(Radiology. 1999;213:59-60.)
© RSNA, 1999
The Fissure Sign1
Jugesh I. S. Cheema, MD
1 From the Departments of Radiology and Nuclear Medicine, Christiana Care Health System, Christiana Hospital, 4755 Ogletown Stanton Rd, Newark, DE 19718. Received April 30, 1998; revision requested June 25; revision received August 6; accepted December 7. Address reprint requests to the author (e-mail: jcheema@rocketmail.com).
Index terms: Embolism, pulmonary, 60.721 • Lung, perfusion, 60.12171 • Lung, radionuclide studies, 60.12171 • Pleura, fluid, 66.76 • Signs in imaging
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APPEARANCE
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The linear area of reduced or absent radionuclide uptake along the distribution of the major and/or minor fissure on a lung perfusion scan is referred to as the fissure sign (Figure).
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Figure 1. Perfusion scan (left lateral view) obtained in a 56-year-old woman shows linear areas of hypoperfusion consistent with the fissure sign (arrows). On a chest radiograph obtained in this patient on the same day this scan was obtained, a small pleural effusion was seen on the left side. Ant = anterior, L = left, Post = posterior.
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EXPLANATION
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Many conditions can cause the fissure sign, but peripheral lung hypoperfusion adjacent to the fissures is the common explanation (1,2).
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DISCUSSION
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The normal separation of the pulmonary lobes by the pleura cannot be detected by using imaging devices because of limited spatial resolution (1). However, an increased separation of the lobes due to pleural thickening or pleural effusion will contribute to the depiction of the fissure sign. The reduction in peripheral perfusion in two adjoining lobes might be large enough to be identified as a photopenic area by using a scintillation detector.
The most common entity associated with the fissure sign is pleural effusion. Pleural fluid eliminates the normally negative pressure of the pleural space. The resultant positive pressure increases the peripheral blood flow resistance and thus causes hypoperfusion (1). A logical extension of the fissure sign is "segmental contouring," which is an extension of the fissure sign to the segmental level, with peripheral hypoperfusion occurring at the boundaries of the two adjacent segments. The fissure sign is more commonly seen on perfusion scans obtained with the patient in a supine position. When the patient undergoes scanning in the upright position, the pleural fluid is more likely to be dependent beneath the lung owing to the effect of gravity. Congestive heart failure and pulmonary edema also might cause the fissure sign because of the associated pleural effusions (1).
Pleural thickening leads to the fissure sign by causing increased separation and severely reducing peripheral compliance, with the result being hypoperfusion. Chronic obstructive pulmonary disease results in a compression of the septal capillaries that is most marked in the subpleural area and thus leads to the development of the fissure sign (1).
In a few autopsy-proved cases, microemboli have been associated with the fissure sign (1). Some of these cases were proved also by using clinical data and pulmonary angiography (3). In these cases, the fissure sign was a result of a mechanical blockage of small vessels.
Radiographic confirmation of fluid in the fissure may not always be possible, either because of a small volume of fluid or because of factors relating to the patient's position immediately before and at the time of chest radiography.
Pulmonary angiography has not been very useful in explaining this perfusion pattern. This is not surprising, because lung scans depict capillary flow better, whereas angiograms depict larger vessels better. In addition, embolism involving less than 40% of the lung is better seen by using perfusion scanning; this is also the case with the fissure sign (4).
In most cases, the fissure sign is not associated with pulmonary embolism. In the majority of cases, its presence indicates pleural effusion; however, in a minority of cases, it may indicate the presence of microemboli. Such emboli are uncommonly thromboembolic materials; they more often consist of fat, tumor, or infected particulates (eg, those in drug abusers).
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Footnotes
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A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
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References
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James AE, Conway JJ, Chang CH, Cooper M, White RI, Strauss HW. The fissure sign: its multiple causes. Am J Roentgenol Radium Ther Nucl Med 1971; 111:492-500.[Medline]
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Chandler HL. Fissure sign in lung perfusion scintigrams. J Nucl Med 1971; 12:326-327.[Free Full Text]
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Gize R, Dizon M, Mishkin F. Analysis of the fissure sign. J Nucl Med 1971; 12:822-824.[Abstract/Free Full Text]
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Fraser R, Pare JAP, Fraser R, Pare PD. Synopsis of diseases of chest 2nd ed. Philadelphia, Pa: Saunders, 1994; 539-573.
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