak in lobar fusion. The surgeon can avoid air leak by properly clamping the fused pulmonary lobe segments (22). The bronchopulmonary segments associated with incomplete major fissure have been well described (3,21).
Spread of Disease
Incomplete major fissure is of major clinical concern because disease processes such as pneumonia may spread from one lobe to the adjacent lobe (Fig 19) via the pores of Kohn and canals of Lambert in the fused area.
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Figure 19a. Pneumonia. (a) CT scan demonstrates pneumonia primarily involving the right upper lobe and extending into the lower lobe across the incomplete part of the major fissure (arrow). (b) CT scan obtained just caudad to a clearly delineates incompleteness of the right major fissure (arrow).
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Figure 19b. Pneumonia. (a) CT scan demonstrates pneumonia primarily involving the right upper lobe and extending into the lower lobe across the incomplete part of the major fissure (arrow). (b) CT scan obtained just caudad to a clearly delineates incompleteness of the right major fissure (arrow).
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Collateral Air Drift
Like disease processes, collateral air drift is believed to occur via the pores of Kohn and canals of Lambert in the fused area. Collateral air drift becomes evident and clinically significant if one lobe is collapsed due to obstruction of the lobar bronchus. The portion of the collapsed lobe adjacent to and fused with the unobstructed lobe may be kept aerated by the airflow across the incomplete major fissure (Fig 20).
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Figure 20a. Squamous cell carcinoma with right upper lobe collapse. (a) Posteroanterior chest radiograph shows a large tumor associated with collapse of the right upper lobe. (b) CT scan shows a portion of the right upper lobe with aeration (*) from the right lower lobe through the incomplete major fissure (arrow).
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Figure 20b. Squamous cell carcinoma with right upper lobe collapse. (a) Posteroanterior chest radiograph shows a large tumor associated with collapse of the right upper lobe. (b) CT scan shows a portion of the right upper lobe with aeration (*) from the right lower lobe through the incomplete major fissure (arrow).
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Reconfiguration of Pleural Effusion
The complex radiographic appearances of pleural effusion have inspired much debate. Perihilar lucency, curvilinear demarcation, and lateral opacity indicate fluid extending into the major fissure and have a composite appearance that has been classically described as the "incomplete fissure sign." This term implies that the pleural effusion is bound by the edge of the incompleteness of the fissure. The incomplete fissure sign was first described by Dandy in 1978 (23); however, some authors have pointed out that this sign may be seen even when the major fissure is complete. In such cases, the presence of the sign may be due to the fact that the pleural effusion is bound by the lateral border of the lower lobe (Fig 21) (24).
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Figure 21a. Incomplete fissure sign. (a) Posteroanterior radiograph shows a curving interface in the lateral part of the left lower lung field (arrows), with lateral opacity and medial lucency. (b, c) High-resolution CT scans (c obtained at a lower level than b) show bilateral pleural effusion. The left pleural effusion is bounded by the lateral border of the lower lobe (arrow), which may explain the appearance on the radiograph. The major fissures are complete; thus, "incomplete fissure sign" may also be seen in cases of complete major fissure.
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Figure 21b. Incomplete fissure sign. (a) Posteroanterior radiograph shows a curving interface in the lateral part of the left lower lung field (arrows), with lateral opacity and medial lucency. (b, c) High-resolution CT scans (c obtained at a lower level than b) show bilateral pleural effusion. The left pleural effusion is bounded by the lateral border of the lower lobe (arrow), which may explain the appearance on the radiograph. The major fissures are complete; thus, "incomplete fissure sign" may also be seen in cases of complete major fissure.
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Figure 21c. Incomplete fissure sign. (a) Posteroanterior radiograph shows a curving interface in the lateral part of the left lower lung field (arrows), with lateral opacity and medial lucency. (b, c) High-resolution CT scans (c obtained at a lower level than b) show bilateral pleural effusion. The left pleural effusion is bounded by the lateral border of the lower lobe (arrow), which may explain the appearance on the radiograph. The major fissures are complete; thus, "incomplete fissure sign" may also be seen in cases of complete major fissure.
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The appearance of right-sided pleural effusion may be very complicated, and the terms fissural complex and middle lobe step have been suggested. The latter term implies the presence of pleural effusion having a flat top and lying inferior to the minor fissure. This may occur when fluid accumulates lateral to the middle or lower lobes and extends to the undersurface of the upper lobe (24). This complex configuration of right-sided pleural effusion requires further investigation, including that of the relationship between the effusion and the incompleteness of the fissure.
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Summary
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Normal major fissures may appear as superolateral or superomedial fissure lines or edges, vertical fissure lines, or intrafissural fat. They may also be visualized when they abut infiltrates; are rotated out of their normal planes by volume loss or overinflation of adjacent pulmonary segments; contain effusion, air, or tumor; or are involved by tumors, granulomatous disease, or fibrosis. The major fissures may be incomplete or absent, thus complicating identification of various pathologic conditions. A disease process may spread to adjacent lobes through the area of pulmonary fusion, and part of a collapsed lobe may demonstrate collateral air drift. Interlobar pleural effusion may have a characteristic appearance (incomplete fissure sign) when the major fissure is incomplete, although this sign may also be present in cases of complete fissure.
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Footnotes
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** indicates entire organ system or region.
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References
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M. G. Lubner
The Incomplete Fissure Sign
Radiology,
May 1, 2008;
247(2):
589 - 590.
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Abstract
LEARNING OBJECTIVES
