DOI: 10.1148/radiol.2401031739
(Radiology 2006;240:299-300.)
© RSNA, 2006
The Cord Sign1
Ram K. P. Vijay, MBBS, MRCS
1 From the Sheffield Teaching Hospitals NHS Trust, SPR Radiology, Royal Hallamshire Hospital, Glossop Rd, Sheffield S10 2JF, South Yorkshire, England. Received October 29, 2003; revision requested January 19, 2004; revision received May 16; accepted June 2.
Address correspondence to the author (e-mail: rkpvijay{at}gmail.com).
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APPEARANCE
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The cord sign is a homogeneous, hyperattenuating, cordlike appearance on a unenhanced transverse computed tomographic (CT) scan of the brain.
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EXPLANATION
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The cord sign appears as the result of increased attenuation in either the dural sinuses or a vein filled with thrombus (Figure). Thrombosis in veins can be visualized directly on unenhanced scans as foci of increased attenuation in the distribution of the affected veins. For veins perpendicular to the transverse plane (including the cortical vein, the vein of Galen, and parts of the superior sagittal sinus), a round focus of increased attenuation may appear on the successive sections. In veins parallel to the scanning plane (including internal cerebral veins, medullary veins, and the straight sinus), the linear nature of the high attenuation may represent a thrombosed blood vessel or venous sinus. This latter finding, referred to as the cord sign, reflects the fact that intraluminal venous thrombosis behaves much like parenchymal hemorrhage and is therefore hyperattenuating for the 1st week or more after formation.
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DISCUSSION
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The diagnosis of cerebral venous thrombosis (CVT) is often made through analysis of images. The diagnosis is based on confirmation of thrombus within the venous sinuses and identification any secondary effects of thrombosis on brain parenchyma or cerebrospinal fluid pathways.
Wendling reported the diagnosis of CVT with CT, and this method of diagnosis was later confirmed with angiography (1,2). CT made it possible to diagnose CVT noninvasively. With CT, it is possible to identify thrombosed veins, collateral venous channels, and complications of CVT directly (3). Unenhanced CT is the most common initial imaging modality for many of these clinical diagnostic considerations. Unfortunately, unenhanced CT scans may often show only subtle findings or appear normal. Before the advent of magnetic resonance (MR) imaging, conventional CT was the best noninvasive method of diagnosing CVT.
The cord sign was originally found in only a minority of images of patients with cerebral venous thrombosis, and there were doubts about the sign's value in routine diagnosis (4,5). With thinner CT sections, however, this sign was detected much more frequently (4,5). Since the cord sign demonstrates a newly formed thrombus, it will seldom be seen in patients with subacute or chronic disease because the thrombus ages, usually becoming isoattenuating and then hypoattenuating after the first 7–14 days. In other cases, the thrombosed veins are too small for the thrombi to be visualized or are obscured by artifacts from adjacent bone.
In a study by Teasdale (5), the cord sign signifying acute thrombus was present in 18 of 32 (56%) patients with CVT who underwent unenhanced head CT. In a similar study, which looked at 76 cases of CVT (4), venous intraluminal thrombus was demonstrated in 25% cases on unenhanced CT scans. The precise prevalence of different signs of CVT—including parenchymal hemorrhage, infarction, and the empty delta sign—varies greatly from study to study, which reflects the small size and selected nature of the population evaluated. Despite these differences, all investigators report that the diagnosis of CVT is dependent on the detection of a combination of these CT findings because no single finding is present in a majority of cases. It is also agreed that images taken both before and after contrast enhancement are necessary for accurate diagnosis (3). After administration of contrast material, CT may help illustrate further clues toward the diagnosis of CVT, such as venous filling defects (empty delta sign), and venous collateral flow with gyral and tentorial enhancement.
The finding of a cord sign on an unenhanced CT scan has been thought to have low sensitivity and specificity for diagnosis of CVT because flowing venous blood also often appears to be mildly hyperattenuating, especially in children and young adults (6). In addition, because of volume averaging, unenhanced axial CT can fail to depict a hyperattenuating sinus in the horizontal segment of the superior sagittal sinus or a transverse sinus.
The most common finding on MR images is replacement of the flow void within an affected dural sinus by abnormal signal intensity. The signal intensity characteristics on T1- and T2-weighted images vary with the age of the clot. MR imaging is more sensitive than CT with regard to detection of the associated parenchymal changes (such as microhemorrhage), which is one of the hallmarks of venous infarction (5).
MR venography is often used for evaluation of CVT. There are many different MR venographic techniques, including time-of-flight, phase contrast, and dynamic contrast-enhanced MR venography. However, various technical problems—such as motion sensitivity artifacts and insensitivity to slow flow—make the interpretation of images produced by MR venography more complex (5–7).
Cerebral CT venography is a rapid, useful method that consistently yields detailed, high-quality images of intracranial venous circulation. CT venography is less complex than MR venography and is practicable in many acute situations to confirm or exclude a diagnosis of thrombosis. CT venography is also a helpful problem solver when MR imaging or MR venography give equivocal results for thrombosis.
Conventional digital subtraction angiography has been considered the standard modality for diagnosing CVT. Advantages of CT venography over digital subtraction angiography include reduced cost, reduced invasiveness, and a minimized diagnosis time. CT venography is free of the flow-related contrast material–mixing phenomena that can be seen in conventional angiography.
In summary, the cord sign is one of the important clues in the diagnosis of CVT on an unenhanced CT scan.
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ACKNOWLEDGMENTS
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My sincere thanks to D. J. A. Connolly, MBChB, BSc, MRCP, FRCR, for his support in the preparation and editing of the manuscript.
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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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- Wendling LR. Intracranial venous sinus thrombosis: diagnosis suggested by computed tomography. AJR Am J Roentgenol 1978;130(5):978–980.[Medline]
- Patronas NJ, Duda EE, Mirfakhraee M, Wollmann RL. Superior sagittal sinus thrombosis diagnosed by CT. Surg Neurol 1981;15(1):11–14.[CrossRef][Medline]
- Zimmerman RD, Ernst RJ. Neuroimaging of cerebral venous thrombosis. Neuroimaging Clin N Am 1992;2:463–485.
- Virapongse C, Cazenave C, Quisling R, Sarwar M, Hunter S. The empty delta sign: frequency and significance in 76 cases of dural sinus thrombosis. Radiology 1987;162(3):779–785.[Abstract/Free Full Text]
- Teasdale E. Cerebral venous thrombosis: making the most of imaging. J R Soc Med 2000;93(5):234–237.[Free Full Text]
- Provenzale JM, Joseph GJ, Barboriak DP. Dural sinus thrombosis: findings on CT and MR imaging and diagnostic pitfalls. AJR Am J Roentgenol 1998;170(3):777–783.[Free Full Text]
- Connolly DJ, Birchall D, Gholkar A. Current theory in imaging of intracranial vascular disease. Imaging 2002;14:396–408.[Abstract/Free Full Text]
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